DE1924585A1 - Methods and devices for the production of holograms - Google Patents

Description

Dr. Klaus Biedermann May 13, 1969

8025 Unterhaching 10-hu-kl

Ter-Meer-Str. 28

Method and device for producing holograms

The invention relates to methods and devices for manufacturing of holograms with an object beam reflected from the object to be holographed and one coherent with this Reference beam.

When recording holograms, special attention must be paid to the correct exposure if one is satisfactory Should get results. Compared to exposure control in conventional photography Please note that both the intensity of the field reflected by the object and the intensity of the Reference field is to be taken into account. However, it is not just the sum of the two quantities, but also their relationship cease in a certain way. The intensities vary with the size and degree of reflection of the recordings Objects and especially with changes in the shooting distance. In addition, the characteristics of the recording material go very well heavily in the result.

009848/1500

The aim of the invention is to provide a method and devices for correct exposure control in the production of Indicate holograms. According to the invention, the intensities of the object beam and / or of the reference beam are used for this purpose automatically controlled by a programmed control device so that in the layer support level, in the the two beams interfere, one taking into account the modulation transfer function of the recording material at least a minimum value reaching modulation occurs, and the total amount of light is about the Exposure time according to the sensitometric data of the Recording material and its processing, if necessary using a non-interfering additional exposure automatically regulated by the control device so that the modulated part of the recording in each the most favorable range of the characteristic curve of the recording and playback process.

The method according to the invention provides a teaching how to proceed with the exposure control. First is by tuning the intensity of the lens field and reference field on each other either by direct measurement of the interference patterns resulting from the superposition or by measuring the intensity of the interfering wave fields the modulation to the desired value

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po 125 / MO 174

brought. After knowing the irradiance on the Recording material or the total available energy in the case of pulsed lasers can now, if necessary using a non-interfering additional exposure, e.g. by a thermal light source, determines the exposure time that the exposure is in the most favorable range of the characteristic curve of the recording process. In this way holograms can be produced in large numbers and with a consistently high degree of reconstruction efficiency while avoiding rejects. By entering more Dates can also be the exposures for multiple recordings on the same hologram or for the special circumstances the reconstruction, e.g. combination of hologram and object or of superimposed holograms be adjusted.

Further details and advantages of the invention result from the subclaims in connection with the "description of exemplary embodiments which are explained in detail with reference to figures. Show it

Fig. 1 is a schematic arrangement for exposure control in the production of Holograms with direct measurement of the modulation and '

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po 123 / MO 174 1924 58 b

Pig. 2 shows an arrangement for exposure control based on measurement of the individual interfering light wave fields »

f, ^ exposure control for image plane holograms and ^ a

When calculating the superposition of two light wave fields of known intensity under simplified assumptions, it results that the modulation p _Q follows

Size has: 2 • ^Vl o • v. 2. "Vl
P = O T ¹ O ^{+ I} ,

where I is the local intensity of the first field, e.g. of the field reflected by the object and I the intensity of the second field, e.g. the reference field. By recording in the layer, the modulation of the imposed exposure by the spatial frequency-dependent modulation transfer factor M (V-) decreased.

The exposure mean value is determined by the subsequent treatment, e.g. developing and, if necessary, bleaching and its modulation via the characteristic curve of the recording material into corresponding values of transparency or the optical " Transfer path length difference. Since the luminous flux in the reconstructed image is proportional to the first approximation Square of the modulation in the hologram, the angular modulation should be large and the implementation characteristic of the material

00 9 8 48/1508

² = cP ² .

po 123 / Mo 174 1924585

be steep. The gradient G (E) of the characteristic curves goes through a maximum that becomes higher, but also narrower, the more the curve is steeper. The gradient G goes into the efficiency of a hologram according to a rough calculation quadratically, since the luminous flux is £ in the reconstructed image

/ «? C · Fp ₀ · M (Y-) · G (E)]

As a first approximation, P is the modulation in the hologram grating. This means that the efficiency of a hologram is all the more Can be higher, the more precisely you get the working point on the Can adhere to the characteristic curve. It is therefore an object to control the mean value of the exposure to a certain one given value E.

