Illumination system having improved contrast and light field near-eye display comprising the illumination system

Abstract

The present disclosure concerns an illumination system, comprising a LC SLM comprising LC pixels, a light source comprising a plurality of point light sources configured to emit a plurality of spatially separated and / or sequentially in time input light beams that are projected on the LC SLM with different angles of incidence relative to an optical axis, and an output polarizer having an output transmission axis. The illumination system further comprises an input polarization component configured to set an input SOP to the input light beam and, in the dark state configuration of the illumination system, orient the input SOP of the input light beams parallel to the extraordinary axis of the LC pixel, such that the output SOP of the output light beams perpendicular to the polarization axis of the output polarizer. The present disclosure further concerns a light field near-eye display comprising the illumination system.

Description

Illumination system having improved contrast and light field neareye display comprising the illumination systemField

[0001] The present disclosure concerns an illumination system using a liquid crystal (LC) spatial light modulator (SLM). More specifically, the present disclosure concerns an illumination system that has improved contrast and a light field near-eye display comprising the illumination system.Background

[0002] In the context of a nematic LC SLM, contrast (the ratio of bright pixel brightness over dark pixel brightness) has always been a critical parameter, challenging to satisfy. Not only is the "on axis" (0° angle of incidence) contrast hard to optimize (for some types of LC), but it also quickly deteriorates as the angle of incidence increases ("off axis" contrast). In the case of a direct view display, the angle of incidence is simply the viewing angle. In the case of a projector display, the angle of incidence is related to the interaction of a cone of light with the LC SLM which depends on the overall optical architecture.

[0003] Low contrast is mainly caused by light leakage in the "dark state" of the LC SLM (when the LC are aligned for minimum optical power, i.e., the "off pixel") due to non-ideal crystal optical interaction or off-axis geometric misalignment. Depending on the type of LC SLM (ex: twisted nematic TN, vertically aligned VA, in-plane switching IPS), the problem is more or less severe. Since the democratization of nematic LC SLM, many solutions have been used to increase the contrast. The solutions revolve around using specifically designed waveplate retarders film / plates to compensate for unwanted polarization rotation, limiting dark state leakage. Unfortunately, not only such components are usually expensiveCreal3-16-PCTand challenging to manufacture, but they need to function over the whole range of incident angles of the system, further increasing the complexity of using such components.

[0004] Moreover, in some applications, nematic LC SLMs are too slow (limited by the relaxation time of the liquid crystals in the scale of ms). Smectic LC SLMs such as ferroelectric liquid crystal on silicon (FLCoS) can have switching time in the range of ps. Even though FLCoS technology has been around since the 1980's, it never reached the popularity level of its nematic competitor. While some of the solutions found for nematic LC SLMs can be used for FLCoS, the cost and complexity problem of manufacturing remains.

[0005] Fig. 1 shows an illumination system 30 comprising a light source 5 configured to emit a plurality of input light beams 110, 111, 112, a LC SLM 3, and a polarizer 20. The LC SLM 3 is arranged in a normal incidence configuration where the LC SLM 3 is in the plane formed by axis "x" and "y", perpendicular to an optical axis 1000 that extends along an axis "z". One input light beam 110 is shown propagating along an optical axis 1000 and two input light beams 111, 112 are shown incident on the polarizer 20 with an incident angle that is non-zero relative to the optical axis 1000. In Fig. 1, only the input light beam 110 propagating along an optical axis 1000 is shown transmitted through the polarizer 20 and incident on the LC SLM 3. The input light beams 110-112 have an input state of polarization (SOP) Y defined by the polarizer 20.

[0006] The LC SLM 3 comprises LC pixels which can be individually driven to generate an SLM image pattern. Each LC pixel comprises molecules that can be described as a uniaxial birefringent plate with a controllable extraordinary axis angle. When one of the input light beams 110-112 is incident on an anisotropic LC pixel, it is split between an ordinary ray (along an ordinary axis) and an extraordinary ray (extraordinary axis) having a velocity that is dependent upon the propagation direction within the LC pixel. If the extraordinary axis of the LC pixel is driven to be alignedCreal3-16-PCTwith, or perpendicular to, the input SOP (dark state configuration, or dark state extraordinary axis), the LC SLM 3 reflects the input light beam 110, generating an output light beam 120 (Fig. 1 shows one output light beam along the optical axis 1000) that undergoes no input SOP change and is reflected towards the polarizer 20. This results in an "off pixel".

[0007] The illumination system 30 further comprises an output polarizer 21 is configured to transmit only the output light beams 120 having a specific output SOP orientation, generating transmitted output light beams 130. In Fig, 1, the input and output polarizers 20, 21 forms a single optical component.

[0008] If the extraordinary axis of the LC pixel is driven to be misaligned with the input SOP (bright state configuration), depending on the thickness of the LC pixel, the output beam 120 undergoes an output SOP change (for example from Y to YX) such that the output light beam 120 is at least partially reflected at the polarizer 20 as a transmitted output light beam 130 having an output SOP X. The transmitted output light beam 130 is reflected substantially perpendicular to the optical axis 1000. This creates an "on pixel". The intensity of the transmitted output light beam 130 depends on the misalignment angle between the output SOP of the input light beam 110 and the extraordinary axis of the LC pixel. An optimum misalignment angle can be about 45° for a LC pixel thickness designed for a half wave plate. The difference between the intensity of the "on pixel" and of the "off pixel" corresponds to the contrast.

