Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
2000-09-29
2003-06-24
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C359S627000
Reexamination Certificate
active
06583938
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical device and to a projection display incorporating such an optical device.
2. Description of the Related Art
GB 9 811 782.3 discloses a projection display in which a transmissive spatial light modulator (SLM) in the form of a liquid crystal device (LCD) is illuminated by an illumination source. The display is of the single panel type and includes a holographic field element and a projection optical system. Each picture element (pixel) of the SLM is associated with a holographic field element reflector which reflects light from the source to the entrance aperture or pupil of a projection lens. The field function of the display is thus incorporated in the holographic reflectors. The holographic reflectors may be patterned in order to generate a spatially multiplexed image without the use of absorptive colour filters.
This arrangement has the inconvenience that either the holographic field element reflector in the form of a volume hologram must be incorporated immediately adjacent the liquid crystal layer which involves manufacturing difficulties, or the holographic reflectors may be incorporated inside the counter substrate of the SLM, which reduces the aperture ration and hence the display brightness.
FIGS. 1 and 2
of the accompanying drawings illustrate a known type of single panel projection display as disclosed in H. Hamada et al, IDRC, 1994, pp 442-423 “A new bright single panel LC-projection system without a mosaic colour filter ” and in U.S. Pat. No. 5,164,102. An illumination source (not shown) directs collimated white light to a set of relatively titled dichroic mirrors
1
for reflecting red, green and blue light with a relative angular separation. The resulting colour component beams are directed to a microlens array
2
disposed on the surface of a monochrome thin film transistor (TFT) LCD
3
. Each microlens of the array
2
is disposed above three pixels and focuses the red, green and blue light beams onto the apertures of respective ones of the three pixels. The LCD
3
is of the transmissive type and the modulated light therefrom passes through a field lens
4
and a projection lens
5
such that the image is projected onto a screen
6
.
The size of the image of the light source produced at each pixel is determined by the system etendue, the pitch of the LCD pixels and the thickness of the glass substrate. Etendue is a term representing brightness at any point in a system and is defined as the product of the beam area and solid angle of the beam divergence. For efficient coupling of optical radiation through a projection system, the etendue should be matched at each point through the system. Etendue mismatch causes a loss of brightness. In this transmission panel system, the light must be focused into the relatively small aperture of the pixel. Thus, the solid angle of the optical beam will be required to increase to compensate in order to maintain brightness. If the glass substrate is too thick then such a solid angle cannot be achieved by the microlenses and light is lost around the edge of the pixel. Also, green light may spill from the aperture of the green pixel into the red pixel and so on.
This type of arrangement works well for large low resolution LCD panels. However, if it is required to provide a higher resolution display for a given size of LCD panel, smaller pixel sizes must be provided and it may not be possible to illuminate such smaller pixels accurately with the available etendue. Alternatively, a larger panel may be used but this results in an increase in the bulk of the system.
C. Joubert et al, “Dispersive holographic microlens matrix for single LCD projection”, SPIE vol 2650 pp 243-249 discloses a projection display as illustrated in
FIG. 3
of the accompanying drawings. An arc lamp
7
and parabolic reflector
8
supply collimated white light to a phase volume holograph
9
which separates the illuminating white light into red, green and blue beams with a small angular separation between each adjacent pair of beams. A cylindrical microlens array
10
focuses the R,G,B beams into the apertures of pixels in an LCD
11
. This display is of the same type as that disclosed in Hamada but with the dichroic filters replaced by the hologram
9
. Accordingly, this display suffers from the same problems associated with beam etendue, panel size and glass thickness.
JP 9 015 626 A discloses a reflection mode projection display as shown in
FIG. 4
of the accompanying drawings. An LCD
12
is provided with a microlens array
13
having microlenses formed on its upper and lower surfaces. The LCD
12
comprises composite pixels, each of which is aligned with a respective microlens
14
formed on the front surface of the array
13
. Each composite pixel comprises individual sub-pixels
15
,
16
and
17
aligned with respective microlenses such as
18
on the lower surface of the array
13
. The sub-pixels
15
,
16
and
18
modulate red, green and blue light respectively, so as to provide a single panel colour display.
The LCD
12
incorporates plane reflectors, each of which forms part of a respective colour component sub-pixel. The green reflector
19
is disposed in the plane of the LCD
12
whereas the red and blue reflectors
20
and
21
are tilted in opposite directions.
Incident collimated colour component light beams are shown at
22
B,
22
G and
22
R. Each of the light beams is focused by the microlens array
13
to the aperture the corresponding sub-pixel and onto the corresponding reflector
19
to
21
. The reflectors
19
to
21
have no optical power and reflect the colour component light beams substantially back along the incident light paths. Thus, light is reflected back through the same microlenses so that the input and output pupils are at the same location. It is therefore necessary to provide a beam splitter in the optical system so that the output light from the LCD
12
for projection can be separated from the path of the input light from the illumination system. This results in increased bulk and weight together with light and contrast losses. Also, as described hereinafter, vignetting occurs and this results in further light losses.
JP 10221681 also discloses the use of a microlens array on the front surface of a reflective LCD. Each colour component sub-pixel of he LCD has a plane reflector which gives rise to vignetting and loss of light. Also, the microlens array is of the two dimensional type and further loss occurs because of lens edge effects and the reduced aperture ratio of lenses. However, separate input and output pupils are provided so that a beam splitter is not required.
U.S. Pat. No. 5,825,443 discloses an arrangement which is similar to that disclosed in JP10221681 and which therefore suffers from the same disadvantages.
EP 0 953 865 discloses a projection display which includes an optical modulator comprising an LCD of reflective type and two microlens arrays. This microlens arrays are aligned with each other and have a pitch which is three times the pitch of the individual colour component pixels of the LCD. Each aligned pair of microlenses is said to constitute a relay optical system having a magnification of 1. The rear electrodes of the LCD pixels constitute plain reflectors. The use of the second microlens array between the first microlens array and the LCD results in increased chromatic aberration which reduces the coupling efficiency of the device. Also, there are several interfaces between materials of different refractive index resulting in substantial losses due to Fresnel reflection which reduces the efficiency of the device. Further, the separation between the second array of microlenses and the reflective pixels results in loss of brightness because of vignetting. Also, it is necessary during manufacture to align the two microlens arrays in he counter substrate and then align this to the back plain of the LCD, which makes the device difficult and expensive to manufacture.
SUMMARY OF THE INVENTION
According
Anderson Duncan James
Khazova Marina Vladimirovna
Slack Jason
Woodgate Graham John
Epps Georgia
Hindi Omar
Renner Otto Boisselle & Sklar
Sharp Kabushiki Kaisha
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