Graded index aspheric combiners and display system utilizing same

GRADED INDEX ASPHERIC COMBINERS AND DISPLAY SYSTEM UTILIZING SAME

HUGHES AIRCRAFT COMPANY

C: 04176

GRADED INDEX ASPHERIC COMBINERS AND DISPLAY SYSTEM UTILIZING SAME

This application is a continuation-in-part of U.S. Patent No. 4,545,646 filed September 2,

1983, entitled Process for Forming a Graded Index Optical Material and Structures Formed Thereby,“ and U.S. Patent No. 5,035,474,־filed April 16,

1984, entitled Biocular Holographic Helmet Mounted Display, each application assigned to the same assignee as this application.

BACKGROUND OF THE INVENTION

This invention relates generally to reflective optical materials, and more particularly to reflective diffraction and interference-type optical elements, such as optical filters and combiners, which are used, for example, in head-up displays or helmet-mounted visor displays .

I . 79391/3.

i.

k . - ‘ / ן In various optical systems» It 18 often necessary to provide a filter in order to remove undesired / radiation while at the same time allowing desired radiation to be efficiently transmitted or reflected.

Such filters and coatings are used» for example, to provide protection from laser radiation for personnel, for electro-optical detectors, and for optical mirrors in a laser system, as a holographic lens In a head-up display system, or In night vision devices. The optical filters currently used for such purposes include absorption filters, reflective multiple layer dielectric filters, and diffraction filters generated by optical holographic techniques. However*,, each of these approaches for providing optical filters has} certain disadvantages, as discussed below.

The absorption filter comprlsds’a material which THIS PAGE: . ¹³ impregnated ^wlth absorption dyes or materials with

July 11, 1906 intrinsic absorption at the wavelength of the incoming laser radiation, as described, for example, in the book entitled Handbook of Optics, W. G. Driscoll, ed., McGraw-Hill .Book Co., New York, 1970, in Section 8 (Coatings and Filters), at pages 7. to 32. This type of protection has the serious disadvantage that the absorbing dye decreases the amount of transmitted radiation to unacceptably low levels. In addition, for laser applications, as the laser radiation energy increases, the radiation can damage the protective filter itself. *'!

ΐ . . 1

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reflective multiple layer dielectric filters consist of alternate layers of two dielectric materials of different refractive indices, which are ί THIS

July

PAGE: 11, 1986 formed on the surface of a substrate by known deposition techniques, such as chemical vapor deposition, sputtering, or thermal evaporation. When the optical thickness of each layer is chosen to be one-quarter of the wavelength of the radiation being reflected, such a structure is referred to as a quarterwave stack, as discussed, for example, in U.S, Patent No. 4,309,075 and in the book entitled Handbook of Optics, previously referenced, in particular in Section 8. However, there are limitations on the spectral bandwidths which can be achieved by such structures, because of the limited material combinations available and the resulting restriction on the choices of index modulations. Moreover, defects at the’abrupt interfaces between the layers in a multilayer, structure can cause unwanted optical scattering. In addition, these defects can cause excessive absorption of radiation by the dielectric material, which can result in thermal damage to the optical filter. Furthermore, in a multilayer dielectric coating, the electric field is strongest at the interface regions between the high index material and the low index material. This highly localized field occurring at the abrupt interfaces can produce maximum temperature increases. Since the thermal expansion coefficients are different for the different dielectric materials of adjacent layers, high thermal stress is developed at the interface regions, which could cause delamination of the successive layers in the film. In addition, the high thermal stress could create microscopic dislocations which result in unwanted optical scattering by the film. Further, substrate roughness, pinholes and contaminants in the conventional multilayer structures formed by evaporation or sputtering techniques increase absorption and scattering, generate localized heating, reduce maximum reflectivity, and increase radiation damage. Finally, these multilayer coatings exhibit reflectance peaks at multiple wavelengths, which causes reduced optical transmission.

Diffraction optical elements have been generated using known methods of optical holography in photosensitive gelatin material, as discussed, for example, in the book entitled Optical Holography״,. by Collier, Burckhardt, and Lin, Academic Press, New York, 1971, Chapter 9 (Diffraction from Volume Holograms) .and Chapter 10 (Hologram Recording Materials), as well as in the book entitled Handbook of Optical Holography, ;by. Caulfield, Academic Press, New York, 1979, Chapter 10 (Application Areas). However, gelatin diffraction elements have environmental stability problems and are susceptible to degradation by humidity and heat. In order to overcome this problem, a protective layer such as glass or a glass-like coating can be used, but such a layer complicates the manufacturing process and adds to unit cost. Moreover, such gelatin filters are limited to use for radiation in the wavelength range from the visible to the near infrared (i.e., up to about 2 microns) since sensitized gelatin is not sensitive to longer wavelength exposures. Consequently, filters for infrared applications cannot be fabricated in a gelatin structure, in addition, the index modulation in the gelatin, which is produced by exposure to the holographic interference pattern and subsequent development, is limited to a shape approximating a sinusoidal

79M./Z configuration or a roughly superimposed multiple sinusoidal configuration. Furthermore, the fabrication of a gelatin filter requires numerous steps, in particular numerous wet chemical steps for development, which are 5 sensitive to processing variables, such as temperature or vibration, that affect the efficiency and peak wavelength of the final structure. In addition, since the resistance of gelatin to damage by heat־or radiation is relatively low, gelatin filters are limited to low 10 power applications. Finally, fabrication of a filter which reflects radiation at two selected wavelengths requires multiple exposure of the gelatin-to two holographic patterns, which produces an irregular , index profile that reduces the efficiency of the filter. T5 One general application in which gelatin filters have heretofore been employed is. that of the optical combiner element of a reflective display, such as a head-up׳ display (HUD) or helmet visor display (HVD) commonly used in aircraft display systems. U.S. Patent 20 3,940,204 discloses exemplary HUD and HVD systems. The laminated gelatin holographic combiner employed for these applications typically comprises a spherical plastic substrate to which are bonded successive layers of . glass, the gelatin hologram, glass, plastic and an 25 antireflective (AR) coating. The glass layers sandwiching the gelatin are required to protect the gelatin from degradation by humidity. As a result of the multiple layers, strong undesirable ghost images may be produced by the gelatin holographic combiners.

Combiners for display systems can be designed to compensate or balance aberrations in the display system. The compensation may comprise the implementation of aspheric reflective layers or surfaces. With the state of the current technology it is not economically ׳.feasible, on a production basis, to provide glass layers or substrates with aspheric surfaces. Instead the required asphericity is incorporated into the gelatin hologram itself,' which means that the fringes will be slanted varying degrees with respect to the gelatin surfaces. This creates a grating at the hologram surface and results in a phenomenon known as chromatic dispersion, wherein the direction of light diffracted from the ^hologram is wavelength dependent. In a holographic display such as the HUD or HVD, if the display light source has any appreciable spectral bandwidth, chromatic dispersion will blur the image at the exit pupil, perhaps to an unacceptable level. Even with narrow band light sources, such as a cathode ray tube (CRT) with P43 phosphor, the fringe slant in some areas of the hologram may be large enough to cause significant dispersion-induced degradation of the image. Slant fringes may also result in flare, a condition in which extraneous diffraction images are produced. The extraneous diffraction may obscure the field of view.

A gelatin holographic combiner for a HUD or visor display is relatively complex and expensive to fabricate. For example, a typical gelatin holographic visor having impact resistance consists of a multi-layer laminant in which the gelatin hologram is sandwiched between two pieces of glass for humidity protection and then laminated between two pieces of polycarbonate visor for impact requirements. Antireflective coatings are applied to the respective outer surfaces of the polycarbonate pieces. The multiple laminate adds weight and complexity to the system. The gelatin holographic HUD combiner is similarly complex and heavy.

The weight of the combiner is an important consideration in the weight-critical cockpit environment. As a result of the relatively high weight of the visor gelatin hologram combiner in an HVD, the visor display center of mass is moved away from the pivot point of the pilot's head» so that the burden on his neck is increased. The increased cantilevered mass in the HUD gelatin hologram combiner decreases the combiner stiffness and resistance to vibration.

SUMMARY OF THE INVENTION Λ

The present invention provides an improved op_tical combiner which is useful in many applications, including HVD~and HUD applications. The combiner comprises a substrate having at leas.t_o.ne')aspheric surface and a ״ diffraction-type ^reflective coating) formed on this surface for selectively reflecting radiation within one ✓ך or more predetermined narrow band ranges of wavelengths impinging on the coating. The asphericity of the I surface may be selected to compensate or balance optical \ aberrations in the display system. The coating may 1 comprise a gelatin hologram, but preferably is a graded- 1 index coating which eliminates the potential degradation of' gelatin holograms by a humid environment and eliminates the significant difficulties encountered in attempting to apply an even gelatin coating to an aspheric surface.

The variation in the index of refraction may occur throughout the thickness of the coating and across the horizontal and lateral extent of the coating as well. A non-sinusoidal variation in the index profile throughout the thickness of the coating can produce a broadened peak in the spectral reflectivity function, as well as multiple peaks. This feature provides the capability of combiners with multiple color reflectivity responses, or which allow use of display light sources of wider spectral bandwidth, resulting in a brighter image and improved angular bandwidth.

In one embodiment the combiner is incorporated in .a biocular helmet visor display resulting in improved optical performance, significant weight savings and a simpler; lower cost combiner structure. In another embodiment the combiner is incorporated into a head-up display for an aircraft resulting in improved optical performance, lower weight, improved safety and greater look-up capability. \

Accordingly, it is a further purpose of the present invention to provide a new and improved optical ! combiner which substantially eliminates־ the chromatic| dispersion and flare characteristics of slanted-fringe\ hologram combiners.!