[0009] Figs. 2a to 2d show the input SOP interaction with the LC pixels oriented along the dark state and bright state extraordinary axis. The interaction between the input light beams 110-112 having a non-zero incident angle relative to the optical axis 1000 and the LC SLM 3 can result in non-ideal dark state output SOP of the output light beams 120, yielding a decrease in contrast.Creal3-16-PCT

[0010] Fig. 2a shows the interaction of the input SOP of the input light beams 110-112 with the LC pixels having an "ideal" dark state extraordinary axis 33. The input SOP 35 of an input light beam (110 for example) is represented in the xy reference of the input light beam. In the dark state extraordinary axis 33 of the LC pixel is controlled such as to be parallel (or perpendicular) to the input SOP 35. In that case, the input SOP 35 undergoes no change in SOP, resulting in a similar output SOP 36 of the output light beam 120. The output SOP 36 having no component on the output polarizer 21 transmission axis 40 (they are perpendicular to each other), very little or no light is transmitted through the output polarizer 21.

[0011] Fig. 2b shows the interaction of the input SOP of the input light beams 110-112 with the LC pixels having an "ideal " bright state extraordinary axis. In this case, the LC pixel is controlled such as to be oriented along bright state extraordinary axis 333 that makes a certain angle relative to the input SOP 35 (the ideal angle being 45° as shown). Depending on the thickness of the LC pixel, the input SOP 35 undergoes a rotation 333r, resulting in an output SOP 36 of the output light beam 120 rotated by 90° compared to input SOP 35. The output SOP 36 is substantially aligned with the transmission axis 40 of the output polarizer 21, resulting in a large transmission of light. Since the dark state transmitted power through the output polarizer 21 is low, and the bright state transmitted power throughout the output polarizer 21 is high, the contrast can be good.

[0012] Fig. 2c shows the interaction of the input SOP of the input light beams 110-112 with the LC pixels having an "non-ideal" LC dark state extraordinary axis 33. The input SOP 35 of an input light beam incident on the LC SLM 3 with a non-zero angle of incidence relative to the optical axis 1000 (111 this time for example) is represented in the xy reference of the input light beam. Here, the dark state extraordinary axis 33 of the LC pixel is not aligned with the input SOP 35 (due to geometrical off axis projections for instance). This will result in an unwanted SOP rotation 33r, leading to an output SOP 36 different from the input SOP 35. Compared to the case of Fig. 2a, the output SOP 36 has a non-negligible componentCreal3-16-PCTalong the transmission axis 40 of the output polarizer 21, resulting in optical power leakage.

[0013] Fig. 2d shows the interaction of an input SOP of the input light beams 110-112 with the LC pixels having a "non-ideal" LC bright state extraordinary axis. The bright state extraordinary axis 333 is slightly misaligned compared to Fig. 2b. This results in a slightly different SOP rotation 333r, turning the input SOP 35 into the output SOP 36. The component of the output SOP 36 along the axis of transmission 40 stays substantially the same in reference to the case of Fig. 2b.

[0014] In most practical cases, the dark state configuration of the LC SLM 3 has a non negligeable impact on the output SOP of the output light beam 120, even for the input light beams 110 having normal incidence on the LC SLM 3, due to imperfection in the orientation of the LC pixels. Imperfections in the orientation of the LC which leads to dark state leakage reflection on the polarizer 20, decreasing the contrast. Ideally, the contrast of the output light beam 130 should be infinite.

[0015] The contrast is dominated by the dark state configuration. In other words, a small increase in dark state intensity has a large impact on the contrast. For that reason, it is important to minimize the dark state intensity.

[0016] Fig. 3a shows the input SOP of the input light beams 110-112 incident on the LC SLM 3 extending in the plane formed by axis "x" and "y". The angle of incidence of the input light beams 110-112 may be between -15° and +15° relative to the optical axis 1000, although other angles are possible. The plain lines represent the input SOP, and the dashed lines represent the crystal orientation of the LC pixels (or dark state extraordinary axis of the LC pixels).

[0017] Fig. 3b shows a detailed view of the specific cases x=15° and y=15° from Fig. 3a and reporting the input SOP 201 (plain line) of the inputCreal3-16-PCTlight beams 110-112 and the corresponding angle of the input SOP in the x, y reference plane, i.e., the orientation of the input SOP known as the azimuth. The angle of the input SOP is shown in bold. The dark state extraordinary axis of a LC pixel viewed by the input light beams is represented by the dotted line 202, as well as its angle of the extraordinary axis in the x, y plane (not in bold). In the case the delta angle A between the angle of the input SOP in the x, y reference plane and the angle of of the extraordinary axis in the x, y plane differs from zero, the output SOP state of the output light beam 120 undergoes unwanted rotation, leading to dark state leakage. When the delta angle A is equal to zero, the input SOP is not aligned with the extraordinary axis.

