Light modulating device

July 7, 1942.

2,289,205

P. NAGY ET AL

LIGHT MODULATING DEVICE Filed Feb. 20, 1940

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Patented July 7, 1942

2,289,205

UNITED STATES PATENT OFFICE

2,289,205

LIGHT MODULATING DEVICE

Paul Nagy, London, and Marcus James Goddard.

Newbury, England

Application February 20, 1940, Serial No 319 85R In Great Britain Janui^Mw ’

Claims.

This invention relates to light modulating devices more especially but not exclusively for use in television systems.

In an arrangement according to the present invention there is provided a series of transparent elements which are capable of substantially independent mechanical oscillation, a device associated with each element for producing the said oscillations and an electron discharge tube for successively exciting, and for controlling the excitation of, the oscillation producing devices.

Most suitably the transparent elements are constituted by portions of a transparent solid or a liquid contained within a suitable chamber, the oscillations in the various portions of the transparent medium being produced by piezo crystal elements. These crystal elements may themselves be separate crystals, or they may be portions of a continuous crystal with a series of electrodes by which the various portions of the crystal may be set in independent oscillations.

In the accompanying drawing is shown two examples of a light modulating device according to the invention, and a diagram indicating the manner of using such a device.

In said drawing, Figures 1 and 2 are sectional views, taken at right angles, of a device according to a first embodiment. Figure 3 is a diagram showing how such a device is employed in association with other apparatus. Figure 4 is a sec- tional view, similar to that of Figure 1, but show- ‘ ing a modified form of construction.

Referring to Figs. 1, 2 and 4, the device comprises an evacuated envelope 10, having a general shape similar to that of a cathode ray tube of conventional construction though it may be rather smaller. At the one end of the envelope is an electrode assembly the purpose of which is to produce an electron beam of controllable electron current, which beam is focused upon the 40 end of the tube and which may be deflected by suitable deflection control means. For a reason which will be hereinafter apparent, the beam in cross section is narrow, but elongated.

The electrode assembly comprises a cathode 1(45 of long, narrow shape, in front of which is a modulating or control “grid” 12. The purpose of this electrode is to vary the electron current in the beam in well known manner; as the beam must be long and narrow the control grid is pro- 50 vided with a slot of corresponding shape. The succeeding electrode 13 is an “anode” serving to accelerate the electron current emerging from । the grid opening, and is maintained at the same high positive potential with respect to the cath- 55 1 (Cl. 178—7.5) ode as a conductive coating 14 on the internal walls of the envelope 10.

Means are provided for the purpose firstly of ^foc_i^{using the} electron image of the ' θ .^{UPOn the end of the tube}> ^and secondly of deflecting the image over the end of the tube as desired. It will be readily apparent that the means for this purpose may be readfly selected from various well known means, as may be the ) means for producing the electron beam, but the means herein shewn comprise electromagnetic focusing means and electrostatic deflecting ?l^eans· The focusing means consists of a coil 15, and the deflecting means of plates 16 By the application of appropriate potentials to the iwates 16, the beam may be swept over the end of the discharge tube in known manner.

Located at the end of the discharge tube is the modulating device which is under the control of the electron beam. This comprises a transparent elastic medium which may be a solid but which is indicated as a fluid contained within a suitable container 17. This elastic medium is adapted to be capable of carrying trains of mechanical oscillations therein, under the influence of a series of oscillation producing devices. To this end there is arranged in the base of the container a piezo-electric crystal system, in this preferred construction, of a device according to the present invention each element consists of a portion of a transparent solid or part of a transparent liquid within a chamber, one side of the solid or chamber being bounded by a piezo-electric crystal or by a system of piezo-electric crystals

