Optics: image projectors – Picture carrier
Reexamination Certificate
2003-07-22
2004-08-03
Dowling, William C. (Department: 2851)
Optics: image projectors
Picture carrier
C359S506000
Reexamination Certificate
active
06769779
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to digital imaging apparatus and more particularly relates to a frame for and method for mounting polarization components and a reflective LCD spatial light modulator.
BACKGROUND OF THE INVENTION
Initially introduced as small-scale imaging devices for business presentation markets, digital color projectors have steadily improved in overall imaging capability and light output capacity. In order for digital motion picture projectors to compete with conventional motion picture film projectors such as those used in theaters, however, a number of significant technical hurdles remain. Unlike conventional motion picture projectors, high-quality digital projection systems provide separate color modulation paths for red, green, and blue (RGB) color image data. The design of digital color projection apparatus requires that monochromatic light beams carrying images formed on each of the individual color channels be combined, with proper intensity and registration, in order to project a full color image.
Referring to
FIG. 1
, there is shown a simplified schematic for a digital motion picture projection apparatus
10
as described in U.S. patent application Ser. No. 10/050,309, incorporated herein by reference. Each color channel (r=Red, g=Green, b=Blue) uses similar components for forming a modulated light beam. Individual components within each path are labeled with an appended r, g, or b, appropriately. For the description that follows, however, distinctions between color channels are specified only when necessary. A light source
20
provides unmodulated light, which is conditioned by uniformizing optics
22
to provide a uniform illumination, directed through an illumination relay lens
80
to a dichroic separator
27
. Dichroic separator
27
splits the white light into red, green, and blue color channels. Following any of the three color channels, light goes to a light modulation assembly
38
in which a relay lens
82
directs light through a prepolarizer
70
to a polarizing beamsplitter
24
. Light having the desired polarization state is transmitted through polarizing beamsplitter
24
and is then modulated by a spatial light modulator
30
, which selectively modulates the polarization state of the incident light over an array of pixel sites. The action of spatial light modulator
30
forms an image. The modulated light from this image, reflected from polarizing beamsplitter
24
, is transmitted along an optical axis O
r
/O
g
/O
b
through an analyzer
72
and is directed by a magnifying relay lens
28
, through an optional folding mirror
31
, to a dichroic combiner
26
, typically an X-cube, Philips prism, or combination of dichroic surfaces in conventional systems. An optional color-selective polarization filter
60
may also be provided in the modulated light path. Dichroic combiner
26
combines the red, green, and blue modulated images from separate optical axes O
r
/O
g
/O
b
to form a combined, multicolor image for a projection lens
32
along a common optical axis O for projection onto a display surface
40
, such as a projection screen.
The reflective liquid crystal device (LCD) of
FIG. 1
is a type of spatial light modulator that is widely used in digital projector design. This device accepts polarized light and modulates the polarization of the incident light to provide colored light beam as output. For obtaining polarized light, a polarizing beamsplitter prism, such as a McNcille prism, is typically employed along with the support of one or more polarizing elements, configured as polarizers and analyzers.
Because modulated light must be combined from each of three color channels in order to synthesize a color image, correct registration of the modulated light is important. When the modulated light is reflected from the surface of spatial light modulator
30
, angular errors in the relative alignment of each LCD surface can cause significant shifts in resolution, yielding unsatisfactory image quality. Further image quality problems, such as loss of contrast, can be the result of imperfect alignment of polarization support components, particularly for polarizing beamsplitter
24
. Moreover, thermal expansion effects can cause further drift in registration and degrade polarization components performance. Thermal expansion becomes a particular concern with high-end projection apparatus, since high brightness is required in these applications. At the same time, compact optical packaging is desirable, with minimized optical path length between image-forming components and the projection lens. These conflicting requirements complicate the design of high-brightness projection apparatus.
The negative impact of thermal expansion on image registration is well known in the art. In response to this problem, U.S. Pat. No. 6,345,895 (Maki et al.) discourages use of a mounting base for supporting reflective spatial light modulators, polarizing beamsplitters, and related polarization support components. Significantly, the U.S. Pat. No. 6,345,895 disclosure even teaches away from the use of a mounting base formed from metals or composite materials having low coefficients of expansion. Instead, the approach proposed U.S. Pat. No. 6,345,895 mounts spatial light modulator components directly to glass prism components used for beamsplitting or color combining, so that components in the optical path remain in alignment with thermal expansion. This same overall type of approach is also taught in U.S. Pat. No. 6,375,330 (Mihalakis); U.S. Pat. No. 6,053,616 (Fujimori et al.); and U.S. Pat. No. 6,056,407 (linuma et al.).
One recognized problem with attachment to prism components is in achieving the initial alignment itself. As one example, U.S. Pat. No. 6,406,151 (Fujimori) describes methods for adhesively affixing LCD components to a prism with alignment. While attachment directly to a glass or plastic prism surface may have advantages for minimizing thermal expansion effects, there appear to be a number of drawbacks with solutions that use adhesives, compounding thermal dissipation concerns for the LCD itself and making component replacement a costly and time-consuming procedure.
Recently, as is disclosed in U.S. Pat. No. 6,122,103 (Perkins et al.), high quality wire grid polarizers have been developed for use in the visible spectrum. While existing wire grid polarizers may not exhibit all of the necessary performance characteristics needed for obtaining the high contrast required for digital cinema projection, these devices have a number of advantages. Chief among these advantages are the following:
(i) Good thermal performance. Wire grid polarizers do not exhibit the thermal stress birefringence that is characteristic of glass-based polarization devices, as was noted above.
(ii) Robustness. Wire grid polarizers have been shown to be able to withstand anticipated light intensity, temperature, vibration, and other ambient conditions needed for digital cinema projection.
(iii) Good angular response. These devices effectively provide a higher numerical aperture than is available using conventional glass polarization beamsplitters, which allows relatively higher levels of light throughput when compared against conventional devices.
(iv) Good color response. These devices perform well under conditions of different color channels. It must be noted, however, that response within the blue light channel may require additional compensation.
U.S. Patent Nos. 6,234,634 and 6,447,120 (both to Hansen et al.) and U.S. Pat. No. 6,585,378 (Kurtz et al.) disclose image projection apparatus using wire grid polarizing beamsplitters. The wire grid polarizing beamsplitter offers advantages over conventional prism-based polarizing beamsplitters, particularly due to its small size and weight. It can be appreciated that there could be advantages for light modulation in a combination using wire grid polarizer and analyzer components. However, as with the more conventional beamsplitter and polarizers employed in prior art projection appa
Ehrne Franklin D.
Silverstein Barry D.
Blish Nelson Adrian
Dowling William C.
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