As is known, it is often necessary to have a polarization discriminating device between the LC device (panel) and projection optics used to form the image. Often, these polarization discriminating device are MacNeille-type polarization beamsplitters, which <BR> <BR> ideally completely reflect light in one polarization state (e. g. , S-polarized light) and<BR> completely transmits light in an orthogonal polarization state (e. g. , P-polarized light). In projection systems incorporating reflective LCD displays, the PBS, or similar device is used to separate'bright-state'light reflected from the device, which enters the projection lens and is imaged onto a screen, from'dark-state'light that is ideally reflected back towards the light source.

The preferred transmission (P-polarized) and reflected (S-polarized) polarization states of such a PBS are geometrical in nature and are defined by the normal direction of the tilted optical coating surface and the propagation direction of the ray. A projection system illuminates the LC device over a finite range of angles as defined by the pupil of the system, and hence the transmitted p-polarization plane is a function of the pupil location.

Unfortunately, upon reflection from a perfect"mirror-like"LC device in the dark state, the symmetry of the system is not preserved and most of the pupil has a polarization state with a significant S-polarization component. This S-polarization component is not rejected by the PBS and is transmitted to the projection lens and onto the screen resulting in a significant drop in contrast. These effects are known as skew- angle effects, and may be partly compensated by incorporating a quarter wave-plate retarder, referred to as a skew angle compensator. In order to effect this compensation, the slow axis of the skew angle compensator is aligned to the nominal p-polarization direction of the PBS.

While the use of a quarter-wave retarder has been incorporated to help in the separation of the"bright"and"dark"polarization states reflected from the LC device, there are complications that have an adverse impact on image quality, particularly contrast. For example, to achieve a substantially maximum contrast, the LC device should act as a'mirror'for light in the black state. As such, the net retardance of the LC device should be zero. However, in reality the net retardance of the LC device is not always zero, and the contribution of the device retardance further exacerbates the problem of separating the polarization states reflected from the LCD. Ultimately, this impacts the contrast of the projected image.

What is needed therefore is a method and apparatus that addresses at least the deficiencies of the art described above.

In accordance with an exemplary embodiment, a light valve device having a first optical retardance is optically coupled to a modified skew angle compensator, which has a second optical retardance. The sum of the first optical retardance and the second optical retardance substantially equals a retardance of a quarter-wave retarder over a prescribed wavelength range.

The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion.

Fig. 1 is a perspective view of a reflective LCD device in a projection system in accordance with an exemplary embodiment.

Fig. 2 is a conceptual view of the sequence of retarders representing the device compensator and the strongly anchored liquid crystal at the substrate interfaces of an LC device in accordance with an exemplary embodiment.

Figs. 3a and 3b are perspective views showing the residual retardance of an LCOS device that either adds to or subtracts from the retardance of a skew angle compensator in accordance with an exemplary embodiment.

Figs. 4a and 4b are theoretical and actual graphical representations, respectively, of the optical retardance (nm) versus wavelength of modified skew angle compensators in accordance with an exemplary embodiment.

In the following detailed description, for purposes of explanation and not limitation, exemplary embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as to not obscure the description of the present invention.

Briefly, an exemplary embodiment compensates for the non-zero retardance of an LC device while operating in the"black"state by providing a skew angle compensator of a chosen retardance such that the net retardance of the skew angle compensator/LC device combination is substantially that of a quarter wave retarder.

Fig. I shows a reflective LCD device 100 in accordance with an exemplary embodiment. An input beam 101 from an optical source 109 is predominantly P- polarized, and is incident on a PBS 110, which is illustratively a MacNielle-type beamsplitter. In the ideal case, all light from the input beam 101 would be incident on the PBS 110. In this case, the PBS 110 transmits light 103 of a first polarization state (e. g. , P polarized light), and reflects light 102 of a second polarization state (e. g. , S polarized light), which is orthogonal to the first polarization state.

In accordance with an exemplary embodiment, the light 103 comprises primarily P-polarized light. This light traverses a skew angle compensating retarder 104, and a reflective LC device 105, which is illustratively an LCOS device. The light 103 is transmitted by the skew angle compensator 104, is reflected by the LCOS device 105 and further is transmitted again by the skew angle compensator 104. The light 112 exiting the skew angle compensator 104 has had its polarization state transformed relative to the input polarization state of the light 103.

In accordance with an exemplary embodiment, upon reflection and transmission through the skew angle retarder 104, the sum of the retardance of the skew angle retarder 104 and the LC device 105 (that is voltage driven into its"black"state) is substantially equal to that of a half-wave retarder over a chosen wavelength range. It is noted that the slow axis of this effective"summed"retarder is aligned to the nominal P-polarization direction of the PBS. The effective half-wave retarder comprised of the skew angle compensator 104 and the LC device 105 transforms the polarization field across the illumination pupil such that the light exiting the modified skew angle retarder 104 is substantially P-polarized and hence is transmitted (i. e. , rejected) by the PBS 110 back towards the light source 109. The polarization transformation effected by the modified skew angle compensator 104 and the retardance of the LC device substantially prevents the dark-state light from being reflected by the PBS to the display optics 111, and thus improves the contrast of the image.

