WO2008015090A1

WO2008015090A1 - Apparatus for eliminating undefracted directly reflected light of a light modulation device

Info

Publication number: WO2008015090A1
Application number: PCT/EP2007/057185
Authority: WO
Inventors: Philippe Renaud-Goud; Pauline Colas
Original assignee: Seereal Technologies S.A.
Priority date: 2006-07-29
Filing date: 2007-07-12
Publication date: 2008-02-07
Also published as: TWI384338B; DE102006035167B4; DE102006035167A1; TW200928625A

Abstract

The invention relates to an apparatus for eliminating undefracted directly reflected light that emerges from at least one light modulation device (1) formed in reflective fashion. The light modulation device (1) has a multiplicity of pixel elements which are arranged in a manner separated with respect to one another by interpixel regions. At least one optical element (2) and the light modulation device (1) are arranged with respect to one another in such a way that coherent light emerges on the optical element as well as on the light modulation device and has light beams with a path difference after reflection at the optical element (on the one hand) and the interpixel regions of the light modulation device, on the other hand. The reflected light beams contain undeflected and modulated light and interfere in such a way that the directly undeflected reflected light is extinguished in the interpixel regions.

Description

Device for eliminating unbent direct reflected light of a light modulation device

The invention relates to a device for eliminating undeflected directly reflected light which emerges from at least one light modulation device of reflective design, the light modulation device having a multiplicity of pixel elements which are arranged separately from one another by interpixel regions. Furthermore, the invention also relates to a method for eliminating non-diffracted directly reflected light by means of the device.

In holography, light-modulation devices which code a hologram are used to reconstruct two-dimensional and / or three-dimensional scenes. In this case, the light modulation device has a multiplicity of pixel elements, between which interpixel regions are located. In the pixel elements, the required information for the reconstruction of the scene is encoded. The interpixel areas have electrodes for driving the pixel elements. These are areas where the incident light is not modulated onto the light modulator, that is, the interpixel regions are static regions on the light modulator. The trend, however, is towards miniaturization, so that the pixel elements must therefore be made smaller, whereby the interpixel areas are becoming increasingly important.

In transmissive light modulators, the interpixel areas absorb the incident light, causing a loss of intensity. However, in reflective light modulating devices, the incident light is reflected at the interpixel areas. This results in the reconstruction of scenes in the Fourier plane to a distortion of the reconstructed scene. The noise is expressed by a very bright area in the middle of the Fourier plane. This means that the relative intensity of the bright area is very high compared to the intensity of other object points. The intensity of the bright area does not depend on the modulation of the pixel elements, but it depends on the filling factor (ratio of codable or photosensitive area to Total area of the light modulation device). However, such effects of the interpixel areas greatly reduce the quality of the reconstructed scene. The reason, however, for the use of reflective üchtmodulationseinrichtungen lies in the low light losses through absorption compared to transmissive displays, the relatively large filling factor for a high light efficiency, the short switching time and their low-cost production.

In the publication by S. Junique et al. "GaAs-based multiple-quantum spatial light modulators fabricated by a wafer-scale process", Applied Optics 2005, Vol. 44, No.9, pp. 1635-1641 discloses that the reflectance of the interpixel region is set to nearly 10% In addition, it is mentioned that absorbing layers are applied to these areas of the surface of the light modulator to reduce the influence of reflections from the interpixel areas on the light modulator.

However, such absorbent layers are difficult to apply to the interpixel areas and add extra expense.

It is the object of the invention to provide an apparatus and a method with which light can be eliminated from interpixel areas of a reflective light modulation device by simple means.

According to the invention the object is achieved in that at least one optical element is provided, which is arranged to the light modulation means such that incident coherent light is reflected to a part of the optical element and to a part of the light modulation means, so that light rays are present with a retardation in that the light beams contain reflected and modulated light and interfere in such a way that the light reflected at the interpixel areas is extinguished. Modulated light here means light that is modulated and reflected by the pixels. Reflected light here means light which is reflected at the interpixel areas and at the optical element.

