Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
1999-06-09
2003-10-14
Chowdhury, Tarifur (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C362S035000
Reexamination Certificate
active
06633350
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a display apparatus suitable for use in a computer display, a view finder for a video camera, a light valve for a video projector, a head-mount display, etc., and particularly an illumination device for such a display apparatus.
An illumination device can be required to have a specific luminance distribution depending on its use and is typically required to show a uniform luminance distribution, e.g., a two-dimensionally uniform distribution as desired in a display apparatus. Particularly, display apparatus including an optical modulation device such as a liquid crystal device frequently use surface illuminant devices, a representative example of which may be one disclosed in Japanese Laid-Open Patent Application (JP-A) 4-86620.
An image display apparatus, such as a head-mount display (HMD) may include a small-sized liquid crystal display device (LCD) of, e.g., ca. 0.7 inch, for which a planar fluorescent lamp has been principally used as a backlight (i.e., illumination device). The backlight may typically show a luminance distribution having a maximum at its center and falling toward the peripheries as represented by a dot and dash line shown in FIG.
1
.
As another conventional example of display apparatus requiring an illumination device, a liquid crystal view finder for use in video camera recorders, etc., will now be described.
FIG. 2
illustrates a manner of illuminating such a liquid crystal view finder.
The liquid crystal view finder
101
includes a liquid crystal panel (liquid crystal display device) P of the transmission type for displaying various data by utilizing the liquid crystal. The panel is provided with polarizers
102
and
103
applied to both surfaces thereof. On the backside (illustrated as the lower side) of the liquid crystal panel P, a cold cathode lamp (illumination device)
105
having an areal size almost equal to that of the panel P so as to illuminate the panel P. On the front side (illustrated as the upper side) of the panel, a virtual image-focusing optical system
106
is disposed, so that light L
1
having passed through the liquid crystal panel P reaches human eyes E via the virtual image-focusing system
106
to recognize the data displayed on the liquid crystal panel P.
The light issued from the above-mentioned cold cathode lamp
105
should desirably have a uniform light quantity distribution. For this reason, the cold cathode lamp
105
is provided with surface unevennesses to form a scattering surface
107
whereby the light from the cold cathode lamp
105
is scattered.
However, such a cold cathode lamp
105
has not been able to provide a uniform light quantity or luminance distribution by the scattering surface
107
alone. According to our observation, the luminance distribution has been found considerably ununiform as represented by a dashed line A in
FIG. 3
such that a quantity I
1
reaching at edges of the liquid crystal panel is only about 30% of a light quantity I
2
reaching the center. Herein, Y
1
on the abscissa represents the size (width) of the image display area of the liquid crystal panel P, and the ordinate I represents a light quantity introduced into the NA (numerical aperture determined by the entrance pupil and focal length of the optical system), which light quantity is actually in the form of a three-dimensional cone.
Incidentally, the reason why the light quantity I
1
reaching edges of the liquid crystal panel P is low, may be attributable to a factor that a liquid crystal panel P has an areal size almost equal to that of the cold cathode ray tube and the edges of the liquid crystal panel P coincide with those of the cold cathode lamp
105
so that light supply to the panel edges becomes insufficient. On the other hand, the center is supplied with a sufficient quantity of light from the cold cathode lamp
105
compared with the edges.
Then, in a case where such a cold cathode lamp
105
is used for illuminating a liquid crystal panel P as mentioned above, there occurs an image quality degradation due to a luminance irregularity. This difficulty becomes pronounced for displaying, e.g., an image of a wide angle object taken at a panoramic wide angle.
As a solution to the above-mentioned problem, it has been proposed to use a filter having a non-uniform transmittance distribution by the above-mentioned JP-A 4-86620. The filter is designed to have a transmittance at the center which is lower than those at edges so as to provide a transmitted light quantity (corr. to I
2
) lower than a transmitted light quantity (corr. to I
1
) at edges, thus providing a uniform light quantity distribution. According to this method, however, the emitted light quantity per se from the illumination device is lowered, thus resulting in a new problem of image quality degradation due to dark illumination. The method further involves a lowering in electric power efficiency causing an increased drive cost and difficult and expensive production of the filter, thus being liable to provide an obstacle to practical application.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an illumination device capable of moderating a remarkable lowering in luminance that occurs at the edges or sides thereof to provide illumination light having a uniform luminance distribution and also a display apparatus including such an illumination device.
Another object of the present invention is to provide an illumination device capable of providing illumination light having a uniform luminance distribution without remarkably lowering the luminance at the center thereof and a display apparatus including such an illumination device.
Another object of the present invention is to provide a display apparatus capable of preventing a lowering in display image quality due to ununiform light quantity distribution.
According to an aspect of the present invention, there is provided a display apparatus, comprising at least: a liquid crystal display device for image display, a backlight for illuminating the liquid crystal display device, and a reflection member having a size almost equal to an outer frame of the liquid crystal display device and disposed insertable between the liquid crystal display device and the backlight.
According to the present invention, it is possible to reduce the difference between the light quantities at the peripheries and the center of the backlight by inserting a reflection frame having a reflection surface between a liquid crystal display device and a backlight.
Further, according to the present invention, two types of light quantity (luminance) distributions from the backlight by providing two states wherein the reflection frame is inserted between and retreated from between the liquid crystal display device and the backlight.
Further, by providing a position sensor for detecting the position of the liquid crystal display device and the backlight and controlling a reflection frame support and control mechanism based on the detection result, it becomes easy to control the insertion and retreat of the reflection frame.
In a first state in the above instance, the reflection frame may be inserted to provide an increased peripheral quantity. In a second state, the reflection frame may retreat and the backlight may be disclosed close to the liquid crystal display device so as to prevent the lowering in light quantity at the center. These two states may be appropriately realized.
By using at least two display apparatus, it becomes possible to realize an image display apparatus, such as HMD.
By moving the liquid crystal display devices and the backlights in the at least two display apparatus in association with each other or in an interlocked manner so that all the display apparatus are in the first state when the display devices and the backlights are in the first position and in the second state when the display device and the backlights are in the second position, it is possible to provide an optimum light quantity (luminance) distribution for plural d
Hoshi Hiroaki
Mochizuki Noritaka
Sasakura Takao
Chowdhury Tarifur
Nguyen Dung
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