Non-linear transmission via the characteristic curve and intermodulation of the object field result in "optical noise", ie scattered light, in the reconstruction. For this reason, the effective input modulation must be limited. The other task is therefore to _{coordinate the intensities I 0} and I _{r of} the two wave fields, taking into account T = I + I and M (V), in such a way that a given effective modulation ρ results. In the case of the recording material, the operating point for quasi-linear transmission does not necessarily coincide with the operating point for maximum

009848/1808

po 123 / MO 174- Ί 9 2 4 ο 0 b

Gain together, it is usually a compromise between the efficiency of the hologram and the impairment caused by it to find nonlinear effects.

The overall task is therefore to determine the local intensities I for each point (x, y) in the hologram plane (x, y) and I (x> y) and to regulate the exposure time t in such a way that that, if necessary, including or with the aid of an incoherent (or in general: non-interfering) Additional exposure I (x, y) t = E (x, y) the modulation P (x, y) receives a given value:

. t

t + i _z (x, y). t _z | = p.

Without additional exposure, the fraction (the modulation of the imposed exposure ρ) is a simple function of the Ratio k = I / I:

P ₀ ⁰

k + A '

M (v) is a material size of the photosensitive layer

009848/1508

po 123 / MO 174- ι 9 2 4 5 ο b

and changes with the spatial frequency V, which is dependent on the angle of view • uniformly and usually slowly over the surface of the hologram in the plane of the direction of propagation of both fields. Ghost also a material size that is also determined by the processing (development, possibly bleaching).

In Fig. 1, an arrangement for exposure control in hologram recordings is shown schematically. With 1 is denotes a reference wave field, which in the plane of the layer carrier 4 with a reflected from the object 2 to 1 coherent object wave field 3 interferes.

A microscope objective 5 is arranged behind the plane of the layer carrier 4, which creates an image of the interference structure in a plane 6, greatly enlarged. The magnification is such that the average interference fringe spacing is still well resolved by a photoresistor 6a arranged in plane 6, for which purpose approximately three strips per mm should not be exceeded with currently commercially available photoresistors. By arranged transversely to the strip direction cylindrical lens 7, the interference fringes are focused in its longitudinal direction and smeared and so obtain higher intensity and uniformity. The photoresistor 6a is oriented so that the electrodes are parallel to the interference fringes.

009848/1508

Via a beam splitter, a partially transparent mirror 10, the same area is also mapped onto a second photoresistor 8, which is optically in the same plane 6, in front of which a diffuser 9 is located. The two photoresistors have a non-linear characteristic and are known to form Way an arrangement for measuring in the plane 4 or 6 existing modulation. Due to the non-linear ^ there are different at the two photo resistors Measured values, the difference of which can be measured using an instrument 13, which represents a measure of the modulation.

For the regulation, a difference corresponding to the required modulation is set at the external bridge resistance 11. By changing the intensity of the reference beam or the object field 5, for example by changing the intensity of the field illuminating the object 2, the ratio ψ of these two intensities is changed so that the modulation assumes the specified value, which is achieved as soon as the bridge is balanced to zero .

To determine the mean value of the irradiance can the diffusely illuminated photoresistor 8 can be used. Its resistance value can e.g. via a separate Power source and a display instrument 14 are determined.

009848/1508

According to this measured value on the instrument 14 can then a timer for a shutter or for the light source can be set or according to a zero method, an adjustable gray wedge coupled to the timer 15 can be shifted perpendicular to the beam path until the instrument 14 displays the predetermined value. A swash plate 12 in the measuring beam path is used to destroy the interference pattern for calibration the bridge circuit 11. The modulation-dependent non- · linearity of the recording material can. in an analogous way let the consideration of the measured value of photo resistor 6a be devices.

Another embodiment of the invention is shown in Fig. shown. The radiant flux of the laser beam (i.e. the Power output of the laser) is controlled at 16 and given to the control center 17. After the closure A beam splitter 19 divides the beam into object illumination beam 20 and reference beam 21. In the example, the object is illuminated from two directions, the splitting is done by beam splitter 22, e.g. in any ratio without loss if the beam splitter has variable splitting ratio. After mirror 23 or 24, the beam is through Lens 25 or 26 with pinhole 27 -or. 28 made divergent and illuminates object 29. By moving the lenses

along the rays, the cone of illumination is the object. size adjustable. Reference beam 21 is on after reflection Mirror 30 through lens 31 with pinhole 32 divergent the photosensitive layer 33 is projected. Here can analogously by "shifting the lens 31 the size of the Holograms and the requirement for the uniformity of the illumination are taken into account. In particular is the distance between the recording layer and the object can also be adapted to the size of the object. The schematic sketch of FIG. 2 is omitted to the representation of arrangements that adapt the lighting and measurement geometry accordingly.

