DE2245408A1 - HOLOGRAPHIC DEVICE - Google Patents

Description

Holographic device
(Priority: September 16, 1971, Japan, No. 71 196/71)

The invention relates to a holography device, in particular a holography device with a phase plate for the production of Holograms not only of digital information, but also of analog sample or image information such as drawings, Chinese Characters, letters and numbers.

For a good quality hologram, the following will be made Properties required: The storage density must be high, the reproduced image must and may be of good quality contain only slight disturbances, the diffraction or diffraction efficiency must be high and the maldistribution factor the information must be small. A method currently widely used for using holography to store information is the Fourier transform holography process in which lenses or objectives are used. Fig. 1 of the attached Drawing 2 shows the schematic view of an apparatus for carrying out this method. This is done by a laser source 11 outgoing parallel light beam (hereinafter referred to as "object light") 13 and into a reflected light

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beam (hereinafter referred to as "reference light") 14 means a beam splitter 12 divided. The object light beam 13 is amplified by means of a beam expander 18 into a light beam of parallel light of large diameter and by means of a focusing lens 17 focused on a hologram 15 which is in the rear focal plane of the lens 17 is arranged. The reference light beam 14 is reflected by a reflector 19 and onto the same area of the hologram medium 15 as the object light beam 13 projected. If the phase, amplitude or polarization of the Object light beam 13 corresponding to information to be recorded by means of an information transmission device or a modulator 16 is modulated immediately behind the Focusing lens 17 is arranged, so the object light beam interferes 13 with the reference light beam 14 «The information is stored on the hologram 15.

An example of the information transmission device is the well-known bit information card »as it is in Pig. 25 shown is. The card is provided with round holes that correspond to a digital information pattern in the form of a matrix are arranged in suitable places. A round hole corresponds to one bit of information. As shown in Figure 9 of the accompanying drawings is shown in which the light intensity is plotted as a function of the distance from a center, there is a difficulty in using the card in that the energy is located on very narrow parts of the spectrum. This is due to the mutual interference of the light rays passing through the respective round holes. Ss is therefore impossible to make the object light beam very intense compared to the reference light beam with fixed energy. That has the disadvantage the consequence that the reconstruction or reproduction quality cannot be very high. When suppressing the intensity of the peak of a sharp spectrum to set an appropriate level is to be, the signal energy falls on the cattle

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of the spectrum "when using a dry photographic plate as Hologram diura in the insensitive part of the photo plate, like that that the quality of the reproduced image is deteriorated. On the other hand, if the energy of the reference light is made very low, so the upper part of the spectrum falls into the saturation range of the dry photographic plate, so that also the quality of the reproduced image is deteriorated. According to a known method, in order to avoid these disadvantages, that the signal energy is formed on the hologram in a sharp spectrum, in such a way that a phase plate according to the positions of the round holes is placed on the card showing the phases of the through the round holes The light rays passing through the card shifts arbitrarily or arbitrarily, and which absorbs the light only slightly scatters. That is, the maximum value of the signal energy is compared made sufficiently low with the case in which no phase plate is used, so as to improve the reproduction quality of the holographic device to improve".

One method prior to using the phase plate employed a method in which the hologram plane was intentionally shifted out of focus in order to prevent that the maximum light intensity is localized too strongly on the hologram plane. This method is suitable for avoiding concentrations of light energy, however, it is then inexpedient if an information store is used with high storage density is to be produced and the size of the hologram is limited to a predetermined area is. When the hologram plane is shifted from the focal plane, the Fourier transform hologram is produced not possible. This leads to the further disadvantage that Fresnel transformation holograms have to be used, for which purpose generally a hologram with a higher resolution than Fourier transform holograms is necessary, another

: i 0 y 8 1 ti / 1 (J /, H

The disadvantage is that in the manufacture of the hologram an effect occurs on the focal plane through which all one-bit information is essentially uniformly distributed across all Distribute points of the hologram plane, while this effect is weakened when the hologram plane is out of the focal plane is shifted. In contrast, the phase plate described above is a very effective means of reduction the light energy concentration on certain spectra and to reduce the maldistribution of information on the Hologram plane. In fact, the well-known phase plate is very effective for improving the quality of the hologram, when it is applied to a holographic device in which the information to be stored consists of a digital signal, and in which the object light beam is modulated by bit information in the form of a matrix.