[0018] Fig. 4 shows the output SOP of the reflected light beams 120 reflecting from the LC SLM 3 extending in the plane formed by axis "x" and "y". The output light beams 120 may be incident on the output polarizer 21 with an angle of incidence between -15° and +15° relative to the optical axis 1000, although other angles are possible. Fig. 4 reports the azimuth angle a (corresponding to the orientation of the output SOP,) and the ellipticity s of the output SOP. In the case where the ellipticity is 0° the output SOP is linear. For any other value, the output SOP is elliptical or circular.

[0019] An optimum contrast can be obtained when the ellipticity s is close to 0° and when the azimuth angle a is perpendicular to the transmission axis of the polarizer 20. Depending on the angle of incidence of the input light beam 110-112, the transmission axis of the polarizer 20 (seen by the incident light beam) changes. Thus, an output SOP azimuth varies with the angle of incidence of the input light beams 110-112. Any deviation from these values may result in increased dark state leakage, and thus contrast drop. Fig. 4 shows that the contrast worsens in the corners of LC SLM 3.

[0020] The polarizer 20 functions as a polarizer that transmits only the input light beams 110 having a specific input SOP. The input light beamsCreal3-16-PCT110 having other polarizations are blocked by the polarizer 20. Thus, the polarizer 20 determines the input SOP of the input light beams 110.

[0021] The polarizer 20 further functions as a second polarizer that reflects only the output light beams 120 having a specific output SOP. The output light beams 120 having other polarizations are not reflected by the polarizer 20. Thus, an "off pixel" is generated by the illumination system 30 when the output SOP of the output light beam 120 is the same as the input SOP of the input light beam 110. An "on pixel" is generated by the illumination system 30 when the output SOP of the output light beam 120 differs from the input SOP of the input light beam 110.

[0022] Fig. 5 shows a variant of the illumination system 30 of Fig. 1 where the LC SLM 3 is in a tilted incidence configuration, i.e., the LC SLM 3 lies in a plane forming a tilt angle relative to the optical axis 1000. In the example of Fig. 5, the output light beams 120 are reflected from the LC SLM 3 in a direction along a reflection axis 1200 substantially perpendicular to the optical axis 1000. The illumination system 30 of Fig. 5 comprises an input polarizer 20 arranged between the light source 5 and the LC SLM 3, and an output polarizer 21 arranged on the side of the LC SLM 3 of the output light beams 120.

[0023] The input polarizer 20 is configured to transmit only the input light beams 110-112 having a specific input SOP. The input light beams 110-112 having other polarizations are blocked by the input polarizer 20. Thus, the input polarizer 20 determines the input SOP of the input light beams 110-112. Similarly to the configuration of Fig. 1, the polarization axis of the input polarizer 20 should be aligned with the LC pixel dark state extraordinary axis. Moreover, the polarization axis of the input polarizer 20 should be preferably perpendicular to the output polarizer 21 polarization axis.

[0024] The output polarizer 21 is configured to transmit only the output light beams 120 having a specific output SOP orientation, generatingCreal3-16-PCTtransmitted output light beams 130. The output light beams 120 having an output SOP orientation perpendicular to the polarization axis 21 are not transmitted by the output polarizer 21. Any other output SOP orientation results in the output light beams 120 being transmitted, depending on the angle of the output light beams 120 relative to the polarizer 21 polarization axis. An "off pixel" is generated by the illumination system 30 when the output SOP of the output light beam 120 is the same as the input SOP of the input light beam 110-112. An "on pixel" is generated by the illumination system 30 when the output SOP of the output light beam 120 differs from the input SOP of the input light beam 110-112.

[0025] Fig. 6 shows input SOP on the LC SLM 3 for the illumination system 30 configured as in the example of Fig. 5. Fig. 6 also shows the projected dark state extraordinary axis of the LC pixel for different angles of incidence of the input light beams 110-112. Comparing Fig. 6 with Fig. 3a, the delta angle A (corresponding to the misalignment between the input SOP (plain lines)) and the projected dark state extraordinary axis (dashed lines) can be higher. The distribution of the delta angle A values is also different between Fig. 6 and Fig. 3a. In Fig. 3a, the delta angle A remains low for most of the angles of incidence and slightly increases for angles of incidence close to -15° and +15°. In Fig. 5, the delta angle A increases rapidly as the angle of incidence y increases. More generally, Fig. 6 shows that the illumination system 30 of Fig. 5 has good contrast only in the central area of the LC SLM 3 (line y=0° where the contrast can be high than 1000:1) and the contrast worsen rapidly when going towards the corners (where the contrast can be below 100:1).Summary

[0026] The present disclosure concerns an illumination system, comprising: a LC SLM comprising LC pixels and controllable by an SLM control unit; a light source, controllable by an illumination control unit and comprising a plurality of point light sources configured to emit a plurality of input light beams that are projected on the LC SLM with different anglesCreal3-16-PCTof incidence relative to an optical axis, the plurality input light beams being spatially separated and / or sequentially in time; and an output polarizer having an output transmission axis. The LC SLM is configured to interact with the input light beams and project output light beams having an output SOP. The output polarizer is configured to receive the input light beams from the LC SLM and transmit output light beams.

[0027] The illumination system further comprises an input polarization component configured to set an input SOP to the input light beam. The input polarization component is further configured to, in the dark state configuration of the illumination system, orient the input SOP of the input light beams parallel to the extraordinary axis of the LC pixel, such that the output SOP of the output light beams are perpendicular to the polarization axis of the output polarizer.

