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  Lighting system and projectorUSPTO Application #: 20070121310Title: Lighting system and projector Abstract: The invention provides a compact, thin, and lightweight lighting system and projector. A lighting system included in a projector according to the present invention is provided with an LED array having LEDs and a reflective plate at the rear side of the LEDs in the light emitting direction, retardation films, a taper rod lens array, a rod lens array, and a reflective polarizing plate. (end of abstract)
Agent: Oliff & Berridge, PLC - Alexandria, VA, US Inventors: Hidefumi Sakata, Takashi Takeda USPTO Applicaton #: 20070121310 - Class: 362019000 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20070121310. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This is a Division of application Ser. No. 10/434,288 filed May 9, 2003. This application claims the benefit of Japanese Patent Application No. 2002-135641, filed May 10, 2002. The disclosures of the prior applications are incorporated by reference herein in their entirety. BACKGROUND OF THE INVENTION [0002] 1. Field of Invention [0003] The present invention relates to a lighting system and a projector. Particularly, the invention relates to a structure of a lighting system having a high efficiency for light utilization. [0004] 2. Description of Related Art [0005] A related art projector combines imaging light with a light modulation unit, such as a liquid crystal light valve, and projects the combined and enlarged light image on a screen from a projection optical system including a projection lens. In a lighting optical system used in such a projector, light emerging from a light source, such as a metal halide lamp, is randomly polarized light. However, a liquid crystal projector including a liquid crystal light valve using polarized light utilizes only polarized light in one direction for display. When light from the light source is incident on the liquid crystal light valve, merely a half quantity of light is absorbed in a polarizing plate at the light incident side, the absorbed light being not utilized for display. [0006] In order to enhance the efficiency for light utilization, a related art liquid crystal projector is provided with a polarization conversion device provided between the light source and the liquid crystal light valve to convert randomly polarized light from the light source to unidirectionally polarized light that is used for display. A polarization conversion device that is generally used is a polarized beam splitter (PBS) array. The PBS array is a combination of a plurality of PBSs, each having a polarization beam separation film and a reflective film, and a retardation film such as a 1/2 wavelength film. The PBS array has a function that one of the p-polarized light component and the s-polarized light component contained in the light from the light source is converted to the other. SUMMARY OF THE INVENTION [0007] Unfortunately, the polarization conversion device having the PBS array of the related art projector is subject to the following problems. [0008] Since each PBS element has a measure of area, the PBS array including PBS elements inevitably has a considerable size and does not meet recent requirements of reduction in the size, thickness, and weight of the projector or does not sufficiently meet these requirements. Furthermore, the light must be incident on the polarization beam separation films of the PBS array. Thus, a lens system is required to converge the light to the polarization beam separation films. Accordingly, the structure of the polarization conversion device is complicated. In addition, the p-polarized light and the s-polarized light separated by the polarization beam separation film have different optical path lengths, resulting in shifting of the lighting condition on the light valve and thus a decrease in efficiency for light utilization. [0009] In order to address or solve the above and/or other problems, the present invention provides a compact, thin, and lightweight lighting system having a high efficiency for light utilization and a simple structure. The invention also provides a compact, thin, and lightweight liquid crystal projector provided with the lighting system. [0010] To address or achieve the above, a lighting system according to the present invention includes at least one light source, a polarization conversion device that polarizes light from the light source in one direction, and at least one reflection plate that is disposed at the rear side of the light source in the light emitting direction such that the reflective face of the reflection plate is opposite to the light source. The polarization conversion device is provided at the light emitting side of the light source, the polarization conversion device having a reflective polarizing plate that transmits polarized light having a predetermined vibration direction and reflects polarized light having vibration directions other than the predetermined vibration direction. [0011] According to this structure, the reflective polarizing plate that transmits one type of polarized light (for example, p-polarized light) and reflects the other type of polarized light (for example, s-polarized light) is provided at the light emitting side of the light source. When randomly polarized light from the light source is incident on the reflective polarizing plate, for example, the p-polarized light passes through the reflective polarizing plate while the s-polarized light is reflected toward the light source. The reflected light toward the light source is re-reflected by the reflection plate provided at the rear of the light source toward the reflective polarizing plate. If the light that did not pass through the reflective polarizing plate repeatedly travels between the reflective polarizing plate and the reflection plate as long as the polarization state of the light does not vary. In fact, the polarization state of the reflected light gradually varies during the repeated reflection. Thus, part of the reflected light passes through the reflective polarizing plate, and the unabsorbed light finally passes through the reflective polarizing plate. When the lighting system of the present invention is applied to a projector, the light that is first reflected by the reflective polarizing plate finally passes through the reflective polarizing plate toward a light modulation device if the polarization of the light is oriented to a predetermined direction. The lighting system of the present invention thus has a higher efficiency for light utilization than related art systems. [0012] In the lighting system of the present invention, the reflective polarizing plate is used as an element having a polarization conversion function, in