In the arrangement shown in Figs. 1 and 2 this crystal system 18 may consist of small elongated and separate piezo-electric crystals as indicated at 18α, equal in number to the number of elements of the device and for television uses equal to the number of picture elements in a fine, each crystal corresponding to one of the elements; m this case the side of the crystals adjacent to the transparent medium of the device is covered with a continuous metal layer 19 which covers all the crystals, while the other side of each crystal is coated with an individual metallic layer or strip 20 electrically insulated from the corresponding layers on the other crystals. In the modified construction shown in Fig. 4, the crystal system comprises a single crystal as indicated at 18b covering the whole of one boundary of the transparent medium. A mosaic of mmute crystal particles, or any intermediate form of crystal system may be used provided that the metallic layers covering the crystal system remain similar to that already described, that is the side of the crystel system adjacent to the transparent medium is covered by a continuous metal layer, while the other side is covered with parallel strips of metal, each strip corresponding to an element of the light modulating device.

The electron beam which is to produce the modulation of the elements makes contact, either directly or through suitable electrically conductive leads, with the metallic strips 20 on the side of the crystal system. The strips 20 may be scanned directly by the electron beam, but this entails complication as the crystal system must be mounted within the envelope 10. Preferably, therefore, contact is made from a contact row of metallic strips 21 provided within the cathode ray tube and similar to the row of strips 20 on the crystal system, each strip of the contact row being connected to the corresponding strip on the crystal system, which may be outside the cathode ray tube. The cathode ray beam makes contact with the strips of the contact row. The electron image formed by the cathode ray beam is of such dimensions that it makes contact with one only of the metallic strips at any one time, i. e., the electron image is restricted in one dimension to the size of the width of the strips, although in the direction of the length of the strips the electron image may conveniently be elongated to permit a fairly high beam current to be employed ₈₀in the cathode ray beam without unduly increasing the current density of the beam. It therefore makes contact in turn with each metallic strip on the crystal system, and conveys to that strip an electrical impulse of intensity corre- 35 sponding to the brightness of the picture element to which the strip corresponds.

The electrical impulse may be produced by the direct charging of the strip 20 (or 2i) negatively by the cathode ray beam. This direct 40 charging is, however, inefficient due to unavoidable secondary emission from the strip which reduces the effective charging current. Furthermore if the modulation level of the picture element corresponding to that one of the strips be- _4ging charged by the cathode ray beam is such that the strip becomes charged to a more negative potential than neighbouring strips, then secondary electrons tend to pass from said strip to the neighbouring strips, causing an effective spread- _c0ing of the charging area of the cathode ray beam, with consequent loss of definition of the picture reproduced by means of the light modulating device. The charging of the strip is effected much more efficiently and with much less loss of definition when secondary emission is utilised to produce positive charging. In this case a grid system consisting of wires 22 running parallel to the direction of scanning is mounted as near as practicable to the metallic strips 21 with which the cathode ray beam makes contact. The grid system is connected to the most positive potential of the cathode ray tube for example by a lead 23 to the coating 14, and is always more positive than any strip with which the cathode ray beam makes contact. The grid system 22 allows almost all the fast primary electrons of the cathode ray beam to pass through, but collects all the secondaries emitted by the strips. The said strips 21 are composed of or coated with a substance of high secondary electron emission, and so become positively charged to an extent proportional to the current intenS crystal

2,289,205 width of the electron image produced by the cathode ray beam, the electrostatic field produced by said grid system does not produce any defocusing of the electron image. This method of charging is very efficient because the secondary emission of a highly emissive surface is several times the primary current, so that the charge acquired by the strip is several times that actually carried by the cathode ray beam, whereas when direct negative charging is employed the charge acquired by the strip is less than that carried by the cathode ray beam.

There are two alternative methods of exciting the crystal system. In one method, only one electrical impulse is supplied in any one scan to each crystal element, in which case a pure picture frequency modulation of the cathode ray beam is sufficient. The continuous metallic layer 19 on the one side of the crystal assembly 18 is maintained at a convenient constant potential. The potential of any given metallic strip 20 on the other side of the crystal system is rapidly brought by the charge reaching it when the cathode ray beam makes contact with it to a value proportional to the intensity of the cathode ray beam at that time. This results in a sudden change of potential gradient across the portion of the crystal system lying between the given metallic strip and the continuous metallic layer. This causes that portion of the piezo-electric crystal system to expand or contract suddenly. The mechanical damping of the crystal system is made as small as possible, and the sudden expansion or contraction therefore causes the section of the • crystal system concerned to oscillate at its natural frequency. The oscillation of the crystal causes a train of supersonic waves to pass into the portion of the transparent medium adjacent to the section of the crystal which is oscillating, I and the intensity of the supersonic waves is proportional to the intensity of the oscillation of the crystal, that is to the intensity of the cathode ray beam when it makes contact with the metallic strip on the crystal system.

