Abstract:
A device for polarization of a video sequence to be stereoscopically viewed has a beam splitter, cells, mirrors, and a control circuit. The beam splitter receives an incident beam and separates it into two beams with perpendicular polarizations. It has four prisms, each with perpendicular faces. The first face of each prism has a phase-delaying plate, and the second face of each prism has a layer that reflects the first polarization and transmits the second. The prisms are arranged such that a first face of each prism is next to a second of an adjacent prism. The cells have variable polarization rotation and are crossed by the beams. Each mirror reflects a beam. The control circuit defines a polarization rotation of the cells such that the beams, after having crossed corresponding cells, have a common polarization that alternates between two perpendicular states.

Description:
RELATED APPLICATIONS 
       [0001]    This application is the national stage entry under §371 of PCT/EP2012/064069, filed on Jul. 18, 2012 which claims the benefit of the Jul. 29, 2011 priority date of French application 1156941. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to the display of stereoscopic video sequences, and in particular the display of stereoscopic video sequences in temporal multiplexing visible with passive glasses. 
       BACKGROUND 
       [0003]    The display of stereoscopic video sequences in cinemas generally uses the alternate projection of two video sub-sequences taken at separate viewing angles. The two video sub-sequences are therefore temporally multiplexed. A first video sequence is thus intended for the left eye, a second video sequence being intended for the right eye, thus creating an impression of relief. The sampling frequency imposed by the cinema standard for a video sequence being greater than 48 Hz (so that the rate of progression of the images is not perceptible by the eye), the projection frequency on a cinema screen is of at least 96 Hertz because each eye must see only the sequence that is intended therefor. 
         [0004]    In a known operating mode, a high-speed video projector is used to emit the two sub-sequences in alternation without any particular polarization. According to the principle described in the U.S. Pat. No. 7,857,455, the light from the projector is separated into two beams with orthogonal polarizations in a beam splitter. The beam splitter is transmissive for the light with a first polarization, and reflective for the light with a second polarization. Thus two light paths are formed. Polarization modulators are arranged on the two light paths. The beam reflected by the splitter is sent back onto a mirror and superimposed on a screen with the beam having crossed the splitter. The screen is, for example, a metallized screen configured for reflecting the projected images while conserving the polarization of the latter. 
         [0005]    For the first sub-sequence, the polarization modulators are controlled so that the beams of the two light paths have a polarization called P on the screen. For the second sub-sequence, the polarization modulators are controlled so that the beams of the two light paths have a polarization called S on the screen. The polarizations P and S are perpendicular. The polarization modulators are thus synchronized with the sub-sequences emitted by the projector. Thus, the two sub-sequences are displayed in alternation on the screen  4  with perpendicular linear polarizations. 
         [0006]    The user himself possesses passive polarized stereoscopic glasses. In practice, a first lens of the glasses possesses a transmissive filter for the polarization S. This filter blocks the first sub-sequence and is transmissive for the second sub-sequence. The second lens of the glasses possesses a transmissive filter for the polarization P. This filter is transmissive for the first sub-sequence and blocks the second sub-sequence. Thus, each eye views only the sub-sequence that is intended for it. 
         [0007]    This type of display has the advantage of relying on glasses that are particularly simple and not very sensitive to damage, which is a useful feature for glasses that are to be used by the public. 
         [0008]    The device described in this patent makes it possible to obtain a high brightness for a given projector power. However, the image seen by the user has insufficient sharpness and the polarization device has a relatively high cost as well as being complicated to focus. Indeed, to compensate for an inequality in length between the two optical paths, this patent relies on a deformation of the reflective mirror to improve the superimposition of the two beams on the screen. 
       SUMMARY 
       [0009]    The invention aims to solve one or more of these drawbacks. The invention thus relates to a device for the polarization of a video sequence to be viewed in stereoscopy, the device comprising: a beam splitter intended to receive an incident light beam so as to separate it into first and second beams with first and second perpendicular polarizations respectively, the beam splitter having four prisms each having first and second perpendicular faces, the first face of each prism having a phase-delaying plate, the second face of each prism having a layer reflecting light with the first polarization and transmitting light with the second polarization, the four prisms being arranged so that the first face of each prism is placed next to the second face of an adjacent prism; first and second cells with variable polarization rotation, respectively crossed by the first and second beams output by the beam splitter; a control circuit defining the polarization rotation of the first and second cells so that the first and second beams having crossed the first and second cells respectively have one and the same polarization simultaneously, and so that this same polarization alternates between two perpendicular states; and first and second mirrors respectively reflecting the first and second beams output by the beam splitter. 
