Patent Application
20060256263
This invention pertains to the field of display devices, and more particularly, to liquid crystal display devices having birefringent compensators.
Liquid crystal display (LCD) devices continue to grow in popularity and in sales. LCDs are increasingly being used not only as display devices for computers, but also in televisions and video monitors. A liquid crystal on silicon (LCOS) device is a type of liquid crystal device that is increasingly being used in projection display systems, such as projection televisions and projection video monitors. More specifically, a projection display system utilizing a reflective LCOS panel is described in U.S. Pat. No. 5,532,763 to Janssen et al., the entire disclosure of which is incorporated herein by reference. An exemplary LCOS device that may be used in such a projection display system is described in U.S. Pat. No. 6,545,731 to Melnik et al., the entire disclosure of which is also incorporated herein by reference.
The reflective LCOS device 100 generally operates as follows. A high intensity, polarized light beam is directed onto at least a portion of the LCOS device 100. The polarized light beam passes through transparent cover glass layer 130, the transparent electrode 126, and liquid crystal layer 122. The polarized light beam is reflected by the reflective pixel electrodes 120, passes back through liquid crystal layer 122, and out through transparent cover glass layer 130. Where a voltage is applied across the liquid crystal material, the polarization of the light beam is altered, for example from one linear polarization to an orthogonal linear polarization. That is, the liquid crystal layer 122 acts as a polarization modulator, depending on a voltage difference applied between the pixel electrodes 120 and the transparent electrode 126. The polarization-modulated light beam emerges from the reflective LCOS device 100 and is passed through an analyzer or polarizing beamsplitter that filters out a certain polarization. The polarization-modulated light beam may then be passed though imaging lenses onto a screen to display an image.
Meanwhile, image contrast is a key parameter for any display device, including LCD devices and particularly reflective LCOS devices used in a projection display systems. Unfortunately, when driven to the dark state, the reflective LCOS device 100 still introduces a residual retardance on light impinging thereon, thereby limiting the contrast of the displayed image.
To compensate for residual retardance and thus achieve a desired contrast ratio, as shown in
Indeed, although the present discussion focuses on the specific context of a reflective LCOS device, it should be understood that the problem of residual retardance, and the contrast-limiting effect thereof, applies generally to LCD devices, and compensator foils are also commonly used with direct view LCD devices. In the case of a direct view LCD device, a compensator foil also may improve the viewing angle characteristics of the display.
In practice, the compensator foil 150 is laminated between two pieces of high quality glass, 152 and 154 to maintain its shape and to provide structural support. Furthermore, each piece of glass 152 and 154 must be provided with an anti-reflection (AR) coating to minimize reflection that can further reduce the display's contrast. Moreover, this also requires that the transparent cover glass layer 130 be provided with an AR coating to minimize reflections at the interface between the transparent cover glass layer 130 and the air.
Further discussion of the problems of residual retardance and skew-angle compensation in an LCD and the use of compensation foils may be found in Jepsen U.S. Pat. No. 6,307,607, the entirety of which is hereby incorporated herein by reference for all purposes as if fully set forth herein.
Unfortunately, there are problems and disadvantages associated with such compensator foils as discussed above. As noted above, the desired retardance is induced into the compensator foil 150 by deforming (e.g., stretching) it a predetermined amount in a predetermined direction. However, the required retardance can be relatively low (e.g., 20-30 nm), and therefore a great deal of precision is required. Accordingly, it is difficult to consistently and repeatably produce compensator foils with the required amount of retardance, so the manufacturing yields are often low. Furthermore, since the compensator foil is located near the image plane of the device, its cosmetic quality must be high. Also, the high quality AR glass sheets between which the compensator foil is sandwiched add to the cost of the device. Finally, packaging and compensation foil attachment are post-semiconductor-fabrication that complicate the overall device fabrication.
Accordingly, it would be desirable to provide an improved method and device for compensating for residual phase shift in an LCD device to improve contrast It would also be desirable to a compensating device for an LCD that can be consistently and repeatedly be produced with a high yield. It would be further desirable to provide a method and device for compensating for residual phase shift in an LCD device that simplifies overall device fabrication. The present invention is directed to addressing one or more of the preceding concerns.
In one aspect of the invention, a reflective liquid crystal device comprises: a semiconductor substrate; a plurality of reflective pixel electrodes disposed above the semiconductor substrate; a liquid crystal layer disposed above the reflective pixel electrodes; at least one transparent electrode disposed above the liquid crystal layer; and a transparent cover disposed above the transparent electrode, wherein the transparent cover has formed in a surface thereof a plurality of gratings having a pitch that is less than a lowest wavelength of visible light.