In the other method of exciting the crystal, several impulses are applied in succession to each section of the crystal system, the frequency of the impulses being preferably near the natural frequency of the crystal. In this case the crystal will be made to oscillate at its natural frequency irrespective of the damping, and a train of supersonic waves will be produced, but the damping should nevertheless be made as small as possible so that the section of the crystal will continue to oscillate after the cathode ray beam has ceased to make contact with the corresponding metallic strips. The succession of pulses may be produced by interrupting at the necessary frequency the current reaching the metallic strip via the cathode ray beam. This may be achieved by actually interrupting the cathode ray beam at that frequency, e. g., by applying signals of that frequency to the grid 13 controlling the current intensity of the beam, or by any other suitable method, e. g., by deflecting the cathode ray beam laterally on ⁸⁵ to and off the row of metallic strips in contact with the crystal system. It may be found advantageous to charge the metallic strip during only a small fraction of each charging cycle, e. g., by using class C carrier modulation of the cathode ray beam. This has the effect of applying sharp impulses to the crystal.

The action of the cathode ray beam in exciting these oscillations is not to produce a pure 1 · « ___ — — XT— — 1 λ1 Λ ATI ί

2,21 but an alternating potential superimposed on a D. C. potential, which is gradually built up as the impulses are applied; a similar D. C. potential is built up in the case where only a single impulse is applied. Each crystal element represents an electrical condenser. One plate of all the crystal elements is common. The other plates of these condenser elements are connected to the common plate through resistances which are of equal value for all the elements. The time constant belonging to each crystal element which constitutes the condenser is defined as is well known by the product of C and R, where C is the capacity and R the leakage resistance of each element. This time constant is preferably chosen 18 so that the potential across the condenser elements will be retained as long as is compatible with the periodic scanning action, i. e., the D. C. potential becomes negligible in a time of the order of one line period; if the time constant 20 were less, leakage across the condenser would absorb part of the energy to the supersonic waves, which are reflected as will be described below, while if it were more, an appreciable part of the signal due to one scan would be retained at 05 the commencement of the next scan.

The metallic strips on the one side of the crystal system may be replaced by a mosaic of mutually insulated metallic particles. In this case the contacting area of the cathode ray beam 30 itself at any one instant defines a crystal element.

In the conventional use of a crystal executing mechanical vibrations through its piezo-electric properties, an electrical alternating potential is 35 usually placed across the crystal, this alternating potential having usually a frequency corresponding to the natural frequency of the crystal. In the present case a potential difference between the plates of the crystal elements is introduced 40 by the cathode ray beam, which charges one plate to a potential corresponding to the intensity of the cathode ray beam, the other plate (the common plate) being maintained at a constant potential of such a value with respect to the cath- ._ ode of the cathode ray scanner as to ensure the best operation of the device. In the usual case mentioned above the positive and negative potential changes can be regarded as a pure AC problem, the alternations lasting sufficiently long for the considerations regarding a steady state to “° hold good. In the present case, however, we are concerned rather with transient impulses.

The light which is to be modulated is passed through the transparent medium in a direction at right angles both to the direction of propagation of the supersonic waves and to the direction of scanning. Modulation of this light. j_s produced by the action of the supersonic waves, either by the Debye-Sears effect, or by any other suitable means, e. g., by utilising the birefringence produced by supersonic waves in conjunction with polarised light. The intensity of the modulated light leaving any element of the transparent medium is proportional to the intensity of the supersonic waves in that element of the medium, and therefore also to the received picture signal’ corresponding to that element.

