Patent Application
20090268109
Projectors for large screen displays to replace conventional film based projectors in motion picture viewing applications must provide high contrast ratios and high light intensities. One class of projector is based on image modulators such as liquid crystal displays (LCDs). When a polarized illumination source is projected on the LCD, the polarization of the light at each pixel is rotated depending on the signal applied to that pixel. A polarization filter processes the light leaving the LCD to block the light whose polarization has been altered.
Polarization filters based on polarization dependent beam splitters have problems in high illumination intensity applications such as projectors for large screen displays. Such beam splitters are based on a coating at a diagonal plane through a glass cube. The light must pass through the glass both before and after the separation of the light into the two linearly polarized components. Stress in the glass can be created by the machining and polishing operations used in the fabrication process and by non-uniform heating of the glass during the operation of the projector. Some of the light is absorbed in the glass and results in a heating of the glass structure. The heat from the absorbed light must be dissipated through the outer edges of the cube, and hence, a thermal gradient is present. The thermal gradient gives rise to stress birefringence that varies across the polarization filter. The stress birefringence converts the linearly polarized light to elliptically polarized light. As a result, some of the light is passed by the LCD pixels when the pixels are set to block light, and some of the light is blocked when the pixels are set to transmit light. This leads to a decrease in the contrast ratio of the LCD. In addition, the contrast ratio varies over the surface of the LCD panel.
To provide high contrast ratio LCD panels, the material from which the beam splitting prism is constructed must exhibit low stress birefringence and/or reduced light absorption. Glass that provides these properties is expensive. In addition, some of these glass compositions include large amounts of lead, and hence, are subject to environmental restrictions.
The problems of stress birefringence can be reduced by utilizing polarization filters that do not require the glass structures discussed above. One such polarization filter is known as a wire grid polarizer. Wire grid polarizers are known to the art, and hence, will not be discussed in detail here. An example of such a polarizer is taught in U.S. Pat. No. 5,986,730, which is hereby incorporated by reference. For the purposes of the present discussion, it is sufficient to note that a wire grid polarizer can be configured to transmit light of a first linear polarization and reflect light of the orthogonal polarization. The polarizer does not require the glass components discussed above, and hence, avoids the stress birefringence problems discussed above.
In an LCD projector system, three LCD panels are required to provide a color projector. The light from an incandescent or other white light source is split into three component bands that are processed by the LCD panels. The modulated light from the LCD panels must then be recombined to produce a color image that is projected onto the screen. The path lengths traversed by each color of light from the light source to the projection screen is preferably the same. If the path lengths are not the same, additional optical components such as lenses are required to compensate for the path differences. The lenses preferably are surrounded by air to provide the optical imaging function without requiring high index of refraction glass. Hence, a number of components with air interfaces are needed, which complicates the construction of the projector and increases the cost.
The present invention includes a light modulation assembly and an image projector utilizing a plurality of such light modulation assemblies. The light modulation assembly includes an optical element, a pre-polarization filter, an image modulator, and an analyzer polarization filter. The optical element has an input port for receiving a light beam. The optical element directs the light beam onto the pre-polarization filter that removes light having a linear polarization in a first direction. The light leaving the pre-polarization filter illuminates the image modulator. The light leaving the image modulator is filtered by the analyzer polarization filter to remove light having a linear polarization with a predetermined direction relative to the first direction. The light leaving the analyzer polarization filter exits the optical element through an output port. In one embodiment, the light beam traverses an input optical path from the input port to the image modulator and an output optical path from the image modulator to the output port, the input optical path being substantially equal to the output optical path in length. The space between the pre-polarization filter and the light modulator and the space between the light modulator and the analyzer polarization filter are devoid of any material that substantially rotates the polarization of light.
A plurality of light modulation assemblies can be combined to provide a projector. A projector according to the present invention includes first and second light modulation assemblies and a beam splitting assembly. Each of the light modulation assemblies includes an optical element, a pre-polarization filter, an image modulator, and an analyzer polarization filter. The optical element has an input port for receiving a light beam. The optical element directs the light beam onto the pre-polarization filter that removes light having a linear polarization in a first direction. The light leaving the pre-polarization filter illuminates the image modulator. The light leaving the image modulator is filtered by the analyzer polarization filter to remove light having a linear polarization with a predetermined direction relative to the first direction. The light leaving the analyzer polarization filter exits the optical element through an output port.
The beam splitting assembly includes an optical element having an input port for receiving an input light beam and an output port for transmitting a spatially modulated output light beam. The beam splitting assembly generates a first light beam having light in a first optical band and a second light beam having light in a second optical band from the input light beam and directs the first light beam into the input port of the first light modulation assembly and the second light beam into the input port of the second light modulation assembly. The first light beam follows a path having a first optical path length from the input port of the beam splitting assembly to the first image modulator, and the second light beam follows a path having a second optical path length from the input port of the beam splitting assembly to the second image modulator. The first optical path length is substantially equal to the second optical path length.
In one embodiment, the beam splitting assembly also combines light leaving the output ports of the first and second light modulation assemblies to form the spatially modulated output light beam. The first light beam follows a path having a third optical path length from the output port of the first modulation assembly to the output port of the beam splitting assembly, and the second light beam follows a path having a fourth optical path length from the output port of the second modulation assembly to the output port of the beam splitting assembly. The third optical path length is substantially equal to the fourth optical path length.
The manner in which the present invention provides its advantages can be more easily understood with reference to
The LCD panel can be viewed as a two-dimensional array of small pixels that operate on the light passing through each pixel. The panel is illuminated with linearly polarized light. In one state, each pixel merely passes the light without altering the direction of polarization of the light. In the other state, each pixel rotates the polarization of the light to the orthogonal direction. The incident light is typically linearly polarized in the desired direction by a polarization filter. The light that leaves the LCD panel with its polarization rotated is processed by a second polarization filter that is oriented to pass only light of the desired polarization, thereby eliminating light that was processed by pixels in one of the two states. As noted above, any material between the polarization filters and LCD panel that alters the polarization of the light results in a reduced contrast ratio for the imaging system.
