Liquid crystal cells – elements and systems – Liquid crystal system – Projector including liquid crystal cell
Reexamination Certificate
2000-05-12
2004-03-09
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal system
Projector including liquid crystal cell
C359S490020
Reexamination Certificate
active
06704065
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to image formation, and more particularly, to optical systems which employ color-selective polarizing elements for forming color images.
2. Background of the Related Art
Optical projection systems in the related art use transmissive thin-film-transistor (TFT) liquid crystal display (LCD) panels. Multi-layer evaporated thin-film dichroic beamsplitters that are tilted with respect to the axis of incident light are used to create physically distinct paths, each representing the spectral power in one portion of an additive primary color band red-green-blue (RGB). An LCD in each path controls the local light transmission level of a particular primary color band. Modulated or imagery light is recombined with additional tilted isotropic coatings and full-color imagery is projected onto a front or rear projection screen.
In transmissive systems, the LCD is positioned between crossed polarizers as an approach to obtain high contrast ratios for most LC electro-optic modes. In reflective systems, where light is incident substantially normal to the LCD panel, the analogous configuration is to position a polarizing beamsplitter (PBS) directly in front of the panel, as described in a parent application, incorporated by reference above.
One type of polarization beam splitter is a tilted thin-film stack with four ports, which reflects or transmits a light spectrum based on its polarization. A PBS ideally functions as a broad-band reflector for a light spectrum polarized along one axis, and as a transmitter for a light spectrum polarized along an orthogonal axis. A dichroic beamsplitter ideally reflects or transmits a light spectrum based only on the wavelength of the light.
A full-color split-path projector may use reflective LCD panels, with dichroic beamsplitters for creating three color paths, a polarizing beamsplitter for each reflective LCD panel, and additional optics for recombining the imagery before the projection lens. Such implementations are cumbersome and expensive.
An alternative is to use a single polarizing beamsplitter, followed by a Philips prism for separating and combining the three color paths. In this architecture, the color splitting/combining structure is effectively positioned between the polarizer and an analyzer. A benefit from a component count standpoint is that the three color paths share the same PBS.
However, high contrast ratio mandates that the Philips prism preserves the input state of polarization for each color path so that light efficiently exits the input port. This condition must be maintained such that the contrast ratio averaged over the f-number of the system is 100:1, and ideally exceeds 200:1. Clean up polarizers, and in some cases additional polarization optics, such as quarter-wave plates, between the LCD panel and prism can then be used to improve the contrast ratio.
In the Philips prism, red and blue are first reflected by a dichroic coating and then total internally reflected (TIR) before impinging on an LCD. Polarization modulated light then returns along the same path. The spectral characteristics of the dichroic coating are strongly dependent on incidence angle, creating a significant cross-talk between color channels. To help overcome problems with cross-talk, a “double notch” filter (DNF) is frequently inserted which substantially blocks interband light, such as true cyan and true yellow. The DNF is also a multi-layer coating, but because it is used at near normal incidence, it is less sensitive to changes in incidence angle. Nonetheless, when averaged over the f-number of the system, the density of light at the notch is reduced.
Accordingly, related art three-panel reflective projection systems use a PBS, a DNF, and a Philips prism, each of which consists of three prisms, two of which have dichroic mirror coatings. To achieve the performance of transmissive systems using reflective LCD panels, an architecture is required that reduces the complexity and cost, while increasing contrast ratio and throughput.
Multi-layer thin-film coatings are used in the related art for manipulating color in projection display systems. This technology is well matched to the high efficiency and high power handling requirements of projection. Moreover, the steep transition slopes desired to maximize luminance, while meeting color coordinate standards, can be achieved. However, tilted isotropic coatings can degrade polarization quality, particularly in low f-number systems. In LCDs, polarization must be accurately preserved in order to achieve low dark state leakage. Furthermore, dichroic mirrors have an angle sensitive half-power wavelength that shifts substantially with incidence angle.
In order to create physically distinct color paths using a dichroic mirror, the layers are often substantially tilted with respect to the axis of incident light. This significantly increases the spectral shift covered by small angular excursions with respect to the bias angle. At a worst case bias angle of 45°, the wavelength (spectral) shift is approximately linear with angular change.
Fundamentally, the apparent thickness of each layer in a thin-film stack is reduced with an off-normal incidence angle, resulting in a blue shift of the spectrum. When a bias angle is present, both blue and red shifts are present in the plane of incidence. Such angle sensitivity can limit the f-number in color management systems and, in particular, LCD projectors.
Reflective silicon display panels are readily known in the related art. The most common reflective silicon display panels are VLSI-based active-matrix panels that are processed to have a high or flat fill factor, and high visible reflectivity. Alternatively, polysilicon panels can be made to function as reflective displays. In VLSI-based panels, a thin liquid crystal film is sandwiched between the silicon chip and a cover glass coated with a transparent conductor, typically indium tin oxide (ITO). The liquid crystal can be either a nematic or smectic material, both of which are well documented in the art. The liquid crystal is an anisotropic medium, which responds to an electric field by changing its orientation. This in-turn changes the polarization state of light propagating through the liquid crystal.
FIGS.
1
(
a
) and
1
(
b
) illustrate related art reflective display architectures where light having a single polarization state is introduced. As shown in FIG.
1
(
a
), light enters a polarizing beamsplitter (PBS)
10
through a first port
12
, and is reflected out a second port
14
towards a reflective LCD panel
20
. The LCD panel
20
reflects the light back through the second port
14
and the PBS
10
, where the light exits via a third port
16
. In
FIG. 1
b
, the light enters the PBS
10
through the third port
16
, travels through the PBS
10
, and exits through the second port
14
. The LCD panel
20
reflects the light back through the second port
14
, where the light is reflected by the PBS
10
and exits via the first port
12
.
The polarization state of reflected light is locally modulated via the voltage dependent distribution of the LC molecules at each pixel of the LCD panel
20
. This polarization encoded imagery is converted to an actual gray shade image using an analyzing polarizer. In the retroreflecting arrangements shown in FIGS.
1
(
a
) and
1
(
b
), light is introduced and analyzed using the PBS
10
. The PBS
10
effectively positions the LCD panel
20
between crossed polarizers , and also directs light through the system and ultimately to projection lenses.
SUMMARY OF THE INVENTION
An object of the present invention is to substantially obviate one or more of the problems due to limitations and disadvantages of the related art.
Another object of the present invention is to reduce the complexity and cost while increasing contrast ratio and throughput of reflective LCD systems.
Still another object of the present invention is to reduce the f-number in color management systems while maintaining contrast ratio.
It is re
Birge Jonathan R.
Robinson Michael G.
Sharp Gary D.
Colorlink, Inc.
Fleshner & Kim LLP
Parker Kenneth
LandOfFree
Optical system for producing a modulated color image does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical system for producing a modulated color image, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical system for producing a modulated color image will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3189291