Since, however, when using the phase plate, the accidental Phase changes of the object light beam are generated discontinuously, the Fourier spectra are shown in FIG the light intensity is shown as a function of the distance from a center, subject to separations and the effective hologram diameter becomes large when the phase boundary lines are superposed within the round bit hole, in particular when analog information such as letters and patterns are to be recorded on the hologram. As a result arise when reproducing, according to the boundary lines of the phase, light and dark areas in the reproduced image, so that the quality of which is deteriorated.

The present invention is therefore based on the object to create a holographic device with a phase plate, which is particularly suitable for recording analog information on a hologram, through which the concentration the light energy is reduced to certain spectra and thereby the reproduction of a hologram is improved »by the

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Maldistribution of information on a hologram level is reduced which improves the quality of all kinds of analog information holograms, and with the one Information storage with very high density is possible.

The holographic device according to the invention is distinguished by an object light beam of the holographic device, for example Phase plate used with a phase surface on which several different continuous or continuous functional forms are randomly distributed. This determines the phase of the object light beam passing through the plate according to the invention arbitrarily shifted, the shift being distributed between 0 and 2 nji (rad), this shift The absorption and scattering of light are low.

The following are the prior art and the present The invention is explained in more detail with reference to the accompanying drawings, some of which have already been mentioned. Show it: 1 is a schematic view of a known hologram storage device;

2 shows a diagram of the light intensity distribution on a hologram when a known disk and a bit information card are arranged displaced one above the other;
Fig. 3 shows the waveform of an input signal of a known one

Fourier transform holograms; Fig. 4 shows the waveform of an input signal using the

known phase plate;
Fig. 5 is a diagram showing the waveform of an input signal

when using the phase plate according to the invention; 6 is a diagram showing the intensity distribution of the light on a hologram in the case of the input signal of FIG. 3;

Fig. 6 shows diagrams with the intensity distribution of the light on '* a Hologrammedium in the case of the signals of Figure 3 _f 4 and 5, respectively.

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9 shows a diagram of the intensity distribution of the light on a holographic medium in the known holographic device in the case of bit information;

10 is a diagram showing the intensity distribution of the light on a hologram with a known one, transferred to the card for bit information ben placed known phase plate;

11 is a diagram showing the intensity distribution of light on a hologram for bit information .in an inventive, on the card laid phase plate;

Flg. 12 shows the intensity distribution in a diagram! of light, ' if a randomly arranged information, for example a letter according to a known method is recorded on a hologram;

13 shows a diagram of the intensity distribution of the light on a hologram medium when the known ones are used Phase plate in the case of FIG. 12;

14 shows, in a diagram, the intensity distribution of the light on a hologram medium when the inventive method is used Phase plate;

Flg. 15 are perspective views and a sectional photos 19 ^{of erfindun} g ^ß ß ^{EMAE en} Phasenplatteι to 23

18a shows a known phase plate for digital signals; and 18b

24 shows, in a diagram, the dependence of the maximum permissible steepness on the effective hologram diameter; and

25 shows the representation of a known bit information map.

To make the advantages of the present easier to understand Invention over the known method is one one-dimensional information from only one bit is available. Without a phase plate, the input signal has a Fourier transform hologram the form shown in FIG. The undressed

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Lines in the figure represent the shape of the information (object light beam) which passes through a round hole. Will be a If the known phase plate for digital signals according to FIGS. 18a and 18b is applied, the input signal receives that shown in FIG Shape. In this figure, the dashed lines represent the phase changes. Here / there is a discontinuous phase point within the bit. The input signal in the case of an applied phase plate according to the invention is shown in FIG. The intensity distribution of the refracted light U (x) is calculated in each case according to the equations listed below, when the signal from an information transmission device follows the following equation:

[o s | £ | »F (D

where S is the diameter of the round hole.

