Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-11-26
2001-09-04
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S062000, C349S063000, C349S117000, C359S487030
Reexamination Certificate
active
06285423
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of optics, and more particularly to polarizing light sources and liquid crystal displays.
BACKGROUND OF THE INVENTION
Currently, most of liquid crystal displays (LCD) are based on the twisted nematic (TN) scheme that require the use of polarized light. A typical direct-view LCD panel consists of many optical components, such as a back lighting system, a liquid crystal panel with addressing electronics sandwiched between two plastic polarizers, a colour filter mosaic for forming full colour displays, etc. Each component has limited optical efficiency. For example, the plastic polarizers absorb at least 50% of light. The addressing electronics has a certain size aperture that limits the amount of light passes through the liquid crystal cell. The colour filters absorb at least two thirds of the light. As a result, the efficiency of such directview liquid crystal panel is very low; only about 5% of total light is used.
In order to enhance the efficiency of the direct-view LCDs, several polarization recovering approaches have been proposed in the past. The first approach is based on the use of the Brewster angle effect to separate s- and p-polarized light, such as the art taught by M. F. Weber in the U.S. Pat. No. 5,422,756, by M. Suzuki in “Reflective polarizer sheet on the backlighting unit”, SID'97 Digest, 813(1997), by H. Tanase et al in “A new backlighting system with a polarizer light pipe for enhanced light output from LCDs”, SID'97 Digest, 365(1997). However, this approach suffers a poor angular performance and low extinction ratios due to the inherited property of the Brewster angle effect. As a result, the gain from converting unwanted polarization to the wanted polarization is partially lost because of the poor performance of the polarizers. No practical system based on this approach is available for the direct-view LCD market.
The second approach is to use reflective cholesteric liquid crystal polarizers, such as the art taught by D. J. Broer et al in “Reflective cholesteric polarizer improving the light yield of back-and side-Lighted flat panel liquid crystal displays”, SID'95 Asia Display Digest, 735(1995), by D. Coates et al in “New applications of liquid crystals and liquid crystal polymers”, SID'96 Eurodisplay Digest, 91(1996), and by L. Li et al in “A single-layer super broadband reflective polarizer”, SID'96 Digest, 111 (1996). Although such polarizers may have a broad band, their extinction ratios are low, about 10:1 to 20:1. A second “cleanup” polarizer is required to absorb the unwanted polarized light and to bring the extinction ratio to a desired level greater than 100:1. In addition, the performance of such polarizers is sensitive to temperature and UV radiation.
The third approach is to use co-extruded reflective plastic polarizer, such as the art taught by A. J. Ouderkirk et al in “Reflective polarizer display,” U.S. Pat. No. 5,828,488. The polarizer consists of a few hundred to a few thousand stretched films made of two plastic materials. One material has birefringence due to the stretching and the other does not. At normal angle of incidence, light polarized in one direction passes because the refractive indices of the two materials are matched. Light polarized in the other direction sees a refractive index difference because of the birefringence. As a result, this polarized light is partially reflected. Its reflectance depends on the refractive index difference as well as the number of layers. Since the refractive index difference is rather small, in order to achieve high reflectance over a broad band of wavelengths, a large number of plastic films with different thicknesses are required. This polarizer is broad band and wide angle. One disadvantage of this approach is that the extinction ratio is small. Second, there is some light loss due to absorption by the films and scattering at layer interfaces.
Therefore, the objective of this research is to develop a high efficiency polarizing light source, more particularly a back-lighting system for direct-view LCDs.
SUMMARY OF THE INVENTION
According to the present invention there is provided a polarizing light source for providing polarized illumination over a two-dimensional surface, comprising a slab-like light guide having an entrance face at one end thereof for admitting unpolarized light from a linear source, a planar front face for emitting polarized light, a planar back face, and an end face; a polarizing device on said back face for reflecting light of a first polarization and transmitting light of a second polarization at non-normal angles of incidence, said reflected light being directed in said light guide to said end face; a reflective arrangement under said polarizing device for returning said transmitted light to said light guide in a direction substantially normal to said polarizing device such that it passes unaffected through said polarizing device for exit through said front face; and a polarization converter on said end face for changing the polarization of said reflected light and directing it into said reflective arrangement through said polarizing device as light of said second polarization for return to said light guide through said polarizing device in a direction substantially normal to said polarizing device such that it passes unaffected through said polarizing device from said reflective arrangement for exit through said front face.
The device in accordance with the principles of the invention employs both polarizations and therefore has substantially increased efficiency relative to the prior art.
Various forms of polarizing devices can be employed, including a thin film polarizing device based on interference and frustrated total internal reflection, or one employing metaldielectric films.
The reflective arrangement is typically in the form of a single prism or prism array with facets inclined so as to return the reflected rays in the normal direction. These rays are unaffected as they return through the polarizing device.
The invention also provides a method of providing polarized illumination over a two-dimensional surface, comprising the steps of directing light from a linear source through an entrance face into a slab-like light guide having a planar front face for emitting polarized light, a planar back face, and an end face; directing said light in said light guide at non-normal angles of incidence to a polarizing device on said back face for reflecting light of a first polarization and transmitting light of a second polarization such that said reflected light is directed in said light guide to said end face; returning said transmitted light to said light guide in a direction substantially normal to said polarizing device such that it passes unaffected through said polarizing device for exit through said front face; and changing the polarization of said reflected light striking said end face and directing the light with changed polarization through said polarizing device at a non-normal angle of incidence for return into said light guide through said polarizing device in a direction substantially normal to said polarizing device such that said light with changed polarization passes unaffected through said polarizing device for exit through said front face.
The invention still further provides a liquid crystal display comprising an addressable liquid crystal array; a polarizing layer in front of said liquid crystal array; a linear light source; a slab-like light guide having an entrance face at one end thereof for admitting unpolarized light from said linear light source, a planar front face for emitting polarized light toward said addressable liquid crystal array, a planar back face, and an end face; a polarizing device on said back face for reflecting light of a first polarization and transmitting light of a second polarization at non-normal angles of incidence, said reflected light being directed in said light guide to said end face; a reflective arrangement under said polarizing devic
Li Li
Pang Zhengda
(Marks & Clerk)
Chowdhury Tarifur R.
National Research Council of Canada
Sikes William L.
LandOfFree
Polarizing back-lighting system for direct view liquid... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Polarizing back-lighting system for direct view liquid..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polarizing back-lighting system for direct view liquid... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2485324