         [0010]    In a variant, the prisms have a right-angle triangle section. 
         [0011]    In another variant, the beam splitter and the mirrors are configured so that the light path of the first and second beams is symmetrical with respect to a plane. 
         [0012]    In another variant, the first and second mirrors reflect the first and second beams in the direction of the incident beam 
         [0013]    In yet another variant, the prisms each have an edge arranged in a plane including the optical axis of the beam splitter. 
         [0014]    In a variant, the delay plates are half-wave plates, the optical axis of which is inclined at 45° relative to to the first polarization. 
         [0015]    In another variant, the control circuit controls the alternation of polarization at a frequency greater than 50 Hz, and preferably less than 250 Hz. 
         [0016]    In yet another variant, the cells with variable polarization rotation are liquid crystal cells. 
         [0017]    In a variant, the cells with variable polarization rotation are interposed between the beam splitter and the mirrors. 
         [0018]    The invention also relates to a system for projecting a video sequence to be viewed in stereoscopy, the system comprising a device as described above, a projection device, the optical axis of which is merged with the optical axis of the beam splitter, and a polarization conservation screen intersecting the first and second beams reflected by the mirrors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Other characterizing features and advantages of the invention will appear clearly from the description of them below, for information purposes and in no way limiting, with reference to the appended drawings, in which: 
           [0020]      FIG. 1  is a schematic representation of a viewing system in stereoscopy according to one embodiment of the invention; 
           [0021]      FIG. 2  is a schematic representation of a section of a polarization device and of light rays crossing it; 
           [0022]      FIG. 3  is a schematic representation of a section of various optical components of the polarization device; 
           [0023]      FIG. 4  is a schematic representation of the light beams and their polarization for a first video sub-sequence; and 
           [0024]      FIG. 5  is a schematic representation of the light beams and their polarizations for a second video sub-sequence. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  is a schematic representation of a stereoscopic display system  1  in which the invention is implemented. The display system  1  comprises a high-speed projector  2 , capable of projecting images at a frequency greater than 50 Hz (generally 144 Hz). The projector  2  can thus project a stereoscopic sequence. The projector  2  thus projects in temporal multiplexing two video sub-sequences of the stereoscopic sequence. The light at the output of the projector  2  does not have any particular polarization, the projector  2  forming an incoherent light source. The luminous flux can cross a collimating lens inside the projector  2 . 
         [0026]    A stereoscopic polarization device  3  is connected to the projector  2 . The projector  2  transmits a synchronization signal to a control module  31  of the polarization device. The stereoscopic sequence projected by the projector  2  crosses a polarization module  32 , which is intended to differentiate the two video sub-sequences by generating respective perpendicular polarizations. The light output by the projector  2  thus crosses the polarization module  32 . The polarization module  32  forms two beams F 1  and F 2  with one and the same polarization. The polarization of the beams F 1  and F 2  changes alternatively between two perpendicular states, respectively called “p” and “s” in the following text. The beams F 1  and F 2  are projected in superimposition onto a screen  4 . The metallized screen  4  has the property of reflecting the luminous flux by conserving the same polarization as the incident luminous flux. 
         [0027]    A spectator equipped with so-called passive stereoscopic glasses  6  views the video sequence in stereoscopy. The glasses  6  have a frame  600  on which first and second passive shutters  601  and  602  are mounted. The first shutter  601  has a transparent lens surmounted by a transmissive linear polarizer for the polarization “P”, and the second shutter  602  has a transparent lens surmounted by a transmissive linear polarizer for the polarization “S”. Thus, each lens is transmissive for the video sub-sequence that is intended therefor, and each lens is shuttering for the video sub-sequence not intended therefor. 
         [0028]      FIG. 2  is a schematic representation of a section of the polarization module  32  and of the light beams crossing it. The polarization module  32  comprises a box in which various optical components are housed. The polarization module  32  comprises a beam splitter equipped with prisms  321  to  324 . The optical axis of the beam splitter is defined by the perpendicular to the input faces of the prisms  323  and  324  and passing by a common edge between the prisms  321  to  324 . The optical axis of the beam splitter is merged with the optical axis of the projector  2 . The polarization module  32  also comprises polarization modulators  331  and  332 . The polarization modulators  331  and  332  are arranged horizontally, symmetrically on either side of the beam splitter. The polarization at the output of the polarization modulators  331  and  332  is controlled by way of the control circuit  31 . The polarization module  32  also comprises reflective mirrors  341  and  342 . The reflective mirrors  341  and  342  are inclined and arranged symmetrically with respect to the beam splitter. The polarization modulator  331  is arranged between the prism  321  and the mirror  341 . The polarization modulator  332  is arranged between the prism  322  and the mirror  342 . The polarization module  32  further comprises output windows  351  and  352 . The output windows  351  and  352  are arranged in vertical planes and face the mirrors  341  and  342  respectively. 