If no phase plate is placed, the result is:

U ₁ (x) = c / t ₀ <£) e * p {-ik ££ Jd £ (2)

When the known phase plate according to FIG. 18 is applied, the result is:

U ₂ (x) * c / t ₀ (£) exp j-ik ^ -irf ^ dp. (3)

When the plate according to the invention is in place, the result is:

U ₃ (x) = c / t ₀ (£) exp [-ik% -i ^} d | (4)

Here, C is a quantity proportional to the amplitude of the incident light *, which is regarded as a constant, β the. Coordinate axis on the plane of the information transmission device, χ an axis on the Fourier plane. φ _Λ and φ _ο mean the size

Ben of the phase shifts due to the phase plates. They could be expressed by the following equations: ^. ^ -. .

. . '; jf s / ft -: f _t ν-

15/1 CJA 5

St: - |

TT ι 0

2rr

r <r <S

where 0 <r <S.

Iu ₁ (X) I ² , | U ₂ (x) | ² and | u ₅ (x) | ² are shown in FIGS. 6, 7 and β, respectively. Since the phase _{changes discontinuously at U 2} (x), a spread of the spectra occurs according to FIG. 7 on the high-frequency side of the Fourier plane. The reason for this will be explained below. The Fourier spectra of the object light beams subjected to the phase changes, which correspond to a straight line with the steepness or inclination a, are shifted from the center according to the following equation:

F (x) - / exp (- i aj *). exp (i The following applies to the putties of the spectra

This means,

(6)

If the phase changes discontinuously, as in U ₂ (x), and a is very large, the spectrum of the object light beam at this point is brought much closer to the high-frequency side on the Fourier plane. The opposite is the case with U «(x) when the phase changes continuously and a is not very large. The shift in the Fourier spectra is therefore weakened. Although at low fre-

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If spectral shifts occur, there is no overall result noticeable spread. If the effective hologram diameter is chosen to be sufficiently large, the information becomes on the high frequency side also reproduced, so that the steepness a can be chosen to be relatively large. In this case it will however, the information density is low.

When reproduction is performed with the effective hologram diameter set to a small value, so the density becomes high. However, the high frequency part of the information is lost and so is the quality of the reproduced picture will worsen. If an image is to be expected, its parameters (signal-to-noise ratio, contour, unevenly bright and dark parts, etc.) exceed certain conditions, so the minimum value of the effective hologram diameter, which is required for a certain steepness, is determined. this is shown in FIG. As can be seen from this figure, when an effective hologram diameter becomes a required information density is satisfied, determines the maximum steepness required for a reproduced image of good quality. That is, a phase plate corresponding to the information can be produced. On the other hand, with the one described above, must Process in which the hologram plane is shifted from the position of the focal point (hereinafter referred to as "defocusing process" the shift must be large in order to obtain a reproduced image of good quality. As a resultjwill the magnitude of the phase change due to the shift in the vicinity of the edge of the information transmission device (of the map) relatively large in terms of the steepness of the straight line. Therefore, the effective hologram diameter should be also be great. When using the phase plate according to the invention, the information density becomes ten times or more greater than in the known embodiment. For example, let the edge length of the card be 40 mm, f = 100,

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- ίο -

λ s 6 · 10, the defocusing or unbundling A »if / f = 0.05 and the information size b = 0.1 mm. Then one will

2. Phase change, with which the condition y * A ♦ χ is met, superimposed on the information according to the defocus process, where y is the size of the phase plate, χ is the distance from the center of the card and A is a coefficient »the calibration

from ■ "■■" ■ ■. "■■■. '■' ■ '■■ ■■> ■■ '■. ■ ''

. πΔ

results. _v

The steepness of the phase change is shown at the edge of the map to

a = y '= 2Ab = 100.

The maximum steepness for producing a .playback image of the same Quality at the highest density using the phase plate according to the invention is

K = S = 31.4

According to Fig. 24, the relationship between the effective hologram diameters according to the two methods is:

r _r : Rg. s 4: 1

where R _R and R ^ mean the effective hologram diameter when using the phase plate or when using the defocussing process.