The measurement of the intensity of the object field I _Q (see above), which varies from object to object according to its size and degree of reflection, can be measured directly in the exposure plane 33 using known methods, for example. During the measurement, the layer is protected from exposure, for example by a blind 100. In the example in Figure 2, the measurement takes place indirectly via a mirror 34-, which can either be swiveled out or rigid and partially reflective, by imaging the object 29 using an objective 35, the photo receiver 36. In the example shown, the photo receiver 36 is in three spatially adjacent areas divided, whereby the angular extent of the object and thus the approximate distribution of the object

- ΛΛ -

energy measured over the spatial bandwidth in the hologram can be. To control the distribution of the intensity of the object field over the surface of the light-sensitive Layer can in the arrangement outlined in the example, the measuring part with lens 55 intd photo receiver either shifted in the plane corresponding to layer 35 or it is available several times over the area. Also the intensity I of the reference field can be measured in a corresponding way, but this will usually not be necessary, since changes provided they occur, ob j ect shifts are introduced and removed by filters or the beam diameter changing the magnitude of these mechanical movements can be taken can.

Only the component of the electrical vector in the object field that is parallel to the electrical vector of the reference field contributes to the interference in the hologram recording. A polarization filter 57 in front of the measuring arrangement only determines the component capable of interference as the object intensity I _Q ; by rotating the polarization filter by 90 ° or by means of a second measuring arrangement with such a filter, the non-interfering portion of the object field intensity is obtained and when determining the modulation ρ

009848/1508

attributed to the intensity of the additional exposure I _2. The last-mentioned component does not interfere with the reference field, but its individual elementary waves interfere with one another and form interference fringe systems which generate scattered light during the reconstruction, which is known as "intermodulation noise". If the radiation which is not capable of interference is switched off by a polarization filter 58 between object 29 and hologram plate 33, it does not occur either as additional exposure or with a proportion of "intermodulation noise".

Metallic surfaces do not change the polarization of the light when reflected, non-metallic diffuse reflective. depolarize animal surfaces; since only the component of the electrical vector parallel to the reference field on the _ Hologram is recorded, non-metallic surfaces appear in the reconstruction only about half as bright as corresponding ones metallic surfaces. The effect does not occur when the object is illuminated with unpolarized light, as e.g. the application, a gas laser without Brewster-Penster is advantageous, the unpolarized light and thus about double Emits radiation flux. The reference field is then through

-0 09848 / T508

Polafilter 46 linearly polarized. The one occurring here "Loss is avoided if a polarizer, for example a Nicol prism, is used as the beam splitter 19. For unpolarized illumination are λ / 4 plate 39 and possibly Diffuser 40 required. For polarized lighting, the object illumination beam has to be rotated by 90 °, with which the Normally produced, but with the higher power of the unpolarized laser. Without polarization filter 38 and / or 37, with unpolarized illumination, an object intensity is measured which is divided equally from the interference-capable part I and the non-interference-capable part Part I composed. Because of the "intermodulation Noise, however, the use of polarizer 38 is more advantageous.

When the lighting on the object 29 as in conventional photography on the most appropriate light-shadow effect etc. is set (e.g. by illuminance distribution on the variable beam splitter 22 and by hard Illumination of point source 28 and soft brightening of diffuser 40) is the illuminance distribution over changed the object plane so that the effect of the modulation transfer function M (v) is precompensated. In FIG. 2, viewed from the hologram, are the left

U 0 9 Π ^; > 8 / IBOB

Parts of the object in the hologram are V-coded in higher spatial frequencies, where M (ψ *) is lower. So the input modulation ρ (-ν) must be higher there so that the effective modulation ρ (y) = p _Q (JV) · MO *) is approximately constant for the entire object. This is achieved, for example, by inserting masks 41 and 42 with an adapted, approximately wedge-shaped permeability profile in the object illuminating beams. The masking can also take place in the object plane, for example by a corresponding partial lengthening of the exposure time by controlled insertion of a mask 43 drawn in the sketch as a backdrop. Licher change of the transmission-reflection ratio achieves the same purpose without loss of laser. energy, masks running in the opposite direction to 41 and 42 are then to be used during the visual alignment of the lighting.