Another purpose is to provide an optical combiner comprising a substrate having an aspheric surface contour to compensate or balance optical aberrations.

Another purpose is to provide an improved. optical combiner in which the diffraction coating is formed directly on a substrate and does not require glass protective layers.

Yet another purpose is to provide an optical combiner which minimizes strong second images.

Additional purposes of the invention are to provide an optical combiner haying a relatively simple, lightweight structure which is suitable for fabrication by relatively low-cost techniques and with Improved optical tolerances.

Further purposes are to provide improved helmet visor displays and head-up displays employing the improved optical combiner»

The foregoing and other Advantages and features of 10 the present invention will become more readily apparent from the following more particular description of the preferred embodiments of the invention,as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a graph showing the change in the refractive index of a 310χ film which wps deposited by a photochemical vapor depositidn process, as a function of the flow rate of the silane reactant.

FIG. 2 is a schematic representation of the continuously graded index material in a sinusoidal pattern, formed in accordance with the present invention.

FIG. 3 presents a graph showing the dependence of: the thickness of the deposited !oxide on the relative intensity of the ultraviolet light used to Initiate the photochemical vapor deposition₍reactlon of one embodiment of the present invention.

FIGS. 4a - 4c are schematic diagrams of a structure having an aspheric substrate surface dind a diffraction optical element formed on the surface thereof in accordance wlt;h the present invention. !

July 11, 1986

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FIG. 5 shows a curve indicating the measured spectral reflectance of a filter formed in accordance with the present invention, while FIG. 6 presents the theoretical reflectance for such a structure.

FIG. 7 presents the refractive index profile for

9S6__ ^a composite index filter formed in accordance with the present invention which reflects three separate wavelengths, of radiation,, while FIG. 8 presents the spectral response for such a composite . index filter.

FIG. 9 is a schematic diagram of an optical combiner having an aspheric substrate structure and a diffraction coating formed on the surface-thereof .

FIG. 10 shows curves of the spectral reflectivity function of a typical gelatin hologram (broken line) 15 with a sinusoidal index profile and a graded-index coating (solid line) with a non-sinusoidal index profile.

FIG. 11 shows'curves of the spectral reflectivity function of typical gelatin holograms with sinusoidal index profiles (broken line), one designed to have a 20 broad spectral bandwidth relative to the other, and a graded-index coating with a non-sinusoidal index profile (solid line) designed to have a broad spectral bandwidth.,

FIG. 12 is a schematic representation of a combiner structure comprising a graded-index coating in which the 25 periodicity (and therefore the peak efficiency wavelength) of the index profile function varies as a function of the surface position coordinates.

FIGS. 13 and 14 depict structures used in a preferred method to fabricate a plastic combiner 3q substrate having an aspheric surface.

FIG. 15 is a simplified schematic drawing of a helmet visor display system incorporating a novel optical combiner in accordance with the invention.

FIG. 16 is a side view of the HVD system depicted 35 in FIG.15.

FIG. 17 is an angled side view of the HVD system depicted in FIG. 15, from a plane parallel to a reference line of the system.

FIG. 18 is another side view of the HVD system depicted in FIG. 15, illustrating the diffraction of image source light rays.

FIG. 19 is a perspective view of the HVD of FIG. 15 shown mounted oh a helmet.

FIG. 20 is a simplified schematic view of a headup display incorporating a novel optical combiner in accordance with the invention.

<td> ' י</td><td> DETAILED DESCRIPTION OF THE INVENTION ' Γη order to form the continously graded index</td>

<td> 15</td><td> optical material of the present invention, it is required that the process for forming such a material be capable of a high degree of control over the composition of the deposited material. For the deposition of a continuously . graded index oxide material, a particularly useful</td>

<td>²⁰</td><td> process is the low temperature, charge-free photochemical vapor deposition process' described in U.S. Patent No.</td>

<td> July 11, 1986 _</td><td> 4,371,587, assigned to the present assignee, and the details of which are incorporated herein by reference. In one embodiment of the latter invention, neutral,</td>

<td> 25</td><td> charge-free atomic oxygen is generated by the mercurysensitized photodissociation of a chosen oxygen- containing precursor, such as nitrous oxide, as shown “־ in Equations (1) and (2) below. Then, the atomic oxygen is reacted with a selected vapor phase reactant,</td>

<td> 30</td><td> such as silane, to form the desired oxide, silicon monoxide (SiO) or silicon dioxide (SiO2), as shown in Equation (3) below.</td>

/ 12

1) 1 ׳) Hg + hv(2537A) ------> Hg* (photoexcited) where h = Planck's constant v = frequency of absorbed radiation (2) N₂0 + Hg* ------> N₂ + 0 + Hg

SiO₂ + H₂0 — (3) S1H4 + 0

------> gio + ₂0_״ • 2Q In an alternative process embodiment disclosed in

U.S. Patent No. 4,371,587, the required atomic oxygen, may be generated by the direct photodissQciation of a chosen oxygen-containing precursor, such as.nitrous oxide (N₂0), as shown .in Equation (4) below. The 15 atomic oxygen so formed reacts with .the chosen vapor phase reactant as shown in Equation (3)' above.

THIS PAGE:

July 11, 1986 _{(4) N20 +} h_v(1750 - 1950A ) -----—> N₂ + 0

The composition of the oxide product of Equation (3) above depends, in.part, on the steady state concentration of atomic oxygen, which, in turn, depends on the amount of nitrous oxide present for a given amount of silane. Thus, by varying the ratio of the silane and nitrous oxide reactants present at a given point in time, the composition of the silicon oxide (SiO_x) product can be controlled, to produce a corresponding control of the refractive index of the oxide material deposited. The composition of the SiO_x can range . 30 from SiO with a refractive index of 1.9 to SiO₂ with a refractive index 1.45.

' 13

The composition of the oxide, as well as the rate of deposition, depends on the mass flow of each of the reactants, the pump throughput, and the intensity of the reaction-inducing radiation. For a constant value for pump throughput and radiation intensity, the effect of changes in the flow rate of one of the reactant gases can be determined. FIG. 1 presents a graph showing the change in the refractive index of a S1O_Xfilm which was deposited as described above, as a function of the flow rate of the silane,reactant.for a constant flow rate of N20 at 62.0 standard cubic centimeters per minute (seem) . As can-.be,. seen in. FIG. 1, the refractive index and composition of . the 'SiO_Y film have a well-defined dependence on the 15 gas flow rate ratio of the reactants. As shown in ׳FIG. 1, the index of the S1O_X film was varied .from ’ THIS PAGE:' 1.46 to 1.60 by varying the S1H4 flow rate by as much

July 11, .1906 _{as a}־ f_actor.of five, while keeping a fixed. Ν2θ flow rate. The large variation in flow rate required to 20 achieve a change in refractive index (An) of 0.14 indicates that the index can be changed precisely and reproducibly by the above-described process. Consequently, in accordance with the process of the present invention, by accurately controlling the 25 gas flow rate ratio of the silane reactAnt to the nitrous oxide reactant as a function of tifae, the composition of the SiOx product can be controllably and continuously altered as a function of time' of deposition or thickness of the deposited material.

/ 79391/2 / 14

In particular, the gas flow rate of the silane can be varied in a sinusoidal pattern to produce. a.correspending sinusoidal variation in the composition of the 3ϊθχ deposited as a function of distance above the ' surface of the substrate, as shown in FIG. 2. The substrate 10, shown in FIG. 2 has. a layer 12 of a continuously graded optical material, such as SiOx, formed on the surface thereof by the above-described process. The composition of the SiOx material at a particular point along the depth or thickness 14 of the film 12 depends on the particular ratio of the reactant gases and the gas flow rate of silane at. the time that particular material was deposited. Since the gas flow rate, of the silane is varied in a sinusoidal pattern, the composition .of the SiO_x material in the layer 14 will vary in approximately the same sinusoidal pattern, THIS PAGE: taking into account small deviations from linearity.

July 11, 19.86 p_urther, since the refractive index of the SiOx material varies as. the composition thereof, it follows that the refractive index (n) of the SiO_x material likewise varies in a sinusoidal pattern 16 as a function of the thickness 14 of the layer 12, as shown in. FIG. 2.

Thus, there is a gradual change in the refractive index of the deposited layer 12 between upper and lower limits of n and 'in־ a predetermined pattern. The difference between the highest and lowest values of the refractive index of the SiOx material is Δη, the index modulation. The notation Λ in FIG. 2 refers to fringe periodicity, the significance of which is discussed below with regard to wavelength of reflected light and Equation (5).

/ ¹⁵

V 1 Furthermore, the gas flow rate of the silane may . W be varied in some other pattern besides a sinusoidal $ . pattern, such as a quasi-sinusoidal, triangular, sawtooth,

, .׳׳£?

i squarewave, or predetermined irregular pattern, to produce corresponding variations in the composition and refractive index of the deposited material.

In addition to the effect of the reactant gas flow rate ratio discussed above, it should also be noted that the intensity of the reaction-inducing ultraviolet 10 radiation affects the rate of deposition of the Oxide.

FIG. 3.presents a graph showing the dependence of the thickness of the deposited oxide in .er Ewenty-minute period on the relative intensity.of the ultraviolet (UV) light used to initiate the photochemical reaction.