Since the information density is inversely proportional to the square of the effective hologram diameter, can be compared with the defocussing method, by using the Phase plate according to the invention, the information density can be increased to sixteen times. ^

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When using the phase plate according to the invention, through which continuous or steady phase changes arise, there is no large spread of the Fourier spectra. At the same time, the height of the spectrum can be adjusted to the maximum intensity be lowered. In this way, an improvement can be made the quality of the reproduced image and an improvement in the reproduction quality.

The invention is to be explained below with reference to a preferred exemplary embodiment. In order to make the advantages of the present invention over the known methods clear, it is assumed that the information is one-dimensional . ■ ^y

In Pig. 1, let the focal length f of the focusing lens 17 be equal to 200 mm. As the information transmission device 16 used a card. First of all, consider a bit formation, in which round holes of the same diameter at equal intervals are arranged from each other. The diameter of the holes is, for example, 250 / rev, the pitch or the distance 500 / U, the number of information is 41 and the card is lying immediately behind the focusing lens 17 for the object light beam. The distribution of the light intensity on the hologram medium 15 is shown in FIG. 9 for the case where the medium 15 lies on the focal plane of the focusing lens 17, and that no phase plate is present. The distribution has the shape sharp spectra. The information energy exists only in extremely narrow areas, which is extremely disadvantageous.

Fig. 10 shows the light intensity distribution on the hologram 15 under the same conditions when a known one discontinuous phase plate is placed so that the discontinuous phase point is within the round opening is superimposed. The arbitrary unit of the ordinate axis represents the intensity of the light transmitted through the card The maximum intensity is compared with that of Fig.

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improved by an order of magnitude. In addition, the distribution widened. However, the spectra are separated into different peaks, with the diameter of the holograms on the High frequency side are stretched.

Fig. 11 shows the distribution of light intensity on the hologram 15 under the same conditions when the according to the invention, continuously changing bubble plate placed is. The maximum light intensity is improved to the same extent as in Fig. 10, but no spectrum separation occurs on and the local presence of the distributed energy is completely avoided.

Figs. 12, 13 and 14 show light intensity distributions on the hologram 15 in the event that the object light beam through the information transmission device accordingly one-dimensional information is modulated, which correspond to randomly arranged letters. In Fig. 12, no phase plate is used, in FIG. 13 the known discontinuous one Phase plate and, in FIG. 14, the continuous phase plate according to the invention. As can be seen from the figures, is the phase plate according to the invention is not only very effective in the case of regular or uniform bit information, but also in the case of general analog information that is not special Show regularity.

The information consists of a two-dimensional Analog pattern, the phase shifts correspond to those of the parallel light beam when passing through the inventive Phase plate is subjected to an arbitrary one or two-dimensional continuous or continuous function (Fig. 20 or 19) which is not regular and in which the distribution of the displacements changes between 0 and 2ττ (rad). The continuous functions are, for example, sine functions, repeating Gaussian distribution functions, polygon functions (approximately straight functions) etc.

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The phase plate according to the invention "consists of a AufWu, which, as shown in FIG. 15, contains a plurality of unit plates which each cause a one-dimensional phase change and on one Plane are arranged in parallel so that their phase change directions are identical. There is an advantage with this arrangement in that since the phase changes are in one dimension, the phase plate can be made more easily than in two Dimensions. In addition, according to the invention, a phase plate arrangement is to be used in which two such one-dimensional Phase plates are arranged one above the other, so that the one-dimensional Phase change directions can intersect each other. An advantage in this case is that the one-dimensional Phase plates can be easily manufactured and used in a two-dimensional pattern by using can be used in the cut state.

As mentioned above, the polygonal line function belongs to the continuous functions - those on the one-dimensionally variable Phase plate are distributed. The invention includes a phase plate with which the phase shifts, accordingly of the polygonal line function when passing through the plate between 0 and 2 ηπ (rad) (n is a positive integer) continuously will be changed. An advantage of this phase plate is that the polygonal line function is very useful for uniform phase change between O and 2 ηπ (rad). In addition, the function is continuous. The polygonal line function includes those shown in Figs are where the oblique lines of isosceles triangles with arbitrary base length and equal heights are connected to each other are. - ·.