In the case of the example shown, the reference beam needs a divergent shape to ensure uniform exposure Compensation for the 1 / (distance) ^ decrease, e.g. through a transparency mask 44, and in each case a correction of the intensity curve that prevails in the cross section of laser beams and roughly follows the normal distribution. This can be done, for example a mask 4-5 can be done. If the mask 45 is not collimated in FIG

3 49/1509

192458S

po 123 / MO 174

widened beam path arranged displaceably, then the intensity distribution can be overcompensated adjustable * resulting in, for example, illuminance falling towards the edge from a nearby small object due to higher reference beam intensity partially compensate with regard to the modulation and largely with regard to the average exposure value leaves. When passing through several optical elements due to birefringence, any deviations from of the linear polarization are switched off by the polar filter 46.

After measuring the original object field intensity, e.g. by means of arrangement 35 »36, the control center .17 sends the Modulation brought to the target value, e.g. by regulating the object field intensity with a polarization or transparency filter 47. In this case, then, illuminance is constant produced and shutter 18 is not changed; for each film material there is accordingly its Sensitivity a fixed exposure time. The disadvantage is that the proportion of the object illumination rays in the Laser power is unnecessarily high for most objects. It is more economical to use medium-sized objects and average reflectance and, in the case of very poorly reflecting objects, attenuation of the reference field

009848/150 8

- 16 PO 123 / MO 174 "

by. Filter 48. The exposure time is the same to extend at the lock 18. Lossless utilization the laser power is achieved by shifting a beam splitter 19 with a continuous transmission-reflection ratio achieved. Here, the target value for the object intensity to be measured in 35/36 becomes a function of the shift position the beam splitter 19; the exposure time is calculated from the reciprocal of the sum of the measured Object field intensity and reference field intensity, those from the position belonging to the beam splitter 19 Transmission follows.

If for picking up moving objects, e.g. plants or animals, a pulsed laser is used, the modulation according to the measured value of a test release or after the modeling light, e.g. by a gas laser beam reflected in the beam path. With especially weakly reflecting objects, the measurement can indicate that the energy of the pulse is not the desired one Overall exposure will result. In this case, the deficit goes into the control as incoherent additional exposure I, which Modulation is set accordingly higher and the remaining exposure is then from a thermal Filled up light source. Likewise, sensitivity and, if necessary, characteristics can be used for longer exposures

009848/15 0 8

PO 123 / MO 174

of photographic plates can be improved by flash pre-exposure ("Weinland effect"). In this case it is 49 a flash lamp that lasts for a certain time emits a constant total energy adapted to the rate behavior of the respective material. The control center is only affected insofar as there is modulation and exposure according to those applicable to pre-exposed material Sets characteristics.

It can happen that a sequence of Recordings are to be made on the same plate, e.g. front, side and rear views, mechanized by a turntable 50 and in known technology for separating several hologram images, e.g. by rotating the photosensitive Layer 33 around its center normal between the recordings. Or an object should be constructed synthetically, also from real individual parts ("phantom picture",. "explosive sketch"). A particularly important case is the holographic one Recording of the wave field of an object at two points in time under different circumstances, e.g. different Temperatures or loads, for interferometric Determination of the previous changes. For such N-fold holograms, the control determines the exposure value, the ^ the in the case of multiple exposure

009848/15 08

PO 123 / MO 174

optimal overall exposure (the optimal overall exposure is somewhat lower when adding exposures according to the linear scale than with a simple exposure with amplitudes increasing proportionally to the time in the interference grating). When controlling the modulation, all other exposures are included in the calculation as incoherent additional exposures I, whereby the input modulation of the individual images is greater ^{by the factor Vn 1 as a first approximation.}

The control principle can also be applied to other types of hologram with modifications.

