Optical arrangement with Fresnel lens

The invention relates to an optical arrangement with at least one Fresnel lens.

Stepped or Fresnel lenses go to the French Physicist Augustin-Jean Fresnel back, which already nineteenth century this as ring lens Marked optical element created. staged or Fresnel lenses have, otherwise in contrast to the used optical lenses with full body concentric, substantially perpendicular to the main plane of the lens disposed on stages, between which annular Portions are. The optically effective surfaces of the annular sections corresponding in shape in about the form of surface portions of a normal lens full-bodied, but are much closer to the opposite surface of the respective lens. Further are the optically substantially inactive surfaces the stages as possible in parallel with the arranged main light propagation direction to possible small reflections or little undesirable scattered light to produce. Therefore, a Fresnel approximation, to on through the stages caused disturbances similar Figure properties as a normal lens. Despite This interference has the Fresnel lens but significant Advantages over conventional lenses that these Lens Type in many applications to significantly preferred can be or even the only choice. Fresnel lenses have a smaller thickness require less of optical material, hence are lighter and have a reduced absorption and therefore a less heating in particular when they are used in lighting equipment with high light intensities.

Fresnel lenses are very advantageous in, for example, Stage spotlights for theater, stage, studio, film or used for architectural lighting purposes.

The reduced thickness of the Fresnel lens but is also their preparation is often much easier. For the Embossing, injection molding or hot molding is a thinner Fresnel lens in which cooling and mold release much better control than their counterpart full volume. fall with increasing size of these lenses These advantages all the more significant. Consequently, preferred applications the lighting equipment, especially in theater, studio, especially for the film on stage and in the architecture, in which a high amount of light often also a high thermal load with but brings disturbances in the imaging properties less consequence.

is from the signaling of rail-transport a Fresnel lens with a centrally arranged, the light preferably directing in the lower half-space parallel Known prism array, which is used for a Part of the light entering the Fresnel lens for the light to provide signal visibility at close range available.

The invention addresses the problem of providing a Fresnel lens in their usability, particularly for lighting Applications continue to improve.

This object is achieved in a surprisingly simple manner with the Features of claim 1.

With the invention, the light scattering element, in particular a diffusion plate, an additional Degree of freedom in designing optical properties be won.

The combination of geometric optical imaging of Fresnel lens with a superimposed thereon scattering lobe of at the lens scattered light permit, lighting technology highly interesting To achieve illumination light distributions. So can not suppresses only the light source or illuminant image but it can with a suitable choice of the scattering Structure and its geometrical dimensions even errors of illuminating beam paths greatly reduced or avoided will.

A particularly interesting application gives to himself Reflector arrangements with a relative to their Replaced by relatively small light source, such as a high-pressure discharge lamp emission regions in the order of a few millimeters and clearly have larger diameter version. In such Light sources may cause a darkening of the central Light field by coming that by the reflector passing replaced an opening within the reflector required, which is significantly larger than the light source and therefore light rays near the optical axis within this opening can not be reflected. By suitable choice of the forward scattering lobe of light scattering Device, preferably a circular central Lens, can surprisingly in Essentially, the geometric optical characteristics of the get Fresnel lens and can nevertheless a central will drop in intensity avoided.

Advantageously, this is the optical arrangement integral to both the Fresnel lens and the lens in a single stamping operation generate low production engineering.

Preferably, the stepped lens is an aspherical lens in order to compensate for spherical shape error and a possible To get good imaging performance.

optically If the Fresnel lens has a base body with a beam shaping effective, concave substantially having surface capable of such complex optical Accounting requirements are supported, as this For example, concave-convex or bi-concave lenses can be defined, in which the Fresnel lens and and their bases are geometrically-optically effective.

Furthermore, the Fresnel lens may have a basic body with a have substantially convex surface, thereby to create convex-concave or biconvex lenses.

Approximation, the shape of the body independently be used optical beam shaping and can beam-forming properties of the Fresnel lens in Combination or used superimposed.

Here, as the base of the Fresnel lens that part understood, which would arise if the Fresnel lens their levels would be removed, this means the Volume of material to which the steps of the stepped lens applied or in which these steps are imprinted.

It is thus technically possible production, initially in the form of calculate the desired stepped lens and additional optical beam-forming properties by the further Refinement "of the base, preferably in plano-concave, plano-convex, bi-concave, bi-convex or concave-convex shape to obtain.

