Optical image amplifier.

00014. The invention relates to a device for amplifying the intensity of an optically generated image.

0002One of the first such devices, generally referred to as optical image intensifiers, is. described, for example, in CH-A 301 222. The basic principle of this known device is that a system of parallel bars spaced apart from one another is mapped onto an associated second bar system via a reflecting surface, the reflecting surface being located on a control layer deformable by electrostatic field forces, which together with a photoelectric conductor layer is arranged in an electrostatic field. The image to be amplified is mapped onto the photoelectric conductor layer in a raster form, whereby the electrostatic field is locally changed in terms of image and a corresponding deformation of the control layer and thus also of the reflecting surface is brought about. The deformed reflecting surface is imaged between the bars of the second bar system on a projection screen, on which an image of greater brightness corresponding to the image to be amplified can then be seen.

0003To generate the electrostatic field, there are two transparent, homogeneous electrode surfaces in the device known from CH-A 301222, which are connected to the photoelectric conductor layer or the control layer and connected to an electrical voltage source. When the photoelectric conductor layer is exposed imagewise, its electrical resistance changes. Due to the change in resistance in the longitudinal, ie The voltage distribution between the photoelectric conductor layer and the space between it and the control layer changes locally to the electric field parallel direction, which in turn causes a corresponding deformation of the control layer and with it the reflecting surface.

0004This principle of utilizing the change in resistance of the photoelectric conductor layer in the longitudinal direction has a number of disadvantages, which are explained in detail, for example, in CH-A 378 432. Among other things, a relatively large minimum thickness of the photoelectric conductor layer is required for a reasonably usable control effect, which on the other hand always brings with it the risk of disruptive space charge effects.

0005In CH-A 378 432 an optical image intensifier is described in which the disadvantages of the known image intensifier explained above are to be avoided by not using the longitudinal but the transverse local electrical conductivity or resistance change of the photoelectric conductor layer. For this purpose, the electrode which is in contact with the photoelectric conductor layer is designed as a strip grid, the electrically conductive strips which are arranged next to one another at regular intervals run orthogonally to the bars of the first bar system and are connected in alternating sequence to one and the other pole of an electrical voltage source are. Furthermore, an optical stripe grid with grid stripes running preferably at 45 ° to the electrode grid is placed in front of this electrode grid on the exposure side.

0006With this electrode design and arrangement, the potential distribution in the photoelectric conductor layer changes during exposure, and it is only this potential distribution in the layer that determines the forces acting on the control layer or the reflecting surface. The photoelectric conductor layer can therefore be as thin as desired, as long as its electrical resistance is not impaired by shunting its base. In addition, the difficulty of a high specific electrical resistance of the layer, which is present in the known longitudinal resistance modulation, is also eliminated, since a suitable choice of the voltage between the electrode strips and the thickness of the layer allows any practically available value.

0007Despite these obvious improvements compared to the image intensifier working with longitudinal modulation, however, the image intensifier described in CH-A 378 432 could not prevail in practice. One of the main reasons is primarily that in the electrode configuration of this image intensifier with alternating polarity of the individual electrode strips, an undesired basic deformation of the control layer occurs, which is much larger than the intended useful deformation caused by the imagewise exposure. This unfavorable ratio between basic and useful deformation is synonymous with low sensitivity and low efficiency of the entire arrangement.

0008A device which is significantly improved in terms of efficiency and short-circuit sensitivity compared to the system of CH-A 378 432 is described in EP-B 29 006 (2 US 519 682). In this device, all strips of the electrode adjacent to the photoelectric conductive layer, which is designed as a strip grid, are at the same potential with respect to the counterelectrode. The image to be amplified is exposed onto the photoelectric conductor layer in a latched form, the grid being orthogonal to the electrode grid. The latter is oriented orthogonally to the bars. The image is optically screened by a mirror screen, by an absorption screen on the surface of a fiberboard or by the fact that the photoelectric conductor layer itself consists of individual strips. Furthermore, the image to be amplified can also be illuminated or rasterized. The system has a very low basic deformation of the control layer and therefore has a relatively high sensitivity and high efficiency.

0009However, this device known from said EP-B 29 006 (sUS-A-4 519 682) still has certain shortcomings. For example, the relatively high dielectric constant of the glass carrier layer located above the electrode grid causes a homogenization of the electric field, which requires relatively high voltages for a given deformation of the gel layer and a relatively large grid spacing for a given air gap between the two electrodes. Further disadvantages are space charge effects in the photoelectric conductor layer, which lead to burn-in of the image (permanent deformation of the gel layer), and the fact that the gel layer is always deformed at the same points, which is also undesirable.