place of a related art PBS array. The reflective polarizing plate may be, for example, a multilayer polarizing film, which contributes to achievement of a compact, thin, and lightweight display, unlike the PBS array. The reflective polarizing plate can receive light on the entire surface, and does not require convergence of light to a specific position that is essential for the PBS array. Thus, the polarization conversion optical system needs no optical lens, resulting in a simplified structure. Furthermore, a wavelength plate, which is essential for the PBS array, is not required. Hence, the number of the component can be reduced. [0013] The lighting system according to the present invention preferably further includes a retardation film between the light source and the reflective polarizing plate. [0014] According to this structure, in addition to a spontaneous change in polarization when the light is reflected by the reflective polarizing plate or the reflection plate, in this structure, the retardation film actively changes the polarization state. Hence, the change in the polarization is enhanced when the light reciprocally travels between the reflective polarizing plate and the reflection plate. As a result, the light transmittance of the reflective polarizing plate increases so that the efficiency for light utilization can be enhanced. Preferably, the retardation film has various phase differences at positions transmitting the light, rather than a uniform phase difference such as a half-wave or quarter-wave phase difference. According to this structure, the light reciprocally traveling between the reflective plate and the reflective polarizing plate is subjected to different types of phase modulation in the forward path and the backward path, resulting in an enhanced change in the polarization state. [0015] The lighting system preferably further includes a rod light guide or a tubular light guide between the light source and the reflective polarizing plate, the tubular light guide having a reflective inner face. Herein, "the rod light guide" or "the tubular light guide having a reflective inner face" indicates a related art rod lens. [0016] According to this structure, the light guide (rod lens) not only introduces the light emerging from the light source to the reflective polarizing plate, but also reflects the light at the inner face of the light guide when the light passes therethrough. The light reflected at various angles is superimposed at the light emitting face of the light guide to uniform the illuminance of the light. That is, when the light is emitted from the illumination unit, the polarization state is oriented in one direction and the illumination distribution is uniform. In general, a projector is often provided with a uniform illumination device, for example, a fly-eye integrator or a rod integrator to uniform the illumination distribution of light from the light source. The illumination unit having the above structure functions as a uniform illumination device and a polarization conversion device. [0017] The reflective polarizing plate may include a grid polarizer having a plurality of reflective elements arranged in a striped pattern at a pitch that is smaller than the wavelength of incident light. [0018] In this structure, the reflective polarizing plate can be composed of an inorganic material, therefore having high light resistance and high heat resistance. Thus, this lighting system is particularly suitable for a projector that emits high-brightness light. [0019] A projector according to a first aspect of the present invention includes the above-described lighting system. The lighting system is a planar lighting system including a plurality of the light sources that are arranged in a flat or curved plane and that emit color light beams of different colors in order of time. The projector further includes a light modulation device including a light valve that is driven in a time division mode in synchronization with emerging timings of the color light beams emerging from the light source in order of time, and a projecting device that projects the light modulated by the light modulation device. [0020] In this structure, the lighting system of the present invention contributes to achievement of a compact, thin, and lightweight projector, and enhances the efficiency for light utilization, resulting in higher brightness and lower electric power consumption. Furthermore, this projector employs a driving mode referred to as a "color field sequential system." Thus, this projector requires only one light valve (single-plate structure), unlike related art three-plate projectors having three liquid crystal light valves for three color light beams. Accordingly, the projector requires only one illumination optical system for optical modulation device. Furthermore, the projector requires no color separation optical system and no color combining system, resulting in a significantly reduced number of the components, a simplified structure, and reduced cost. [0021] A projector according to a second aspect of the present invention includes a plurality of the above-described lighting systems. Each of the lighting systems is a planar lighting system including a plurality of the light sources that are arranged in a flat plane and the lighting systems emit color light beams of different colors. The projector further includes a plurality of light modulation devices including light valves that modulate the color light beams emerging from the lighting systems via the polarization conversion device; a color combining device that combines the modulated color light beams from the plurality of light modulation devices; and a projecting device that projects the light combined by the color combining device. [0022] Also, according to the projector of this structure, the lighting systems of the present invention contribute to achievement of a compact, thin, and lightweight projector, and enhance the efficiency for light utilization, resulting in higher brightness and lower electric power consumption. This projector requires a plurality of light valves, unlike the projector according to the first aspect. Since this projector emits different color light beams from the plurality of light sources and has the light valves for these color light beams, it does not require a color separation device, which is essential for any related art display. As a result, it has a simplified structure compared with the related art structure. Furthermore, synchronization of driving of the light sources with the light valves is not required, unlike the first aspect. Hence, the drive mechanism is simplified, and liquid crystal light valves having a lower response rate may be used. Continue reading... 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