When the substantially annular, optically effective Surfaces of the steps as a circular arc surface portions are designed to produced easily to realized geometries are used, which nevertheless optically still have relatively good properties.

In are a simple, inexpensive embodiment the substantially annular, optically effective Surfaces of the steps formed a conical envelope.

The optimum optical imaging performance is, however, in Substantially at a collecting Fresnel lens, thus a Lens with a positive focal length and focal point reellem, achieved when the substantially annular, optically effective surfaces of the respective stages shaped are that an approximately plane wave with the optical axis vertical phase fronts the lens then leaves when in this light enters, which from a single real Focus comes. In the case of a diffusing lens, thus a lens with a negative focal length and virtual Focal point, the optimum when the light is achieved, a plane wave, which enters the Fresnel lens in a spherical wave is converted, the center of a single virtual focus appears to come.

In a particularly preferred embodiment, the Lens only in a central region of the Fresnel lens and preferably arranged on the side of the steps, as this embodiment even with a single Hot-forming step can be produced with high precision.

It is very advantageous if the lens in a central region of the Fresnel lens is arranged demarcated, because then can hereby surprisingly variable Intensity distribution when lighting Lighting devices are produced. So can for example by use of diaphragms or by a amended focusing the incoming light field whose varying diameter and a variably adjustable Transition from scattered to geometrically-optically imaged Light are created. As long as only the inner light Lens hits, define their properties the Shape of the emerging and illuminating light box. If increasingly in enlargement of the diameter of the light field geometisch-optical imaging properties will emerge, For example, a very uniform enlargement of the illuminating light beam can be achieved.

An even steadier and smoother transition to change the Light distribution can be achieved when the light scattering element differ widely scattered areas, preferably a centrally more stray and having edge less strongly scattering region.

Preferably the lens is depending on the material to their scattering behavior adapted by hot forming, in particular, embossing, and / or injection molding manufactured.

Preferred materials for the Fresnel lens and / or the Lens are glass and glakeramische materials. A particular advantage of the glass-ceramics high Thermal shock resistance.

Further, the optical assembly with Fresnel lens and Lens of several elements assembled to For example, different manufacturing processes and their to take advantage.

For one, in particular embossed plastic stepped lens connected to an existing glass Lens are to give a hybrid composite of glass and Plastic results.

If the stepped lens includes a material having a first Dispersion behavior and a further lens opposite refractive power, preferably a Fresnel lens with a material with a second Dispersion behavior can even chromatically corrected or achromatic lens systems are created.

As optical path length is the purpose of this description, Wavelength of a central portion of each Light spectrum used previewed.

If the stepped lens is an embossed plastic lens, it can be very beneficial when an optical Path length difference at the respective level of less than about 1000 optical wavelengths since then in generally realized a relatively flat Fresnel lens may be, through which only small disturbances in the caused geometric-optical light propagation will.

Further, it may in locally high light intensities depending on the construction to be very useful, not as conventionally to use gelatin filter, which is in the range strong light intensity, such as near real hot spots fade quickly or even melt and can inflame but coated or dyed to use glasses.

Thus, the stepped lens and / or the diffusing screen as Filter, in particular as UV, IR or colored bandpass filter and / or adapted conversion filter, a much more reliable and more accurate filtering of light to be provided. Furthermore, it is within the scope of this Embodiment, sets of optical arrangements produce, which, preferably with dichroic or Interference filter layers on defined Voted light temperatures for defined light sources are.

For example, a defined color shift in The direction of lower color temperature values ​​of a door High-pressure discharge lamp, the spectrum of a Black body radiator, such as a light bulb to lend.

Furthermore, spectrally dominant bands of excited Discharge lines are reduced and thus a defined homogeneous spectral distribution are achieved.

In addition, with such filter assemblies for predetermined spectra of light sources and lighting scenes are simulated in their spectral distribution, as For example, morning light, night light, thunderstorm or Storm light, so that with a single light source an associated set according to the invention of optical Arrangements most requirements in the studio, theater, Film and architecture can be satisfied.

Since dichroic or interference filters high Radiation intensities with a large spectral precision withstand permanently, they may not, depending on application only spectrally better but by their long life even also cheaper than conventional color filters be. Further, harsh environmental conditions, such as for example, in architectural lighting or Exteriors another reason for use Such optical assemblies.

It is particularly advantageous when using Plastic lenses and / or -streuscheiben if these bear coated mechanical scratch protection layer.

