Optics: image projectors – Reflector
Reexamination Certificate
2001-09-27
2003-03-04
Dowling, William (Department: 2851)
Optics: image projectors
Reflector
C353S099000
Reexamination Certificate
active
06527396
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical device as well as a projector unit and a rear projector unit using the same.
2. Description of the Related Art
A projector system is drawing attention lately as a large screen display unit and a CRT projector system using a small, high definition and luminous CRT, a liquid crystal projector system using a liquid crystal panel, a DMD projector system using DMDs (Digital Micro-mirror Device) and others are manufactured. Among them, the DMD is one of light bulbs for the projector system which is being expected most because it is congenial to displaying computer information and to digital TV broadcasting in principle because it is a digital driving element.
The structure of the DMD will be explained at first. The DMD is fabricated by arraying a large number of micro-mirrors of 16 &mgr;m×16 &mgr;m for example on a silicon substrate so as to control reflecting directions of incident light per each mirror by electrically controlling them. The respective micro-mirrors correspond to pixels and image information may be displayed by projecting the light on a screen via a projector lens.
FIG. 1
is a perspective view showing the structure of two pixels of DMD elements. The figure shows micro-mirrors
510
and
511
which are inclined from the normal line of an element
600
by +10 degrees or −10 degrees, respectively.
Aluminum is evaporated on the surface of the micro-mirrors
510
and
511
so that the micro-mirrors operate as square mirrors for example having high reflectivity. The direction in which light is reflected may be switched by the tilting angle of the micro-mirrors and the light intensity from DMD is controlled by pulse width modulating (PWM) the micro mirrors. Such micro-mirrors are arrayed two-dimensionally in accordance to a predetermined format of 480×640 or of 600 ×800 for example to construct as a light bulb. The DMD is detailed in Larry J. Hornbeck, “Digital Light Processing for High-Brightness, High-Resolution Applications”, SPIE Vol. 3013, pps. 27-40 and others, so that a further detailed explanation thereof will be omitted here.
Next, the principle of an optical system of a video projector using the DMD as a light bulb will be explained with reference to FIG.
2
.
FIG. 2
is a conceptual diagram for explaining the principle of display of the DMD by means of a single plate DMD projector unit which comprises an illuminant
150
such as a metal halide lamp, a DMD
601
, a light absorber
602
for absorbing unnecessary light and a projector lens
80
. In the figure, micro-mirrors
512
and
513
which are turned (tilted) by +10 degrees and a micro-mirror
514
which is turned (tilted) by −10 degrees in the DMD
601
are conceptually enlarged. In
FIG. 2
, light emitted from the illuminant
150
enters the DMD elements from the direction tilted by +20 degrees from the normal direction of the DMD
601
. The light reflected by the micro-mirror
514
which is turned by −10 degrees deviates from the projector lens
80
and is absorbed by the light absorber
602
, thus becoming a pixel of a black point on a screen not shown. Meanwhile, the light reflected by the micro-mirrors
512
and
513
which are turned (tilted) by +10 degrees are condensed by the projector lens
80
, thus becoming pixels of bright points on the screen. Thus, the micro-mirror image on the DMD
601
is enlarged and projected on the screen as an image. It is also possible to display a color image by a single plate projector unit in which a rotary color filter is disposed within the optical path or by a three plate type projector unit which separates light into three primary colors of RGB and modulates per each color by arraying a dichroic prism and a dichroic filter.
By the way, as it is apparent from the operating principle of the DMD described above, the most characteristic condition in illuminating the DMD is that the illuminating light must be inputted with a predetermined angle from the normal direction of the plane-to-be-illuminated.
FIG. 3
is a schematic diagram showing the structure of a prior art color image display unit for displaying color images by using the DMD. It has been disclosed in JP-A-10-039240 for example. In
FIG. 3
, the unit comprises a DMD
603
, micro-mirrors
515
and
516
composing the DMD, a parallel white color illuminant
151
emitting parallel white light, an optical thin film
400
causing transmission light of specific wavelength corresponding to an incident angle, an imaging lens
800
and a screen
900
.
What should be noticed most here is the disposition of the parallel white color illuminant
151
with respect to the DMD
603
. That is, the parallel white illuminant
151
is disposed in the direction inclined by a predetermined angle &agr;
1
with respect to the DMD
603
so as to input the parallel white light to the respective mirrors arrayed two-dimensionally on the DMD
603
. The white light incident on the DMD
603
is reflected by the mirrors
515
and
516
whose tilting angle &thgr;
1
is controlled and is then guided to the screen
900
by the imaging lens
800
. While the optical thin film
400
is inserted to display color images, an explanation of its operation will be omitted here. The oblique illumination described above is required not only in the single plate projector unit of this example but also in a DMD projector unit of the type of a plurality of plates in common.
It is apparent that when the plane-to-be-illuminated is illuminated by concentric illuminating light fluxes, i.e., fluxes having a distribution of intensity rotationally symmetric about an optical axis from the direction tilted from its normal line, a distribution of the illuminated light is not concentric on the illuminated plane.
FIG. 4
is a diagram schematically showing the DMD
604
illuminated by the illuminating light fluxes having the concentric distribution of intensity and seen from the normal direction. Curves within the figure conceptually show equi-intensity lines by connecting points of the equal light intensity. Here, the flux of the illuminating light is inputted to the DMD
604
from the direction tilted by 45 degrees from one edge as indicated by an arrow in the figure and so that the optical axis of the illuminating light flux is tilted by 20 degrees from the normal line of the surface of the DMD. Accordingly, there arises a problem that the distribution of light intensity on the DMD surface is not concentric about the center point of the DMD
604
as shown in the figure and uneven illumination asymmetric in the up and down and right and left directions which is very inappropriate as a projector unit occurs on a screen. It is noted that the distribution of intensity in the present specification means the distribution of intensity of light within a plane vertical to the optical axis of the light flux.
The uneven illumination maybe relatively readily reduced by digital signal processing in case of the DMD projector unit. That is, it may be achieved by standardizing the intensity of projecting light per pixel by decreasing the gradation within the screen more than the original gradation for example so that the distribution of illuminance is uniformed on the whole screen based on a pixel to which the illuminating light of the least intensity enters. However, the improvement of the image quality by means of such light reducing process is not desirable from the aspect of the utility factor of the light. Because the DMD is a reflecting type light bulb, it has a merit that it is relatively strong against heat as compared to a transmission type light bulb and allows a high output illuminant such as a xenon lamp and a metal halide lamp of several hundreds W to 2 or 3 kW classes to be used. However, the rate of the light of the illuminant reaching to the screen in the end, i.e., the utility factor of the light, has stayed around several % with respect to the original output of the light of the illuminant similarly to the pro
Okamori Shinji
Shikama Shinsuke
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