Optical: systems and elements – Lens – Multiple component lenses
Reexamination Certificate
2000-10-27
2002-10-29
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Multiple component lenses
C359S793000, C359S651000
Reexamination Certificate
active
06473244
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection lens, and, more particularly, to a projection lens suitable for use in, for example, a projector of a projection display device.
2. Description of the Related Art
In recent years, the use of a projection display device is becoming widespread.
What is called a rear projection display device is known as one type of projection display device. In the rear projection display device, image light is projected onto a transmissive screen from the back surface side thereof in order to display an image.
What is called a three-plate projection display device which includes two-dimensional image display devices (that is, light valves) provided in correspondence with three colors, red (R), green (G), and blue (B), is known as one type of rear projection display device. In the three-plate projection display device, light beams from a white light source and collimated by, for example, a reflector is separated into light beams of the three different colors, a red light beam, a green light beam, and a blue light beam, by a color-separating mirror.
Then, the light beams of the three different colors impinge upon the corresponding two-dimensional image display devices (such as liquid crystal displays (LCD)) formed in correspondence with a red (R) image electrical signal, a green (G) image electrical signal, and a blue (B) image electrical signal. Image light beams obtained at the two-dimensional image display devices formed in correspondence with their respective colors, red, green, and blue, are synthesized into a white image light beam by a color synthesizing optical system in order to be projected in enlarged form onto a transmissive screen through a projection lens.
Another type of three-plate projection display device is known. In this type, light beams emitted from light sources (such as light-emitting diodes (LED) or lasers) which emit light beams of three different colors, a red light beam, a green light beam, and a blue light beam, are collimated. The collimated light beams are made to impinge upon corresponding two-dimensional image display devices formed in correspondence with a red (R) image electrical signal, a green (G) image electrical signal, and a blue (B) image electrical signal. Image light beams obtained at their corresponding two-dimensional image display devices formed in correspondence with their respective colors, red (R), green (G), and blue (B), are synthesized into a white image light beam by a color synthesizing optical system in order to be projected in enlarged form onto a transmissive screen through a projection lens.
What is called a single-plate projection display device which includes one two-dimensional image display device is also known.
In one type of known single-plate projection display device, light sources which emit light beams of three different colors, a red light beam, a green light beam, and a blue light beam, are made to emit or transmit light based on time division. Then, in accordance with the timings of the passages of the light beams of the three different colors from the light sources, driving operations using a red (R) image electrical signal, a green (G) image electrical signal, and a blue (B) image electrical signal corresponding thereto are performed on one two-dimensional image display device in order to project an image in enlarged form onto a transmissive screen through a projection lens.
In another type of known single-plate projection display device, color filters for passing light beams of three different colors, red, green, and blue, are formed with every one pixel on a panel. A white light beam is made to impinge upon one two-dimensional image display device. Then, the light which has passed through the two-dimensional image display device is projected in enlarged form onto a transmissive screen through a projection lens.
In still another type of known single-plate projection display device, white light is made to impinge upon three dichroic mirrors disposed at very small angles. The light beams are separated into three different color light beams with each angle in order to obtain a red light beam, a green light beam, and a blue light beam. The obtained light beams are made to impinge upon one two-dimensional image display device. On the two-dimensional image display device, the light beams are collimated by very small lenses formed in correspondence with sets of three image pixels formed in correspondence with the three colors, red (R), green (G), and blue (B). Each color pixel is driven by its corresponding color image electrical signal. The light beams which have impinged upon their corresponding pixels are projected in enlarged form onto a transmissive screen through a projection lens.
Considering, for example, the limitations caused by a quick return mirror, various lenses having structures similar to that of the projection lens installed in each of the above-described projection display devices have been proposed. They include a wide-angle photographic lens for a single-lens reflex camera having a long back focus and a wide-angle projection lens for a projection television using a CRT (cathode ray tube).
In the structure of each projection display device described above, an optical element, such as a dichroic prism or a dichroic mirror, may be disposed as a color-light synthesizing system. In the case where a reflective, two-dimensional image display device is used, an optical element, such as a polarization beam splitter prism or a polarization beam splitter mirror, may be disposed. In these cases, the back focus which corresponds to the distance from the two-dimensional image display device to the last end of the projection lens needs to be made long.
In a projection display device which forms an image in enlarged form on the entire transmissive screen using one projector, in order to make the projection display device itself more compact, it is necessary to reduce the projection distance (for example, the length of the center light beam measured from a light-exiting end of the projection lens to the transmissive screen through a projection lens). To reduce the projection distance, it is necessary to increase the divergence angle of the exiting lens as a result of forming the projection lens into a wide-angle lens in order to form a large screen.
In order to improve color uniformity on the screen onto which image light beams are projected, it is preferable that the angle width of the light beams which strike the coated surfaces of, for example, a polarization beam splitter prism or a polarization beam splitter mirror (used when a dichroic prism or a dichroic mirror, used as a color synthesizing optical system, and a reflective two-dimensional image device are used) be fixed.
Therefore, the projection lens needs to be telecentric so that a principal ray from abaxial point to the projection lens is perpendicular to the two-dimensional display device. Here, the projection lens is symmetrical with respect to the optical axis which passes through the center of the two-dimensional display device, whereas the two-dimensional display device itself has a high contrast in only one direction. Therefore, the light beam itself which is shed upon the two-dimensional display device needs to be shed at an angle.
For a two-dimensional image display device, a liquid crystal display (LCD) or the like is ordinarily used. The liquid crystal display is driven using a matrix electrode. Therefore, unlike the case where a cathode ray tube (CRT) is used, it is difficult to correct distortion of the projection lens. More specifically, when a cathode ray tube is used, distortion of the projection lens can be relatively easily corrected by using a function for correcting a raster form such as a pincushion distortion correction function. However, when a liquid crystal display device which performs a dot-matrix display operation is used, corrections of raster distortion are ordinarily not carried out.
From the above, it is preferable that distortion of the
Epps Georgia
Thompson Timothy J
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