Further, undesired reflections, in particular to the Step faces not only cause that light from the Main light flux is lost, but it can even in the illumination plane brighter circles or dots form, which on by an anti-reflective coating this be stepped surfaces greatly reduced or even suppressed can.

The invention is described with reference to preferred Embodiments and with reference to the Drawings accompanying explained in more detail.

Show it:<dl tsize="6" compact="compact"><dt>Fig. 1</dt><dd>a first embodiment of the optical Arrangement of a stepped lens with a centrally arranged roughly in Essentially circular diffusing screen which individual, mutually slightly twisted facets having,</dd><dt>FIG. 2</dt><dd>a second embodiment of the optical Arrangement of a stepped lens with a centrally arranged roughly in Essentially circular diffusing screen which has facets that by a Monte Carlo method displaced from their regular position were,</dd><dt>Fig. 3</dt><dd>a third embodiment of a Fresnel lens with a centrally arranged roughly in Essentially circular diffusing screen in which the individual facets of the lens lying on an Archimedean spiral,</dd><dt>Fig. 4</dt><dd>a cross section through a plano-convex lens with central Lens, whose base body Substantially plan and its Fresnel lens convex is trained,</dd><dt>Fig. 5</dt><dd>a cross section through a biconcave Fresnel lens arrangement which geometrically optical strahlerweiternde or light scattering and has characteristics in which both the Base as well as their geometrical optics effective Fresnel lens system essentially concave is configured</dd><dt>Fig. 6</dt><dd>an enlarged detail of an upper Portion of the cross-sectional view of FIG. 4,</dd><dt>Fig. 7</dt><dd>a cross-sectional view of a convex- Fresnel lens assembly whose base concave and their geometric-optically active Fresnel lens system substantially convex is adapted</dd><dt>Fig. 8</dt><dd>a cross-sectional illustration of an Hybrid lens array of an embossed plano plastic Fresnel lens arrangement, which on an existing glass Lens is attached,</dd><dt>Fig.9</dt><dd>a cross-sectional view of a hybrid lens achromats in which a glass-made Plano with a plastic or existing glass with other dispersion Biconcave stepped lens connected.</dd></dl>

Detailed description of preferred embodiments

The invention is described below with reference to preferred embodiments described in greater detail.

In this description, it is generally assumed that in the light entering the lens in the drawings of the left spreads coming to the right side.

Further, in the description of the various Embodiments, the same reference numerals for the same or in Substantially equivalent components of the optical Arrangement 1 used.

Reference is now made to FIG. 1, reference, which has a first embodiment of the optical arrangement of a Fresnel lens with a centrally arranged roughly in Essentially circular diffusing screen which individual, slightly twisted facets each having, shows

The covers throughout denoted by 1 optical assembly a Fresnel lens 2, and a medium in the region arranged diffusing screen. 3

The Fresnel lens 2 has concentrically arranged, annular steps with optical effective Surface areas, which only in FIG. 1 example provided by the numeral 4, 5 and 6 are.

The illustrated in Fig. 1 as well as in the figures 2 and 3 Lens 3 are exemplary lenses, as these in the German patent application DE 103 43 630.8 of the same Applicant of 19 September 2003 entitled "Lens" describes the content by Referring to the full extent also to the content of the present Disclosure is made.

In this particularly preferred embodiment, in Heissformungebungsschritt a single, substantially in an existing plastic embodiment, from a substantially planar base body 7, the optical Arrangement 1 produced.

Below are initially only commonality in Figs. 1, 2 and 3, the optical assemblies 1 described and then in detail their respective Differences explained.

The circular diffusion plate 3 is on the Light exit side of the main body 7, and covers the entire surface within the first annular portion 8, which clearly distinguished itself, preferably seamlessly adjoins this.

The base 7 is reellem at a lens, right side, thus a positive focal point preferably in Area of ​​the lens 3 and the field of annular surfaces 4, 5, 6 and 8, convex or after arched exterior shaped as for example schematically in the cross sectional illustrations in Figs. 4 and Fig. 6 is shown.

The basic body 7 is at a lens with virtual or negative, the left-side focal point preferably in the range the lens 3 and in the annular curved surfaces 4, 5, 6 and 8, concave or inwardly shaped as for example schematically in a Cross-sectional view in Fig. 5 is shown.

