The invention relates to a multiple imager projection system using a large arc lamp with a projection system having a smaller imager.
Microdisplays using Digital Light Processing (DLP) and/or Liquid crystal display (LCD), and particularly liquid crystal on silicon (LCOS), imagers are becoming increasingly prevalent in imaging devices such as rear projection television (RPTV).
Digital Light Processing (DLP) imagers use an array of micro-mirrors, each acting as a pixel, which are pivoted at a very high rate of speed to temporally modulate light intensity on a pixel-by-pixel basis.
Liquid crystal displays (LCD's), and particularly liquid crystal on silicon (LCOS) systems use a reflective light engine or imager. In an LCOS system, projected light is polarized by a polarizing beam splitter (PBS) and directed onto a LCOS imager or light engine comprising a matrix or array of pixels. Throughout this specification, and consistent with the practice of the relevant art, the term pixel is used to designate a small area or dot of an image, the corresponding portion of a light transmission, and the portion of an imager producing that light transmission.
Each pixel of the DLP or LCOS imager modulates the light incident on it according to a gray-scale factor input to the imager or light engine to form a matrix of discrete modulated light signals or pixels. The matrix of modulated light signals is reflected or output from the imager and directed to a system of projection lenses which project the modulated light onto a display screen, combining the pixels of light to form a viewable image. In this system, the gray-scale variation from pixel to pixel is limited by the number of bits used to process the image signal. The contrast ratio from bright state (i.e., maximum light) to dark state (minimum light) is limited by the leakage of light in the imager.
One of the major disadvantages of existing LCOS and DLP systems is the difficulty in reducing the amount of light in the dark state, and the resulting difficulty in providing outstanding contrast ratios. This is, in part, due to the leakage of light, inherent in these systems.
In addition, since the input is a fixed number of bits (e.g., 8, 10, etc.), which must define the full scale of light, there tend to be very few bits available to define subtle differences in darker areas of the picture. This can lead to contouring artifacts.
One approach to enhance contrast in LCOS in the dark state is to use a COLORSWITCH™ or similar device to scale the entire picture based upon the maximum value in that particular frame. This improves some pictures, but does little for pictures that contain high and low light levels. Other attempts to solve the problem have been directed to making better imagers, etc. but these are at best incremental improvements.
In microdisplay systems, a general, very desirable tendency is the reduction of the imager area. This is desirable because of improved yields on the imager, and smaller optical components, thus reducing the cost of the system. Reducing the imager area places increasing constraints on the arc lamp design. As the imager shrinks the arc lamp must also be scaled down in size to keep the etandue constant. The reduction in size of the arc lamp results in increasingly shorter arc lamp life, causing increased maintenance and cost to operate the microdisplay.
The invention provides a projection system that provides improved contrast and contouring of a light signal on a pixel-by-pixel basis using a two-stage projection architecture, thus improving all video pictures. The projection system uses two imagers, the first being larger to accommodate a large lamp, sized to the first imager and the second being smaller. The first imager has a matrix of pixels for modulating light on a pixel-by-pixel basis to form a first modulated matrix of light. The second imager has a matrix of pixels corresponding to the pixels of the first imager for modulating the first modulated matrix of light on a pixel-by-pixel basis to form a second modulated matrix of light. The second imager having a size smaller than the size of the first imager. A relay lens set provides a magnification of less than 1.0 to relay each pixel of light in the first modulated matrix of light onto a corresponding pixel of the second imager.
The invention will now be described with reference to accompanying figures of which:
The present invention provides a projection system, such as for a television display, with enhanced contrast ratio and reduced contouring, while providing good lamp life. This is accomplished by using a larger imager 50 for the first stage to maintain a larger lamp 10, and a smaller image 60 for the second stage. In the embodiment illustrated, lamp 10 may be an arc lamp generating white light 1, suitable for use in an LCOS system. For example a short-arc mercury lamp may be used. The white light 1 enters an integrator 20, which directs a telecentric beam of white light 1 toward the projection system 30. The white light 1 is then separated into its component red, green, and blue (RGB) bands of light 2. The RGB light 2 may be separated by dichroic mirrors (not shown) and directed into separate red, green, and blue projection systems 30 for modulation. The modulated RGB light 2 is then recombined by a prism assembly (not shown) and projected by a projection lens assembly 40 onto a display screen (not shown).