The crystal element corresponding to each element of the transparent medium is energised by the cathode ray beam for a small fraction only of the period of each line scan. Therefore it produces only a short train of supersonic waves, which, furthermore, quickly passes across the part of the transparent medium through which

39,306 \ 3 the light is passing. This short train of supersonic waves would, if unaided, modulate a very small fraction of the total light flux passing through the transparent medium. According to 6 a further feature of the present invention, in order to produce efficient modulation of the light the transparent medium is divided into sections by surfaces indicated at 24 in Figures 1 and 2 which reflect a proportion of the energy of the 10 supersonic waves and which lie in planes parallel to the surface of the crystal system. The distances between adjacent surfaces are preferably all equal. In order to give the maximum efficiency, the surface 25 forming the boundary of the transparent medium on the side remote from the crystal system should be as nearly as possible a perfect reflector for the supersonic waves, and the partitions between the sections of the medium should themselves absorb no substantial portion of the energy of the supersonic waves. These partitions should reflect a fraction, preferably not more than half, of the energy of the supersonic waves incident upon them, and transnut the remainder, if the transmission coefficients of the partitions are suitably chosen, then after a few reflections the energy of the supersonic waves is divided almost uniformly between the sections of the transparent medium. Preferably the height of each section is so chosen, with reference to the frequency of the waves employed, that resonance is produced in each section. If there are (say) n sections of the medium separated by (n-1) partitions, then the time taken to produce a uniform distribution of energy is a minimum if the transmission coefficients of successive partitions are η—1 n-2 n-3 n ’ n— 1’ n—2’ · · i where the first named transmission coefficient refers to the partition nearest to the crystal system, and so on. If the energy supplied in the form of supersonic waves by the crystal system is n times the energy needed to modulate fully the light passing through one section of the device, then after a few reflections the light passing through all sections of the device is fully modulated. Reflection from the surface of the crystal system and from the total reflector forming the opposite boundary of the transparent medium prevents energy from escaping from the device, so that the light remains modulated until the supersonic energy is absorbed by the various components of the device. The device is most efficient if the 55 energy decays to zero in the time of scanning _Of one picture line, so that each element has just lost its modulation when the cathode ray beam returns to produce a fresh level of modulation, ine attenuation time cannot exceed this value 00 ^W1‘nout causing the modulation level of any element in one line to affect the modulation level °^f ‘’“'responding element of the next line. If the attenuation is insufficient to reduce the modulation to zero in the period of line scan05 ^ning? ^as “ay be the case particularly if solid media are employed in the device, then additional attenuation may be introduced by arranging that the partitions between sections of the medium and/or the reflector which forms one boundary 70 of the medium should absorb a proportion of the supersonic energy; the leakage resistance of the condenser elements of the crystal system may also be adjusted to produce greater damping When solid media are employed, the partitions between 75 the sections may be formed by the interfaces be2,289,205 is the simple and automatic way in which storage is effected. The supersonic waves, excited by the cathode ray beam for a maximum of one picture element time, travel in the transparent medium belonging to that picture element for a time of the order of one line period. The light is thus modulated by the same impulse for a time of the order of one line period, so that the modulation of the light is stored until the cathode ray beam is ready to modulate it afresh in the next line. The total number of crystal elements corresponds to a whole line, so that a complete line, or almost a complete line, is always present on the screen. Thus in the direction of line scanning perfect storage is achieved. Furthermore, as a given portion of the cell always corresponds to the same picture element of any one line, no line scanning device apart from the cathode ray beam is necessary, thus eliminating the “high-speed scanner” and motor used in conventional mechanical optical television systems, and the only mechanical scanning means is the low speed frame scanner such as a mirror drum or an oscillating mirror.

In contradistinction, to many light modulating devices, the device described herein transmits light most efficiently in the direction of line scanning. The light efficiency at right angles to line scanning is, however, also high, and is capable 30 of being made high enough to enable the device to transmit an amount of light to the picture which approaches within measure of the amount supplied to a like picture by direct projection.