The basic body 7 may, however, particularly when using a hybrid lens, which in cross-section in the Figure 8 9, be formed in two or more pieces and includes both having the Fresnel lens 2 Body portion 7 and another illustrated body portion 9, which as shown in FIG. 8 shown respectively just schedule or as in FIG. 9 can be configured for example plano.

Preferably, the main body portion 9 is at produced hybrid glass lenses of a first material and is the main body section 7 of a second glass Material with a different dispersion than that of the The body portion 9 or of a heat-formable Plastic.

Reference is now made to Fig. 4 with respect which a planoconvex stepped lens with a central diffusing screen 3 shows as well as to FIG. 6, which shows a detail of Fig. 4 enlarged representation reproduces.

In the one-piece in Fig. 4 and Fig. 5 shown Fresnel lens 2, the respective optically active Surface 11, 12, 13 part of an aspherical or may be a spherical lens and the optical Arrangement 1 having an edge region 10, which for Holder in a zugeorndeten mechanical recording may be of plane parallel.

As part of an aspherical lens, the annular, optically effective surfaces of these levels (for example, 4, 5, 6, 11, 12, 13) shaped such that a plane in approximately Shaft perpendicular to the optical axis in phase fronts a reellem focus is united.

Here, the optical axis is through the center of the optical assembly is substantially perpendicular to the Main planes extend.

In the case of the illustrated in Fig. 5 biconcave lens stages the respective annular, optically effective Surfaces so shaped that, from a left- incoming plane wave, the phase fronts of Spherical wave generated whose virtual focal point or their apparent origin on the optical axis to the left to be of the Fresnel lens 2 shown in Fig. 5 seems.

Whereby these geometrical optics conditions exactly for a wavelength in a medium wavelength range of the Light spectrum used apply.

For ease of manufacture can complex in place of aspherical annular geometries also an aspherical Lens can be approximated by spherical ring portions.

Here, as well as possible approximate spherical Sections, thus arc surface portions used for the respective surfaces of the rings to a simpler manufacturing the necessary stamping tools to reach.

A repeated simplification consists example for Fresnel with a very high number of stages and only small optical path length differences of each between the edge of adjacent stages, to use cone-shaped optical surfaces which then only in their tendency to the average slope of the Asphere be adjusted.

Here, the individual annular portions and the central circular portion of the Fresnel lens, depending on on whether or light-diffusing properties desired are to be designed either concave or convex.

To illustrate the extremely variable applicability of the inventive concepts is shown in Fig. 5 is a biconcave lens stage, 7, in FIG. a convexoconcave Lens and are shown in Fig. 8 and Fig. 9 hybrid lenses, shown in FIG. 9 of which the lens chromatically having corrected properties.

Subsequently, the hybrid lens shown in FIG. 9 Referring, in which a plano-convex lens 14 with a real focal point on the right side of Lens 14 with a concave-convex diffusing stepped lens 15 connected is.

The respective powers or focal lengths and the Brechungsidizes of the two lenses 14 and 15 are a total of chosen so that still gives a convergent effect. This means it results in an overall positive lens with a right shifted focus.

Here, the material of the Fresnel lens 15 is so chosen such that the effect of the dispersion in the entire assembly of the effect of the dispersion of plano lens 14 runs against so that overall lower chromatic aberrations for this Lens system arise.

In an alternative embodiment, the Fresnel lens 15 also consist of an embossed plastic, which on the Lens is laminated 14th This plastic lens 15 can be provided with a scratch-resistant layer 21st

If embossed glass lenses used, the optical Path length difference in the respective stage preferably more than 100 optical wavelengths.

is When using embossed plastic Fresnel preferably an optical path length difference at the respective step of less than approximately 1000 optical Wavelengths preferred.

Furthermore, the around the central circular portion of the Fresnel lens arranged in the annular portions Substantially the same radial extension 16, this means the same step width 16 exhibit, see particularly Fig. 6. It is consequently different height levels, since typically the Angle of inclination of the respective annular, optically active Surface portions with increasing distance to the center change.

Alternatively, in order of production engineering high precision in difficult to accomplish to be molded materials, the height 17 of the optically effective surface portions of constant are maintained, so that in this way with rings different size width result, see in particular FIG. . 6

Furthermore, the Fresnel lens 2 and / or the diffusing screen 3 be designed as a filter, in particular as a UV IRoder colored band filters and / or conversion filters be formed.