0010Further image intensifiers are in CH-A-454 296, US-A-3 638 027 (<img file="EP0247972A1_D0001.tif" /> DE-A-2 011 575) and US-A-4 023 969. The image intensifiers known from these publications also have more or less many of the deficiencies of the other known image intensifiers explained above. The object of the invention is now to improve a device of the type known from EP-B 219 006 (: zUS-A-4 519 682) with regard to the above-mentioned defects which still adhere to them. According to the invention, this object is achieved by the measures specified in the characterization of claims 1 and 4. Particularly advantageous and expedient refinements of the invention are described in the dependent claims.

0011The invention is explained in more detail below with reference to the drawing. Show it:<ul id="ul0001" list-style="none"><li>1 is a schematic representation of the overall device,</li><li>2 shows a control element designated SE in FIG. 1 according to EP-B 29 006 (US-A-4 519 682),</li><li>3 shows a simplified illustration of a first embodiment of a control element according to the invention,</li><li>4 shows a second embodiment of a control element according to the invention,</li><li>5 shows a third embodiment of a control element according to the invention with the associated electrical components,</li><li>FIG. 6 shows a diagram for explaining a compensating layer shown in FIG. 5,</li><li>7 shows a further embodiment of a control element according to the invention with the associated electrical elements and</li><li>Fig. 8 shows a detailed variant.</li></ul>

0012The overall arrangement of the device essentially corresponds to the device known from EP-B 29 006 (= US-A 4 519 682). The light coming from a light source 1 passes through two lenses or Lens systems 2 and 3 onto a first bar system 4, is thrown by this via a prism 5 onto a control element SE to be explained, which contains the control layer a lens system 8 directed onto a projection surface 9. The image 10 to be amplified is illuminated by a light source 11 and via a lens or a lens system 12 is imaged on the photoelectric conductor layer contained in the control element SE. The longitudinal edges of bars 4 and 7 run perpendicular to the plane of the drawing in FIG. 1.

0013As said, the device according to the invention corresponds to the known image intensifiers. More about their structure and their function can be found in the relevant literature or, for example, the already mentioned CH-A Nos. 301 222 and 378 432 and EP-B 29 006 (: 2 US-A-4 519 682).

00142 shows a known control element SE according to EP-B 29 006 (<img file="EP0247972A1_D0002.tif" /> US-A-4 519 682). The parts that are irrelevant to the understanding, such as frames, brackets, etc. are omitted.

0015The control element SE comprises a transparent lower carrier plate 20, on which there is a transparent conductivity coating 21 and the control layer 22 already mentioned, and a transparent upper carrier plate 23, on which in turn a metallic conductive electrode grid 25, a photoelectric conductor layer 26 and a black one , opaque and non-conductive protective layer 27 are arranged. There is an air gap with a width W of about 10-30 µm between the upper and lower carrier plates with their respective layers.

0016The electrode grid 25 consists of a series of transparent, electrically conductive strips 25a, which are arranged in parallel next to one another at regular intervals and are aligned parallel to the longitudinal edges of the bars 4 and 7. The grid period of the electrode grid, designated c, is approximately 50-200 μm and is preferably as small as possible. The width d of the conductor strips 25a is approximately 1/6 to 1/4 of the raster period c, preferably approximately 1/5. With a practical grid period of approximately 150 μm, the strip width is then preferably approximately 30 μm and the distance between two conductor strips is accordingly approximately 120 μm.

0017The individual conductor strips 25a of the electrode grid 25 are connected at their ends by a busbar 28, which is only shown schematically in the drawing, and connected to the one pole of an AC voltage source 29. The other pole of the voltage source 29 is connected to the conductivity coating 21 serving as the counter electrode on the lower carrier plate 20. The electromotive force of the voltage source 29 is approximately 200-300 V._eff at a frequency of about 100-500 Hz, depending on the photoelectric conductor layer.

0018The individual conductor strips 25a of the electrode grid 25 are therefore all at the same potential with respect to the counter electrode 21. Without exposure of the photoelectric conductor layer 26, it is insulating or poorly conductive, so that relatively flat potential troughs form between the individual conductor strips 25a, that is to say between the conductor strips, there is a smaller amount of potential with respect to the counterelectrode 21 than conductor strips 25a at the location. These potential valleys lead to a wave-shaped deformation of the control layer 22, these waves being directed parallel to the longitudinal edges of the bars 4 and 7 in accordance with the orientation of the electrode grid. The light originating from the light source 1 and totally reflected on the surface 22a of the control layer 22 is deflected transversely to the bars 7 and therefore passes the bars 7 onto the projection surface 9. The latter thus appears homogeneously brightened. If the photoelectric conductor layer 26 is now illuminated homogeneously, the potentials on and between the conductor strips 25a of the electrode grid 25 are equalized and the control layer 22 does not experience any deformation. The light totally reflected by the control layer can now not pass the bars 7 onto the projection screen, and the latter remains dark.