It is noted that bright-state light undergoes a polarization transformation from P- light to S-light by the LC device and is reflected by the PBS 110 as light 108 to the display optics 111. This process follows from well-known optical principles, and follows from the arrangement of the LCD device 100. As such, it is not further discussed so as to not detract from the focus of the present embodiments.

Finally, it is noted that the arrangement of elements of the LCD device 100 is merely illustrative, and other arrangements could be used. For example, the structure of the embodiment of Fig. 1 alternatively could be arranged so that the light first reflected from the PBS onto the LC device and subsequently transmitted through the PBS towards the projection lens. Of course, the embodiments described herein could be readily adapted to such an arrangement.

Fig. 2 shows schematically the orientation of the slow axes of various retarders that comprise a simplified retarder model of the device 105 of an exemplary embodiment.

Illustratively, the LC device is a normally white 45TN0 LCOS mode, which may be used as the device 105. The LCOS device of the exemplary embodiment has two main sources of retardance when voltage driven into the"black"state. The first source is associated with the top substrate LC alignment layer (top rub) of the device and the second source is associated with the silicon substrate LC alignment layer (bottom rub). In particular, when the LCOS device is fabricated, the molecules of the LC material are given a preferred orientation. For example, the molecules in the 45TN0 are oriented with a relative twist angle of 45° between the bottom and top substrates. Upon application of an electric field, the bulk of the molecules are homeotropically aligned with the field (i. e. parallel to the device normal direction). However, at the top and bottom alignment interfaces, a few layers of the LC molecules are strongly anchored to the alignment layer and are largely unaffected by the applied field, which results in a residual retardance at these interfaces.

With the applied electric field driving the LC into the"black"state the LCOS device may be represented by the two retarders 201 and 202, having respective slow axes 207 and 206 as shown. An additional device compensator (retarder) 203 is included with the device to compensate largely for the back rub 201 resulting in a resultant residual device retarder 204, which has a slow axis 208 in a direction that is either parallel or perpendicular to the incident polarization direction (P-polarized).

Accordingly, the resultant residual device retarder 204 representing the compensated device is combined with a modified skew-angle compensator to produce an effective quarter wave retarder (half-wave in reflection). In particular, the modified skew angle compensator accounts for the anomalous birefringent behavior of the LC device, and together the LC device and the modified skew angle compensator substantially compensate for the geometrical skew angle effects at the PBS.

In the exemplary embodiment of Fig. 2, the compensator 203 substantially 'cancels'the retarding effect of the back rub 201 of the LC device producing a residual retardance 204. This residual device retardance is shown in Figs. 3a and 3b, where depending on the orientation of the slow axis of the device (301,304) and slow axes of the modified skew angle compensator 302,303, the device retardance either adds to (Fig.

3a) or subtracts from (Fig 3b) the modified skew angle compensator 302,303. Hence the skew angle compensator must be modified to account for the residual birefringence of the device.

From the above discussion, it is clear that it may be necessary to add to or subtract from the retardance of the'ideal'quarter-wave retarder to compensate for the retardance of the LC device. Fig. 4a shows the residual retardance 401 vs. wavelength of a 45TN0 device operating in its"black"state that includes a compensator to cancel the back rub <BR> <BR> retardance (i. e. , essentially only the front rub retardance remains). Depending on whether the slow axis of the skew angle is parallel or perpendicular to the slow axis of the front rub, the retardance of the skew angle retarder retardance should be subtracted from or added to, respectively. Stated differently, it may be necessary to increase the retardance relative to the retardance of the ideal quarter wave device 402, to have a greater retardance as at 403, or it may be necessary to decrease the retardance to have less retardance than the quarter wave as at 404.

Fig. 4b shows the retardance dispersion curves of two sample skew angle retarders 405,406 over a prescribed spectral range. If the device front rub slow axis is perpendicular to the ideal QW slow axis, a skew angle retarder such as that of curve 405, may be used to approximate a retardance as at 403 over a wavelength range of approximately 460 nm to approximately 580 nm. Contrastingly, if the device front rub slow axis is parallel to the ideal QW slow axis, the skew angle retarder of curve 406 may be used over a wavelength range of approximately 550 nm to approximately 570 nm to approximate a retardance as shown at curve 404. Of course, these are merely illustrative, and not limiting of the exemplary embodiments. The invention being thus described, it would be obvious that the same may be varied in many ways by one of ordinary skill in the art having had the benefit of the present disclosure. Such variations are not regarded as a departure from the spirit and scope of the invention, and such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims and their legal equivalents.