It is particularly advantageous if this, a page, such as Example in Fig. 9 by way of example on the left side of Plankovexlinse 14 represented an interference filter layer 20 is applied.

This interference filter layer system can alternatively for shifting the color temperature or to compensate use of spectral lines.

It is furthermore particularly advantageous if at least the in each case a light source facing surface of the optical assembly 1 is made of glass and biased, preferably toughened as this one significantly increased thermal resistance is achieved.

It may the lens 3 generally left both on the, - Thus, the light entry side and on the right, thus the light exit side of the optical arrangement 1 be arranged.

Further, 7 it is possible, as shown in Fig. Only schematically shown, on both the light entry and on Light output side respectively to arrange a diffusion plate 3, so that their scattering effect superimposed defined.

Further, the lens 3, instead of a sharp radial boundary also differ strongly scattering exhibit areas, for example, a centrally more scattering region and a, preferably steady expiring edge less strongly scattering region exhibit.

For this purpose, the lens, for example, a defined having grain size which in a central region 22 a finer grain structure and with increasing radial Distance in a peripheral area 23 a coarser includes grain structure, see this issue shown schematically also Fig. 8.

In the embodiments described below of an alternative to simple grits or matted areas usable lenses 3, the new approach under alia, by the regular arrangement of facets a regular Lens departing.

This occurs at a first, in Fig. 1 shown Embodiment in that a diffusing screen 3 there is provided a transparent base body 7, 9, wherein the optically effective surface of the Diffusing plate 3 in the facets 24, 25, 26, which only are exemplified by reference signs, divided and wherein each facet 24, 25, 26 or an elevation depression formed with a second, arched Surface and is associated with the facets 24, 25, 26 are arranged relative to each other or twisted assume different geometric shapes.

Under a facet here is an area understood are the geometric of the edge contour of the respective Form is spanned. Depending on the design of the first Surface, ie the surface of the base body 7 of the Diffusing plate 3, as a flat or convex surface, the Facet 24, 25, 26, which by the geometric shapes spanned, also be plane or arched.

The facet 24, 25, 26 associated with collection or Depression is an element of the lens. 3 The Elevation or depression has the facet 24, 25, 26 as Area and located at least substantially above or below this base surface. The assessment or depression can act in case lighting as a lens.

This solution results in a superposition of Plurality of differently contoured light fields and thus wish according to a circular light field.

Depending on each of the facets and the configuration Nature of the facets associated elevations or depressions can be a light field with a providing selectable gradient of illuminance, or such which expires predetermined soft or hard.

A soft-edged light field is one having a low gradient of illuminance to the edge of Light field out. Conversely, a strong gradient of the Illuminance at the edge of the light field to a hard expiring Lichtenfeld. Another advantage achieved is that with this facet configuration marginal discoloration when using discharge lamps can be avoided.

To the diversity of the individual to the superposition to increase contribution of light fields and over the top to obtain said advantages, various Measures are taken.

So it can be provided that the facets of a polygonal have edge contour. Here is the number of vertices of the Polygons variable.

The facets with a polygonal edge contour should Cover surface completely, otherwise no local Scattering effect is given.

Furthermore, lenses can be provided, in which the facets 24, 25, 26 different have surface areas, as exemplified in Fig. 2 is shown.

As polygons three-, four-, five-, six- and / or Seven-corner are elected. The links between adjacent corners of the polygons may be straight or be curved lines.

Another consequence of the irregularity of the facets yields that these different orientations exhibit.

Another measure, with which one is the target round Light boxes, and the terms of illuminance Edge toward soft or hard leveling light fields approaches, is the selection and any variation of the respective buckle Projections or depressions. The buckle may be spherical, and the elevation or depression be designed according to a spherical cap. alternative , the curvature can be selected aspherical. furthermore is to ensure the abovementioned objective, the possibility the depth of the recesses or the height of the lands to vary.

From the foregoing it follows that the listed measures alternatively or cumulatively may be provided. For the practical implementation of above solution is in a first Solution variant a diffusing screen provided that a transparent base body having a first surface , wherein the first surface divided into facets is, and in every facet an elevation or depression formed with a second, arched Surface is associated, and wherein the vertices S the elevations or depressions along a spiral are arranged.

The apex is S of the elevation or depression defined as the intersection point of the passing facets Priority Surface normal of the facet with the curved surface the elevation or depression.

at two adjacent facets are the collection of the radius and / or the depth of the elevation or depression are different, in general, the common edge curved, and it will be apparent to the wells edges different in a plan geometric shapes accept.

