Optical: systems and elements – Single channel simultaneously to or from plural channels
Reexamination Certificate
2000-09-22
2003-06-03
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
C349S009000
Reexamination Certificate
active
06574046
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a time-division image projector, and more particularly to a time-division image projector that obtains a color projected image by rotating a color wheel, which is a color selection element in which a plurality of color filters are arranged in radiating form.
2. Description of the Related Art
The prior art includes time-division image projectors that, as a basic construction, are provided with a rotatably driven transmission color wheel in which transmission color filters that transmit each of red, green and blue light are arranged in radiating pattern. By transmitting white light that is radiated from a light source through the transmission color wheel, such an image projector isolates and projects each of the colors red, green and blue in time slots.
Japanese Patent Laid-open No. 211358/96 discloses a liquid crystal projector that uses a color filter disk. The technique disclosed in this publication is described as the first example of the prior art.
FIG.
1
(
a
) is a side view showing the construction of time-division image projector
500
of the first example of the prior art, and FIG.
1
(
b
) is an enlarged frontal view of a portion of the prior-art example of FIG.
1
(
a
).
In the time-division image projector
500
shown in FIG.
1
(
a
), white light from white light source
501
is irradiated onto color filter disk
502
shown in FIG.
1
(
b
), whereby red, green and blue light is transmitted in succession synchronized with the rotation of color filter disk
502
. The light that is transmitted by color filter disk
502
is condensed by condenser lens
504
and irradiated onto dot matrix liquid crystal display panel
505
. Dot matrix liquid crystal display panel
505
, synchronized to the timing of the switching of red, green and blue colors of color filter disk
502
, displays images corresponding to the red image signal, green image signal, and blue image signal. The thus-formed color image is then projected by way of projection lens
506
onto screen
509
.
When this type of color filter disk
502
is used, however, the diameter of color filter disk
502
must be sufficiently greater than the outside diameter of motor
503
, with the resulting problem of unwanted bulkiness of the device.
To solve this problem, a method can be considered in which, in place of the color filter disk, a reflective color wheel is used in which reflecting color filters that reflect each of red, green, and blue are arranged in radiating form.
FIG.
2
(
a
) is a side view showing the construction of time-division image projector
600
of the second example of the prior art, and FIG.
2
(
b
) is an enlarged frontal view of a portion of the prior-art example shown in FIG.
2
(
a
).
In time-division image projector
600
shown in FIG.
2
(
a
), white light from white light source
601
is irradiated onto reflective color wheel
602
shown in FIG.
2
(
b
), and red, green, and blue light is successively reflected in synchronism with the rotation of reflective color wheel
602
. The light that is reflected by reflective color wheel
602
is condensed by condenser lens
604
and irradiated onto dot matrix liquid crystal display panel
605
. Dot matrix liquid crystal display panel
605
displays images corresponding to the red image signal, green image signal, and blue image signal in synchronism with the timing of the switching of red, green and blue of reflective color wheel
602
. The color image that is formed in this way is projected onto screen
609
by way of projection lens
606
.
A more compact time-division image projector is thus realized through the use of reflective color wheel
602
, which is more compact than color filter disk
502
. However, the following problems were encountered in the above-described examples of the prior art.
The first problem is a reduction in the color purity of the projected image.
This problem occurs because light in unwanted wavelength bands tends to be mixed in the projected image because the reflectance of unwanted wavelength bands cannot be adequately suppressed in the characteristic of reflective dichroic filters
602
R,
602
G, and
602
B as shown in FIGS.
3
(
a
)-(
c
).
A second problem is large irregularities in color in the projected image.
This problem occurs because minute discrepancies inevitably occur in the angles of mounting reflective dichroic filters
602
R,
602
G, and
602
B, with the result that the optical axes of each of the reflected beams fail to accurately match.
A third problem is that, when a rod lens is added between reflective color wheel
602
and condenser lens
604
in the construction of
FIG. 2
for the purpose of decreasing irregularities in color over the entire screen of a projected image, the white balance of the projected image is degraded.
This problem occurs because minute discrepancies inevitably occur in the angles in which reflective dichroic filters
602
R,
602
G, and
602
B are mounted, which in turn causes deviations in the axes of the incident light of each of the colors to the rod lens, whereby the projected images of each color fail to accurately match.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a compact time-division image projector in which the projected image has high color purity, few irregularities in color, and a high level of white balance.
To solve the above-described problems, the present invention adopts the following novel and distinctive construction.
The time-division image projector of the present invention features the provision of: light source (
1
in
FIG. 4
) that radiates white light; transmission color wheel (
31
), in which are mounted a plurality of color filters (
31
R,
31
G, and
31
B) that each transmits only light of a specific wavelength band, that selectively transmits only light of specific wavelength bands of the white light from light source (
1
); reflection element (
33
), for which the normal of the reflecting surface is arranged at an inclination with respect to light that is irradiated into transmission color wheel (
31
), that reflects the light of specific wavelength bands that is transmitted by color filters (
31
R,
31
G, and
31
B) in a different direction; optical modulation element (
41
) that is supplied, in time divisions, with a plurality of image signals corresponding to light of each of the specific wavelength bands that has been reflected by reflection element (
33
), and that modulates, in pixel units, the intensity of light of the specific wavelength bands that has been transmitted by color filters (
31
R,
31
G, and
31
B) according to the plurality of image signals; projection lens (
5
) that projects the light that is emitted from optical modulation element (
41
); and motor (
32
) that is linked with axis of rotation (
31
a
) of transmission color wheel (
31
) and that causes transmission color wheel (
31
) to rotate in synchronism with the timing of supply of the plurality of image signals that correspond to the light of specific wavelength bands.
In other words, the chief characteristic of the present invention lies in the combined use of transmission color wheel (
31
) and reflection element (
33
) to constitute a pseudo-reflective color wheel, thereby combining the advantages of both the transmission and reflective types.
By adopting this construction, the intensity of unwanted wavelength components can be easily reduced through the use of transmission color filters (
31
R), (
31
G), and (
31
B), in which the attenuation rate of the unwanted wavelength bands is characteristically greater than in a reflective color filter.
In addition, the light that is irradiated onto transmission color wheel (
31
) is transmitted by transmission dichroic filters (
31
R), (
31
G), and (
31
B) and then reflected by a single reflection element (
33
) that is shared by each of the beams, whereby the optical axes of each of the beams can be made to accurately match. By adopting a means as described hereinabove, the time-division image projector o
Epps Georgia
Harrington Alicia M
McGinn & Gibb PLLC
NEC ViewTechnology Ltd.
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