As shown in FIG. 3, as the intensity of the UV light י is increased, the thickness of the oxide deposited in a

THIS PAGE: given time period or the deposition rate increases.

July 11, .1986

The source of the UV light used to generate the data of . FIG. 3 in accordance with the first process embodiment־ 20 Present invention was a bank of four mercury vapor arc lamps with a major output at a wavelength of 254 nanometers, at a distance of2,75 ׳ inches (6.99 centimeters) from the substrate. Alternatively, a scanning laser beam may be used as the source of 25 reaction-inducing r.adiation in. accordance with the second process embodiment of the present invention, as discussed, in further detail herein. In addition to the’ “ reaction parameters discussed above with regard to FIGS. 1 and 3, control of the gas-flow pattern and the 3q gas pressure inside the deposition chamber is also necessary for producing reproducible and uniform oxide layers.

. 35

In accordance with the above-described process of the present invention, the modulated index layer is formed parallel to the substrate surface, to provide a zero-degree (non-slanted fringe) diffraction optical

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element,. However, the modulated index layer of the present invention may alternatively be formed on an i

I 15 ־IHIS PAGE:

July 11, 1986 aspheric substrate which incorporates optical power, to provide a diffraction optical element which compensates or balances optical aberration while providing undistorted see-through. Such a structure is shown schematically in FIGS, 4a and 4b. In FIG. 4a, there is shown a substrate 20 .having an arbitrarily-shaped aspheric surface and formed, for example, of polycarbonate. On the surface of the substrate 20 and conforming thereto, there is formed וזץ accordance with the process of the. present invention a layer 22 comprising a continuously graded index optical material in a predetermined index pattern. As a separate element, there is shown in FIG. 4a a cover member 24, in which the surface 26 that faces the substrate 20 matches the surface profile of the substrate 20. The cover member 24 is laminated by means of epoxy or other optically transparent adhesive (not shown) to the substrate 20 having the layer 22 formed thereon, to provide the structure shown in FIG. 4b. As represented schematically. in FIG. 4b, incident radiation 28 of a wavelength in the selected reflective band is reflected . ”^Tby the graded index layer 22; while radiation 30 with a wavelength outside the selected reflective band passes through the complete structure. When used in a head-up display system such as described. in U.S. Patent No. 3,940,204, assigned to the present assignee, incident radiation 28 is the light from a cathode ray tube which / Y1 / 1 is diffracted by the graded index layer 22 to the eye / of the viewer, and the radiation 32 is light from the environment outside the cockpit which is transmitted ' through the complete structure of FIG. 4b to the eye of : 5 the viewer. The ability to use such aspheric substrates greatly increases the number of diffraction optics ; applications which may be satisfied by the zero-degree diffraction optical element of the present invention. A structure such as shown in FIG. 4b is useful for 10 head-up display diffraction optics combiners, for example, as previously discussed.

Further, it is anticipated that a; slant fringe diffraction optical element may be formed in accordance with the present invention by positioning the nozzle 15 slits of one reactant gas near the substrates. The other reactant gas flows uniformly across the entire .

THIS PAGE: substrate surface; and the slits are separated at a ;uly H, 1906 distance equal to the fringe spacing on the substrate surface. The slant angle is controlled by the related 20 movement of the substrate and the nozzle slits, as the

׳ . . ¹ j deposition proceeds.- ! The significance of the above-described process

I for forming a continuously graded index optical material in a predetermined pattern is that such a process may 25 be advantageously־used to form a reflective diffraction optical element which performs similarly to a conventional holographic filter, to diffract the incident light and _T_r. produce a preselected diffraction effect, as described, for example, in the book entitled Optical Holography, 30 previously referenced, in Chapter 1 (Introduction to Basic.Concepts) .

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THIS

July

PAGE: 11, '1986

The optical properties of a film consisting of discrete, multiple layers are well-described by multilayer matrix theory, as discussed, for example, by. P. H, Berning in the book entitled Physics of Thin Films, edited by G. Hass, Academic Press, New York, 1963, starting at page 69. This theory may be applied . to the calculations of the optical properties of a graded-index film by approximating ,the graded film as a stack of N very thin discrete-index layers. For N of a very large value, this approximation is sufficiently accurate for the devices of the present invention. Using this approximation and conventional multilayer film optical theory, the reflectance, • transmittance, and absorbance for both s- and ppolarizations, as well as their averages can be calculated, preferably with the aid of a computer program, for graded-index films. These calculations can be made at any wavelength or angle of incidence for any graded-index coating configuration. The electric field and absorption profile within the coating can . also be calculated. Such calculations indicate the feasibility of fabricating narrow band, high reflectance spectral filters by deposition of graded index films. This method of analyzing graded-index films is described, for example, by K’., A. Winick, in the Final Scientific Report on Thick Phase Holograms, Environmental Research Institute of. Michigan, January, 1981.

Calculations of the optical properties of graded index films illustrate that the reflectance characteristic of the film depends primarily on the Fourier composition of the index profile. Thus, for example, a holographically exposed diffraction optical element . with sinusoidal index modulation has the same performance at the designed reflection wavelength (λρ) as that of the multilayer square modulation of equal periodicity '

whose fundamental

Fourier component is of equal module

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July 11, 1906 tion amplitude. Consequently, for some diffraction optics applications where high reflectance at only a single wavelength is desired, the squarewave multilayer film and the sinusoidally modulated . film are equally viable alternatives, in terms of optical properties. However, because of the disadvantages of the multilayer structure previously discussed, the sinusoidal profile or some other graded index profile which the process of the present invention provides, may offer distinct advantages over the multilayer squarewave profile.

Furthermore, the graded index film of'the present invention may be designed to have any peak wavelength or wavelengths desired, barring materials limitations, by designing each sinusoidal component, of the index profile to have a periodicity A described in Equation (5) below.

(5) A! = Xpi/2n where

A! = periodicity of the i^th sinusoidal component in the index profile λρϊ = i*^ peak wavelength n = average index of refraction ' -- ,.:

SiO_v araded index filters are material-limited to a peak wavelength from 0.4 to 2.5 micrometers, since δΐΟχ becomes highly absorptive outside this range. However, • other materials besides 510χ, such as aluminum oxide or

3q zirconium oxide, may be used to form a layer with a .

graded index profile which reflects radiation at higher or lower wavelengths. Thus, by the process of the present ,invention, diffraction optical elements may be formed to. reflect radiation in the ultraviolet, visible and infrared ranges. . ״ ־ / .

/ 20 / 1 In addition, in accordance with an alternative to the first process embodiment of the present invention, the peak reflection wavelength (λ_ρ) may be varied across the horizontal surface of the diffraction optical 5 element by.varying the localized reactant gas flow rate across the substrate surface as desired or by varying the intensity of the reaction-inducing radiation, as discussed herein, to produce variations in the thickness, refractive index, periodicity, and λρ of the deposited ' material. Such devices with horizontal variations of λρ have been heretofore unattainable.

Moreover, the process of the present-־^,invention may be used to form layers which exhibit modulation in either absorptivity or refractive index or both, since 15 there is a known relationship between absorptivity and refractive index. Absorptivity a is defined in Equation

THIS PAGE: ₍₆₎ below, and the relationship thereof to refractive

July 11, 1986 .- index is defined׳ in Equation (7) below.

(6) a =

Λ . where a = absorptivity k ־ extinction coefficient λ = wavelength of incident radiation (7) N = n-ik .

where. N = complex refractive index , n = real refractive index k = extinction coefficient

-r ♦

Thus, in the same way that variations in n, the real refractive index, of a layer of material may be produced as previously described herein, so may corresponding variations in the extinction coefficient, k, of a

793)1/2 layer of material be produced. For example, photochemically deposited oxides, such as ΒΐΟχ, have a cutoff region (e.g. about 2.5 micrometers for SiO_x) at which they become highly absorptive, and the absorptivity is highly dependent on the stoichiometric composition.

Thus, by varying the stoichiometric composition of SiO_xin accordance with the present invention, a structure may be produced which exhibits modulation of absorptivity as . well as refractive index. Consequently, the detailed 10 discussion herein with respect to ״refractive index is intended to include the complex refractive index described above.

Using the above-described photochemical vapor deposition process as further described in’Example 1, 15 an oxide filter was formed with aה16־period, sinusoidally modulated refractive index that varied

1Q6 between 1.45 and 1.63, producing a peak wavelength at 1,48־ micrometers (pm). The spectral reflectance of the filter was measured for various wavelengths of incident radiation using a spectrophotometer and known procedures., and the curve obtained is shown in FIG. 5. A reflectance of 81,3 percent at.the fundamental wavelength (Xq) of 1.48 micrometers was obtained, as shown in FIG. 5. This reflectance value . is to be compared to the theoretical prediction of 94.1 percent reflectance at 1.48 pm shown in FIG. 6, which was based on the theory and calculations previously discussed.

The fact that the measured reflectance peak of 3g 81,3 percent in FIG. 5 deviated somewhat from the predicted 94.1 percent in FIG. 6 indicates that the modulated-index patterfi the 'deposited film deviated somewhat from the intended sinusoidal pattern. .This

J 79391/2

f. 1 conclusion is supported by the observation of small reflectance peaks in FIG. 5 at higher-order harmonics: 1/2 λ_ο (0.75 pm) and 2/3 λ_ο (1.0 pm). Each reflectance spike corresponds to a particular Fourier 5 component of the refractive index profile, Therefore, a perfect sinusoidally modulated index profile will exhibit only one reflectance peak, at the fundamental wavelength of ₀ג. The fact that the higher-order peaks in FIG. 5 are low-amplitude indicates that the 10 deviation of the film from sinusoidality is relatively small. It is anticipated that an accurate sinusoidal pattern for the refractive index profile׳ can be achieved by using a monitoring and feedback loop control system ' for.measuring refractive index and thickness of the film as deposited. The more accurate the sinusoidally, modulated index profile, the higher the reflectance

July 11 1986 ^va^^{ue at the} fundamental wavelength.