[0028] The present disclosure further concerns a light field near-eye display comprising the illumination system.

[0029] The illumination system has an increased contrast for all angles of incidence of the input light beams. This and other advantages will be apparent from the present application of the embodiments described herein.Brief description

[0030] Exemplar embodiments of the invention are disclosed in the description and illustrated by the drawings in which:Fig. 1 shows an illumination system comprising a light source configured to emit a plurality of incident light beams, a LC SLM, and a polarizer;Figs. 2a to 2d show the input SOP interaction with the LC pixels oriented along the dark state and bright state extraordinary axis;Creal3-16-PCTFig. 3a shows the input SOP of the incident light beams incident on the LC SLM;Fig. 3b shows a detailed view of the specific cases x=15° and y=15° from Fig. 3a;Fig. 4 shows the output SOP of the reflected light beams reflecting from the LC SLM;Fig. 5 shows a variant of the illumination system of Fig. 1 where the LC SLM is in a tilted incidence configuration;Fig. 6 shows input SOP on the LC SLM for the illumination system of Fig. 4;Fig. 7 shows an illumination system comprising a light source, an LC SLM and a polarization compensator, according to an embodiment;Fig. 8 shows a variant of the illumination system of Fig. 7, wherein the light source, the LC SLM and the polarization compensator are integrated into a Photonic Integrated Chip (PIC);Fig. 9 shows another variant of the illumination system of Fig. 7 wherein the light source comprises a single point light source;Fig. 10 illustrates a variant of the illumination system of Fig. 7 with an input polarizer tilted;Fig. 11 is a cross-section view of the LC SLM;Fig. 12 is a cross-section view of the LC SLM, according to an embodiment;Fig. 13a shows an SLM control signal generated by an SLM control unit and controlling the LC SLM;Fig. 13b shows an SLM control signal, according to an embodiment; andFig. 14 illustrates a light field near-eye display comprising the illumination system, according to an embodiment .Creal3-16-PCTDetailed description

[0031] Fig. 7 shows the illumination system 30 according to an embodiment. The illumination system 30 comprises a LC SLM 3 containing LC pixels. The LC pixels of the LC SLM 3 are controllable by an SLM control unit 82 via an SLM control signal 84 generated by the SLM control unit 82. The illumination system 30 further comprises a light source 5 comprising a plurality of point light sources 10, 11, 12 configured to emit a plurality input light beams 110, 111, 112 that are projected on the LC SLM 3 with different angles of incidence relative to an optical axis 1000.

[0032] The light source 5 can be controllable by an illumination control unit 81 via an illumination control signal 83 generated by the illumination control unit 81. The light source 5 can be controlled such that the input light beams 110-112 are spatially separated and / or are emitted sequentially in time.

[0033] The illumination system 30 further comprises an input polarization component 22 having an input transmission axis. The input polarization component 22 is configured to set an input SOP of the input light beams 110-112 projected on the LC SLM 3.

[0034] The LC SLM 3 is configured to interact with the input light beams 110-112 and project output light beams 120-122 with an output SOP that depends on the orientation of an extraordinary axis of the LC pixels of the LC SLM 3 on which the input light beam 110-112 incident. Here, the term "interact" can be used to means that the LC SLM 3 is configured to reflect the input light beams 110-112 in the case the LC SLM 3 is reflective, or that the LC SLM 3 is configured to transmit the input light beams 110-112 in the case the LC SLM 3 works in transmission.

[0035] The illumination system 30 further comprises an output polarizer 21 configured to receive the input light beams 110-112 incoming from the LC SLM 3 and transmit output light beams 130, 131,132. The outputCreal3-16-PCTpolarizer 21 has an output transmission axis, such that the output light beams 120 having an output SOP oriented perpendicular to the polarization axis 21 are not transmitted by the output polarizer 21. Any other output SOP orientation results in the output light beams 120 being transmitted, depending on the angle of the output SOP relative to the transmission axis of the output polarizer 21.

[0036] A dark state configuration of the illumination system 30 corresponds to the extraordinary axis of the LC pixel being aligned with the input SOP of the input light beams 110-112 such that the dark state output SOP (high contrast) is oriented perpendicular to the output transmission axis. For input light beams 110-112 incident on the LC SLM 3 with a nonzero angle of incidence, the extraordinary axis of the LC pixel is not fully aligned with the input SOP, resulting in optical power leakage and low contrast.

[0037] In an embodiment, the input polarization component 22 is further configured to, in the dark state configuration of the illumination system 30 and depending on the angle of incidence of the input light beams 110-112, orient the input SOP of the input light beams 110-112 parallel to the extraordinary axis of the LC pixel, such that the output SOP of the output light beams 120-122 are perpendicular to the polarization axis of the output polarizer 21.

[0038] The input polarization component 22 allows for reducing the delta angle A (possibly down to zero) for all the angles of incidence of the input light beams 110-112, increasing the contrast.

[0039] In one aspect, the input polarization component 22 is arranged between the light source and the LC SLM 3. The input polarization component 22 can comprise a passive or an active component.

[0040] In some aspects, the input light beams 110-112 can be collimated by a collimating optical element 4 arranged between the light source 5 andCreal3-16-PCTthe LC SLM 3 or arranged between the input polarization component 22 and the LC SLM 3.

