MEM devices, such as digital light processing (DLP™) devices disclosed in Texas Instruments' patents and technical publications, have been widely used in projection displays. The MEM devices, also called reflective light modulators or micro-display panels, are made of small electronically controlled micro-mirrors in rows and columns. (In the following sections, the terms “DLP” and “MEM” are used interchangeably to describe MEM display panels). FIG. 1A shows the principle of the MEM device and FIG. 1B shows a typical arrangement of a single DLP panel projection display. Each micro-mirror or pixel in a DLP panel can be titled electronically at two fixed positions +α, or −α, corresponding to “on” and “off” pixels, respectively. In FIG. 1A the light is incident from the side of the micro-display panel at an angle 2α to the panel normal. For “on” pixels, the mirrors are tilted towards the incident beam. The incident beam is reflected along the normal of the panel surface and is directed through an optical arrangement that typically includes a projection lens as shown in FIG. 1B. For “off” pixels, the micro-mirrors will be tilted away from the incident beam, i.e. away from the projection lens direction and this unwanted beam will be absorbed by a light absorber. For “flat pixels” or any non-moving specular surfaces on the panel, the light will be reflected with an angle 2α from the panel normal and will also be absorbed by a light absorber. By controlling the amount of time each micro-mirror is in the “on” and “off” positions an image can be encoded onto the “on” beam and projected onto a screen. In addition, full colour images can be displayed on screen by using a colour filter wheel to generate colour light beams time sequentially at a fast speed.
As shown in FIG. 1A, the incident beam and the image or “on” beam directions are separated from each other. However, in practice the incident light beam contains a cone of light over a range of angles and so therefore does the reflected beam. The two beams are very close to each other. In order to physically separate them in a projection arrangement, sometimes a total internal reflection (TIR) prism, or “light gate”, device as shown in FIG. 1B is used. The light beam with a cone angle from an illumination system is incident upon the TIR surface at angles greater than the critical angle, thus the beam is totally reflected towards the DLP panel. For “on” pixels, the light reflected back from the panel to the TIR surface is incident at angles smaller than the critical angle and thus will pass through this surface. Because of the use of the TIR prism, the cone angle of the “on” pixels, or the aperture of the illumination system and the projection optical system can not exceed the cone angle determined by the mirror tilting angle to avoid overlap of the beams. For a DLP panel with 10° tilting angle, the f-number of the illumination and projection systems will be 2.9, thus limiting the amount of light that can be used from the illumination system and the light efficiency of the system. FIG. 1B also shows a typical illumination system arrangement of DLP projectors having a light source, a colour wheel, and a light pipe. DLP projection systems, such as the arrangement shown in FIG. 1B, are used for two dimensional (2D) image displays, such as in rear projection TVs or front projectors.
A three-dimensional (3D) stereoscopic image display needs to display 2D images that represent both the left- and right-eye perspective views for the viewer. This can be achieved, for example, by displaying a left-eye 2D image in one polarized light and a corresponding right-eye 2D image view in orthogonally polarized light so that a viewer wearing polarizing glasses receives correct 2D images in each eye and thus perceives a stereoscopic 3D image. There are a several approaches to use DLP projectors to project three-dimensional (3D) images. However, in these approaches, the DLP projectors have internal optical arrangements that are similar to 2D-only DLP projectors. They use un-polarized light and achieve 3D operation only in combination with other external components or devices. Because of this they are highly light inefficient for 3D use. In the first 3D approach, a single active DLP projector can alternatively display left- and right-eye 2D images time sequentially. A viewer, wearing active liquid crystal shutter (LC) glasses that are synchronized with the DLP projector, will be able to see 3D images because the left- and right-eye LC shutters open only when the correct eye image is displayed. The LC shutters typically transmit only about 35% of the incident light. In addition, because the left- and right-eye images are each displayed at most half of the time, this arrangement is very light inefficient for displaying 3D images. The 3D images appear very dim in comparison to the same projector displaying a single 2D image. Furthermore, LC shutter glasses are expensive and need electrical power to operate, making them bulky and inconvenient to wear.
In the second approach, two DLP projectors can be used to separately project the left- and right-eye images. Each projector has a sheet polarizer in front of it. One sheet polarizer allows light in one polarization to pass and the other one allows the orthogonal polarization to pass. Thus, the left- and right-eye 2D images are projected simultaneously with orthogonal polarizations onto a polarization-preserving screen. A viewer wearing polarizing glasses will be able to see 3D images because the polarizing glasses only allow the correct image to be seen by the correct eye. However, such an approach is bulky and expensive because it requires two projectors and it is also difficult to align the images from the two projectors. In addition, the light efficiency in 3D is low as well because of the use of external polarizers whose transmittance at most is 42%. Another approach to display 3D images with DLP projectors is to encode the left- and right-eye 2D images using different spectral regions of the illuminating light and to view a stereoscopic 3D image using appropriate colour filter glasses. This approach can be implemented with a colour filter wheel time sequential approach or with dual projectors. Similarly with the polarization encoding approach, this approach also results in expensive and light inefficient 3D displays.
Liquid crystal display (LCD) microdisplay technology represents an alternative to MEMs devices in projection systems. In contrast to MEMs micro-mirror panels that modulate the direction of an incident illuminating beam in order to form an image, LCD devices form an image by changing the polarization of the illuminating light. LCD micro-panels can operate in either transmission or reflection and polarizing beam splitters (PBS) are needed to separate the polarized image beam from the orthogonally polarized illuminating beam. The light sources used in projection systems typically emit un-polarized light and one advantage of MEMs devices is that they can operate with un-polarized incident light whereas LCD devices require polarized incident light. MEMs devices reflect light at different angles for “on” and “off” pixels and use a TIR prism to separate the encoded image beam from the illuminating light. LCD devices modulate the polarization of the illuminating beam and use a PBS to separate out the polarized encoded image beam.