         [0029]    The beam splitter is configured for separating the incoherent light originating from the projector  2  into two beams having polarizations P and S respectively. 
         [0030]    For a first ray Ra arriving at the interface between the prisms  321  and  323 , the light decomposes into a ray R 1  crossing this interface and a ray R 3  reflected by this interface. At the interface, the P-polarized part of the ray Ra is transmitted, whereas the S-polarized part of the ray is reflected. 
         [0031]    For a second ray Rb arriving at the interface between the prisms  322  and  323 , the light decomposes into a ray R 4  crossing this interface and a ray R 2  reflected by this interface. At the interface, the P-polarized part of the ray Rb is reflected, whereas the S-polarized part of this ray is transmitted. 
         [0032]    The reflected and P-polarized ray R 2  is transmitted by the interface between the prisms  321  and  323 . The transmitted and P-polarized ray R 1  is reflected at the interface between the prisms  321  and  324 . The rays R 1  and R 2  cross the polarization modulator  331  and reach the mirror  341 . The rays R 1  and R 2  are reflected by the mirror  341  and cross the output window  351 . A first light beam F 1  is thus formed at the output of the window  351 . 
         [0033]    The reflected and S-polarized ray R 3  is transmitted by the interface between the prisms  322  and  323 . The transmitted and S-polarized ray R 4  is reflected at the interface between the prisms  322  and  324 . The rays R 3  and R 4  cross the polarization modulator  332  and reach the mirror  342 . The rays R 3  and R 4  are reflected by the mirror  342  and cross the output window  352 . A second light beam F 2  is thus formed at the output of the window  352 . 
         [0034]    The beam splitter generates two light beams perpendicular to the incident beam. The mirrors  341  and  342  reflect these beams so that the beams F 1  and F 2  projected onto the screen  4  are parallel with the incident beam. 
         [0035]      FIG. 3  is a schematic representation of a section of the structure of an example of a beam splitter being able to be incorporated into the polarization module  32 . The prisms  321  to  324  have respective transparent elements  381  to  384 . The transparent elements  381  to  384  have a cross section in the shape of a right-angle triangle. The transparent elements  381  to  384  are, for example, made of glass or from any other transparent and optically neutral material, for example a synthetic material. The prisms  321  to  324  are fixed together, for example, by way of an index adaptation sealant. 
         [0036]    The prism  321  has a polarization separation layer  371  on a first face of the transparent element, and a plate of half-wave type  361  on a second face. The polarization separation layer  371  is reflective for the polarization P and transmissive for the polarization S. A plate of half-wave type induces a phase delay of 180° to the polarization along its slow axis. The optical axis of the plate  361  (its fast axis) is inclined at 45° with respect to the direction of polarization S. 
         [0037]    The prism  322  has a polarization separation layer  372  on a first face of the transparent element, and a plate of half-wave type  362  on a second face. The polarization separation layer  372  is reflective for the polarization P and transmissive for the polarization S. A plate of half-wave type induces a phase delay of 180° to the polarization along its slow axis. The optical axis of the plate  362  is inclined at 45° with respect to the direction of polarization S. 
         [0038]    The prism  323  has a polarization separation layer  373  on a first face of the transparent element, and a plate of half-wave type  363  on a second face. The polarization separation layer  373  is reflective for the polarization P and transmissive for the polarization S. A plate of half-wave type induces a phase delay of 180° to the polarization along its slow axis. The optical axis of the plate  363  is inclined at 45° with respect to the direction of polarization S. 
         [0039]    The prism  324  has a polarization separation layer  374  on a first face of the transparent element, and a plate of half-wave type  364  on a second face. The polarization separation layer  374  is reflective for the polarization P and transmissive for the polarization S. A plate of half-wave type induces a phase delay of 180° to the polarization along its slow axis. The optical axis of the plate  364  is inclined at 45° with respect to the direction of polarization S. 
