Optics: image projectors – Reflector
Patent
1997-02-25
1999-02-16
Dowling, William
Optics: image projectors
Reflector
353102, G03B 2128
Patent
active
058712666
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a projection-type display device and, in particular, to a projection-type display device suitable for a passive image creation device, exemplified by liquid crystals and/or by a digital micromirror device (DMD) that creates an image by moving a large number of small mirrors independently.
2. Background Art
The recent implementation of the promotion of the high-definition television has led to an increasing demand for large-screen display devices that can be used in ordinary homes. Although there has been progress in increasing the size of CRT devices, which have become standard as conventional display devices, to answer this demand, it is said that structural problems will limit the size thereof to 40 inches. As alternatives, it has become possible to produce plasma or liquid crystal display devices that are thin and have large screens, but these involve problems concerning reliability, lifetime, and particularly cost, so there is no basic solution for a large screen exceeding 60 inches.
Projection-type display devices, which project an original image produced by a small CRT or liquid crystal display device from a front plane or rear plane onto a screen in front of a viewer, have gradually appeared on the market as another method of implementing a large screen. In particular, improvements in compactness, resolution, and transmittance ratio in apparatuses that utilize liquid crystal display devices have astonished consumers and continue to act as a driving force in market diffusion. DMD apparatuses configured of tiny mirrors are also becoming practicable. These differ from CRTs in that they do not generate light in themselves, but are passive image creation devices that require separate illumination portions.
There are a number of technical problems that will have to be conquered to ensure an even wider market presence for a projection-type display device using such a passive image creation device. Representative problems concern: "brightness and uniformity on screen," "space efficiency," "cost," "weight," and "size."
The description below explains in steps the prior-art techniques used to solve the above problems. First of all, the basic configuration of a prior-art projection-type display device is shown in FIG. 36, to illustrate the technical problems concerning "brightness and uniformity on screen." This device is configured of an illumination portion 1 formed of a light source 1a and a parabolic mirror 1c for converting a principal light ray generated from the imaginary center thereof into a parallel beam; an image creation portion 2 formed of an image creation device 2a that selectively transmits luminous flux from the illumination portion 1 and a field lens 2b that controls the direction of the principal light ray; and an imaging portion 3 for projecting the original image created by the image creation portion 2 onto a screen 4. Reference number 3c of the imaging portion 3 denotes an aperture stop for controlling the brightness on screen and 3a, 3b, and 3d denote lenses.
With this optical system, the luminous flux reaching the screen 4 is reduced as the aperture stop 3c is stopped down and thus the imaging portion 3 reduces the cone angle .phi. from the image creation portion 2, so that the efficiency of utilizing luminous flux generated from the light source 1a is radically reduced. This is particularly dramatic when the light source 1a is large in comparison with the image creation portion 2.
Next, assume that the light source 1a is changed in a state in which the aperture stop 3c has been stopped down, in FIG. 36. This time, the efficiency of utilizing the luminous flux rapidly rises as the size of the light-generating portion of the light source 1a is gradually reduced. However, if it is assumed that the angle with respect to the optical axis, of the principal light ray generated from the imaginary center of the light source 1a, which is placed in the vicinity of the focus of the parabolic mirror 1c, is .omega., it b
REFERENCES:
patent: 5418583 (1995-05-01), Masumoto
patent: 5491525 (1996-02-01), Yamasaki et al.
patent: 5662401 (1997-09-01), Shimizu et al.
patent: 5749642 (1998-05-01), Kimura et al.
Matsuo Eiki
Negishi Masataka
Dowling William
Nissho Giken Kabushiki Kaisha
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