In the case of image plane holograms, the object 5I is projected in coherent light by means of an objective 52 approximately around the plane of the recording material 54-. In the example in FIG. 3, the reference field 55 falls from the rear onto the layer 35. In accordance with the luminance of the projected object, greater intensity variations occur in the object wave field. Therefore, the irradiance distribution in the hologram plane must be scannedj ζ. B. via mirror 56 with a planar arrangement of a large number of photoelectric converters 57 or by scanning line by line. The control sets and selects the intensity ratio according to the areas with the greatest irradiance

009848/1508

the exposure so that the most holy places can be tolerated lie beyond the maximum of the function G (E). The picture contains individual limited parts with extraordinarily high luminance (reflections), a modulation adjustment according to these reflections would reduce the overall efficiency of the hologram too much, but failure to do so would result in overexposure (G (E)) far beyond the Maximum) appear dark in the reconstruction. An improvement is possible by compressing the intensity scope in the object field using the well-known technique of the gradation mask. The production of the mask can be done with the Control device can be controlled if photochromic material is used for it. The mask 58 is in front of the Recording level or in a conjugate level used. A light source 59 of the wavelength at which the photochromic material is at its maximum sensitivity has illuminated the object 51 and simultaneously with the laser light source 61 via a partially reflecting mirror 60 so that the mask material 58, behind which instead of the Receiving layer temporarily the measuring arrangement 57 is located, provided with a -for the laser wavelength, but not the Source wavelength 59 »transparent filter. The tax revenue switches off the source 59 as soon as the measuring arrangement 57 detects a sufficient reduction in the contrasts. Since photochromes Material "is very insensitive, so can the

009848/15 08

PO 123 / MO 174

Laser source 61, possibly supported by a source the same light color, produce the mask with an exposure against which the hologram exposure is then negligible is. After the exposure, the photochromic mask is bleached again by exposure to a suitable wavelength.

The problems with Fourier transform holograms are analogous. Here, the measurement of the object field intensity can be carried out in the area of the hologram in which the spatial frequency of the most important detail size of the object is recorded, which is then with the best efficiency is reconstructed.

The control center 17 »that, on the one hand, the information the "measuring arrangement 16 and 36 and the indirectly from position e.g. beam splitters, filters, lenses, etc. carried data on the intensity of the reference field and further influencing the measurements or exposure Receives values about the respective geometry of the entire arrangement and, on the other hand, the necessary intensity ratios between each ray and the exposure time with both coherent and non-interfering Lighting determined, can e.g. according to Art

0 09848/1508

an analog computer using known technology. The basis for this is the previously given equation for the modulation. It is therefore important that, in addition to the respective internal parameters, the data on the photographic material to be processed is available in the control center in an automatically readable form, for example in the form of a punch card. This punch card contains at least the information about the target exposure average value Έ and the target average input modulation ρ. For an automatic compensation of the transfer function, its course is entered as far as possible in the form of parameters of an analytical approximation for the image angle used. For multiple exposures, special reproduction conditions, etc., the mean values to be strived for are also to be entered, for holograms with irradiance variations, more data via the function G (E) to calculate the amount of contrast to be set, etc.

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Claims (18)