[0041] In the example of Fig. 7, the LC SLM 3 is represented in a tilted incidence configuration, i.e., the LC SLM 3 lies in a plane forming a tilt angle relative to an optical axis 1000. However, the LC SLM 3 can also extend in a plane that is substantially normal to the optical axis 1000.

[0042] In an embodiment, the point light sources 10-12 are spatially separated such that the input light beams 110-112 are spatially separated.

[0043] In such configuration, the input polarization component 22 can comprise a plurality of polarization elements 220, where each polarization element 220 orients the input SOP of one of the input light beams 110-112. In particular, each polarization element 220 is configured to, in the dark state configuration of the illumination system 30, orient the input SOP of one of the input light beams 110-112 perpendicular to the extraordinary axis of the LC pixel, such that the output SOP of the output light beams 120-122 are perpendicular to the polarization axis of the output polarizer 21. Here, each polarization element 220 independently adjusts the orientation of the input SOP of each of the input light beams 110-112, the degree of adjustment depending on the angle of incidence of the input light beam 110-112.

[0044] Since each polarization element 220 acts on a single input light beam 110-112, the polarization elements 220 can comprise a passive element. In that case, the degree of adjustment of each polarization element 220 can be calibrated by considering the arrangement of the point light sources 10-12, input light beams 110-112 and the LC SLM 3. In the case the input polarization component 22 comprises an active component, the calibration can be performed dynamically.

[0045] In one aspect, the polarization elements 220 comprise a wave plate retarder. In that case, adjusting the orientation of the input SOP canCreal3-16-PCTbe such that the output SOP of the output light beams 120-122 is substantially linear and substantially aligned with an axis perpendicular to the transmission axis of the output polarizer 21.

[0046] In another aspect, the polarization elements 220 comprise a linear polarizer.

[0047] In another aspect, the illumination system 30 further comprises an input polarizer 20 (such as described in Fig. 5) configured to set the input SOP of the input light beams 110-112. In such configuration, the input polarization component 22 is configured to, in the dark state configuration of the illumination system 30 and depending on the angle of incidence of the input light beams 110-112, orient the input SOP of the input light beams 110-112 parallel to the extraordinary axis of the LC pixel, such that the output SOP of the output light beams 120-122 perpendicular to the polarization axis of the output polarizer 21.

[0048] When the illumination system 30 comprises both the input polarizer 20 and the input polarization component 22, the latter can be arranged between the input polarizer 20 and the LC SLM 3.

[0049] In yet another aspect, the illumination control unit 81 is configured to control the light source 5 such that the light source 5 emits the input light beams 110-112 sequentially in time. In particular, the illumination control unit 81 can be configured to control the illumination control signal 83 such that the point light sources 10-12 are activated in a specific time dependent fashion.

[0050] In that case, it can be advantageous for the input polarization component 22 to comprise an active component. The active input polarization component 22 can be configured to, in the dark state configuration of the illumination system 30 and depending on the angle of incidence of the input light beams 110-112, dynamically and time sequentially orient the input SOP of the input light beams 110-112 parallelCreal3-16-PCTto the extraordinary axis of the LC pixel, such that the output SOP of the output light beams 120-122 perpendicular to the polarization axis of the output polarizer 21 .

[0051] To that end, the illumination system 30 can comprise a polarization control unit 85 configured to control the active input polarization component 22. The input polarization component 22 can be controlled in synchronization with controlling the light source 5 by the illumination control unit 81. As shown in the configuration of Fig. 7, the SLM control unit 82 can be further configured to control an SLM signal control 84 to control the LC pixels on the LC SLM 3 in synchronization with the sequentially emitted input light beam 110-112 and possible with controlling the active input polarization component 22.

[0052] Fig. 8 shows a variant of the illumination system 30 of Fig. 7, wherein the light source 5, including the point light sources 10-12, and the input polarization component 22 are integrated into a Photonic Integrated Chip (PIC).

[0053] As illustrated in Fig. 8, the point light sources 10-12 can be spatially separated. In this configuration, the input polarization component 22 can comprise a plurality of polarization elements 220, possibly passive, as described above. Here, the passive polarization elements 220 can be integrated in the PIC is configured to separately adjust the input SOP of each input light beam 110-112 in a non-dynamic fashion, i.e., each input polarization component 22 can be configured to, in the dark state configuration of the illumination system 30 and depending on the angle of incidence of the input light beams 110-112, dynamically and time sequentially orient the input SOP of the input light beams 110-112 parallel to the extraordinary axis of the LC pixel, such that the output SOP of the output light beams 120-122 perpendicular to the polarization axis of the output polarizer 21.Creal3-16-PCT

[0054] In another aspect, the input polarization component 22 integrated in the PIC is active. The active input polarization component 22 can be configured to, in the dark state configuration of the illumination system 30, dynamically and time sequentially orient the input SOP of the input light beams 110-112, as described above.

