Optics: image projectors – Unitary plural refracting surfaces
Patent
1997-04-17
2000-08-15
Dowling, William
Optics: image projectors
Unitary plural refracting surfaces
353 98, G03B 2114
Patent
active
061025453
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a liquid crystal display unit excellent in efficiency of use of light and image quality.
Although the present invention is mainly disclosed about a projection type liquid crystal display unit, it can be applied to a direct-view type liquid crystal display unit or a fiber type liquid crystal display unit.
BACKGROUND ART
In order to construct a liquid crystal display unit excellent in image quality, that is, excellent in contrast ratio, light passing through a liquid crystal panel is required to be collimated as much as possible. According to the fruit of researches in recent years, for example, in order to obtain a contrast ratio not lower than 200 to 1, it is necessary that the angle of divergence of light passing through a liquid crystal panel is limited to a range of about 0.15 rad p--p in a first direction (in a direction of narrower directivity) and to a range of about 0.3 rad p--p (about twice as much as the aforementioned 0.15 rad p--p) in a second direction (in a direction of wider directivity).
A typical example of light collimating or collimator means in the prior art is a parabola mirror. A projection type liquid crystal display unit in the prior art is shown in FIG. 1.
In FIG. 1, the reference numeral 1 designates a light source; 2, a parabola mirror; 3, a liquid crystal panel; 4, a projection lens; and 5, a screen. The arrows show paths of light rays. The prior art has at least the following problems. directly without reflection by the parabola mirror 2 are not collimated. Accordingly, the contrast ratio and image quality of reproduced images are deteriorated. improvement of the relative corner illuminance ratio are contradictory to each other. That is, when one is improved, the other is deteriorated. The term "relative corner illuminance ratio" used herein means the ratio of corner illuminance to central illuminance on the liquid crystal panel and is hereinafter abbreviated to "RCI (relative corner illuminance)". respect to the optical axis thereof. Accordingly, the sectional area of output light is shaped like a circle. When the radius of the circle is 1, the area is .pi.. On the other hand, the liquid crystal panel 3 is shaped like a rectangular inclusive of a square. The area of the rectangular inscribed in a unit circle is not larger than 2. Accordingly, a loss of about 36%(1-2/.pi.) occurs in the peripheral region because of mismatched aspect. that the path of an air flow is not linear, it is difficult to improve the efficiency of heat dissipation from the light source.
The result of inventor's analysis on the cause of the aforementioned problems on the basis of phisics or natural science will be described below with reference to FIGS. 1 and 2. FIG. 2 shows a coordinate system in which Z is taken in the direction of the optical axis of the parabola mirror 2 and r is the distance from the optical axis. Assume that the shape of the parabola mirror 2 is given by the following expression: parabola mirror.
The light source 1 is located at the focal point (Z=0.5R.sub.1) of the mirror 2. Accordingly, output light reflected by the mirror becomes collimated light. Upon the assumption that the light source is isotropic, light intensity thereof is denoted by I [cd]. Accordingly, total light flux is 4 .pi.I[lm]. The increase of output light flux from the isotropic light source is proportional to the increase of the cosine of a zenith angle .theta. measured from the optical axis passing through the light source. The total light flux T collimated by the parabola mirror 2 and the efficiency of use of light E(.theta..sub.M) can be obtained as the following expressions. Incidentally, the aspect mismatch loss in the aforementioned item (3) is regarded to be neglected upon the assumption that the liquid crystal panel is shaped like a disk. ##EQU1##
On the other hand, the illuminance J of output collimated light of the parabola mirror 2 is inversely proportional to the square of the distance from the light source to the mirror. Accordingly, the following e
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Dowling William
Hitachi , Ltd.
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