In order to eliminate reflected light from the interpixel areas in reflective light modulation devices, a device is provided according to the invention which has an optical element. This optical element is associated with the light modulating means so that the light incident on the light modulating means and the optical element is reflected such that the light beams reflected from the light modulating means and the light beams reflected from the optical element make a retardation from each other respectively. The path difference is selected such that the light beams from the light modulation device and from the optical element interfere with one another such that the directly reflected light from the optical element and from the interpixel regions of the light modulation device cancel each other out.

In this way, the problem of noise in the Fourier plane, for example, in the reconstruction of two- or three-dimensional scenes can be reduced or eliminated without significant loss of information regarding the scene. The quality of the modulated light by means of the light modulation device after the destructive interference increases substantially, since the directly reflected portion has been eliminated from the interpixel areas. This considerably improves the contrast in the imaging plane of the light modulation device or in the reconstruction plane.

In one embodiment of the invention it can be provided that the light modulation device and the optical element are arranged in series parallel to each other, wherein a portion of the incident light is reflected at the optical element and a part of the light transmitted and modulated by the pixel elements and directly at the interpixel areas is reflected, wherein the reflected light rays destructively interfere with each other. It is advantageous if the optical element has an antireflection coating on a surface. In a further embodiment of the invention can be provided that the optical element is controllable. By a controllable design of the optical element, the elimination of the undiffracted reflected light of the interpixe regions can take place in real time.

Furthermore, it can be provided in one embodiment of the invention that the optical element has a surface shape which corresponds to a surface shape of the light modulation device. This allows a highly precise compensation of the reflected light from the interpixel areas.

Further, the object is achieved by a method for eliminating non-diffracted light by combining an optical element and the light modulation device with each other so that incident light is reflected on the optical element and interpixel regions of the light modulation device such that the reflected light rays are destructive interfere.

Further embodiments of the invention will become apparent from the remaining dependent claims. In the following, the invention is explained in principle with reference to the embodiments described in more detail in the figures. The principle of the invention is described using monochromatic coherent light. However, the subject invention is also applicable to colored holographic reconstructions or other optical applications.

The figures show:

FIG. 1 shows a schematic representation of a light modulation device in FIG

Connection with an optical element for elimination of non-diffracted light according to the invention;

FIG. 2 shows a second possibility for eliminating unbent light according to the invention; 3a shows a third possibility for the elimination according to the invention of up to 3e unbentugtem light with various embodiments of the optical

element; and

Figure 4 shows a schematic representation of a projection device with the device according to the invention for the elimination of non-diffracted light.

FIG. 1 shows a first possibility for the elimination of unbent light. This undiffracted light generally generates a disturbing noise in a Fourier plane of a light modulation device, which manifests itself in a high intensity in the middle region of the Fourier plane. However, such noise occurs only when using reflective light modulating device 1 having pixel elements. Since the light incident on the light modulation device 1 is reflected directly at interpixel regions of the light modulation device 1 and is not modulated, this light is superimposed with the modulated light of the light modulation device 1, resulting in the above-mentioned noise.

In order to reduce or eliminate such aberration, at least one optical element 2 is provided in a device according to the invention. The optical element 2 is formed in this embodiment as a glass plate having on one surface an anti-reflection layer 3, wherein the surface is directed to the light modulation device 1. It is important to ensure that the optical element 2 and the light modulation device 1 are arranged in parallel to each other in series. In order to reduce or eliminate the reflected undiffracted light from the interpixel areas of the light modulator 1, the optical element 2 is to be arranged to the light modulator 1 so that the incident coherent light has light rays after reflection on the optical element 2 and the light modulator 1 , which have a path difference to each other. The light beams have modulated and reflected light. The amplitudes and the Gap difference between the light beam reflected at the optical element 2 and the light beam modulated and reflected at the light modulation device 1 are selected or adjusted such that these two light beams destructively interfere with one another and thus the light which is directly reflected in the interpixel areas is extinguished. To achieve this, the reflectance of the entire optical element 2, the transmissive surface of the optical element 2 or the distance between the optical element 2 and the light modulation device 1 can be adjusted.