In addition, with regard to FIG, 5, the absence of a measured reflectance peak at λ₀/3 (0.5 pm), which 20 i® characteristic of a multilayer structure, indicates that this coating is not a discrete-layer quarterwave stack, and is consistent with the result predicted for a sinusoidally modulated film.

The above-described theory and calculations can 25 be used to determine the feasibility of fabricating other narrow band, high reflectance spectral filters by the process of the present invention. For example, using such calculations, it has been determined that a coating design with reflectivity of 99.97 percent at 3q 0.53pm can be achieved with a sinusoidal index profile .having a modulation of 0.105 and a thickness of 1,5pm.

It has also been determined that as the film thickness increases, the index modulation can be reduced and .

/ 23 applied for an increased number of cycles, while still maintaining the required reflectivity. Another factor for consideration is the reflectance bandwidth. The larger the index modulation, the wider the reflectance 5 bandwidth at a given thickness or at a given efficiency level, which causes reduced photopic see-through or signal transmission. As another example, it has been determined from such calculations that an optical coating design with a reflectivity of 99.93 percent at 10 1.315 micrometers can be achieved with a graded index layer of SiO_x having a sinusoidal profile, with an index modulation of 0.42, a.thickness of 8,.2'micrometers, and a modulation period of 0.396 micrometers.

Furthermore, in accordance with the process of 15 the present invention, there may be fqnned a coating with a composite index profile which is the linear THIS PAGE: ׳superpositioning of a number of sinusoidal index .profiles and. which exhibits high reflectance at multiple wavelengths corresponding to the individual sinusoidal 20 index profiles. This kind of composite profile can be designed analytically as'described above and fabricated • using the graded index process of the present invention. For example, the three separate index profiles required to provide protection against three separate wavelengths 25 of radiation at 0.6 pm, 0.8 pm, and 1.0 pm are combined to form the composite refractive index profile versus film thickness shown in FIG. 7. Using the composite profile index of FIG. 7 for a total film thickness of 16 micrometers on a glass substrate, the theoretical 30 reflectance of such a device at various wavelengths is shown in FIG. 8. The extremely high reflectance at 0.6 pm, 0.8 pm, and 1.0 pm is evident in FIG. 8. In a similar manner, other coatings may be formed with other index profiles which are analytically synthesized 35 to provide particular optical characteristics.

79391/2 /

24 י / f 1 In addition, the photochemical vapor deposition process of U.S. Patent No. 4,371,587 can be used to produce many different oxide films, such as Si02, Ti02_׳ AI2O3, Ta2O5_׳ and Sn02. Further, such a process can be used to provide a film structure that consists of two oxides with compositions that change gradually and continuously from one oxide to another.

For a composite oxide film of Si02» with arefractive index of 1.45, and TiO2, with a refractive index of

2.55, the maximum index modulation achievable is 1.1, as compared to the 0.45 modulation achieved in. the SiO_xoxide system. The indices Of some other useful oxides are 1.76 for AI2O3, and 2.18 for ZrO2.

Finally, as a practical matter, preliminary analysis has shown that the tolerance, on the thickness., and index variation are higher for the graded index film τ זן T ף p Λ G E י »

July 11 1986 structure of the, present invention than for a prior art ־multilayer quarterwave stack coating. It is anticipated that if a reasonably precise monitoring system for 2q thickness and refractive index is installed, the photochemical vapor deposition process of the present invention will produce coatings with excellent reflectance, and showing minimum degradation due to the random error associated with the deposition process.

There are several advantages in forming a reflective diffraction optical element for use as a filter by the process of the present invention. First, . the filter is formed of an oxide material which is inherently stable in high humidity and over a wide temperature range, whereas conventional gelatin holograms are not. In addition, the SiO_x formed by the photochemical vapor deposition process has exhibited superior adhesion on glass, as well as plastics such as polycarbonate, and conforms to the shape of the , substrate surface. The latter two properties make the

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process of the present invention particularly well suited for the fabrication of head-up display combiners or night vision visors on curved substrates. Moreover, the SiOx of the present invention can be deposited at

5. a temperature sufficiently low (e.g. 30 to 200°C) so as to avoid thermal degradation of a plastic substrate, which makes possible the use of light weight: plastic substrates in laser eye protection devices and head-up display devices. Further, the SiO* formed as described herein possesses excellent optical and mechanical . properties, such as good surface morphology and low λ . pinhole or defect'density, which result in reduced optical scattering. In addition, the low defect density . of such an oxide makes it less susceptible to laser radiation damage. Further, by the process of. the tutc oACtf. present invention, a continuously graded index optical July 11, 1986 material is deposited, and, thus, avoids the previously discussed prior art problems, such as reduced transmission, optical scattering, and thermal damage,, caused by the juxtaposition of discrete layers of differing composition. By using the continuously graded index reflective coating of the present invention, the prior art problems of localized concentration of mechanical stresses ab well as a concentration of thb electric field are avoided. It has'been analytically determined that the peak electric field in a filter with a sinusoidal profile is less than the peak electric field encountered in a square-wave multilayer filter having identical optical properties at the desired wavelength. In addition, the gradual change in . composition in the material formed in accordance with the present invention reduces the thermal stress in the film when subjected to the high laser energy flux. This, reduced thermal stress is expected to שן increase the laser damage threshold. The gradual

<img file="IL79391A_D0006.tif" />

CHIS PAGE: July 11, 1986 hange may also reduce the absorptance of the film. In addition, a better thermal match with . the substrate may also be attained by adjusting the composition of the dielectric at the substrate interface so that the substrate and dielectric have compatible thermal expansions.

Moreover, the index of refraction of the material deposited in accordance with the present invention can be adjusted to a predetermined profile, which permits greater flexibility in the design of the optical element. In particular, the optical element of the present invention can be formed as a npn-sinusoidal profile to provide high reflectance at several wavelengths, as previously discussed with regard to composite index profile devices. These latter devices are novel optical devices which have not been possible heretofore, such as a multiple-wavelength narrow band reflective diffrac' tion optical element incorporated in a single deposited layer, which is useful in two-color head-up display combiners, laser eye protection visors,.and reflective coatings on laser mirrors . -In addition, the periodicity of the profile can be changed to reject any predetermined laser line within the spectral band of the deposited material, e.g. a peak wavelength from 0.3 to 2.5 m for SiO_x. Further, because^ the optical devices of. the present invention can be constructed to provide high reflection of radiation within a narrow bandwidth, such devices have high transmission of the signal of interest and enhance the efficiency of signal detection.

In addition, provide high reflection over a wide wavelength region (e.g., 0.3 to 2.5 micrometers or greater for Si Οχ or up to 5 micrometers forA12°3). ’ the devices of the present invention can

. ר2 /

Further, the continuously graded index filters of the present invention can be fabricated by a photochemical vapor deposition process which is charge-free and avoids charge damage or radiation damage to sensitive devices, such as charge-coupled devices and compound semiconductor devices. In addition, such a process is performed at a low temperature, e.g., 30 to 200’C, and avoids thermal damage to temperaturesensitive substrates, as well as stress caused by thermal iO mismatch of the substrate and deposited layer. Moreover, the material deposited by this photochemical vapor deposition process has excellent surface morphology, low pinhole density, low impurity content, low stress, large area uniform thickness, and conformity to substrate shape. The process of the present invention is highly reproducible and capable of large-scale production of ^T״i^S H^GE1q86 uniform deposes- Finally, by the׳ above-described ^Juy ' .process, a filter can be fabricated by a single step .

process, which reduces fabrication complexity ^and cost. It is particularly noteworthy that by the process of the present invention a filter which .protects against multiple wavelengths of radiation can be fabricated in a single device by a single step process.

Further, the process of the present invention may be used to form a refiec.tive coating on the surface of a mirror substrate to provide a surface with high reflectivity, low absorption, and low scattering.׳ Such a highly reflective film is useful on mirrors provided in laser systems to reflect and guide the laser beam.

₃₀ In order to form such a reflective coating, the process previously described herein with particular reference to FIG. 5 is performed on a molybdenum or silicon • mirror substrate, for example. The reflectance of this coating was measured at various wavelengths of incident 35 radiation and the results are as shown in the curve, of

<img file="IL79391A_D0007.tif" />

FIG. 5 previously discussed. In order to. improve adhesion and decrease stress, it may be advantageous in some cases to use a binder material, such as chromium or titanium, between the substrate and the graded index material. Such a metal binder may also be used to reduce the thickness requirement of the deposited film at a particular index modulation.

