Optics: image projectors – Polarizer or interference filter
Reexamination Certificate
2001-12-07
2004-05-25
Adams, Russell (Department: 2851)
Optics: image projectors
Polarizer or interference filter
C353S031000, C353S033000, C353S084000, C353S102000, C349S009000, C348S790000
Reexamination Certificate
active
06739723
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field Of The Invention
The invention relates to data projection systems, and more particularly to providing an efficient and uniform source of polarized light in liquid crystal-based projectors.
2. Description of the Related Art
Data projectors, also known as digital projectors, are used for projecting images, usually color images, onto a screen. These projectors generally employ one of two types of data projection technologies. First, there are projectors using liquid crystal display (LCD) panels that act as the image forming device. LCD panels can be grouped into two categories: transmissive panels or reflective panels. Transmissive panels are constructed with front and back glass substrates so that light can travel through the panel. Reflective LCD panels are constructed with a front glass substrate and a reflective silicon back substrate. Projectors using transmissive LCD panels are generally referred to as LCD projectors. Projectors using reflective LCD panels are sometimes referred to as LCOS projectors. (LCOS stands for Liquid Crystal on Silicon). Second, there are projectors using reflective digital light processing (DLP) chips. These silicon based chips have movable micro-mirrors constructed on them that act as the image forming device. These projectors are generally referred to as DLP projectors.
Transmissive and reflective liquid crystal display (LCD) projectors typically include a light source emitting a randomly polarized white light. The light is transmitted through an LCD panel which modulates the light according to the desired image data. The resultant light is projected onto a screen for display. For color applications, a single panel or a three-panel LCD projector can be used. In a three-panel LCD projector, incident white light is typically separated into three primary color light components (such as red, green and blue) using prisms or color dichroic mirrors. Each color component passes through an associated LCD panel and is thus separately modulated. The modulated color components are recombined using a color combination prism and projected to produce a color image. In a single panel LCD projector, a color LCD panel can be used. Alternately, conventional methods for modulating different color light components onto the LCD panel, such as by using a color wheel, can be used.
FIG. 1
is a schematic diagram of a conventional single panel LCD projector. LCD projector
10
includes a parabolic lamp
12
as the light source emitting randomly polarized white light. A bank of lens arrays
14
collects the white light generated by lamp
12
and directs the light onto a polarizing converting system (PCS) plate
16
. PCS plate
16
includes a column of polarizing beam splitter (PBS) elements for converting randomly polarized light from lamp
12
to a single polarization. In operation, the incident white light is split into s-polarized light and p-polarized light at the 45° reflecting surface of the polarizing beam splitter element. The p-polarized light is transmitted and s-polarized light is reflected at this surface. The p-polarized light is then transmitted through a half-wave retarder which converts the p-polarized light to s-polarized light. This first beam, now s-polarized, exits PCS plate
16
. Meanwhile, the original s-polarized light, separated from the p-polarized light, is reflected a second time in the PCS elements and exits PCS plate
16
as s-polarized light at a distance W from the first beam of s-polarized light. In this manner, PCS plate
16
converts the incoming white light into s-polarized light which is directed to a condenser lens
18
. PCS plate
16
using PBS elements can achieve polarization at about 80% efficiency.
Condenser lens
18
focuses the s-polarized light onto a transmissive LCD panel
20
. LCD panel
20
typically includes an input polarizer, a liquid crystal module, electrodes for applying charges to the liquid crystal material, and an output polarizer (also called an analyzer). For example, LCD panel
20
can be a conventional thin-film transistor (TFT) active matrix LCD panel. Conventional liquid crystal panels use a twisted-nematic liquid crystal orientation. LCD panel
20
modulates the polarized light incident upon it to form an image to be displayed. The modulated light is directed to a projection lens
22
for magnifying and focusing the image onto a screen (not shown).
LCD panel
20
typically includes an input polarizer to further polarize the incident light to achieve a higher degree of polarization. The input polarizer is needed because PCS plate
16
is only capable of polarizing about 80% of the light. Also, some depolarizatoin can occur as light propagates through other optical elements, such as the condenser lens. Therefore, an input polarizer is placed directly before LCD panel
20
to removes light that is not polarized in the desired orientation. In this manner, the input polarizer ensures that the light transmitted through LCD panel
20
is completely and precisely polarized.
While the conventional LCD projectors such as LCD projector
10
is useful in data projection applications, the conventional LCD projector architecture has several disadvantages. First, the lens array used to collect the light from the light source has a narrow acceptance angle, typically less than ±3 degrees. The narrow acceptance angle of the lens array limits light collection, thus introducing inefficiency in the optical system.
Another disadvantage associated with the conventional LCD projector architecture is the color gradient problem that can occur with the use of the lens array/PCS system in a three-panel LCD projector.
FIG. 1
illustrates a single-panel LCD projector. A three-panel LCD projector can be built using any conventional architecture. In one example, one lens array/PCS systems is used and two color separation plates are included in the optical path between the PCS system and the three LCD panels. The color separation plates typically include a color dichroic film for separating the white light from the light source into three separate color components. Most color separation plates are designed to work with light impinging upon it at a 45 degree angle. However, when the lens array system is used, the light transmitted through lenslets on one side (e.g. the left edge) of the lens array go through the color separation plate at different angles than light transmitted through lenslets on the center or right side of the lens array. As a result, the projected image may suffer from a color gradient problem. Attempts at eliminating these color gradients are not satisfactory because they usually involve using “cleanup” filters which cause the loss of final system brightness.
Finally, LCD projectors are typically larger in size than corresponding DLP projectors because they use lens array systems. This is because the lens array, the PCS system and the condenser lens are larger components than the light integrating devices typically used in DLP projectors. The smallest DLP projectors are typically 2-5 lbs. as compared to typical LCD projectors which are 5-8 lbs. LCD projectors are thus not well suited for portable applications. Therefore, it is desirable to provide an LCD projector that is more compact but with improved brightness to improve the portability of the equipment, while avoiding the aforementioned disadvantages.
FIG. 1
illustrates an LCD projector using a transmissive LCD panel. LCD projectors using a reflective LCD panel, such as liquid crystal over silicon (LCOS) panels, are all well known. Reflective LCOS projectors operate by shinning light onto an image forming reflective chip. The light reflected off the image forming reflective chip forms the image to be projected and displayed. In the case of an LCOS projector, the image forming device is a silicon chip with a liquid crystal panel built over the surface of the silicon chip. The liquid crystal cell on the LCOS chip modulates the incident light on a pixel by pixel basis to generate the desired image.
DLP projectors are also wel
Haven Thomas J.
Munson Kurt R.
Adams Russell
Cook Carmen C.
Delta Electronics , Inc.
Koval Melissa J
Patent Law Group LLP
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