Dr. Klaus Biedermann May 13, 1969 Unterhaching 10-hu-kl Ter-Meer-Str. 28 · PO 123 / MO 174 patent claims 1. Method of making holograms with a νοώ. _k to be holographic object reflected object beam Tand a coherent reference beam, characterized in that the intensities of the object beam and / or the reference beam are automatically regulated by a programmed control device so that in the layer support plane in which the two beams interfere, one under consideration the modulation transfer function of the recording material at least a minimum value reaching modulation occurs, and that the total amount of light over the loading f exposure time is automatically regulated by the control device according to the sensitometric data of the recording material and its processing, if necessary using a non-interfering additional exposure, so that the modulated part of the recording is in the most favorable range of the characteristic curve of the recording and playback process. 009Θ4Θ / 1508 po 123 / MO 174 2. The method according to claim 1, characterized in that that the irradiance that the reflected from the object Field in the recording level (33) or a generated conjugate plane to it, evaluated photoelectrically and the intensity of the object beam and / or the reference beam in terms of modulation control is controlled. 3. The method according to claim 1, characterized in that that the modulation is measured in the recording plane or a plane conjugate to it and the object and / or Reference beam intensity can be regulated accordingly. 4. The method according to claim 2 or 3, characterized in that that with hologram types with great variation the relative object field intensity over the recording area the extent of the variations is measured and object and / or reference beam intensity regulated according to a certain value within this scope will. 5. The method according to claim 4, characterized in that that at one or more fixed points the object field intensity is measured and according to this " 009348/1508 . PO 123 / MO 174 Value or one obtained by comparing these values Size object and / or reference beam intensity can be regulated. 6. The method according to claim 4 · or 5 »marked thereby-" shows that if the local intensity variations are too great, the contrasts are shown by a transparency mask be reduced, the mask is preferably fabricated in situ on photochromic material, while the Control device tracks the contrast reduction up to the target value. 7. Apparatus for performing the method according to claim 3, characterized in that the layer support plane is at least partially based on a modulation measuring device known per se is shown enlarged, in particular on two with the same partial image } hit photo resistors (6a, 8) with non-linear ones Characteristic curve, with a diffuser (9) arranged in front of one and the difference between the two measured values Measure for the modulation is. 8. Apparatus according to claim 7 »characterized in that in front of the beam splitter (10) for the two ■ photoresistors (6a, 8) a cylindrical lens (7) with transverse to the direction 009848/1508 po 123 / MO 174 the interference fringe is arranged along the axis and that the electrodes of the 10o resistors are preferably run parallel to the interference fringes. 9. Device according to one of claims 4 and 5 »thereby characterized in that the measured value of one or both photoresistors is used to determine and / or control the exposure time. 10. Apparatus for performing the method according to claim 2, characterized in that a photoelectric Means (36) for measuring the intensity of the wave field reflected by the object (29) is provided, which over suitable means, such as adjustable beam splitters (22), displaceable gray wedges or displaceable in the direction of the beam Objectives (25) adjusts the irradiance on the object (29) so that the irradiance emanating from the object reached a target value on the recording material (33). 11. The device according to claim 10, characterized in that that in the beam path of the light reflected by the object (29) a solid partially transparent or a pivotable one Mirrors (34-) are provided which show the radiation intensity distribution present in the layer support plane (33) transferred to the photo receiver (36). 0 0 3 3 4 8 /] 5 0 8 PO 123 / MO 174 ' 12. The device according to claim 10 for performing the method according to claim 4, characterized in that for line-by-line scanning of the recording plane or “A conjugate level to it and to regulate the vom Irradiance emanating from the object corresponding to the measured value variation, one or more photoelectric Converters are provided. 13. Apparatus according to claim 10 for performing the method according to claim 4, characterized in that the distribution of the irradiance from the object field over the recording plane in this or one to its conjugate plane measured by a grid-like arrangement of photoelectric converters and the regulation is carried out according to a critical variable determined from the individual values. 14. Device according to one of claims 7 to 11, characterized in that in the holographic recording of non-metallic objects for the separation of the depolarized portion of the light reflected by the object (29), a polarization filter (37) is provided in front of the measuring arrangement and a further arrangement for _{Measurement of s} to be added to the additional light with the reference field non-interfering component is present. 48/1508 PO 123 / MO 174 15. Device according to one of the preceding claims, characterized in that a polarization filter (38) is arranged between the object (29) and the layer support (33). 16. Device according to one of the preceding claims, characterized in that a separately controllable Additional light source (49) is provided for uniform exposure of the substrate. 17. Device according to one of the preceding claims, characterized in that transparency masks or their temporal shift during the exposure controlled shading screens the irradiance times change the exposure time over the object space in such a way that the product of input modulation and Transfer function over the field of view angle remains approximately constant, with this compensation in the Control of the intensity ratio and the exposure time is included. 18. Device for performing the method according to one of claims 1 to 3 and 13, characterized in that that a control center (17) designed in the manner of a computer, in particular an analog computer, is provided 009848/1508 PO 123 / MO 174- - contains inputs for the photoelectric measuring devices, for the respective settings of beam splitters, filters, expansion optics, etc. that determine the reference field intensity, for the respective geometric relationships of the arrangement that determine the measurement and / or the irradiance, for the number of recordings provided per hologram, and in particular for the transfer function and the sensito- * metric data of the light-sensitive material, its processing and the reproduction of the holograms, which control center controls the intensities of the object beam and / or reference beam as well as the overall exposure, possibly with the aid of an incoherent additional exposure, so that during the input processing and type of playback reconstructions of the desired characteristic can be obtained. 009848/1508