Furthermore, in accordance with a second process embodiment of the present invention, a continuously graded index optical material may be deposited by the photochemical vapor deposition process previously described herein except that a scanning laser beam or other collimated beam is used as the source of radiation . to initiate the desired chemical reaction.' The laser beam may be scanned across the surface׳of the substrate or other means may be used to effect relative movement

THIS PAGE:

July 11, 1986 of the laser beam with respect to the .substrate, such as‘moving the substrate or adjusting the optical focusing system, as is known in the art and described, for example, 20 in U.S. Patent No. 4,340,617. The laser beam or other collimated beam used has an output at the wavelength of radiation required to induce the desired photochemical reaction, as previously discussed herein. The beam is scanned across the surface of the substrate in a controlled manner so that one segment of the substrate at a time is exposed to the beam, with the segment size being determined by the beam diameter. The rate of deposition of the reaction product depends on the amount of reaction-inducing radiation to which the reactants are exposed. Consequently, if a given segment of the substrate has a longer exposure to the laser beam, that segment will have an increased amount of reactive product deposited thereon. Thus,, a spatial variation of the deposition rate over the horizontal ‘ surface of the substrate can be achieved by controlling the scanning speed or duty cycle laser beam . thickness deposit and the 5 varied over accordance beam exposure . a &lt;----by the sc structure has a

THIS PAGE: July. 11, 1986 and pattern of the

Since the deposition rate determines the _0E the deposited layer, the thickness of the refractive index thereof are likewise . the lateral, surface of the substrate in with the above-described scanning laser -, This variation in thickness may be in continuous manner or in a stepped manner, ־־ anning pattern of the laser beam. The resulting . continuously graded index-layer as a function of thickness as previously described herein, , in addition, the thickness of that layer varies predetermined pattern across the burf0 ־־־ t ־ substrate. Si־־״ th. amount ־Modulation given thickness determines the periodic , y. index modulation, the deposited layer ° ^dsegments of the substrate surface wi periodicities As previously discuss־ Equation (5) herein, the periodicity 0 _ .

modulation determines the peak9 radiation. Consequently, in accordance with radiation ., ״resent invention, second process embodiment of th P^.

_a diffraction optical element may be different peak wavelengths of reflection an ־.

ness at various segments <>f the substrate surface. Such a structure is useful as a display system second process combiner in a head-up

Alternatively, in accordance with the . embodiment of the present invention, .

the amount Of reaction-inducing radiation .

segments of the substrate are exposed, by using a flood source of ultraviolet a mask placed close to the substrate to prevent the radiation from striking the substrate at predetermined segments.

predetermined may be varied radiation and surface t / 30

The first and second process embodiments of the present invention may be used to provide various optical elements with varying thickness and/or refractive index across the horizontal surface thereof, in addition to the modulated refractive index as a function of thickness as previously described. In one case, the refractive index is modulated as a function of thickness to provide.a chosen λρ as described with regard to the first ;process embodiment and, in addition, the thickness of the deposited layer is varied in a desired pattern, such as to form a convex surface, as described above with regard to the second process embodiment. The resulting structure has a horizontal variation in thickness and the «same λρ across the horizontal surface of the structure. A structure having such a variation in thickness provides a change in the efficiency of the July 11, 1986 diffraction element across the horizontal surface.thereof, which is advantageous for compensating for spatial nonuniformity in the intensity of the incident radiation.

In a second case, the thickness of the deposited layer is varied in a desired pattern as described immediately above, and, in addition, λρ is varied across the horizontal surface.of the layer as previously described with respect to an alternative of the first process embodiment of this invention. The resulting structure has a horizontal variation in thickness and variations in λρ across the horizontal surface of the structure.

In a third case, the thickness of the deposited layer is held constant and therefractive index of. the deposited layer is varied across the horizontal surface of the substrate to produce variations in periodicity and. λρ, as described above. In such a structure, variations in λρ' across the horizontal surface of the substrate are

<img file="IL79391A_D0008.tif" />

not dependent on the thickness of the deposited layer.

The constant thickness of such a structure may be achieved by placing a cover with an iris opening over the substrate and opening or closing the iris over a 5 given segment of the substrate to control the amount of radiaton striking the substrate segment, while, at the same time altering the intensity of the radiation or the reactant gas flow rates in order to achieve the required modulated refractive index as a function of IO thickness and as a function of horizontal position on the substrate surface.

In summary, the following are some.of the unique characteristics of the process of the present invention, which provide conventional optical filter devices with improved performance, as well as novel filter devicesheretofor unavailable:

a. arbitrary profile of (complex) refractive index modulation by control of reactant flow rate ratio;

b. high index modulation; . .

c. variable peak wavelength across the surface of the substrate;

d. low temperature deposition;

e. uniform coating conformed to substrate shape; and ־

f. versatility in deposition materials.

I

In particular, some of the new and improved optical devices which may be formed in accordance with the

3q present invention are:

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THIS PAGE: July 11, 1986

a. wide angle optical filters with variable λρ across the filter for wide angle receptions, such as bandpass filter, narrow band transmission or reflection filter, cut-off filter;

b. absorption or transmission type apodizer to provide even intensity across the resultant beam, such as variable ,density neutral density transmission filter, variable reflection filter;

c. surface grating with grating profile shaped by the UV light profile'and/or the variable index through grating depth;

d. variable index coating on substrates or fiber cores to form special optical devices, such as optical fibers or integrated optics elements;

e. ‘ element for replication of masks and computer generated holograms;

f. IR and visible filter for laser hardened IR detector and solid, state components, such as a laser protection filter for detectors, or for personnel laser eye protection;

g. narrow band ,'single color or multi-:color, transmission or reflection or cut-off . type filters;

h. thin lens with variable surface profile and variable index throughout oracross the lenses;

i. anti-reflection, or high reflectivity filters on plastic substrates, or on glass substrates;

<img file="IL79391A_D0010.tif" />

any of the above coatings on aspheric substrates; and slanted fringe optical devices.

Moreover, the process of the present invention is not limited to the use of oxides of silicon, but may include any of the oxides which may be deposited by the process disclosed in U.S. Patent No. 4,371,587, previously referenced herein, in addition, other materials besides oxides may be deposited as described herein by other 10 photochemical processes, such as silicon nitride by the process disclosed in U.S. Patent 4,181,751, assigned to the present assignee , and various sulfides~'by the process disclosed in U.S. Patent 4,447,469, assigned to the present, assignee. Further, combinations of 15 these various materials may be used,.such as silicon, dioxide in combination with silicon nitride. As a .

1986 practical matter, the material deposited .in accordance ,.with the present invention must be stable in the presence of air and water vapor,

Furthermore, the present invention is not’limited to photochemical vapor deposition processes, but includes other known deposition processes in which the reactant gas flow rate ratios are controlled as described herein to provide a continuously graded index optical material 25 with a predetermined index profile. For example, in a thermal chemical vapor deposition process in which reactants are heated to a sufficiently high temperature to bring about a chemical reaction to form a desired product, the gas flow rate .ratio of the reactant׳ gases 30 - is controlled in the manner previously discussed herein.

. A type of thermal chemical vapor deposition of particular interest in this regard is the deposition of an epitaxial layer by metallo-organic chemical vapor deposition.

/ 34

In a thermal evaporation or physical vapor deposition process in which two sources, such as zinc sulfide and zinc selenide, are heated to produce evaporation thereof and subsequent condensation on the substrate, the thermal evaporation of one source, such as zinc sulfide, is held at a constant rate while the thermal evaporation . of the second source is varied as a function of time, in the manner discussed herein. Similar methods can be used in electron beam evaporation techniques and sputter evaporation techniques using two targets. Likewise,, in a molecular beam epitaxial growth process in which an epitaxial layer of a material is grown by causing beams of atoms or molecules to impinge on the target, the relative proportion of the impinging beams can be altered as described herein to provide,.an epitaxial .. layer having a graded refractive index in a predeterTHIS PAGE: . . .

July 11, 1986 ^mined pattern.

EXAMPLE 1

2q This example illustrates the formation of a continuously graded index optical material in a predetermined pattern suitable for use as a filter, in accordance with the first process embodiment of the present invention as previously described herein.

The )photochemical vapor deposition process and apparatus described in U.S. Patent No. 4,371,587, previously referenced herein, were used to deposit a film of SiOx on a glass slide substrate having dimensions of 2 inches (5.08 cm) by 3 inches (7.62 cm) and 40 mils (0.10 cm).

3q thick. The vapor phase reactants were silane (SiH4) and nitrous oxide (N2O) , and mercury was used as a photosensitizing agent. The reaction-inducing radiation was at a wavelength of 2537A; the substrate temperature was 100°C; and the total operating pressure was approxi35 mately one torr (millimeter of mercury). Alternatively, within the range of about 0.1 to

The reactant gases SiH4 and N2O the deposition chamber through which control the flow of the

<img file="IL79391A_D0011.tif" />

THIS PAGE:

July 11, 1986 entered at one end of mass flow controllers reactant _Sas־s. The £10״ of reactant gases was initiate/and followed by adjustment of total pressure, gas £10״ rates, gas flow ratios, and substrate temperature. Gas pressure, during deposition ״as ־pt constant and the substrates ־״re positioned so that there pattern . SiOx was initia inducing data of FIG. 3, prev requi--mercury vapor lamps , of Newark, New Jersey, ’ and was located approx me of Ν2θ was flow rate of SiH₄ was .varied - from 0.90 seem to 3.5 seem. . .