Alternatively, the white light 1 may be separated into RGB bands of light 2 in the time domain, for example, by a color wheel (not shown), and thus directed one-at-a-time into a single LCOS projection system 30.
An exemplary LCOS projection system 30 is illustrated in
In the exemplary embodiment, illustrated in
The first light matrix 5 of s-polarized light is reflected by the PBS 71 through a relay lens system 80, which provides a magnification of less than one to project each pixel of first light matrix 5 onto a corresponding pixel of smaller imager 60. In an exemplary embodiment, illustrated in
As shown in
After the first light matrix 5 leaves the relay lens system 80, it enters into a second PBS 72 through a first surface 72a. Second PBS 72 has a polarizing surface 72p that reflects the s-polarized first light matrix 5 through a second surface 72b onto a second imager 60. In the exemplary embodiment, illustrated in
The second imager 60 then produces an output matrix 6 of p-polarized light. Each pixel of light in the output matrix 6 is modulated in intensity by a gray scale value provided to the imager for that pixel of the second imager 60. Thus a specific pixel of the output matrix 6 (i,j) would have an intensity proportional to both the gray scale value for its corresponding pixel (i,j), in the first imager and its corresponding pixel (i,j)2 in the second imager 60.
The lamp 10 must be sized to the first stage imager to maintain the desired etandue. Using a larger imager 50 in the first stage of the projection system 30 allows the lamp 10 to be larger, resulting in longer lamp life. Moreover a more modest imager (in terms of contrast ratio) can be used for the larger imager 50, because a second, smaller imager 60 will also be used to modulate the projected image. The modest large imager 50 receives the lamp 10 illumination (from a larger arc lamp) and then relays the light using a now less than unity magnification lens to illuminate on a pixel by pixel basis a “high quality” smaller imager 60. In the illustrated exemplary embodiment a ˜0.7″ larger imager 50 is used as an illumination imager, and a ˜0.5″ smaller imager 60 is used as an image making imager. The relay lens system 80, as described above provides one-to-one correspondence between the pixels of the larger imager 50 and the smaller imager 60.
The light output L of a particular pixel (i,j) is given by the product of the light incident on the given pixel of first imager 50, the gray scale value selected for the given pixel at first imager 50, and the gray scale value selected at second imager 60:
L=L0×G1×G2
L0 is a constant for a given pixel (being a function of the lamp 10, and the illumination system.) Thus, the light output L is actually determined primarily by the gray scale values selected for this pixel on each imager 50, 60. For example, normalizing the gray scales to 1 maximum and assuming each imager has a very modest contrast ratio of 200:1, then the bright state of a pixel (i,j) is 1, and the dark state of pixel (i,j) is 1/200 (not zero, because of leakage). Thus, the two stage projector architecture has a luminance range of 40,000:1.
L max=1×1=1;
L min=0.005×0.005=0.000025
The luminance range defined by these limits gives a contrast ratio of 1/0.000025:1, or 40,000:1. Importantly, the dark state luminance for the exemplary two-stage projector architecture would be only a forty-thousandth of the luminance of the bright state, rather than one two-hundredth of the bright state if the hypothetical imager were used in an existing single imager architecture. As will be understood by those skilled in the art, an imager with a lower contrast ratio can be provided for a considerably lower cost than an imager with a higher contrast ratio. Thus, a two-stage projection system using two imagers with a contrast ratio of 200:1 will provide a contrast ratio of 40,000:1, while a single-stage projection system using a much more expensive imager with a 500:1 ratio will only provide a 500:1 contrast. Also, a two-stage projection system with one imager having a 500:1 contrast ratio and an inexpensive imager with a 200:1 ratio will have a system contrast ratio of 100,000:1. Accordingly, a cost/performance trade-off can be performed to create an optimum projection system.
Output matrix 6 enters the second PBS 72 through second surface 72b, and since it comprises p-polarized light, it passes through polarizing surface 72p and out of the second PBS 72 through third surface 72c. After output matrix 6 leaves the second PBS 72, it enters the projection lens assembly 40, which projects a display image 7 onto a screen (not shown) for viewing.
To provide one-to-one correspondence between the pixels of the first imager 50 and the second imager 60, the relay lens set 80 must provide good ensquared light energy. That is, the light from a pixel (i,j) in the first imager 50 must be accurately projected onto the corresponding pixel (i,j) on the second imager 60.
The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/US04/14657 | 5/11/2004 | WO | 11/8/2006 |