In another aspect of the invention, a liquid crystal display device comprises: first and second substrates; a liquid crystal layer disposed between the first and second substrates; means for selectively changing an orientation of liquid crystal molecules of the liquid crystal layer to selectively control a polarization of light passing through the liquid crystal layer; and a form birefringent compensator on a surface of one of the two substrates through which the light exits the device.
In yet another aspect of the invention, a liquid crystal device comprises: a semiconductor substrate; a plurality of pixel electrodes disposed above the semiconductor substrate; a liquid crystal layer disposed above the pixel electrodes; at least one transparent electrode disposed above the liquid crystal layer, a transparent cover disposed above the transparent electrode; and a transparent sheet disposed above a surface of the transparent cover, the transparent sheet including a form birefringent compensator structure.
Further and other aspects will become evident from the description to follow.
In the description and claims to follow, when a first device or structure is said to be “on” a second device or structure, it is understood that this encompasses both the case where the first device or structure is directly on the second device or structure, and the case where there are intervening devices or structures, or even air, between the first device or structure and the second device or structure. When it is intended to state that the first device or structure is directly on the second device or structure, without any intervening devices or structures, then it will be said that the first device or structure is directly on the second device or structure.
The semiconductor substrate 210, insulating layer 215, reflective pixel electrodes 220, liquid crystal layer 222, and transparent electrode 226 are similar to corresponding elements described above with respect to
However, in contrast to the device 100 shown in
For example, suppose that a reflective LCOS device produces a residual retardance of 30 nm that requires compensation. Also assume that the form birefringent compensator structure is formed in glass (n=1.5) at an interface with air as the incident material. With a 50% duty cycle, the refractive index difference is approximately Δn=0.1 when the period of the grating is less than about 0.3 times the wavelength of the incident light beam. In that case, the thickness of the grating would be about 300 nm.
The index difference of the form birefringent compensator structure 300 depends upon the grating period when the period approaches the wavelength of the impinging light beam. Beneficially, this property of the form birefringent compensator structure may be used to tailor the dispersion of the compensator to match the dispersion of the residual retardance of the liquid crystal device that it accompanies.
Other grating profiles can easily be envisioned from the above descriptions. For example, a structure with a sinusoidal cross-section can also provide a monotonically increasing grating profile and thereby eliminate the need for a separate A/R layer or coating.
Beneficially, the form birefringent compensator 250 may be relatively easily and consistently replicated in various ways. The required grating profile can be fabricated into a nickel shim that can be used to stamp the structure into a surface of a desired transparent material. Alternatively, the form birefringent compensator 250 may be patterned onto the surface of a desired transparent material by UV-curing of a polymerizing optically transparent fluid.
In an alternative embodiment, the form birefringent compensator 250 includes a grating that does not have a physical profile. The grating may be created by producing a structure having an index of refraction that is uniform along one direction, but is modulated along a second direction. For example, a form birefringent compensator 250 may be produced by exposing a monomer/liquid crystal mixture to UV light producing an interference pattern (e.g., sinusoidal) to create phase separation resulting in a refractive index/phase grating. In other words, the grating may exist as a pattern (e.g., sinusoidal) of a structural variance within the form birefringent compensator material that results in a corresponding variance in the index of refraction of the material. In that case, the physical surface of the form birefringent compensator may exhibit a flat profile.
Thus, the manufacturing yield can be improved compared to the compensator foil 150 of
The form birefringent compensator 250 may be integral to a separate transparent sheet placed above the transparent cover 230, such as a transparent glass sheet that may have an A/R layer or coating thereon. As explained above, the form birefringent compensator 250 may be stamped into the transparent sheet or it may be patterned thereon, or created by another process. If the form birefringent compensator 250 is patterned onto the transparent sheet, it may comprise a different material structure than the transparent sheet, which then acts as a carrier for the form birefringent compensator 250.
Beneficially, the form birefringent compensator 250 may be integral to the transparent cover 230 of
Although the principles have been illustrated above in the context of a reflective LCOS device, the form birefringent compensator may be more widely applied to liquid crystal display (LCD) devices.
Similarly to the device 200, in the direct view LCD panel 500, the form birefingent compensator 550 may be integral to the second substrate 530, or may be integral to a separate transparent sheet placed above the top surface of the second substrate 530.
While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 60472604 | May 2003 | US | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB04/01669 | 5/14/2004 | WO | 11/21/2005 |