Using the previously discussed .data of FIG. and a graph generated from experimental.data ך show the change in £10״ ratewith time, the £1״־ rate of the ־״־11־ reactant ״as altered with time by manual turning _ of the g״£10 ־־ controller knob at a predetermined rate to produce an oxide film with a refractive index t ־ varied in a continuous sinusoidal pattern from.1.45 1.63, for a total of 16 periods, producing a peak

Alpnnth at 1 48״m. The measured spectral reflectance wavelengtn at x.iupi״. * ₍

Of the holographic filter so formed is presented 1״

FIG. 5, showing an 81.3 percent peak efficiency,, and this data has been previously discussed in detail.

was minimum disruption of the laminar gas flow Upon system equilibration, the deposition of iated by transmission of the reactionradiation into the reaction chamber, using the lously discussed,:to determine the ired intensity of light. A bank .of four low-pressure obtained from Canrad-Hanovia, Inc. was used as the source of light imately 2.75.inches (6.99 centi- . ters) from the substrate surface. '.The .gas flowrate held constant at 62.0 seem and., the gas ... varied in.a controlled manner / 36 / .j / 1 Such a structure 19 useful, for example, as a highly / reflective coating on the surface of a mirror used In a / laser system, in order to provide ¹maximum transmission

0E the laser signal and minimum Idser damage to the mirror. '.

EXAMPLE 2 \ This example Illustrates the formation of a layer ’ » 1,.

of a continuously graded Index optical material on 10 the surface of a substrate in accordance with the process of the present invention, in which a composite oxide film of silicon dioxide (SiO2> and titanium dioxide (Ti02) is formed, to provide a maximum refractive index modulation of 1:1.

₁₅ The process described in Example 1 above .is followed except that titanium tetrachloride (TICI4) THIS PAGE: is used as an additional vapor phase reactant. As

July 11, 1906 _desccibed i_n u.s. patent No, 4,371,507, the .atomic oxygen formed by the photochemical. dissociation of the nitrous oxide reacts with the HCI4 to form titanium dioxide. In accordance With the present invention,. the gas flow rate of nitrous oxide is held constant and the gas flow rates of SiH4 and;TiC14 are varied in a controlled manner with respect to each other and with repect to the nitrons oxide. The required flow rates of the SiH4 and T1C14 are determined from experimental data which indicate the dependence of the refractive index of the deposited ׳material on the flow rates. The SIH4 and TICI4 reactants each react.with 3q the atomic oxygen to form SiO2 and!T102_׳ respectively, which deposit simultaneously on the substrate to provide a composite oxide film comprising S1O2 and T1O2. The composition of the composite oxide may vary from pure S1O2 with a refractive index of 1,45 ’ A .

along the continuum to pure T102 wl;th a refractive.

Index of 2.55. The resulting composite oxide film has a continuously graded refractive index as a function of thickness, with a maximum index modulation of 1.1.

<img file="IL79391A_D0012.tif" />

EXAMPLE 3'

This example Illustrates the formation of a layer of a continuously graded .index optical material on the surface of a glass substrate in which the index also varies in a predetermined pattern Across the.surface of the substrate, in accordance with the second process embodiment of the present ifiventioh as previously described herein. The general procedure described in Example 1 is followed except that the source of radiation Is a scanning laser beam comprising an . argon-fluoride tunable excimer las6r and associated

906 electronics and optics obtained from Lumonics of New 'Jersey and a raster scanning mechanism obtained from General Scanning, Inc. of Watertown, Massachusetts,

Under computer-control, the laser beam is scanned in a predetermined pattern across the horizontal surface of the substrate, as described in U.S',. Patent No. 4,340,617, for example. There is deposited op the substrate a layer of a material, such as 81Οχ, which' has a continuously.

graded refractive index as a function of the thickness of the deposited layer and also a graded refractive index varying radially across the horizontal surface of the substrate In a pattern corresponding to the pattern of the scanning laser beam.

While the present invention has been particularly described with the respect to the !preferred embodiments thereof, it will recognized by those skilled in the art that certain modifications in form and detail may be made without departing from the intention and scope of the invention, in particular, the scope of the invention / 79391/2 // 38 is not limited to the photochemical vapor deposition of a continuously graded index layer of oxides of silicon, which was used merely as an example, but is intended to include oxides, nitrides, sulfides, and other materials and combinations thereof, with suitable optical properties. In addition, while the preferred process embodiment of the present invention has been referred to as a photochemical vapor deposition' process, it is not intended to limit the present invention to the process embodiment of U.S. Patent No. 4,371,587 in which atomic oxygen is phoj;.ochemically generated; rather it is intended to include any oxide fonned by any process embodiment of the latter patent.

Moreover, while the present invention has been 15 described with reference to a particular photochemical HIS PAGE; vapor deposition process which is specifically controlled uly 11, 1986 !_{n or(}j_er to form an oxide having a continuously graded refractive index in a predetermined pattern, it will be recognized that other known deposition processes for 20 forming oxides, as well as .other materials, may be similarly controlled to achieve a deposited layer having the same graded index profile and optical properties as described herein. Other materials which may be used to form the graded index layer are characterized by 25 being dielectrics which are transmissive in the wavelength range of interest.

Further, the present invention is not limited to the particular refractive index profiles specifically disclosed herein, but is intended to include any pre30 determined profile. In addition, it is not intended to limit the present invention to the particular process details described herein, but to include any variations in process parameters as may be required in order to achieve the desired refractive index profile in the 35 deposited material.

Finally, the present invention is not limited to the uses of the continuously graded index materials as specifically set forth herein, but includes structures comprising such continuously graded, index materials

<img file="IL79391A_D0013.tif" />

which may be used for any purpose .

Referring now to FIG. 9, a novel diffraction-type optical combiner is disclosed. The improved performance and relatively simple structure of the combiner.40 are well suited for applications such as HUDs or HVDs. The combiner 40 illustrated in FIG. 9 comprises a substrate 45, fabricated from a material such as glass or plastic. A diffraction coating 50 is. formed on the surface 46 of the substrate, and may comprise a gelatin hologram or a graded-index coating. The graded-index coating is preferably formed in accordance with t,he photochemical vapor deposition process described hereinabove or some other deposition process. An antireflective coating 48 is formed on the other surface 47 of the substrate 45.

The surfaces 46 and/or 47 can have a plane or spherical contour, but preferably have an aspheric contour selected to compensate or balance aberrations in the display system in which the combiner 40 is employed. The coating 50 can provide a narrow band, high reflectivity response, similar to a conventional gelatin hologram formed on־׳a spherical substrate. However, the gelatin hologram coating has improved optical performance over conventional gelatin holograms formed on spherical substrates, This improved performance results from the elimination of the slant fringes, the substrate contour providing the asphericity necessary for aberration compensation rather than the hologram itself. The considerations and principles which enter into determining the specific shaping of the aspherically contoured surface needed to achieve given design criteria are the same well-known considerations and principles which enter into determining the desired orientation of the fringes in a gelatin hologram with, of course, appropriate conventional compensation for any differences in refraction due to changes in the med ium.

The added flexibility in the coating design and fabrication using graded index coatings allows the production of improved performance combiners, such as those used to make high see-through and multiple color displays, and thus has certain advantages over the gelatin hologram coating.

The graded-index coating can be applied directly on plastic surfaces such as polycarbonate, and is inert to environmental effects such as humidity and temperature in the normally specified ranges, unlike the gelatin hologram. These two characteristics enable the gradedindex coating to be applied directly to the surface 46 of the substrate 45, which provides a major optical advantage. Since the surface 46 is the surface facing the observer's eye 58, as illustrated in FIG. 9, the combiner 40 substantially reduces the strong displaced second (ghost) image often encountered with a laminated gelatin holographic combiner. The intensity of the reflection from the second surface 47 is generally several orders of magnitude down from the intensity of the main image reflected from surface 46 and, therefore, is not a significant problem affecting the practical use of the combiner in a HUD or HVD system. Moreover, the ant!reflective (AR) coating 48 is applied only to surface 47 of the substrate 45, while the gelatin type of HUD or visor combiner requires AR coatings on both sides of the combiner.

079391/2 :1

־ ⁴¹ !

The reduction In intensity of the ghost image is Illustrated in FIG. 9. The display light source may typically comprise a CRT 55. The narrow band light generated by the CRT Is incident on the combiner 40 along ray 56. The coating 50 may typically be adapted to reflect 80% of the incident Tight from the source 55 &lt;., back along ray 57 to the observer's eye 58. The small portion of the display source light which is not reflected by the coating 20 is transmitted along ray 56a to the Interface of the substrate 45 and the AR coating 48. A typical AR coating typically reflects only about .5% of the incident light and transmits the remaining light. The reflected light travels along ray 57a; The coating 50 reflects 80% pf the light incident along the ray 57a and transmits only 20% of the incident light. The intensity of the second image along ray 57a is only (20%) (.5%) (20%), or .02% of the intensity of the light Incident on the combiner 40 from source 55. Thus, the intensity of the reflected second image is Ear less than the Intensity of the primary reflected image. p

The ghost image performance jpf the combiner 40 illustrated in FIG. 9 is contrasted with a conventional gelatin holographic combiner, comprising at least five layers, two outer AR coatings formed on outer surfaces of two glass substrates, which, in !turn sandwich the gelatin hologram. Even assuming c^parable performance for the AR coating ( .5% reflectivity) and 80% reflectivity for the gelatin hologram at the wavelength of the source light, there are now five interfaces to be considered, and three significant components of the displaced image. The combined intensity of the three secondary components is .84%, or a factor of 40 higher .as compared with .02% for the combiner shown in Fiq'. 9.

'1. ף 'j

The combiner illustrated in FIG. 9 can also provide improved see-through performance, larger exit pupil size, multiple color and optimized display efficiency. These improved performances directly result from the flexible design and fabrication processes achievable through deposition techniques such as the photochemical vapor deposition process described above, which are capable of depositing coatings with graded-index profiles.