An advantage of this design is that, when the polygonal line function, the shape of inclined lines is isosceles Triangles that have Fourier spectra on the hologram

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plane are distributed symmetrically on both sides in relation to the center, so that no uneven spectra can occur. Since the Base lengths are arbitrarily distributed, are also the inclinations or the steepness of the oblique lines of the isosceles triangles arbitrarily, so that maldistributions of the spectra on the hologram plane are reduced or at least reduced.

The invention further comprises phase plates in which isosceles triangles with certain base lengths are so distributed are that the occupation factor with respect to the entire phase plate can be smaller with larger base lengths. A The advantage of this arrangement is that the spectral intensity is uniform by reducing the more intense spectra can be made. As a result of the weaker steepness of the oblique lines of the isosceles triangles contain namely the Fourier spectra have a more intense spectrum in the low-frequency part of the hologram plane than in the high-frequency part the hologram plane.

The invention further comprises a phase plate (Fig. 17 » 22), in which the polygonal line function consists of successive isosceles triangles with at least one part in which the the phase shift communicated to the light beam passing through is constant. An advantage of this arrangement is that the spectrum is generated at the center of the hologram by adding the part that is parallel to the bases and has a constant phase will. In contrast, in the middle part of the Fourier spectra in the one described above, only isosceles are missing Triangles composed of phase plates make a spectrum.

The present invention also includes a phase plate (Fig. 23) in which the stepwise changing parts of the phase plate shown in Fig. 18 successively straight lines or curves are connected with each other, the steepness of which is less than a certain value. Because in this

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Case the location of the inclined surfaces on the connected parts is randomly distributed, no spectral maldistribution occurs on the hologram plane, even if the same inclinations or steepnesses are present. For example, in a phase plate, with the phase changes of O, π and 2π can be generated arbitrarily in equal divisions are accommodated, the sum of the numbers of the parts with which the phase changes of O and 2π can be effected equal to the number of parts with which the phase change of π is possible, according to the phase changing parts Fig. 3 may be connected to each other by straight lines, the absolute value of their inclinations being within those mentioned above permissible limits is the same, and wherein the slopes can have positive and negative signs.

■ The following is a method for producing the phase plates according to the invention are described.

The respective functional forms are generated by means of an electronic computer and displayed on a cathode ray tube in the form of differences in brightness. A dry photographic plate is exposed using the reproduction. After developing and fixing, the photographic plate is bleached so that the desired properties are shown as positive. This means that after the photo plate has been exposed, developed and fixed, it has a white-black density distribution. The silver deposited on the photo plate is converted into Ag ^ Fe (CN) g, AgCl + HgCl or AgCl + Cr ₂ Cl, i.e. into substances with a low light absorption factor and a high refractive index, for example by means of sodium ferricyanide, chromium brighteners (Kodak) and mercury chloride. In this way, the desired properties can be obtained. Another method that can also be used is to use an electron beam recorder in place of the cathode ray tube to generate the images. A coming also into question method is that the phase plate _is manufactured using gelatinous bichromates for the dry plate. '309815/1045

Of course, the phase plate according to the invention is not a diffuser that is mainly used for light scattering. As is apparent from the above description, the shape of the continuous function can be determined arbitrarily. So she can for example contain a differential quotient which corresponds to the density of the information transmission device communicated Information corresponds. The phase shifts can also fall within a predetermined range, or the shape can combine these two measures.

During the production of the phase plate according to the invention, the case sometimes occurs that the desired shape, for example isosceles triangles, are not preserved in perfect shape, but are somewhat distorted or distorted. Through this however, no problem arises because the purpose of the present invention can be substantially achieved.

The communicated by the phase plate according to the invention Phase distribution can be chosen so that it lies in the range between O and 2 im (n = 1, 2 ...). A similar effect results when a dry plate in which sodium dichromate is dissolved in polymethyl methacrylate is used in place of the above-mentioned dry plate with gelatinized bichromate.

With the device according to the invention, the Fourier transform hologram can of a general analog pattern or a bit pattern with good fidelity, with reduced Maldistribution of information and high density are generated. Furthermore, when using the known phase plate in the Attention was paid to recording a hologram of bit information that the round holes and the positions of the phase plate for the superposition correspond to each other. On the other hand, enough when using the phase plate according to the invention, a not very careful superposition. There is a similar effect without the need in known devices to pay particular attention to the correspondence of the position.