Thus, the amount of reflected light can be adjusted by providing an incident light beam to the light modulation device 1 having a larger area than the active area of the light modulation device 1 and the size of the pupil. By means of the following equation the desired conditions can be determined:

where R is the reflected light, T is the transmitted light, FF is the fill factor, S _bea m is the irradiated area of the light modulator, and S _ac t _IVΘ sL _{M is} the area of the pixel elements of the light modulator.

It is provided that the light collimated on the optical element 2 and the light modulation device 1 falls. In addition, the phase difference between the light beam reflected by the optical element 2 and the light beam reflected by the light modulating device 1 is Δφ = π + 2kπ, where k is an integer. In this embodiment, the phase difference between the two light beams is Δφ = π (k = 0).

However, in this embodiment, there is a problem that multiple reflections between the light modulation device 1 and the optical element 2 come about. However, the intensity of the multiply reflected Rays be very small. Of advantage, however, is the ease of manufacture of the optical element 2 and the arrangement to the light modulation device. 1

Therefore, to avoid the multiple reflections, it is proposed to provide a mirror as an optical element 2, which has an opening 4 in a central region. FIG. 2 shows this possibility of the embodiment of the device for eliminating unbent light from the interpixel areas. A portion of the incident light is reflected at outer regions 2 a of the optical element 2, with a further portion of the incident light falling through the opening 4 of the optical element 2 onto the light modulation device 1. This part of the light is modulated by the pixel elements and reflected back, whereby a part of this light falls on the interpixel areas and is reflected there directly. After re-passage of the modulated and reflected light through the opening 4, the two light components interfere with each other such that the reflected light is extinguished from the interpixel areas. In this way, the quality of the modulated light beam is also significantly improved here.

In the figures 3a to 3e further possibilities of the device are shown. Between the optical element 2 and the light modulation device 1, a beam splitter element 5 for better or easier beam guidance is provided in each exemplary embodiment with respect to FIGS. 3 a to 3 e. The beam splitter element 5 may be formed, for example, as a beam splitter cube. The phase difference between the light beam S ₁ after the reflection at the optical element 2 and the light beam S ₂ after the reflection at the light modulation device 1 is Δφ = π + 2kπ, where k is an integer. Both light beams Si and S ₂ must be coherent with each other.

In FIG. 3 a, the device has the light modulation device 1 and the optical element 2, which are arranged at a distance from one another in the same plane via the beam splitter element 5. The light modulation device 1 here has an almost flat surface. The optical element is formed as a mirror, which has a reflectance which is adapted to the amplitude of the modulated light. To reduce or eliminate the reflected Light from the interpixel regions, the light incident on the beam splitter element 5 is guided by means of this to the optical element 2 and to the light modulation device 1 and reflected there or modulated. Both partial beams Si and S2 interfere destructively with one another after their reflection. This means that the directly reflected portion of the sub-beam S ₂ from the interpixel areas and the reflected sub-beam Si interfere with each other in such a way that they cancel out. For this to be ensured, both partial beams Si and S ₂ must be coherent with one another.

It is also possible, instead of adapting the reflectance, to adapt the area of the optical element 2, in this case the mirror, to a quantity of the reflected light. It is also advantageous here that it is not a prerequisite to use collimated light, since the optical element 2 and the light modulation device 1 are arranged in the same plane. Furthermore, it is advantageous that the inlet beam into the beam splitter element 5 and the output beam from the beam splitter element 5 can be spatially separated from each other after elimination of the directly reflected component in the light.

