Optics: image projectors – Polarizer or interference filter
Reexamination Certificate
2002-06-05
2004-10-19
Mathews, Alan A. (Department: 2851)
Optics: image projectors
Polarizer or interference filter
C353S038000, C353S081000, C359S486010, C359S490020, C349S009000, C349S117000
Reexamination Certificate
active
06805445
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to digital projection apparatus employing liquid crystal devices for image forming and more particularly to an apparatus and method for achieving high levels of contrast by using a slightly rotated wire grid polarization beamsplitter in combination with a liquid crystal display (LCD) and a polarization compensator for minimizing leakage light in the pixel black (OFF) state.
BACKGROUND OF THE INVENTION
In order to be considered as suitable replacements for conventional film projectors, digital projection systems must meet demanding requirements for image quality. In particular, to provide a competitive alternative to conventional cinematic-quality projectors, digital projection systems need to provide high resolution, wide color gamut, high brightness (>10,000 screen lumens), and frame-sequential system contrast ratios exceeding 1,000:1. In addition, the digital systems must also provide constancy of image quality, image data security, low equipment purchase and maintenance costs, and low data distribution costs, to make a switchover from conventional film based systems compelling.
The most promising solutions for digital cinema projection employ one of two types of spatial light modulators as image forming devices. The first type of spatial light modulator is the digital micromirror device (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DMD devices are described in a number of patents, including for example U.S. Pat. Nos. 4,441,791 and 5,600,383 (both to Hornbeck). Optical designs for projection apparatus employing DMDs are disclosed in numerous patents, including U.S. Pat. Nos. 5,914,818 (Tejada et al.) and 6,089,717 (Iwai). Although DMD-based projectors demonstrate some capability to provide the necessary light throughput, contrast ratio, and color gamut, the current resolution limitations (1024×768 pixels), as well as high component and system costs, have restricted DMD acceptability for high-quality digital cinema projection.
The second type of spatial light modulator used for digital projection is the liquid crystal device (LCD). The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. At high resolution, large area LCDs can be fabricated more readily than DMDs. LCDs are a viable alternative modulator technology to be used in digital cinema projection systems. Among examples of electronic projection apparatus that utilize LCD spatial light modulators are those disclosed in U.S. Pat. Nos. 5,808,795 (Shimomura et al.) and 5,918,961 (Ueda). A few years ago, JVC demonstrated an LCD-based projector capable of high-resolution (providing 2,000×1280 pixels), high frame sequential contrast (in excess of 1000:1), and high light throughput (nominally, up to 12,000 lumens). This system utilized three vertically aligned (VA) (also referred as homeotropic) LCDs (one per color) driven or addressed by cathode ray tubes (CRTs). While this system demonstrated the potential for an LCD based digital cinema projector, system complexity and overall reliability remain concerns. In addition, that particular prototype system had a high unit cost that made it unacceptable for broad commercialization in a digital cinema projection market.
JVC has also developed a new family of vertically aligned LCDs, which are directly addressed via a silicon backplane (LCOS), rather than indirectly by a CRT. While these new devices are promising, they have not yet been demonstrated to fully meet the expectations for digital cinema presentation. The JVC LCD devices are described, in part, in U.S. Pat. Nos. 5,652,667 (Kuragane) and 5,978,056 (Shintani et al.) In contrast to most twisted nematic or cholesteric LCDs, vertically aligned LCDs promise to provide much higher modulation contrast ratios (in excess of 2,000:1). It is instructive to note that, in order to obtain on screen frame sequential contrast of 1,000:1 or better, the entire system must produce >1000:1 contrast, and both the LCDs and any necessary polarization optics must each separately provide ~2,000:1 contrast. Notably, while polarization compensated vertically aligned LCDs can provide contrast >20,000:1 when modulating collimated laser beams, these same modulators may exhibit contrasts of 500:1 or less when modulating the same collimated laser beams without the appropriate polarization compensation. Modulation contrast is also dependent on the spectral bandwidth and angular width (F#) of the incident light, with contrast generally dropping as the bandwidth is increased or the F# is decreased. Modulation contrast within LCDs can also be reduced by residual depolarization or mis-orienting polarization effects, such as thermally induced stress birefringence. Such effects can be observed in the far field of the device, where the ideally observed “iron cross” polarization contrast pattern takes on a degenerate pattern.
As is obvious to those skilled in the digital projection art, the optical performance provided by a LCD based electronic projection system is, in large part, defined by the characteristics of the LCDs themselves and by the polarization optics that support LCD projection. The performance of polarization separation optics, such as polarization beamsplitters, pre-polarizers, and polarizer/analyzer components is of particular importance for obtaining high contrast ratios. The precise manner in which these polarization optical components are combined within a modulation optical system of a projection display can also have significant impact on the final resultant contrast.
The most common conventional polarization beamsplitter solution, which is used in many projection systems, is the traditional MacNeille prism, disclosed in U.S. Pat. No. 2,403,731. This device has been shown to provide a good extinction ratio (on the order of 300:1). However, this standard prism operates well only with incident light over a limited range of angles (a few degrees). Because the MacNeille prism design provides good extinction ratio for one polarization state only, a design using this device must effectively discard half of the incoming light when this light is from an unpolarized white light source, such as from a xenon or metal halide arc lamp.
Conventional glass polarization beamsplitter design, based on the MacNeille design, has other limitations beyond the limited angular response, including fabrication or thermally induced stress birefringence. These effects, which can degrade the polarization contrast performance, may be acceptable for mid range electronic projection applications, but are not tolerable for cinema projection applications. The thermal stress problem has been improved upon, with the use of a more suitable low photo-elasticity optical glass, disclosed in U.S. Pat. No. 5,969,861 (Ueda et al.), which was specially designed for use in polarization components. Unfortunately, high fabrication costs and uncertain availability limit the utility of this solution. As a result of these problems, the conventional MacNeille based glass beamsplitter design, which works for low to mid-range electronic projection systems, operating at 500-5,000 lumens with approximately 800:1 contrast, falls short for digital cinema projection.
Other polarization beamsplitter technologies have been proposed to meet the needs of a LCD based digital cinema projection system. For example, the beamsplitter disclosed in U.S. Pat. No. 5,912,762 (Li et al.) has theoretical transmitted and reflected extinction ratios in excess of 2,000:1. This prism offers the potential of using both polarizations with a six LCD system, thereby enhancing system light efficiency. However, size constraints and extremely tight coating tolerances present significant obstacles to commercialization of a projection apparatus using this beamsplitter design.
Alternately, liquid-filled beamsplitters (see U.S. Pat. No. 5,844,722 (Stephens), for example) have been shown to provide high extinction ratios needed for high-contrast appli
Kurtz Andrew F.
Mi Xiang-Dong
Nothhard Gary E.
Silverstein Barry D.
Blackman Rochelle
Blish Nelson Adrian
Eastman Kodak Company
Mathews Alan A.
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