[0055] Fig. 9 shows another variant of the illumination system 30 of Fig. 7, wherein the light source 5 comprises a single point light source 0 configured to generate a single incident light beam 100. The light source 5 further comprises a micro lens array 16 comprising a plurality of lenslets 161 and a steering element 15, configured to sequentially steer the single incident light beam 100 towards one of the lenslet 161. Each lenslet 161 converges the single incident light beam 100 and forms the point light source 10-12 that sequentially generates spatially separated input light beams 110-112 projected on the LC SLM 3. The single incident light beam 100 can be collimated by a collimating lens 14. The spatial separation between the input light beams 110-112 is defined by the size (and separation between) the lenslets 161.

[0056] The steering element 15 can comprise a scanning two (or three) axis (tip-tilt) MEMS mirror (or "micromirror"), a tunable metasurface or a liquid crystal-based phase modulator. The choice of technology for the steering element 15 may depend on the required speed and optical configuration of the illumination system 30. The steering element 15 can be controlled by the illumination control unit 81.

[0057] In the configuration of Fig. 9, the input polarization component 22 can be active and arranged between the single point light source 0 and the steering element 15. The active input polarization component 22 can be configured to, in the dark state configuration of the illumination system 30 and depending on the angle of incidence of the input light beams 110- 112, dynamically and time sequentially orient the input SOP of the input light beams 110-112 parallel to the extraordinary axis of the LC pixel, suchCreal3-16-PCTthat the output SOP of the output light beams 120-122 perpendicular to the polarization axis of the output polarizer 21.

[0058] To that end, the illumination system 30 can comprise a polarization control unit 85 configured to control the active input polarization component 22. The input polarization component 22 can be controlled in synchronization with controlling the light source 5 by the illumination control unit 81. As shown in the configuration of Fig. 9, the SLM control unit 82 can be further configured to control an SLM signal control 84 to control the LC pixels on the LC SLM 3 in synchronization with the sequentially emitted input light beam 110-112, and possibly with controlling the active input polarization component 22. The active input polarization component 22 can comprise a MEMS comprising a rotatable waveplate.

[0059] In particular, the illumination control unit 81 can be configured to sequentially pulse the single incident light beam 100 in synchronization with the LC SLM 3 refresh rate. The SLM control unit 82 controls the SLM control signal 84 such that the LC pixels on the LC SLM 3 are controlled in synchronization with the sequentially generated (pulsed) input light beam 110-112.

[0060] Alternatively, the input polarization component 22 can comprise a passive component. For example, the input polarization component 22 can comprise a plurality of polarization elements 220, each polarization element 220 adjusting the input SOP of one of the input light beams 110- 112 generated by one of the lenslet 161. In that case, the input polarization component 22 (the polarization elements 220) can be arranged between the micro lens array 16 and the LC SLM 3.

[0061] This allows for optimizing the output SOP of the output light beam 120-122 and improve the contrast through the output polarizer 21 (the contrast is defined by the "off" (dark state) and "on" (bright state) amount of light transmitted through the output polarizer 21).Creal3-16-PCT

[0062] As shown in Fig. 9, the illumination system 30 can comprise another collimating lens 4 arranged between the micro lens array 16 and the LC SLM 3 and configured to collimate the input light beam 110-112.

[0063] Fig. 10 illustrates a variant of the illumination system 30 of Fig. 7 (and similar to the configuration of Fig. 5) where the input polarization component 22 is tilted relative to the optical axis 1000. This configuration allows for improving the contrast for all angles of incidence of the input light beams 110-112. This can be explained by the reduced delta angle A.

[0064] Fig. 11 is a cross-section view of the LC SLM 3 showing an input light beam 110 incident on the LC SLM 3 with an angle of incidence x=0° and y different than 0°. The LC SLM 3 comprises a protective layer 31, a LC layer 32 with a dark state extraordinary axis 33 along the axis y (represented by a dot), and a reflective layer (mirror surface) 38. The input light beam 110 has a linear input SOP 35 along the axis y (represented by a dot). At the air / glass interface of the protective layer 31, the input light beam 110 refracts into a refracted light beam 36 and has rotated output SOP 37 (by simple refraction). In the case the LC layer 32 has an ordinary index of refraction close to glass (~1.5), there is almost no refraction at the interface between the protective layer 31 and the LC layer 32. The rotation of the output SOP 37 contributes to the high delta angle A (as shown in Fig. 6) responsible for lower contrast.

[0065] Fig. 12 shows a variant of the LC SLM 3 of Fig. 11 where the protective layer 31 is configured to provide a delta angle A that is minimized such that the contrast can be improved. The protective layer 31 is configured such that the refracted light beam 36 refracts only in the y, z plane (see Fig. 11). The output SOP 37 rotates but remains in the y, z plane, keeping the output SOP 37 of the refracted light beam 36 aligned with the extraordinary axis 33, improving the contrast.

[0066] In one aspect, the protective layer 31 has the shape of a prism having a layer surface 39 arranged such that the input light beam 110 isCreal3-16-PCTincident on the layer surface 39 with an angle of incidence x=0° and y=0°. In other words, the input light beam 110 along the optical axis 1000 is incident perpendicular to the layer surface 39.

[0067] The thickness of the LC layer 32 can be adjusted to consider the angle of the refracted light beam 36. The protective layer 31 allows for avoiding unwanted rotation due to refraction of the input beam SOP, improving contrast through the output polarizer 21.