         [0040]    Thus:
       the P-polarized part of the ray Ra is reflected off the layer  373 , S-polarized by crossing the plate  363 , and transmitted by the separation layer  372 . The ray R 3  therefore reaches the polarization modulator  332  with a polarization S;   the S-polarized part of the ray Ra is transmitted by the layer  373 , P-polarized by crossing the plate  361 , and reflected by the separation layer  371 . The ray R 1  therefore reaches the polarization modulator  331  with a polarization P;   the P-polarized part of the ray Rb, having crossed the plate  363 , is reflected off the layer  372 , S-polarized by crossing the plate  363 , transmitted by the separation layer  373  and P-polarized by the plate  361 . The ray R 2  therefore reaches the polarization modulator  331  with a polarization P;   the S-polarized part of the ray Rb ,having crossed the plate  363 , is transmitted by the layer  372 , P-polarized by crossing the plate  362 , reflected by the separation layer  374 , and S-polarized by crossing the plate  362  again. The ray R 4  therefore reaches the polarization modulator  332  with a polarization S.       
 
         [0045]    For a sub-sequence intended for the left eye, the control module  31  commands the polarization modulator  331  to transform the polarization P of the rays R 1  and R 2  into polarization S by applying an adequate polarization rotation. The rays R 1  and R 2  reflected off the mirror  341 , exiting the window  351  and applied to the screen  4  therefore have a polarization S. The control module  31  commands the polarization modulator  332  to maintain the polarization S of the rays R 3  and R 4 . The rays R 3  and R 4  reflected off the mirror  342 , exiting the window  352 , and applied to the screen  4  therefore have a polarization S. The beams F 1  and F 2  thus have one and the same polarization S arriving on the screen  4 . This polarization S is visible through the shutter  602  of the glasses  6 . 
         [0046]    For a sub-sequence intended for the right eye, the control module  31  commands the polarization modulator  332  to transform the polarization S of the rays R 3  and R 4  into polarization P by applying an adequate polarization rotation. The rays R 3  and R 4  reflected off the mirror  342 , exiting the window  352  and applied to the screen  4  therefore have a polarization P. The control module  31  commands the polarization modulator  331  to maintain the polarization P of the rays R 1  and R 2 . The rays R 1  and R 2  reflected off the mirror  341 , exiting the window  351  and applied to the screen  4  therefore have a polarization P. The beams F 1  and F 2  thus have one and the same polarization P arriving on the screen  4 . This polarization P is visible through the shutter  601  of the glasses  6 . 
         [0047]    By virtue of the symmetry of the optical system of the polarization module  32 , the beams F 1  and F 2  are superimposed on the screen  4  after having travelled one and the same distance. Thus, the sharpness of the image formed on the screen  4  is optimal. Furthermore, the optical system of the polarization module  32  does not necessitate the application of a mechanical deformation to any mirror, the sharpness of the image being thus optimized for reduced cost and complexity. Furthermore, the brightness of the video sequence on the screen  4  is optimal for a given light power of the projector  2 . Indeed, the polarization module  32  does not necessitate the use of a linear polarizer, which does not induce a high light absorption. 
         [0048]    The polarization separation layers  371  to  374  can be implemented in the form of dielectric coatings of so-called MacNeille type. These coatings can be formed by a stack of layers that alternate between a high refractive index and a lower refractive index (for example alternating indices of 2.1 and 1.62 for transparent elements  381  to  384  with a refractive index of 1.815). The polarization separation layers  371  to  374  can also be implemented in the form of networks of grids. 
         [0049]    The half-wave plates  361  to  364  are formed from a material having adequate birefringence properties. 
         [0050]    The polarization modulators  331  and  332  are typically formed from liquid crystal cells. Such liquid crystal cells are voltage-controlled to selectively apply either no polarization rotation or a polarization rotation of 90° to the light rays crossing them. 
         [0051]    The polarization module  32  advantageously comprises a transmissive thermal screen  353  at its input. This thermal screen  353  makes it possible to limit the heating of the polarization module  32  due to the infrared radiation from the projector  2  arranged nearby. 
         [0052]    The invention has been described for an example in which the beams F 1  and F 2  have a linear polarization analyzed by the shutters of the glasses  6 . However, the invention can also be implemented by forming the beams F 1  and F 2  with circular polarizations, by placing a quarter-wave plate in front of the output  351  and a second quarter-wave plate in front of the output  352  (these plates being oriented at 45° to the polarization axis of the beams exiting the polarization modulators) and by equipping the glasses  6  with the corresponding quarter-wave plates.