Optics: image projectors – Color filter
Reexamination Certificate
2000-06-20
2002-10-22
Dowling, William (Department: 2851)
Optics: image projectors
Color filter
C353S031000, C348S743000
Reexamination Certificate
active
06467910
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image projector which is optimum for application to a projector projecting a color image on a screen etc., more particularly relates to an image projector using a color wheel to split white light emitted from a light source into effective light beams of the R (red), G (green), B (blue), and other wavelength bands shifted by time, modulating the split R, G, and B effective light beams by a liquid crystal panel, digital micro-mirror device (DMD), or other spatial light modulator, and projecting them on a screen etc.
2. Description of the Related Art
Conventionally, this kind of image projector may be roughly divided into two systems. The first system of such an image projector uses three R (red light), G (green light), and B (blue light) liquid crystal panels, DMDS, or other spatial light modulators. It passes the white light emitted from a light source through three R, G, and B diohromic mirrors to split it into R, G, and B effective light beams which it then guides to three spatial light modulators, drives the three spatial light modulators, combines the three R, G, and B optical images obtained by modulation by the three spatial light modulators by a prism system, and projects the result on a screen etc.
Next, as shown in
FIG. 21
, the second system of such an image projector, shown by reference numeral
1
, comprises a white light source
2
comprised by a discharge lamp
2
a
, a reflection mirror (reflector)
2
b
, etc. and a condenser lens
3
, a transmission type color wheel
4
, and a single spatial light modulator
5
arranged in series on an horizontal optical axis P of white light L
1
emitted from the light source
2
. As shown in
FIG. 22A
, the transmission type color wheel
4
has formed around the periphery of its wheel surface
4
a
red filter RF, a green filter GF, and a blue filter BF comprised by R, G, and B color filters
4
b
dividing the outer periphery into three sections. Note that, as shown in
FIG. 22B
, there is also a transmission type color wheel
4
with a white filter WF provided in addition to the color filters
4
b
to thereby divide the outer periphery into four and improve the luminance.
Returning to
FIG. 21
, the transmission type color wheel
4
is arranged perpendicular to the optical axis P of the white light L
1
emitted from the white light source
2
. The transmission type color wheel
4
is rotated at a high speed (10 s to 100 s of revolutions per second) by a driving means (not shown). The white light L
1
emitted from the white light source
2
strikes rotating positions of the red filter RF, green filter GF, and blue filter BF of the color filters
4
b
of the transmission type color wheel
4
perpendicularly and passes through the red filter RF, green filter GF, and blue filter BF. As a result, the white light L
1
is split into R, G, B, and other effective light beams L
2
shifted by time by the transmission type color wheel
4
. The effective light beams L
2
strike the spatial light modulator
5
at different times. The spatial light modulator
5
modulates the effective light beams L
2
to three R, G, and B(W) optical images L
3
(and white light for improving the luminance) shifted by time by applying R, G, and B image signals (and a signal for improving the luminance) in synchronization with the time the three or four R, G, and B(W) effective light beams L
2
strike it and emit those optical images L
3
. At this time, the three R, G, and B optical images L
3
(and white light for improving the luminance) can be viewed by the human eye superimposed, so the optical images L
3
are projected by a projection lens
6
on a screen
7
etc. to thereby project a full color image.
Summarizing the problems to be solved by the invention, the first system of image projector using three spatial light modulators and dichromic mirrors suffers from the problem of the large number of parts and a difficulty in space saving, so is not suitable for compact projectors.
The second system of image projector
1
using the transmission type color wheel
4
and the single spatial light modulator
5
as shown in
FIG. 21
has the problem that it is difficult to reduce the height of the overall apparatus.
That is, as shown in
FIG. 23
, when using the transmission type color wheel
4
, the cross-sectional area of the spot SL of the white light L
1
striking the transmission type color wheel
4
must be made sufficiently small with respect to the diameter of the transmission type color wheel
4
. At the instant the spot of light SL passes a dividing point RGP, GBP, or BRP of the red filter RF, green filter GF, and blue filter BF, two colors of effective light end up to be mixed. If the spatial light modulator
5
operates at the instant a mixed effective light beam L
2
strikes it, an optical image L
3
having two mixed colors is projected on the screen
7
.
To prevent this, it is necessary to turn off the spatial light modulator
5
at the times the spot of light SL passes the dividing points RGP, GBP, and BRP of the red filter RF, green filter GF, and blue filter BF to prevent projection of an optical image L
3
of two mixed colors on the screen
7
.
FIG. 24
shows the state in which the spatial light modulator
5
is supplied with R, G, and B image signals SR, SG, and SB in synchronization with the striking of the three R, G, and B effective light beams L
2
. The mixture of two colors of light in an optical image L
3
is prevented by setting times T for turning off the spatial light modulator
5
between times of application of the R, G, and B image signal SR, SG, SB.
The off time T of the spatial light modulator
5
, however, lowers the luminance of the optical image projected on the screen
7
and causes a lowering of the image quality of the projected full color image.
Accordingly, it is desirable that the spot SL of the emitted light L
1
striking the transmission type color wheel
4
be as small as possible. The white light emitted from the discharge lamp
2
a
or other white light source is originally emitted from a light source with great limitations, however. Even if condensing this white light L
1
, the white light L
1
cannot be focused completely to a single point. Consequently, the spot of light SL has a certain size.
For the reasons described above, as shown in
FIG. 25
, the diameter of the transmission type color wheel
4
must be made relatively large (normally the diameter-D is not less than approximately 10 cm). In a conventional image projector
1
of the system where in the transmission type color wheel
4
is arranged perpendicular to the optical axis P of the white light L
1
emitted from the white source
1
, the overall height H of the image projector
1
increases in proportion to the diameter D of the transmission type color wheel
4
. Further, If the overall height of H of the image projector
1
increases, the overall volume of the image projector
1
increases, consequently the image projector
1
becomes large in size and heavy in weight. Further, costs are increased due to the increase of the casing material of the image projector
1
etc.
Accordingly, as shown in
FIG. 26
, it has been considered to make the transmission type color wheel
4
tilt slightly with respect to the optical axis P in order to make the overall height H of the image projector
1
shown in
FIG. 25
as small as possible.
When tilting the transmission type color wheel
4
with respect to the optical axis P, however, as shown in
FIG. 27A
, the spot SL of the white light becomes an ellipse and increases in cross-sectional area. As described above, the time during which the spot of light SL passes the dividing points RGP, GBP, and BRP of the red filter RF, green filter GF, and blue filter BF of the transmission type color wheel
4
becomes longer, the time T for turning off the spatial light modulator
5
shown in
FIG. 24
becomes longer, and consequently the luminance of the optical images L
3
projected on the screen
7
ends up being lowered.
Note that in
Dowling William
Frommer William S.
Frommer & Lawrence & Haug LLP
Sony Corporation
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