In the figure 3b the same device as shown in Figure 3a, wherein the optical element 2 is designed in this embodiment as a deformed plate. If the light modulation device 1 has a non-planar surface, as shown here, an optical element 2 is required, which has an approximately same surface shape as the surface shape of the light modulation device 1 in order to compensate for the effect of the interpixel regions as possible with high accuracy. For this reason, the surface of the optical element 2 is preformed corresponding to the surface of the light modulation device 1, but can not be changed during the operation of the device. Again, the reflectance of the optical element 2 must be adjusted again. The optimal reflectance is: p = 1 - fill factor. The mode of action of this device corresponds to that of FIG. 3 a and will therefore not be further described as in the following exemplary embodiments. A particular advantage of this device is that additional errors of the light modulation device 1 can be corrected in this way. A device similar to that shown in FIG. 3b is shown in FIG. 3c. However, in this embodiment, the optical element 2 is formed as a holographic plate. This holographic plate 2 is static and adapted to the light modulation device 1 and can not be changed during the operation of the device either. The optical element 2 should also compensate for the shape deviation of the light modulation device 1 here. The production or design of the optical element 2 takes place in this embodiment according to the following principle:

Optical element = (- light modulation device) * (1 - fill factor), wherein no activation of the light modulation device 1 takes place. In this case (1-fill factor) leads to the same amplitude of the two light beams and to the π-phase shift. If another embodiment of the optical element 2 is required, the optical element 2 must be replaced by another optical element which is also manufactured. The advantage of such a device is that the holographic plate in comparison to the deformed plate in Figure 3b is much easier to manufacture without additional facilities and without increased effort. Likewise, additional errors of the light modulation device 1 can thereby be corrected.

A further embodiment of the device is shown in FIG. 3d. In this Ausführungsbeispiei, the device has the same structure as in the preceding figures 3a to 3 c, wherein the optical element 2 is designed to be controllable. The optical element 2 is formed here as an active deformable mirror and can be controlled by means of a control device SE not shown in detail in order to achieve a required surface shape. The control can also take place during the operation of the device. In order to eliminate the disturbing reflected light from the interpixel areas, the optical element 2 may be driven so as to have, for example, a surface shape corresponding to a surface shape of the surface

Light modulation device 1 assumes. Thereafter, the operation of the device as already mentioned above. Replacement of the optical element 2 with a change of the light modulation device 1 is thus no longer necessary. By means of such an optical element 2, a real-time compensation is possible. In addition, the optical element 2 can be adjusted more flexibly.

FIG. 3e also shows a device with controllable optical element 2. As optical element 2, a light modulation device is provided in this exemplary embodiment. Such an optical element 2 is designed so that it has a sufficiently identical surface shape as the light modulation device 1, which is also connected here to a control device SE. If such a configuration is present, then the interpixel regions of the optical element 2 can exactly compensate the interpixel regions of the light modulation device 1. As a result, no disturbances from the interpixel areas can influence the beam path, whereby a more accurate reconstructed scene can be generated.

FIG. 4 shows a holographic projection device in connection with the device for eliminating non-diffracted light from the interpixel areas. The device in conjunction with the light modulation device 1 and the optical element 2 is arranged after a light source 6, a lens 7 for expanding a beam and a collimator lens 8. After the modulation or reflection of the light at the light modulation device 1 and the optical element 2 is formed by a lens 9, a Fourier transform FT on serving as a screen optical element 10. The optical element 10, hereinafter referred to as the screen, can as a lens , Mirror or similar imaging element. The light modulation device 1 is thereby imaged via the lens 9 and the screen 10 into a viewer window 11 of a viewer plane 12. The coded hologram on the light modulation device 1 is thus reconstructed in a reconstruction area which spans between the screen 10 and the viewer window 11. The reconstructed scene can be watched by a viewer who looks through the viewer window 11. If the resolution of the light modulation device 1 is sufficiently high, then it is possible that the viewer can observe the reconstructed scene with both eyes through the viewer window 11. However, the resolution is the Light modulation device 1 low, then it is necessary that two viewer windows 11, one for each eye of the observer, are provided. In this case, a second arrangement comprising a light source, a lens 7, a collimator lens 8 and the device for eliminating unbent light with a light modulation device 1, an optical element 2 and a beam splitter element 5 is advantageously necessary.