The imaging assembly includes wire grid polarizer filter 23, LCD panel 24, and wire grid polarizer filter 25. Wire grid polarization filter 23 acts as a pre-light modulation polarization filter that removes light having one linear polarization to provide an illumination source that consists of light of a predetermined linear polarization. The light leaving LCD panel 24 has been spatially modulated by changing the polarization of the light at each of the pixels. The light leaving the LCD panel is processed by a second wire grid polarization filter 25 with its axis set to eliminate light of the unwanted polarization.
The light leaving polarization filter 25 is reflected back along path 27. The optical element is a monolithic glass part that is constructed from a rectangular body 21 and a pair of angular members 22 having reflective surfaces that direct the light through the modulating assembly. However other arrangements could be utilized. It should be noted that stress birefringence in the optical element does not significantly alter the polarization of the light passing through LCD panel 24, and hence, the problems associated with stress birefringence discussed above are alleviated.
Refer now to
Projector assembly 30 can be viewed as having 4 components, a light separation and combining assembly 31 and 3 light modulating assemblies 35-37 that operate in a manner analogous to light modulation assembly 20 discussed above. Beam splitting and combining assembly 31 is a 3 axis rhomb beam splitter that includes two chromatic beam splitters 51 and 52 and two reflectors 53 and 54. A multicolor light beam entering beam splitting and combining assembly 31 in region 32, which acts as an input port, is split into three beams having different colors as shown at 61-63, respectively. Chromatic beam splitter 51 reflects light in a first band of wavelengths while transmitting light in the other bands to generate beam 61. For example, chromatic beam splitter 51 could reflect light in a band of wavelengths around a wavelength in the red region. The blue and green components of the input light beam would then strike chromatic beam splitter 52, which reflects light in a second band of wavelengths, e.g., a band around a wavelength in the green region of the optical spectrum, to create beam 62. The light transmitted by chromatic beam splitter 52 is then reflected by reflector 54 to form beam 63, which includes light in a third band of wavelengths, e.g., a band around a wavelength in the blue region of the optical spectrum. Reflector 54 can be either a chromatic beam splitting surface or a non-wavelength specific reflector.
The light in each of the beams is processed by a corresponding light modulation assembly. The light modulating assemblies corresponding to beams 61-63 are shown at 35-37, respectively. Each light modulation assembly includes a light pipe and an imaging assembly. The imaging assembly for light modulation assembly 35 is labeled in
In addition, the light pipes are also preferably chosen such that the optical path length from the LCD panel in each of the light modulation assemblies to output port 33 is the same for all of the light modulation assemblies. If this condition is not met, projection lens 66 operating on the output light beam 68 will generate an image in which one or more of the component color images is out of focus with respect to another of the component color images. In the arrangement shown in
The embodiments illustrated in
The above-described embodiments utilize a light modulation assembly in which the polarization filters are wire grid polarizers operating in a transmissive mode. That is, the light that is modulated by the LCD panel is the light that passes through the wire grid polarizer. However, embodiments in which the wire grid polarizers operate in a reflective mode can also be constructed. Refer now to
The advantages provided by the wire grid polarizers are the result of two properties of this type of polarization filter. First, the effective thickness of the light modulation assembly in the case of the transmissive polarization filters shown in
Second, the wire grid polarizers make possible designs in which there is no glass between the polarization filters and the LCD panel. Hence, the polarization state of the light leaving the pre-modulation polarization filter is not altered by stress birefringence in the medium between the filter and the LCD panel. Similarly, the polarization of the light leaving the LCD panel is not altered by stress birefringence in the medium between the LCD panel and the analyzing filter.
The above-described embodiments utilize wire grid polarizers as the pre-polarizing filter and the analyzer polarization filter after the LCD panel. However, other forms of polarizers could be utilized for these functions. For example, a reflective surface in which the light is reflected at the Brewster angle generates a linearly polarized reflected light beam. Hence, one or both of the wire grid polarizers shown in
The embodiments of the projector assembly discussed above with reference to
The above-described embodiments of a projector assembly utilize an input light source having three color components that are divided out into three color beams that are each processed by a light modulation assembly. However, projector assemblies having different numbers of component light beams and light modulation assemblies can also be constructed using the present invention.
In the above-described embodiments of the present invention, various optical paths are referred to as being equal in optical path length. It will be appreciated that these optical paths need only be substantially equal in length for the present invention to provide its advantages. For the purposes of this discussion, two output optical paths will be defined as being substantially equal if the images of the LCD panel in each of the light modulating assemblies are both in focus on a projector screen when viewed by human observer. Similarly, two input optical paths will be defined to have substantially the same optical path length if the differences in the solid angle subtended by the two corresponding LCD panels cause intensity differences in the images generated by the two LCD panels that are less than an intensity difference that a human observer can observe.
The above described embodiments of projectors according to the present invention utilize rhomb beam splitters to split the input light beam into the component color light beams and recombine the spatially modulated light beams. However, embodiments that utilize other forms of beam splitter for these functions can also be constructed. Refer now to
The above-described embodiments of the present invention utilize LCD panels as the image modulator. However, embodiments of the present invention that utilize other forms of image modulator that modulate the polarization of the light could be utilized. For example, light modulators based on ferro-electrics are known to the art.
It should be noted that the polarization filters and the above-described filters could be multi-layer filters in which each layer reflects light of a particular polarization and wavelength.
Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.
