Liquid crystal cells – elements and systems – Liquid crystal system – Projector including liquid crystal cell
Reexamination Certificate
2001-12-03
2004-06-08
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal system
Projector including liquid crystal cell
C349S096000, C349S117000, C353S033000
Reexamination Certificate
active
06747710
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of light valve projection systems, and more particularly, to a light valve projection system using a resonant microcavity anode (RMA).
2. Description of Related Art
Currently, one of the major issues with non-CRT projection displays is the lack of an adequate light source for illumination. The existing technology is inefficient, short lived, and requires major optical systems to transfer the light into a usable form.
Conventional cathode ray tube (CRT) displays use electrons emitted from an electron gun and accelerate them through an intense electric field projecting them onto a screen coated with a phosphor material in the form of a powder. The high-energy electrons excite luminescence centers in the phosphors which emit visible light uniformly in all directions. CRT's are well established in the prior art and are commonly found in television picture tubes, computer monitors and many other devices.
Displays using powder phosphors suffer from several significant limitations, including: low directional luminosity (i.e., brightness in one direction) relative to the power consumed; poor heat transfer and dissipation characteristics; and a limited selection of phosphor chromaticities (i.e., the colors of the light emanating from the excited phosphors). The directional luminosity is an important feature of a display because the directional properties influence the efficiency with which it can be effectively coupled to other devices (e.g., lenses for projection CRT's). For direct viewing purposes this is desirable, as the picture can be seen from all viewing angles. However, for certain applications a Lambertian distribution (the normal light flux pattern observed from a luminescent screen where light is emitted uniformly in all directions) of the light flux is inefficient. These applications include projection displays and the transferring of images to detectors for subsequent image processing. Heat transfer and dissipation characteristics are important because one of the limiting factors in obtaining bright CRT's suitable for large screen projection is the heating of the phosphor screen. Chromaticity is important because the faithful reproduction of colors in a display requires that the three primary-color phosphors (red, green and blue) conform to industry chromaticity standards (i.e., European Broadcasting Union specifications). Finding phosphors for each of the three primary colors that exactly match these specifications is one of the most troublesome aspects of phosphor development.
Another consideration is the vacuum in a CRT. To allow the electron beam to travel between the electron gun and the phosphor screen, a vacuum must be maintained within a CRT. Conventional powder phosphors have a high total surface area and, generally, organic compounds are used in their deposition. Both the high surface area and the presence of residual organic compounds cause problems in holding and maintaining a good vacuum in the CRT. Using thin-film phosphors overcomes both of these effects, as the total external surface area of the tube is controlled by the area of the thin-film (which is much less than the surface area of a powder phosphor display) and, furthermore, there are no residual organic compounds present in thin-film displays to reduce the vacuum in the sealed tube.
The thin-film phosphors, though, exhibit one prohibiting disadvantage, however, due to the phenomenon of “light piping.” Light piping is the trapping of light within the thin-film, rendering it incapable of being emitted from the device. This is caused by the total internal reflection of the light rays generated within the thin-film. Since the index of refraction (n) of most phosphors is around n=2, only those light rays whose incident angles are less than the critical angle, will be emitted from the front of the thin-film. The critical angle for an n=2 material is around 30 degrees. Therefore, the fraction of light that escapes from the front of the thin-film is only about 6.7% of the total light. The common design of placing a highly reflective aluminum layer behind the film only doubles the output to about 13% of the light. Moreover, this light is spread in a “lambertian distribution” and is not directional. As a result of light piping, the external efficiency (i.e., the percentage of photons escaping from the display relative to all photons created in the display) is less than one-tenth that of powder phosphor displays. Therefore, in spite of the unique advantages offered in terms of thermal properties, resolution, and vacuum maintenance; the development of commercial CRT devices based on thin-films is held back by their poor efficiency due to “light piping”.
Microcavity resonators, which can be incorporated in the present invention, have existed for some time. Microcavities are one example of a general structure that has the unique ability to control the decay rate, the directional characteristics and the frequency characteristics of luminescence centers located within them. The changes in the optical behavior of the luminescence centers involve modification of the fundamental mechanisms of spontaneous and stimulated emission. Physically, such structures as microcavities are optical resonant cavities with dimensions ranging from less than one wavelength of light up to tens of wavelengths. These have been typically formed as one integrated structure using thin-film technology. Microcavities involving planar, as well as hemispherical, reflectors have been constructed for laser applications.
Resonant microcavities with semiconductor active layers, for example silicon or GaAs, have been developed as semiconductor lasers and as light-emitting diodes (LEDs).
Microcavities have been used with lasers, but the laser microcavity devices work above a laser threshold, with the result that their response is inherently nonlinear near this threshold and their brightness is limited to a narrow dynamic range. Displays, conversely, require a wide dynamic range of brightness. Microcavity lasers utilize stimulated emission and not spontaneous emission. As a result, these devices produce highly coherent light making these devices less suitable for use in displays. Highly coherent light exhibits a phenomenon called speckle. When viewed by the eye, highly coherent light appears as a pattern of alternating bright and dark regions of various sizes. To produce clear, images, luminescent displays must produce incoherent light.
The resonant microcavity display or resonant microcavity anode (RMA) is more fully described in U.S. Pat. No. 5,469,018 (to Jacobsen et. al), U.S. Pat. No. 5,804,919 (to Jacobsen et al), and U.S. Pat. No. 6,198,211 (to Jaffe et al), and in an article written by Jaffe et al entitled “Avionic Applications of Resonant Microcavity Anodes”, all hereby incorporated by reference. The controlled light output of an RMA utilizes a thin film phosphor inside a Fabry-Perot resonator. The structure of a monochrome RMA can consist of a faceplate having a thin film phosphor embedded inside a resonant microcavity. The references mentioned above clearly describe the benefits of using an RMA arrangement over a conventional CRT arrangement using phosphor powders.
As described above, a major problem with non-CRT projection displays is the lack of an adequate light source for illumination of the projection system. The existing technology such as most arc lamps is inefficient, short lived, and requires major optical systems to transform the light into a usable form. Although a ultra high pressure (UHP) arc lamp made by Philips has become the industry standard due to its reasonable lifespan, the Philips UHP arc lamp still has many of the detriments of inefficiency and required overhead for transforming due to the non-coherent nature of the light source. Furthermore, in order to utilize the UHP lamp in such a projection system would require a very small arc to make a sensible etendue, and therefore an efficient optical system.
Hall, Jr. Estill Thone
O'Donnell Eugene Murphy
Duong Tai
Fried Harvey D.
Kolodka Joseph J.
Parker Kenneth
Thomson Licensing S.A.
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