By means of the device according to the invention in the projection device described above, the disturbing reflected light in a central region of the screen 10 is substantially reduced or eliminated.

It is of course also possible to design the projection device such that the light modulation device 1 is imaged on the screen 10 and not in the observer plane 11. Accordingly, the Fourier transform FT arises between the lens 9 and the screen 10 and is imaged by means of the screen 10 in the viewer plane 12. In this way, the contrast on the screen 10 can be improved.

In addition, the projection device can also be designed in such a way that the observer window 11 is tracked when the observer moves in the observer plane 12. Several observers can also observe the reconstructed scene in the observer plane 12. A colored reconstruction of the scene is also possible.

By eliminating the undiffracted light from the interpixel areas upon reflection at the light modulation device 1, the quality of the reconstruction beam is substantially improved in a projection device. This means that the noise is reduced in a central region of the Fourier plane, which greatly increases the contrast in the plane of the light modulation device 1.

Possible fields of application of the device according to the invention for the minimization of undiffracted light are, in addition to holographic projection devices for a two- and / or three-dimensional representation for the private and work area, such as for computers, mobile phones, television, electronic games, automotive industry to display information or entertainment, medical technology or even for military technology, for example, for displaying terrain profiles and lithography, the adaptive optics or for communication purposes. Of course, the present device can also be used in other areas not mentioned here, in which reflected light from the interpixel areas has to be reduced or eliminated.

Claims

claims:

A device for eliminating non-diffracted directly reflected light which emerges from at least one light modulation device of reflective design, wherein the light modulation device has a multiplicity of pixel elements which are separated from one another by interpixel regions, characterized in that at least one optical element is provided which is intended for Light modulation means is arranged such that incident coherent light is reflected to a part on the optical element and to a part of the light modulator, so that light rays are present with a retardation, wherein the light rays contain reflected and modulated light and interfere with that to the Interpixelbereichen reflected light is extinguished.

2. Device according to claim 1, characterized in that the light modulating means and the optical element are arranged in series parallel to one another, wherein a part of the incident light is reflected at the optical element and a part of the light is transmitted and modulated by the pixel elements and at the interpixel areas is directly reflected, the reflected light rays destructively interfere with each other.

3. Device according to claim 2, characterized in that the optical element has on one surface an antireflection coating.

4. The device according to claim 1, characterized in that the optical element is formed as a mirror having an opening in a central region, wherein the incident light is reflected at the optical element and modulated and reflected when passing through the opening at the light modulating means ,

5. Apparatus according to claim 1, characterized in that the optical element is controllable.

6. The device according to claim 1, characterized in that the optical element has a surface shape corresponding to a surface shape of the light modulating device.

7. The device according to claim 1, characterized in that for beam guidance between the optical element and the light modulating means, a beam splitter element is provided.

8. The device according to claim 1, characterized in that the phase difference between the reflected light beams π + 2kπ, where k is an integer.

9. Apparatus according to claim 1, for use in holography, in particular for the holographic reconstruction of scenes that are encoded in the light modulation device.

10. A method for eliminating non-diffracted direct-reflected light emerging from at least one reflective light modulator, wherein a plurality of pixel elements of the light modulator modulate incident light and interpixel regions between the pixel elements directly reflect the incident light, characterized in that an optical element and the optical element Light modulation means are combined with each other so that incident light is reflected at the optical element and at the interpixel areas of the light modulation device such that the reflected light rays destructively interfere.

11.A method according to claim 10, characterized in that the reflectance of the optical element is adapted to a quantity of the reflected light.

12. The method according to claim 10, characterized in that the surface of the optical element is adapted to a quantity of the reflected light.

13. The method according to claim 10, characterized in that the light falls collimated on the optical element and on the light modulation device.