[0068] Fig. 13a shows an SLM control signal 84 generated by the SLM control unit 82 and controlling the LC SLM 3. The SLM control signal 84 comprises a plurality of voltage pulses V of positive and negative voltages (square signal) as a function of time t. When the voltage is positive, the extraordinary axis is in a given position (for example in the dark state). When the voltage is negative, the extraordinary axis is in another position (for example in the bright state). Fig. 13a also shows the illumination signal comprising a plurality of light pulses 900, 901 and 902 corresponding to the input light beams 110-112, respectively. To create an off pixel in the LC SLM 3, the light pulse 900-902 corresponding to the input light beam 110 along the optical axis 1000 should be activated during the dark state of the LC SLM 3.

[0069] In an embodiment, the SLM control unit 82 is configured to control the SLM control signal 84 by adjusting the voltage pulses depending on the incidence angle of the input light beams 110-112 such as to, in the dark state configuration of the illumination system 30, orient the input SOP of the input light beams 110-112 parallel to the extraordinary axis of the LC pixel, independently of the angle of incidence of the input light beams 110-112, such that the output SOP of the output light beams 120-122 are perpendicular to the polarization axis of the output polarizer 21.

[0070] Fig. 13b shows the SLM control signal 84 where the voltage pulse V is adjusted depending on the incidence angle of the input light beamsCreal3-16-PCT110-112. In comparison to Fig. 13a, the SLM control signal 84 has a voltage pulse V for the light pulse 900 that is unchanged. The voltage pulse V for light pulse 901 is increased, and the voltage pulse V for light pulse 902 is decreased. The extraordinary axis of the LC pixel (and delta angle A) can be changed by varying the amplitude of voltage pulse V of the SLM control signal 84.

[0071] In one aspect, the LC SLM 3 comprises a ferroelectric liquid crystal on silicon (FLCoS).

[0072] Controlling the SLM control signal 84 by adjusting the voltage pulses can be advantageously used in the case the LC SLM 3 comprises a FLCoS. The FLCoS SLM 3 provides the individual pixels or subpixels, and the production of intermediate pixel intensities between full transparency and full opacity. When using the FLCoS SLM 3, the input light beams 110 are sequentially separated in time.

[0073] In some embodiments, the input polarization component 22 comprises a transmissive Ferroelectric Liquid Crystal (FLC) cell. The FLC cell functions similarly to the FLCoS but is not reflective and is not on silicon.

[0074] In another aspect, the input polarization component 22 can comprise a first FLC cell configured to adjust the azimuth angle a of the input SOP. The first FLC cell can have a thickness corresponding to the one of a halfwave plate which rotates a linear dark state input SOP, depending on the orientation of the dark state extraordinary axis. The input polarization component 22 can further comprise a second FLC cell configured to adjust the ellipticity s of the input SOP of the input light beams 110-112. The second FLC cell can have a thickness corresponding to the one of a quarterwave plate which transforms a linear dark state output SOP into an elliptical SOP (of same orientation / azimuth). The polarization compensator 22 comprising the first and second FLC cells allows for adjusting the output SOP of the output light beams 120-122 for improved contrast through the output polarizer 21.Creal3-16-PCT

[0075] Fig. 14 illustrates a light field near-eye display 60 comprising the illumination system 30, according to an embodiment. The LC SLM 3 has the tilted incidence configuration for miniaturization purposes. The configuration of the illumination system 30 is similar to the one shown in Fig. 7. Here the illumination system 30 further comprises a relay optics 18 configured to focus and redirect the transmitted output light beams 130- 132 into projected light beams 140, 141, and 142 projected towards an eye box 19.

[0076] The illumination control unit 81 can be configured to control the light source 5 such as to sequentially illuminate the LC SLM 3 with the input light beams 110-112 having different incident angles within the integration time of the eye 90. The SLM control unit 82 can be further configured to adjust the image content on the LC SLM 3 such as to generate different perspectives in a way that a user placing his eye 90 in the eye box 19 can see an authentic 3D rendering of a digital scene.

[0077] In the configuration of Fig. 14, the input polarizer 20 can be arranged between the collimating lens 4 and the LC SLM 3.

[0078] Having described exemplary embodiments of the disclosure, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.

[0079] Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Various elements, which are described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. Other embodiments not specifically described herein are also within the scope of the following claims.Creal3-16-PCTReference Numbers and0 single point light source10, 11, 12 point light source15 steering element16 micro lens array161 lenslet18 relay optics19 eye box100 incident light beam110 input light beam111, 112 input light beam120 output light beam121, 122 output light beam130, 131, 132 transmitted output light beams140, 141, 142 projected light beam1000 optical axis1200 reflection axis20 input polarization component21 output polarizer22 input polarization component201 SOP of the incident light beam202 extraordinary axis of a LC pixel viewed by the transmitted light beam220 polarization element3 LC SLM30 illumination system31 protective layer32 LC layer33 dark state extraordinary axis35 input SOP36 refracted light beam37 output SOP38 reflective layer39 layer surface4, 14 collimating optical element5 light sourceCreal3-16-PCT60 light field near-eye display81 illumination control unit82 SLM control unit83 illumination control signal 84 SLM control signal85 polarization control unit90 eye900, 901, 902 light pulse a azimuth A delta angle e ellipticityCreal3-16-PCT