The arrangement of the vertices S along a Spiral creates a variety of irregular arranged facets with which desired according to a round Light field is created, which in the case of discharge lamps Border area aüfweist no discoloration, and its Gradient of the illumination intensity can be specified.

The height of the elevations and depressions over the can Diffusing plate 3 across be varied, so that the Elevations and depressions of different heights or deep fail. This also contributes to the goal of a round and more or less soft or hard leaking Lichtenfeld provide.

In one embodiment shown in Fig. 3 the Lens 3 are the vertices of S Facets 24, 25, 26 is substantially on a Archimedean spiral.

The individual points are obtained by continued Removing a constant arc length L along the spiral from the inside to the outside. The vertices can equidistantly be arranged to each other. In addition to the equidistant arrangement of the vertices is also a variable arc length L possible. Thus, an inside outside increasing arc length L are chosen. To this Manner, inside the lens small Facets with elevations lower height or with depressions little depth, and thus a small scattering effect. To the The edge facets are larger, the amount of Elevations and the depth of the depressions becomes larger and the scattering effect is also greater. The light field then has a rather small half scattering angle with fairly large illuminance on the center. In contrast this at constant L illuminance would rather plateau-shaped and soft discontinued.

The above-mentioned measures, which alternatively and if necessary can be taken cumulatively, allow to many ways, the lens 3 to the respective An illumination system, for example, the respective reflector, adapt.

Thus, by the choice of the spiral type, the value of Arc length L, but also by varying or constancy of the Arc length, an adaptation to a reflector done. These measures allow the light field in predetermined regions of the illumination system to influence to reinforce it locally or weaken, and thus allow a variety of ways, the light field optimize.

From the foregoing it follows that the Professional with a wealth of possible methods parameters is given to the hand as the light field under We make light of the illumination system and can adapt. Insofar allowed the approach of different geometric shapes for the facets a very diverse and variable adaptation of Light field to the respective conditions.

Examples are shown in FIGS 1 and 2 further preferred Embodiments. In Fig. 1, a first embodiment the optical arrangement of a stepped lens with an approximately centrally disposed, circular substantially Lens, which mutually slightly twisted individual, has shown and Fig. 2 shows a second facet Embodiment of the optical arrangement of a stepped lens centrally disposed with a roughly substantially circular Lens, having what facets by a Monte Carlo method from their regular Location were added.

It should occur, that the realization of a wish according predefined light field or a desire according Scattering behavior also several options are open, which differ in style. So it allow Solution variants also, in terms of the aesthetic Appearance optimized lenses provide. It may for a facet example used diamond pattern or the shape of a cock passage will.

Further, it is within the scope of the invention also possible to non-coaxial or arrangements of nichtkonzeritrische use lens.

LIST OF REFERENCE NUMBERS

<dl tsize="2" compact="compact"><dt>1</dt><dd>optical arrangement</dd><dt>2</dt><dd>fresnel lens</dd><dt>3</dt><dd>Lens</dd><dt>4</dt><dd>annular concentric substantially optically effective surface portions</dd><dt>5</dt><dd>dto.</dd><dt>6</dt><dd>dto.</dd><dt>7</dt><dd>Base (section)</dd><dt>8th</dt><dd>as 4bis 6</dd><dt>9</dt><dd>Body part of the multipart arrangement</dd><dt>10</dt><dd>plane-parallel edge region</dd><dt>11</dt><dd>optically effective surface</dd><dt>12</dt><dd>dto.</dd><dt>13</dt><dd>dto.</dd><dt>14</dt><dd>Plano of solid material</dd><dt>15</dt><dd>convex-concave Fresnel lens</dd><dt>16</dt><dd>radial extent of the optically active Oberflächenabschitte, width</dd><dt>17</dt><dd>Height of the optically effective surface portions</dd><dt>18</dt><dd>Anti-reflective coating</dd><dt>19</dt><dd>Scratch protection layer</dd><dt>20</dt><dd>Interference filter layer</dd><dt>21</dt><dd>Scratch protection layer</dd><dt>22</dt><dd>central, more stray field</dd><dt>23</dt><dd>edge-side, less strongly scattering region '</dd><dt>24</dt><dd>facet</dd><dt>25</dt><dd>dto.</dd><dt>26</dt><dd>dto.</dd></dl>