1. Field of Invention
This invention relates to the recovery of light that might otherwise be unused in projection systems.
2. Description of the Related Art
Projection displays work by projecting light onto a screen. The light is arranged in patterns of colors or brightness and darkness or both. The patterns are viewed by a viewer who assimilates them by associating the patterns with images with which the viewer may already be familiar, such as characters or faces. The patterns may be formed in various ways. One way to form patterns is by modulating a beam of light with a stream of information.
Polarized light may be modulated by filtering it with polarized filters. Polarized filters will pass light, in general, if their polarization matches the polarization of the incident light. A liquid crystal display (LCD) imager may be used to perform the modulation in LCD-type projection displays. The LCD imager may include pixels that may be modulated by altering their polarization to either match or differ from the polarization of incident light. The light input to the LCD imager is polarized such that when the LCD pixels are modulated the polarization of the selected pixels is changed, and when the light output from the imager is analyzed by another polarizer, the selected pixels will be darkened. The pattern may be projected onto a screen as the presence or absence of light. If the polarization of the pixels is modulated with information in a pattern with which a viewer is familiar, the viewer may recognize the pattern projected onto the screen.
One way to polarize light for an LCD imager is with a polarizing beam splitter (PBS). Polarized light may be provided to an imaging system with an array of lenses, such as a fly's eye lens, and an array of polarizing beam splitters. A parabolic reflector may be used with a fly's-eye lens to focus light such that the light is nearly parallel. The beam is split into many sections by the lens array and each section is refocused by another lens array into the polarizing beam splitter array. A parabolic reflector, however, may reduce the brightness of a source of light, such as an arc. Furthermore, the efficiency of a fly's-eye lens recovery system depends critically on the alignment of the two lens arrays and the polarizing beam splitter array. Finally, a polarization recovery system comprised of a parabolic reflector and a fly's-eye lens may not be suited for sequential color single imager systems.
Elliptical reflectors may be used with a light pipe and a color wheel to produce sequential colors as well. Such a system, however, still requires a polarization recovery system and does not solve the intrinsic loss of brightness associated with ellipsoidal reflectors. The light output from the polarizing beam splitter array will then be linearly polarized and focused into the target. Each polarizing beam splitter divides unpolarized light into beams having disparate polarizations. Only one of the beams will be of the correct polarization to input to the LCD imager after the light is polarized. The other beam will be of an incorrect polarization and hence unusable directly.
Polarization recovery systems may be used to recover light of the unused polarization by converting it into usable light with the correct polarization. Various schemes have been developed to convert the incorrectly polarized light to the correct polarization so that it too may be used. One method, shown in FIG. 1, is to transmit light of a first polarization 102 from a polarizing beam splitter 104 directly to an output 106 while reflecting light of a second polarization 108 at an angle to the output 106, such as a 90° angle. The light of the second polarization 108 is then reflected so it is parallel the light of the first polarization 102, heading toward the output 106. A retarder plate 110, e.g. a quarter wave or half wave plate, is placed in the path of the light of the second polarization 108 to rotate it into light of the first polarization 102 such the output consists of light of only the first polarization 102.
Retarder plates rotate light from one polarization to another by slowing light in one plane down while allowing light in the opposite plane to pass relatively unimpeded. The speed at which light propagates through a medium is, in general, related to its wavelength. The degree to which light is slowed down will thus also be related to its wavelength. Since retarder plates that are applied to broadband light must pass light of a range of wavelengths, some light will be retarded more than other light. Retarder plates are, in general, tuned to a particular wavelength. In particular, wavelengths that are longer or shorter than the tuned wavelength will not be completely rotated from the unusable polarization to the correct polarization. Thus some of the light of wavelengths longer or shorter than the tuned wavelength will be lost, or at least not recovered. Retarder plates, furthermore, are relatively expensive and often not reliable. A retarder plate makes a polarization recovery system itself expensive and unreliable.
Although these systems have been used commercially, the cost of the components is high and they require critical alignments and optical designs. As a result, there is a need for a system to perform polarization conversion with high efficiency, simple configurations and lower costs.