0019With image-wise illumination of the photoelectric conductor layer 26, a corresponding but reversed image-wise luminance distribution results on the projection surface, whereby light areas of the image 10 to be emphasized appear dark and dark areas light on the projection surface. The control layer 22 is any elasto-viscous material, the surface or the optical properties of which can be deformed or changed by an electrical field. The control layer is preferably a gel layer with a thickness D of approximately 30-120 μm, preferably approximately approximately 70 μm. As a gel, for example, the quality offered under the name Sil-Gel 604 by the company Wacker, Munich FRG comes into question. The control element SE described above according to EP-B 29 006 (z. US-A-4 519 682) still has the shortcomings explained above. These deficiencies are now eliminated by the improved control elements according to the invention shown in FIGS. 3-8. The basic structure of these control elements according to the invention corresponds to that according to FIG. 2, so that the control elements are only shown schematically in section (parallel to the paper plane in FIG. 2). However, these sectional representations make it easy to see the details important for understanding the invention. The components of the control elements according to the invention which have not been changed from FIG. 2 are given the same reference numerals as in FIG. 2nd are referred to and are not explained again separately below.

0020The control element SE according to FIG. 3 has between the upper carrier plate 23 and the electrode grid 25 a continuous and optically transparent ground electrode 42 which is separated from the latter by an insulation layer 41 and which is connected to the same pole of the AC voltage source 29 as the counter electrode 21. This ground electrode 42 reduces the homogenization of the electric field caused by the carrier plate 23 and thereby allows lower voltages and smaller grid spacings for a given air gap W. The distance a between the electrode grid 25 and the ground electrode 42 is approximately 5 μm to 20 μm, preferably 10 _I.Lm up to 15 µm.

0021FIG. 4 shows a variant of the control element SE of FIG. 3. Here, the ground electrode 42, like the electrode grid 25, is designed as a transparent strip grid, the individual grid strips 42a being at a gap from those of the electrode grid 25. The grid strips 42a are electrically connected at their ends and connected to the voltage source 29.

0022The essential features of the control element SE shown in FIG. 5 is that the photoelectric conductor layer 26 is electrically insulated from the electrode grid 25 by an insulation layer 43 and that the electrically conductive strips 25a of the electrode grid are alternately connected to one and the other pole of the AC voltage source 29 are connected, ie every second strip is at the same potential as the counter electrode 21. Furthermore, a compensating layer 44 is provided between the electrode grid 25, the strips of which are designated A and B here, and the upper carrier plate 23. This has the optical damping behavior shown in FIG. 6, that is to say the optical damping 8 of this layer is lowest in the region of the grid strips A, B of the electrode grid 25 and strongest in between. The damping is preferably brought about by the reflectivity of the compensating layer. Due to the specified damping curve, the compensation layer 44 causes the control light to have the same control effect everywhere.

0023The alternating potentials of the successive strips A, B of the electrode grid 25 have the result that the potential on the gel surface 22a is well modulated. Due to the lack of electrical contact between the electrode grid 25 and the photoelectric conductor layer 26, less undesirable space charges build up and the photoelectric conductor layer 26 remains neutral overall. Space charges in the photoelectric conductor layer tend to build up at twice the spatial frequency of the control field. As a result, the influence of the space charges in the photoelectric conductor layer on the surface 22a of the control layer 22 is much less.

0024The strips A, B of the electrode grid 25 are preferably transparent, for example made of indium tin oxide. As a result, the control light can also be effective under the electrode grid, which reduces moiree phenomena.

0025The insulation layer 43 preferably consists of a high-purity, high-temperature-resistant plastic, for example polyimide, or of an inorganic insulator, for example silicon dioxide or silicon nitride.

0026The photoelectric conductor layer 26 preferably consists of amorphous or crystalline silicon. It is advantageously interrupted by a large number of non-conductive or poorly conductive tracks 26a, which run transversely to the longitudinal direction of the strips A, B of the electrode grid 25 (FIG. 8). With a raster period c of 50 gm, for example, these tracks 26a are each about 1 µm wide and about 6 - 7 µm apart. The mutual spacing and the number and width of the tracks 26a are dimensioned such that their presence, with uniform illumination of the photoelectric conductor layer 26, means that no appreciable influence on the potential profile on the surface 22a of the control layer 22 can be determined. The non-conductive or poorly conductive tracks 26a, which of course can also be simple interruptions in the photoelectric conductor layer 26, prevent or complicate charge carrier transport along the strips A, B of the electrode grid 25, and thus an undesirable phenomenon similar to the so-called "spreading" of picture lines .