For HVD or HUD applications, it is desirable for the viewer to see clearly the external environment through the visor or HUD combiner and that the color of the external scenes not be tinted by the combiner. This requires that only a small portion of the external light incident on the combiner be reflected by the diffraction coating on the combiner. If the reflectivity response of the combiner is not narrow band, or if there are significant sidelobes' in the reflectivity response about the peak efficiency wavelength, then the external environment will appear tinted to the viewer and the see-through performance of the combiner will be degraded .

The index profile of a graded-index coating can be designed to provide a narrow band, high reflectivity spectral response with side-lobe reflection minimized so that the see-through performance is improved. This feature is illustrated in FIGS. 10 and 11.

FIG. 10 is a graph plotting the reflectivity function of both conventional gelatin holograms and graded-index holograms as a function of the wavelength of the incident light. In FIG. 10 the reflectivity of the gelatin hologram is indicated by the broken line and the reflectivity of the graded-index coating is indicated by the solid line. The index profile of the gelatin hologram is sinusoidal. However, the graded-index coating is designed to have a non-sinusoidal profile which will yield suppressed sidelobes in the spectral reflectance profile of the coating. Typical peak reflectivity values of 80%, centered at 543 nm, are readily achievable by both types of coatings. However, the gelain hologram reflectivity exhibits typical sidelobes of appreciable reflectivity for light outside the narrow band wavelength range of interest. The sidelobes in the reflectivity function indicate that considerable light from the external environment will be reflected by the combiner and not transmitted to the observer's eye, thus degrading the see-through performance. On the other hand, the graded-index coating can be designed and fabricated to minimize the sidelobes in the reflectivity response, so that the see-through performance is improved.

By employing a graded-index coating with a particular non-sinusoidal index profile, the peak reflectivity response can be broadened so that a wider bandwidth Δλ of wavelengths is reflected with at least 80% reflectivity. To achieve a corresponding broadening of the reflectivity response for a gelatin hologram coating would typically require higher peak reflectivity, and consequent increase in the sidelobe level, resulting in undesirable refraction in the sidelobe spectral region. This effect is illustrated in FIG. 11 where the solid line depicts the reflectivity response of a graded-index coating with a non-sinusoidal index profile and the upper broken line depicts the reflectivity response of a gelatin hologram designed to provide the same bandwidth but with higher peak efficiency of 95%.

The graded index coating allows great flexibility in balancing the bandwidth, sidelobe and reflectivity response of the combiner according to the requirements of particular applications. For example, a bandwidth Δλ (as depicted in FIG. 11) on the order of 20-30 nm can be designed with acceptable reflectivity, and low sidelobe level. This allows the use of the phosphor P53 for the CRT light source, which emits light whose wavelength is centered at 543 nm within 5-10 nm range. This is in contrast to the P43 phosphor which is used in CRTs to emit light at wavelengths centered at 543 nm but within a 2-3 nm range. Thus, with the broadened reflectivity response, the image brightness may be increased.

The bandwidth increase achievable with nonsinusoidal graded-index coatings provides another performance advantage, an increase in the angular reflectivity function. The angular reflectivity function characterizes the reflectivity as a function of the angle of incidence for a given wavelength of the impinging light. If the reflectivity response is high over a wider band of incident angles, the exit pupil size is correspondingly larger.

The increase in exit pupil size can be maximized through control of two parameters. Increasing the magnitude of the index modulation (i.e., the difference between the highest and lowest index) through the thickness of the coating for a given non-sinusoidal, gradedindex coating is found to result in broadening of the peak reflectivity response, and correspondingly the angular reflectivity- bandwidth. Moreover, the gradedindex coating can be fabricated from substances having a higher average index of refraction than the gelatin hologram. For example, a graded-index coating formed from layers of Si0_x and Si02 can be designed with an average index of 1.75, compared to the typical gelatin hologram with an average index of about 1.50. This results in an exit pupil area which is about 30% larger than the gelatin hologram.

A requirement of a combiner used in a typical display system is that it be able to reflect light from the display source which is incident upon the combiner at angles of incidence which vary as a function of position on the combiner. A holographic or interferencetype coating is designed to reflect a narrow band range of wavelengths and to transmit light of wavelengths outside the narrow band range. The narrow band range, centered at a particular wavelength, shifts as the angle of incidence is shifted from the normal direction. The center frequency at normal incidence is typically referred to as the hologram wavelength. Because the display source light typically is not incident normally at the combiner surface, and in fact the angle of incidence varies .across the surface of the combiner, the peak wavelength must be varied accordingly across the combiner in order to maximize the display efficiency. Both the holographic gelatin combiner and the graded-index ) combiner are able to meet this requirement.

The graded-index coating can be generated on the substrate so that the peak efficiency hologram wavelength varies at different points on the' combiner to achieve maximum display efficiency in the designed viewing area. This effect is illustrated in FIG. 12. The optical combiner 60 comprises a substrate 61 on which a graded-index coating 62 is formed. A display source 63 generates display light at wavelength Xo. Three rays 64, 65, 66 of the display light are shown incident across the surface of the coating 62 formed on the combiner 60. The respective non-equal angles of

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Incidence for rays 64, 65, 66 are 0'j, 0₂, «3. At the points of incidence, the respective hologram wavelengths are _׳2ג A3, designed such that the peak efficiency wavelength reflected back to the observer's eye at the respective non-normal angles of incidence 0j, 0₂, θ₃ is י * ._סג

The same effect can be obtained by using an aspheric substrate contour with any diffraction coating, such as a gelatin coating, or to combine the aspheric substrate contour with a graded-index coating.

Thus, the combiner 60, in accordance with the invention, may comprise a substratd having an aspheric surface, and a diffraction type reflective coating for / reflecting radiation within one or more predetermined narrow band ranges of wavelengths impinging on the layer. The coating can comprise a gelatin hologram or j other type of diffraction coating. The preferred ן embodiment, however, comprises an aspheric substrate i of a lightweight plastic material, on which a gradedindex coating is formed. The aspherlclty can be designed ' in accordance with the requirement? of a specific optical system to compensate or balance optical aberrations in the system. Moreover, the graded-index profile can be designed to provide specific reflectivity responses, to provide multiple color capability, improved see-through, enlarged pupil size, and increased efficiency.

The fabrication of aspheric. plastic substrates is well known In the opthalmlc art, The aspheric surface is formed, for example,. by₍a casting process, utilizing a master (for example, gl'ass or metal) having the aspheric surface formed thereon by conventional grinding and polishing techniques./ The master is then employed to make a nickel plate replica, one for each surfac? of the substrate (each surface may have a different contour) . The nickel plate replicas of the surfaces may then be utilized to ca,st a plastic substrate.

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This method for fabricating the plastic substrate is generally illustrated in FIGS. 13 and 14. A desired aspheric surface contour 106 is formed on a glass substrate 105 to define glass master 125. Using the glass master, the nickel plate replica 110 of the surface contour 106 is formed, also by conventional techniques known to those skilled in the art.

&lt; By the same process, a second nickel plate replica 110a can be formed using a second master. The two nickel replicas 110, 110a may then be employed as cast surfaces to cast a premeasured quantity of an unpolymerized acrylic (or CR-39) compound 115 into a substrate whose contours replicate the contours of the nickel replica 110, 110a. The casting method Includes 15 disposing a compressible gasket retainer 120 around the periphery of the contours 106, 106a, sandwiching the quantity of unpolymerized acrylic compound 115 between the two plates and retained by the gasket 120, setting the plates in an elevated temperature bath so 20 that the acrylic compound polymerizes, usually shrinking by about 13%, to assume the contour^ 110, 110a. The plates may then be removed from the^elevated temperature bath and separated to remove the formed casting.

The casting method provides β₍ relatively low-cost 25 technique to fabricate the asphericjsubstrates in production quantities. The parts may also be fabricated by injection molding. In contrast ,;.fabrication of aspheric glass substrates in production quantities would be prohibitively expensive using conventional techniques.

<img file="IL79391A_D0016.tif" />

<img file="IL79391A_D0017.tif" />

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One application to which a combiner as illustrated in FIG. 9 may be advantageously employed is in helmet mounted visor displays (HVDs) used by aircraft flight personnel. As is known, images from a light source such as a CRT may be used to display symbology information or reticle information on a see-through visor, so that the symbology is presented to the helmet wearer as he is viewing ־the external environment through the visor׳. The general arrangemeht of a biocular visor system in accordance with the invention is illustrated in the schematic block diagram of FIG. 15. In this figure,, light is shown emanating from an object source 210, which may be a CRT, and directed by an image folding prism 211 via a relay lens 212 to a beam splitter.213. The beam path from the source 210 to the image folding prism 211 is shown unfolded for convenienc0 of illustration. At the beam Splitter 213, the incident light is split into two beams by splitter 213, which ruly 11,

L986 directs the beams laterally to a pair of folding members, shown as wing mirrors 214.

The.mirrors 214 redirect the respective beams through a plastic window 216 toward the respective aspheric mirrors comprising the combiners 215 which are optical elements as shown in FIG. 9. The selectively reflective properties of the combiners 215 cause the object source light to be redirected toward the user's eyes through exit pupils 217. Each eye views the image at the corresponding exit pupil as a virtual image at infinity. The intermediate images, represented by the broken lines 216, are developed between the relay lens 212 and the combiners 215.

One specific, preferred HVD embodiment is represented schematically in further detail in FIGS.

16-18. The arrangement of the principal elements of

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July 11, 1986 the biocular system is shown in FIG. 16 in relation to the wearer’s head, with the helmet omitted for simplicity.

The system comprises a minature cathode ray tube (CRT) source having a flat display element 220 bearing a phosphor on its inner surface 221. As indicated;in FIG. 16, the normal to the CRT plate 220 is aligned at 10 27.039° to the reference line 222 of the system.