Claims

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Claims (16)

DA-10065 PATENT CLAIMS r \ J Holography device with a coherent light source ^ for generating an object light beam and a reference light beam, with a modulation device for modulating the object light beam according to information to be recorded, with at least one recording medium, and with an optical device for projecting the modulated object light beam and the reference light beam onto the same Place on the recording medium so that a hologram pattern is recorded as a result of the interference between the modulated object light beam and the reference light beam, characterized by a phase plate made of transparent material and arranged in the path of the object light beam (13) with a phase surface on which several different continuous functional forms in are arranged arbitrarily so that the phase of the object light beam passing through each part of the phase plate can be arbitrarily shifted. 2. Holography device according to claim 1, characterized in that the phase plate is designed so that their continuous functional form has a differential coefficient which is predetermined by a recording density of the information. 309815/1045 3. Holography device according to claim 1, characterized in that the phase shift of the object light beam lies within a predetermined range. 4. Holography device according to claim 1, characterized in that g e k e η η, that the phase plate is designed so. that the continuous functional forms are uniform in one direction and are distributed arbitrarily * perpendicular to this direction. 5. Holography device according to claim 1, characterized in that g e k e η η draws that the phase plate consists of several unit plates, on the individual surfaces of which the different continuous functional forms are designed so that they are distributed in a one-dimensional direction, the Unit plates are parallel to each other. 6. Holography device according to claim 5, characterized in that two of the unit plates are superimposed on each other are arranged so that the phase directions of the object light beam shifted by the two plates are perpendicular lie to each other. 7. Holography device according to claim 1, characterized in that g β k en η that the continuous functional forms of the phase plate are approximated by several straight lines »so that the phase of the object light beam passing through it lies on average within a range between 0 and 2 nn (rad). 309815/1045 8. Holograph! E device according to claim 1, characterized in that g e k e η η ζ ei c h η e t that the continuous functional forms of the phase plate have the shape of isosceles triangles, the heights of which are equal to each other, and which are arranged so that their oblique lines are connected with each other and their basic lines are different and randomly distributed. 9. Holography device according to claim 8, characterized in that the baseline of the isosceles Triangles has a certain length, and that these are distributed so that the degree of occupation thereof with respect to the entire plate is reduced with increasing length. 10. Holography device according to claim 7, characterized in that the phase plate is at least a part has, in which the size 'of the shifted phase of the object light beam is constant. 11. Holography device with a coherent light source for generating an object light beam and a reference light beam, with a modulation device for modulating the object light beam in accordance with one to be recorded Information, with at least one recording medium, and with an optical device for projecting the modulated Object light beam and the reference light beam to the same place on the recording medium, so that as a result the interference between the modulated object light beam 309815 / 1OA5 and recorded a hologram pattern on the reference light beam is characterized by one made of transparent material and in the path of the object light beam (13) arranged phase plate with a phase surface the several different step shapes are randomly distributed, each with the adjacent step shape are continuously connected. 12. Holography device according to claim 11, characterized in that the phase plate is designed so that the size of the shifted phase of the object light beam is arbitrary within the range between 0 and 2 nn (rad) lies. 13 · Holography device according to claim 11, characterized in that the step shapes with the adjoining Step shape connected by a straight line whose steepness is less than a certain value. 14. Holography device according to claim 13 »characterized by g e k en η that the straight lines have the same steepnesses. 15. A method for producing a phase plate for a holographic device according to any one of claims 1 to 14, characterized characterized in that several different 3 0 9 8 1 H / 1 (H 5 continuous, arbitrarily distributed functional forms are recorded so that a dry plate is exposed to light, which indicates the shapes and that the exposed dry plate is bleached. 16. Method of manufacturing a phase plate for a Holography device according to one of Claims 1 to 14, characterized characterized that several different continuous, arbitrarily distributed function formers are recorded, net and a dry plate of gelatinized dichromate with Light is exposed that indicates the shapes » 309815/1045 Blank page