0027The opaque protective layer 27 is non-conductive or in some cases has a significantly lower conductivity than the photoelectric conductor layer 26 without light exposure (dark conductivity). Furthermore, it should also have no photoelectric effect. It can be realized by vapor-deposited pigments, pigments bound in epoxy resin, cermet or amorphous silicon doped with p and n.

0028The air gap between the control layer 22 and the protective layer 27 is preferably a gas with a high dielectric strength, for example SF₆ filled out.

0029The control layer 26 consists of a gel and is preferably doped with additives that make the refractive index as large as possible, but without reducing the transparency. An example of such additives is phenyl silicone oils.

0030Furthermore, the gel layer 22 is preferably doped with additives that make the gel conductive, but without adversely affecting the optical properties. Examples of such additives are polyacetylenes.

0031The voltage source 29 advantageously supplies a square-wave voltage. As a result, the gel layer 22 experiences a uniform force as long as the conductivity of the photoelectric conductor layer 26 is low. This is favorable for the light efficiency of the bright field.

0032Furthermore, an automatic switching device 45 is advantageously provided, which periodically swaps the polarity of the strips A and B of the electrode grid 25 (frequency 0.1 to 0.5 Hz). As a result, the gel layer 22 is not always deformed at the same point and does not undergo plastic deformations. Any remaining space charge effects that may still exist are also symmetrized. For interruptions in the strips A, B of the electrode grid 25 or To prevent their effects, the stripes of the same polarity are preferably electrically connected to one another at their two ends.

0033Between the insulation layer 43 and the photoelectric conductor layer 26 or between this and the protective layer 27 there can also be a thin passivation layer 46 made of silicon dioxide or silicon nitride (FIG. 7), which suppresses disruptive surface effects on the photoelectric conductor layer 26.

0034A practical exemplary embodiment of a control element SE according to FIG. 5 has the following data:<ul id="ul0002" list-style="none"><li>Screen period c: 30 µm</li><li>Width of the conductor strips d: 6 µm</li><li>Insulation layer thickness 43: 5 µm</li><li>Thickness of the photoelectric conductor layer 26: 0.7 µm Thickness of the protective layer 27: 3 µm</li><li>Air gap W: 20 µm</li><li>Thickness of the gel layer 22: 40 µm</li></ul>

0035The transparent electrodes are preferably made of indium tin oxide. All other data, unless otherwise stated, according to the prior art, for example EP-B 29 006 (<img file="EP0247972A1_D0003.tif" />US-A-4 519 682).

0036FIG. 7 shows a further variant of a control element SE according to the invention, which essentially differs from the variant according to FIG. 5 only in that the strips of the electrode grid 25 designated A, B and C now alternate cyclically to one pole each three different AC voltage sources 29a-c are connected, the other pole of which is connected to the counter electrode 21 (ground, reference potential). Otherwise, this variant corresponds in all details to the embodiment according to FIG. 5. Furthermore, a phase shifter stage 47 is also provided.

0037The three AC voltage sources 29a-c generate three identical, amplitude-modulated AC voltages, the modulating functions of which are phase-shifted from one another by 120 °, according to the following formulas:<maths id="math0001" num=""><img file="EP0247972A1_D0004.tif" /></maths><maths id="math0002" num=""><img file="EP0247972A1_D0005.tif" /></maths><maths id="math0003" num=""><img file="EP0247972A1_D0006.tif" /></maths>

0038Are in it _{Among other things}(t), _Ub(t), u_c(t) the instantaneous voltages, Uo the peak value of the amplitude, ω the frequency, (p_O the phase of the modulation and k a parameter between 0 and ≧ 1, in practice preferably k = 1.

0039The advantage of this 3-phase control of the electrode grid 25 is that the location of the greatest deformation of the control layer 22 is no longer bound to the grid, but can be set electrically via the modulation phase (po. If (po, for example, 0, then is located the location of the largest gel layer deformation at the location of the grid strips A. At (p_O = 60 ° it lies between stripes A and B. Depending on (po it can run through all points from stripe A to stripe C).

0040By suitable variation of the modulation phase ϕ over time₀ the gel layer 22 can be stressed uniformly over time at all locations, thereby avoiding permanent deformation. With a suitable choice of k (e.g. k = 1), this displacement can take place in such a way that the gel deformation never becomes zero.

0041The temporal variation of the modulation phase ϕ₀ is carried out by the phase shifter stage 47 in such a way that (po with a frequency of, for example, 0.5 to 50 Hz, periodically runs through all possible values ​​from 0 ° to 360 ° Improve resolution and average moiree effects and thus suppress them.

0042In this embodiment variant, a raster period c of 20 μm with a width d of the raster strips of 5 μm has proven itself for the electrode raster 25. All other data and design features are the same as in the exemplary embodiment according to FIG. 5 or, if not specified in more detail, as in the known devices of this type.