Claims

Claims1. An illumination system (30), comprising: a liquid crystal (LC) spatial light modulator (SLM) (3) comprising LC pixels and controllable by an SLM control unit (82); a light source (5), controllable by an illumination control unit (81) and comprising a plurality of point light sources (10-12) configured to emit a plurality of input light beams (110-112) that are projected on the LC SLM (3) with different angles of incidence relative to an optical axis (1000), the plurality input light beams (110-112) being spatially separated and / or sequentially in time; and an output polarizer (21) having an output transmission axis; wherein the LC SLM (3) is configured to interact with the input light beams (110-112) and project output light beams (120-122) having an output SOP; wherein the output polarizer (21) is configured to receive the input light beams (110-112) from the LC SLM (3) and transmit output light beams (130-132); wherein the illumination system (30) further comprises an input polarization component (22) configured to set an input SOP to the input light beams (110-112) and, in the dark state configuration of the illumination system (30), orient the input SOP of the input light beams (110-112) such that the output SOP of the output light beams (120-122) are perpendicular to the polarization axis of the output polarizer (21).

2. The illumination system according to claim 1, wherein point light sources (10-12) are spatially separated such that the input light beams (110-112) are spatially separated.

3. The illumination system according to claim 2, wherein the input polarization component (22) comprises a plurality ofCreal3-16-PCTpolarization elements (220), each polarization element (220) orienting the input SOP of one of the input light beams (110-112).

4. The illumination system according to claim 3, wherein the polarization elements (220) can comprise a passive element.

5. The illumination system according to claim 4, wherein the polarization elements (220) comprise a wave plate retarder or a linear polarizer.

6. The illumination system according to any one of claims 1 to 5, wherein the illumination control unit (81) is configured to control the light source (5) such that the latter emits the input light beams (110-112) sequentially in time.

7. The illumination system according to claim 6, wherein input polarization component (22) comprises an active component controllable by a polarization control unit (85); and wherein the active input polarization component (22) is controllable to, in the dark state configuration of the illumination system 30 and depending on the angle of incidence of the input light beams (110-112), dynamically and time sequentially orient the input SOP of the input light beams (110- 112) such that the output SOP of the output light beams (120-122) perpendicular to the polarization axis of the output polarizer (21).

8. The illumination system according to claim 7, wherein the input polarization component (22) is controllable in synchronization with controlling the light source (5) by the illumination control unit (81).

9. The illumination system according to claim 8, wherein the SLM control unit (82) is further configured to control the LCCreal3-16-PCTpixels on the LC SLM (3) in synchronization with the sequentially emitted input light beam (110-112).

10. The illumination system according to any one of claims 7 to 9, wherein the polarization compensator (22) comprises a MEMS comprising a rotatable waveplate.

11. The illumination system according to any one of claims 1 to 10, wherein the input polarization component (22) is arranged between the light source (5) and the LC SLM (3).

12. The illumination system according to claim 1, wherein the light source (5) comprises a single point light source (10) configured to generate a single incident light beam (100); and wherein a steering element (15) is configured to sequentially steer the single incident light beam (100) towards one of a plurality of lenslets (161) of a micro lens array (16), such that the lenslets (161) form the point light sources (10-12) sequentially generating the input light beams (110-112).

13. The illumination system according to any one of claims 1 to 12, wherein the LC SLM (3) comprises a ferroelectric liquid crystal on silicon (FLCoS).

14. The illumination system according to any one of claims 1 to 13, wherein the input polarization component (22) comprises at least one transmissive Ferroelectric Liquid Crystal (FLC) cell.

15. The illumination system according to claim 14, wherein the input polarization component (22) comprises a first FLC cell configured to adjust the azimuth angle of the input SOP and a second FLC cell configured to adjust the ellipticity of the input SOP.Creal3-16-PCTT116. The illumination system according to claim 1, wherein the input polarization component (22) is further configured to, in the dark state configuration of the illumination system (30), the SLM control unit (82) is controllable such that the SLM control signal (84) comprises voltage pulses that are adjustable to orient the extraordinary axis of the LC pixel parallel to the input SOP of the input light beams (110-112), depending on the incidence angle of the input light beams (110-112) such that the output SOP of the output light beams (120-122) perpendicular to the polarization axis of the output polarizer (21).

17. The illumination system according to any one of claims 1 to 16, wherein the LC SLM (3) comprises a protective layer (31) having the shape of a prism with a layer surface (39) arranged such that the input light beam (110) along the optical axis (1000) is incident perpendicular to the layer surface (39).

18. The illumination system according to any one of claims 1 to 17, wherein the light source (5) and the polarization compensator (22) are integrated into a Photonic Integrated Chip (PIC).

19. The illumination system according to any one of claims 1 to 18, wherein the LC SLM (3) lies in a plane forming a tilt angle relative to the optical axis (1000).

20. The illumination system according to any one of claims 1 to 19, wherein the input polarizer (20) lies in a plane forming a tilt angle relative to the optical axis (1000).

21. A light field near-eye display (60) comprising the illumination system 30 according to any one of claims 1 to 20.

22. The light field near-eye display according to claim 21, comprising relay optics (18) configured to focus and redirect theCreal3-16-PCTtransmitted output light beams (130-132) into projected light beams (140- 142) projected towards an eye box (19).Creal3-16-PCT

Images