Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1996-12-17
2001-06-05
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S010000, C349S096000, C349S009000
Reexamination Certificate
active
06243152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to projection displays, and more particularly to an improved contrast polymer dispersed liquid crystal display projection system.
2. Description of the Related Art
The cathode ray tube (CRT) has for many years been the cornerstone of computer and television display technology. It is not without its drawbacks. Larger sizes necessarily require larger vacuum tubes, more powerful magnets, and other components which increase in cost at a greater rate than the size of the display. It is not possible to make an arbitrarily large CRT display.
Because the display now represents a significant percentage of the cost of computer systems, and because the need for larger displays has increased with the graphical user interfaces that now prevail, alternatives to the CRT are very desirable.
A number of such alternatives have been developed. One technology is the liquid crystal display (LCD), which when used in a flat panel system presents a number of advantages over CRTs. But again, LCDs cannot be made arbitrarily large because of manufacturing limitations.
One promising technology involves projection displays. Projection televisions are well known, but have typically required three projection engines, one for each color. They do present the advantage, however, that an arbitrarily large surface can be used as the display surface, dependent on the intensity of the light source and optics of the projection system. That is, for large screens, projection display systems present unique advantages.
One type of projection system that has been developed employs polymer dispersed liquid crystals (PDLCs). Such liquid crystals have a polymer matrix that includes bubbles of liquid crystal. If no field is applied, the liquid crystal bubbles take on many different orientations. Because at least one of the indices of refraction of the liquid crystal differs from the index of refraction of the polymer, incoming light will be scattered.
If an electric field is applied, the liquid crystal bubbles will instead align with the field. If the materials are carefully controlled so the index of refraction of the liquid crystal for light polarized perpendicular to the electric field direction matches the index refraction of the polymer, then light will propagate through the material without being reflected and scattered by the bubbles.
A PDLC is typically provided with a reflective backing, and is used to reflect an intense light source when energized. Such a system is illustrated in FIG.
1
. As is discussed below, in such a system, a light source is collimated onto the PDLC, which when fully energized passes the collimated light. The passed light is reflected by the backing, refocused through a pinhole aperture, and transmitted to the display surface. When the PDLC is not energized, the collimated light is instead scattered, so that very little light is reflected and refocused through the pinhole aperture onto the display. PDLC display systems are generally disclosed and discussed in U.S. Pat. No. 5,404,171 to Nague, et al., issued Apr. 4, 1995, which is hereby incorporated by reference. However, PDLC-based projection systems have a relatively low contrast ratio, at least because some of the scattered light will be transmitted through the pinhole aperture. Improvement of the contrast ratio of PDLC-based projection systems is desirable.
SUMMARY OF THE INVENTION
According to the invention, a method and apparatus is provided for creating a high contrast image for a projection display. A source of polarized light is provided to a polymer dispersed liquid crystal. The polymer dispersed liquid crystal reflects a polarized image. Those portions of the polymer dispersed liquid crystal that are not energized instead scatter the light, in the process randomizing the polarity of that scattered light. The polarized image is then transmitted to the display screen, but in the process is first passed through a polarizing film that only transmits light of the polarity of the polarized image. By passing through the polarized film, half of the scattered light is eliminated, because its polarity has been randomized. Therefore, substantially all of the polarized image is transmitted to the display, but half of the scattered light is eliminated, thus doubling the contrast of the display.
In alternative embodiments, a projection system is provided where polarized light is first passed through a collimating lens onto the PDLC at a slight offset, and the returned image passes through the same collimating lens, but aligned with an aperture. It is passed through the aperture and then focused onto the display screen. In this embodiment, a polarizing filter is placed in the light path between the display screen and the collimating lens.
In other alternative embodiments, the polarized light is provided by a lamp that emits only polarized light, or is instead provided by a reflecting film that reflects the light from the lamp to the collimating lens. In this latter embodiment, the reflecting film is polarity specific and only reflects light of a first polarity to the collimating lens and transmits the remainder of the light.
In further alternative embodiments, a reflecting polarizing film reflects polarized light to the polymer dispersed liquid crystal display. A retarder is provided in the path between the reflecting polarizing film and the polymer dispersed liquid crystal, so that the polarity of the image is shifted by an appropriate amount such that the image is then transmitted back through the polarizing film. Alternatively, the polarizing film can pass the initial light and reflect the polarized image. As a further alternative, an elementless lamp (or other lamp capable of reabsorbing and reemitting light) can be used as a light source, and the light of the unused polarization reflected back to the elementless lamp for reabsorption and reemission.
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Knox Richard M.
Zeinali Mehdi
Duke University
Fleshner & Kim LLP
Sikes William L.
Ton Toan
LandOfFree
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