Folding prism 223 is adjacent the CRT 220. The entrance face 224 of the prism 223 is orthogonal to the , reference line 222, while the exit face 225 is at a wedge angle of 4.554° to the reference line 222. The 15 light from the cathode ray tube is internally reflected, within the prism 223 which is spaced so that the entrance face 224 is located at a point which is .411 inches from a zero reference line 226, which is orthogonal to the reference line 222 at the outer surface of the CRT face 220. ״ ________________ __________

Next to the folding prism 223 is the relay lens

227 comprising three lenses 228, 229, 230 in a modified Cooke triplet, together with a fourth lens 231. The

Jluy 11, 1936_____{tw0 ou}ter lenses 228 and 231 of the relay lens 227 have aspheric surfaces while the lens 230 is a meniscus lens with spherical surfaces. The mounting of the relay lens off-axis serves to bend the axial ray 232 into coincidence with the reference line 222.

793)1/2

The light transmitted by the,relay lens 227 is then directed to a beam splitting prism 233. The front surfaces 235 (FIGS. 16 and 17) have a concave curvature . The prism 233 splits the light transmitted by the relay lens 227 into a pair of laterally directed images. These Images are reflected by wing mirrors 236, 237 (shown in FIG. 17) on opposite sides of the beam splitting prism 233 and sent to the respective combiners^,240 through window 230 employed to seal the relay optics from the external environment. Rays from a particular field point are reflected by the combiner 240- toward the user's eye through an exit pupil 248 (represented by.lines 248 in FIG. 16 and FIG. 17).

• The view in FIG. 17 is taken from the upper right of FIG. 16, parallel to the reference'line 222 and the aligned faces of the folding prism 223 and beam splitting prism 233 . For simplicity, the wing mirrors .236 and 237 are not shown in FIG. 16.

The axis (represented by line 227a) of the relay lens 227 comprising the lens elements 228-231 is at an angle of 8.630 degrees to the reference line 222, and the point at which the axis 227a intersects the surface 228a is displaced from the reference line 222 by 0.22 inches.

As indicated in FIG.16, the .;aspheric lens 228 is mounted such that the point at which its surface 228a is intersected by the axial ray 232 is located 1.645 inches from the zero reference line 226. . The face 234 of the prism 233 is 2.941 inches fifom the zero reference line 226, while the point at which,the reflected axial ray 232 exits the combiner 240 is 7,»526 inches from the zero reference line 226. The combiner 240 is oriented such that its element axis in the plane of

FIG. 16 forms an angle of 30.431 degrees with the path of the axial ray exiting the combiner 240 (extending in the direction of the exit pupil 248) .

The location of the wing mirrors 236 and 237 is limited in the disclosed embodiment by a requirement for a 40’ see-through capability and the necessary head clearance. This in turn fixes the location of the beam splitting prism 233 . ־| First order and packaging consideration (especially the need for a long back focus to allow the inclusion of the fold prism- 223) dictate that the aperture stop fall slightly before the prism 233. This is also one of the positions of the aperture stop for which tho relay lens 227 can most readily be designed for superior performance. However, with minor modifications of the design, the aperture July 11, 193£_ _{gtop may be lo(?ated} before or within the relay lens

227 if desired .

Sufficient eye relief has been, incorporated into the display system to allow the user to wear glasses and a standard oxygen mask. Given the eye relief and aperture stop location desired, the ‘focal length of the combiner 240 and the magnification of the relay lens 227 are: readily determinable. Because the user looks through the combiner 240, see-through distortion is carefully controlled. I;

<td colspan="3"> A summary of the optical characteristics of the</td>

<td colspan="3"> particular embodiment of the display system depicted FIGS. 15-19 is set forth in Table I.</td>

<td> TABLE</td><td></td><td></td>

<td> Parameter</td><td></td><td> Value</td>

<td> System Horizontal field of view Vertical field of view Exit pupil width (truncated Exit pupil height (truncated Effective focal length Horizontal f-number Vertical f-number Eye Relief CRT diameter Exit pupil separation Wavelength</td><td> circle) circle)</td><td> 40’ 30° 15mm 10mm 21.6mm 1.44 2.16 98mm 19mm 62.5mm 542-550</td>

Internal

<td> Combiner focal length</td><td> 50 .8mm</td>

<td> Combiner f-number</td><td> 3.4</td>

<td> Combiner bend angle</td><td> 60.8°</td>

<td> Relay lens f-number</td><td> 1.01</td>

<td> Relay lens field of view</td><td> 37°</td>

<td> Approximate relay lens width</td><td> 29mm</td>

July 11, 1986 .

It will be apparent to those skilled in.the art that the various parameters set forth above with respect to the particular embodiment depicted in FIGS. 15-18 are selected in accordance with acceptable design principles and may be varied in accordance therewith, in a manner well known to those skilled in the art, to adapt the design to various applications and specifications without departing from the scope of the present invention. Other parameters of the system, not specified, such as the prescriptions for the lens surfaces of the relay lens 227 and the surface of combiner 240, may.^be chosen in accordance with such principles to satisfy any particular design specifications. These principles are understood to those skilled in the art, as evidenced by optical engineering textbooks such as Modern Optical Engineering, by Warren J. Smith (McGraw-Hill, Inc, _____

1966) . .It will also be apparent to those skilled in the art that separate optical channels, each with a design similar to that illustrated in FIGS. 15-17, can be used, to provide a separate display to each eye by simply eliminating the foldng mirror 236 and prism 233. This dual separate channel configuration provides the capability for a stereoscopic display system.

The HVD system, depicted in FIGS. 15-19 has substantial advantages over the HVD systems employing gelatin holograms. The enhanced optical performance achieved by graded-index combiners is one such advantage,. While the specific performance parameters .to be optimized are application dependent, it is apparent that the aspheric combiner can be optimized to provide improved image quality substantially resulting from the minimization of the flare and chromatic dispersion inherent in a gelatin holographic combiner, larger exit pupil size, improved see-through and efficiency, larger field of view and elimination of ghost images. Moreover, the combiner can be fabricated from lightweight polycarbonate, substantially reducing the weight of the combiner and the burden on the visor wearer.

Another application for a combiner as shown in FIG. 9 is in head-up displays (HUDs) for aircraft cockpit use. A general schematic diagram of a HUD employing the novel combiner is depicted in FIG. 20. The aircraft has a canopy 260 and a surface 266. A combiner 270 comprises a substrate 269 and a diffractiontype coating 268. A cathode ray tube 278 provides an image source at object plane 276. The object plane is imaged through a relay lens 280 to a folding prism 282 having a surface angled to direct the rays over the surface of the combiner 270. The rays are then diffracted by the combiner 270 to the pilot's eye 284.

A head-up display employing the combiner comprising an aspheric substrate and graded-index diffractive coating has several advantages over systems employing gelatin holographic coatings. One advantage is the weight reduction, resulting from the elimination of the glass layers which are typically employed to protect the gelatin hologram. With a plastic substrate, the weight savings can be significant. Further, the plastic substrate does not present the safety risk of shattered glass inherent in gelatin holograms combiners; with a plastic substrate the combiner is birdstrike safe. This in turn permits the combiner to be located closer to the canopy than is permitted with glass combiners, allowing the design of head-up displays with greater look-up capability. Other advantages of the head-up display system employing the aspheric combiner are reduced see-through distortion and ghost images and improved image quality. Moreover, the lower cantilevered mass of the lightweight combiner increases the stiffness of the combiner and reduces its sensitivity to vibration.

The following summarizes some of the advantages resulting from combiners fabricated in accordance with the principles discussed herein:

a. the strong double image due to the first surface reflection in a gelatin combiner is eliminated because the graded-index coating is located on the outermost surface. This placement of the graded-index coating also eliminates the need for an antireflective coating on one side of the combiner;

b. the combiner can provide improved optical performance such as higher see-through and efficiency and larger exit pupil area and field of view due to the flexibility in coating design and fabrication;

c. the combiner is a lightweight assembly because only one substrate is used, which may be lightweight polycarbonate instead of glass. No cover lamination is needed, as compared with the gelatin HUD or visor display;

d. it is easier to fabricate a multiple-color HUD combiner or visor display combiner;

e. the combiner eliminates image-degrading flare and chromatic dispersion caused by slanted fringes in holographic gelatin combiners, which allows the use of broader-band display sources (CRTs) resulting in brighter, more efficient displays;

f. thegraded-index combiner with an oxide coating is environmentally stable and may, therefore, be applied on plastic, as well as on glass substrates;

g . the fabrication of the combiner is more cost-effective on a production level than gelatin combiners. Tooling costs may also be lower;

h. the reduced combiner thickness results in less see-through distortion even when both surfaces are asperic;

i. with only one wedge tolerance to consider instead of several, there is less difficulty in eliminating fabrication related see-through distortion, boresight error and, in the case of helmet visor displays, binocular disparity;

j. for head-up displays, the lower cantilevered mass of the lightweight combiner makes the combiner stiffer and less vibration sensitive;

k. the plastic substrate of the combiner provides a quality of being birdstrike-safe, thereby providing the capability of designing a head-up‘display with enhanced look-up capability;

l. the combiner when used in a helmet visor display is lighter weight than gelatin hologram visors, and has reduced see-through distortion and ghost images, with an overall improved optical MTF and image resolution.

Although the invention has been described with reference to specific embodiments, the exact nature and scope of the invention is defined in the following claims.