1. Field of the Invention
An apparatus consistent with the present invention relates to a projection system and, more particularly, to a projection system which can improve contrast using a wire grid polarization beam splitter.
2. Description of the Related Art
In a conventional projection system, a light valve, such as a liquid crystal display (LCD) or a Digital Micro-mirror Device (DMD), controls the on/off operation of light emitted from a light source on a pixel-by-pixel basis and forms a picture, and a magnifying projection optical system enlarges the picture to be displayed on a large screen.
Projection systems are classified into 3-panel projection systems or single-panel projection systems according to the number of light valves that are used. A 3-panel projection system provides better light efficiency than a single-panel projection system but is more complicated and expensive than the single-panel projection system. The single-panel projection system can include a smaller optical system than the three-panel projection system but provides only ⅓ of the light efficiency of the three-panel projection systems, because red (R), green (G), and blue (B) colors, into which white light is separated, are sequentially used. More specifically, in the single-panel projection system, white light radiated from a white light source is separated into three color beams, namely, R, G, and B color beams, using color filters, and the three color beams are sequentially sent to a light valve. The light valve operates according to the sequence of color beams received to create images. Since the single-panel projection system sequentially uses color beams, the light efficiency is reduced to ⅓ of the light efficiency of a three-panel projection system.
A color scrolling method has been recently developed in which the light efficiency of the single-panel projection system is increased. In the color scrolling method, R, G, and B beams, into which white light is separated, are simultaneously sent to different locations on a light valve. Since an image cannot be produced until all of the R, G, and B beams reach each pixel of the light valve, the R, G, and B color beams are moved at a constant speed by a color scrolling means.
FIG. 1 is a schematic diagram of a single-panel scrolling projection system disclosed in U.S. Publication No. 2002/191154 A1. Referring to FIG. 1, white light emitted from a light source 100 passes through first and second lens arrays 102 and 104, a polarization conversion system (PCS) 105, and a condenser lens 107, and is separated into R, G, and B color beams by first through fourth dichroic filters 109, 112, 122, and 139. More specifically, the red beam R and the green beam G, for example, pass through the first dichroic filter 109 and travel along a first light path L1, while the blue beam B is reflected by the first dichroic filter 109 and travels along a second light path L2. The red beam R and the green beam G on the first light path L1 are separated by the second dichroic filter 112. The red beam R continues along the first light path L1, passing through the second dichroic filter 112, and the second dichroic filter 112 reflects the green beam G along a third light path L3.
The red, green, and blue beams R, G, and B are scrolled while passing through first through third prisms 114, 135, and 142, respectively. The first through third prisms 114, 135 and 142 are disposed in the first through third light paths L1, L2, and L3, respectively, and as the first, second, and third prisms 114, 135, and 142 rotate at a uniform speed, R, G, and B color bars are properly scrolled. The green and blue beams G and B, which travel along the second and third light paths L2 and L3, respectively, are transmitted and reflected by the third dichroic filter 139, respectively, and combined. The red, green, and blue beams R, G, and B are then combined by the fourth dichroic filter 122. The combined beam is transmitted by a polarization beam splitter (PBS) 127 and forms a picture using a light valve 130.
The scrolling of the R, G, and B color bars due to rotation of the first through third prisms 114, 135, and 142 is illustrated in FIG. 2. Scrolling represents the movement of color bars formed on the surface of the light valve 130 when the first, second, and third prisms 114, 135, and 142 corresponding to R, G, and B colors, respectively, are synchronously rotated.
A color image obtained by turning the pixels of the light valve 130 on or off according to an image signal is magnified by a projection lens (not shown) and projected onto a screen.
Since the conventional projection system uses different light paths for different colors, a light path correction lens must be included for each of the colors, components for unifying the separated light beams must be further included, and separate components must be included for each of the colors. Hence, the conventional optical system is bulky, and the manufacturing and assembly thereof is complicated, thus decreasing the yield.
Three motors for rotating the first, second, and third scrolling prisms 114, 135, and 142 generate much noise during operation. Thus, the projection system adopting three motors is manufactured at a greater cost than a color wheel type projection system which utilizes a single motor.
In order to produce a color picture using a scrolling technique, color bars as shown in FIG. 2 must be scrolled at a constant speed. Hence, the conventional projection system must synchronize the light valve 130 with the three prisms 114, 135, and 142 in order to achieve proper scrolling. However, controlling the synchronization is not easy. Due to the circular motion of the scrolling prisms 114, 135, and 142, the color scrolling speed by the three scrolling prisms is irregular, consequently deteriorating the quality of the resultant image.
The conventional projection system uses a dielectric-coated MacNeille type PBS. Since the dielectric-coated MacNeille type PBS does not properly split a beam with a wide incidence angle according to polarization direction, contrast is degraded.