Television – Video display – Projection device
Reexamination Certificate
2000-05-08
2004-04-06
Miller, John (Department: 2614)
Television
Video display
Projection device
C348S786000, C348S832000, C348S842000, C359S456000, C359S459000, C359S460000
Reexamination Certificate
active
06717626
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to projection television sets, particularly to projection television sets having improved optical characteristics, and more particularly to a protection panel for a rear projection television set that has a spectral transmittance that enhances the quality of a color image projected through the protection panel.
BACKGROUND
Projection television sets, particularly rear projection television sets, are a popular alternative to picture tube television sets, as they provide relatively large viewable screens compared to conventional picture tubes. As shown in
FIG. 1
, a rear projection television set
10
generally includes a cabinet
12
with a set of internal projection tubes
14
, a mirror
16
, and electronic circuitry (not shown) for receiving broadcast signals and the like, and/or for controlling the projection of an image onto a screen assembly
18
mounted to the front of the cabinet
12
.
Turning to
FIG. 2
, each projection tube
14
includes a cathode ray tube (“CRT”)
20
, a spacer
22
attached to the CRT
20
, and a lens assembly
24
mounted to the spacer
22
. The lens assembly
24
includes one or more individual lenses, such as a condenser lens
26
and a “C-lens”
28
, mounted in a lens barrel
30
, generally with a cooling fluid
34
provided between the CRT
20
and the C-lens
28
, as is known in the art. The CRT
20
receives signals from the electronic circuitry that excite a fluorescent substance
32
in the CRT
20
such that the projection tube
14
emits an image (not shown), generally of a single color of light, towards the mirror
16
. Generally, a set of three projection tubes
14
are provided (only one shown), each projection tube
14
emitting an image in one of the primary colors, i.e., substantially red, green, and blue light, respectively.
Returning to
FIG. 1
, the mirror
16
reflects and/or focuses the images (represented generally by ray of light
36
) from the projection tubes
14
towards the screen assembly
18
. The screen assembly
18
may include a fresnel lens
38
that may further condense the images emitted by the projection tubes
14
, and a lenticular screen
40
that may correct and/or control the directional projection of the images, as is known in the art. Finally, an outer protection panel
42
is provided that protects the internal components, such as the lenticular screen
40
, and allows the images from the projection tubes
14
to be seen from the front of the cabinet
12
.
The fluorescent substances provided in each of the CRT's of a conventional set of projection tubes generate emission spectra that are well known.
FIG. 3
shows the relative intensity or luminance of light emitted by a set of commonly used CRT's as a function of the wavelength of the light. For example, a blue CRT generally emits light that corresponds to curve
51
, having a peak around a dominant wavelength of about 450 nm. A green CRT generally emits light that corresponds to curve
52
, having a dominant wavelength of about 545 nm, and including several sidebands
52
A-
52
D. Finally, a red CRT generally emits light that corresponds to curve
53
, having a dominant wavelength of about 610 nm, and including sidebands
53
A-
53
B.
The sidebands
52
A-
52
D,
53
A-
53
B are undesirable emissions generated by the fluorescent materials, and generally reduce the overall color purity of the resulting images that are projected onto the screen assembly
18
. To correct for these sidebands, it has been suggested to provide filters in the red and green projection tubes. For example, a color absorbing material may be provided in one of the lenses of the projection tubes, such as the C-lens
28
, as is disclosed in U.S. Pat. No. 5,010,396 issued to Hanyu et al. Alternatively, a color absorbing material may be included in the cooling fluid
34
within the projection tube
14
, as is disclosed in U.S. Pat. No. 5,055 922 issued to Wessling.
For example, a colored C-lens for a red CRT may provide a filter characteristic such as that shown FIG.
4
. The “spectral transmittance” (i.e., the ratio of the intensity of the light exiting from a material as compared to the intensity of the incident light entering the material, as a function of wavelength of the light) for this colored C-lens may provide a transmittance above about 600 nm that is substantially greater than ninety percent (90%),e.g., about ninety two percent (92%). Below 600 nm, however, the transmittance decreases steeply, thereby substantially removing, for example, light from the sideband
53
A (shown in FIG.
3
), and consequently improving the purity of the red image.
In addition to problems of color purity, rear projection televisions may also experience problems with contrast due to light sources external to the television set. “Contrast” is defined as a ratio of the intensity of light that is received by an observer from a screen displaying one hundred percent (100%) white, and the intensity of light that is received from a screen displaying zero percent (0%) black. If undesired light is seen when the projection television is displaying zero percent (0%) black, it may appear to be brighter than normal black, resulting in a contrast that is lowered and a picture quality that appears to be deteriorated.
For example, as shown in
FIG. 5
, light from a light source
44
may interfere with the images projected through the protection panel
42
, such as from a lamp in the room where the projection television set is located or sunlight from a nearby window. The light source
44
may emit rays of light
46
, some of which may be directed towards the screen assembly
18
, and be at least partially reflected back towards an observer
48
trying to watch the projection television
10
.
An exemplary ray of light
46
A from light source
44
is shown that may strike an outside surface
42
A of the protection panel
42
. As the ray
46
A enters and passes through the protection panel
42
, first and second reflected rays
46
C,
46
E may be reflected off of the outside and inside surfaces
42
A,
42
B of the protection panel
42
. In addition, a portion of the ray
46
F may leave the protection panel
42
and strike the outside surface
40
A of the lenticular screen
40
. Although the ray
46
F may be diffused by the outside surface
40
A of the lenticular screen
40
, it may generate a third reflected ray
46
G, a portion of which may exit the protection panel
42
as ray
46
I and be directed towards the observer
48
.
All of these reflected rays
42
C,
42
E,
42
I may interfere substantially with the images being projected by the projection television
10
, represented by image ray
36
that passes through the screen assembly
18
towards the observer
48
as ray
36
E. For example, a conventional protection panel may reflect about four percent (4%) of incident light striking its surface. Thus, a substantial amount of undesired light (as much as eight percent (8%) or more of the incident light) from a light source
44
may be reflected from the screen assembly towards an observer
48
, thereby substantially reducing the contrast of the images being viewed on the protection panel
42
.
To reduce the effects of this undesired external light, it has been suggested to add color absorbing material to the protection panel, resulting in what is known as a “dark tint” protection panel. Such dark tint protection panels generally have a spectral transmittance that is reduced by about twenty percent (20%) as compared to non-tinted protection panels. The spectral transmittance is substantially uniform across all wavelengths, such that the protection panel does not substantially affect the color of the images projected through it.
FIG. 6
comparatively illustrates an exemplary spectral transmittance of a nontinted protection panel (line
61
) and a dark tint protection panel (line
62
). The spectral transmittances shown include a four percent (4%) reduction in intensity of the light passing through the respective panels due to reflection from each of
Hibi Taketoshi
Kondo Taiji
Nakano Yuzo
Miller John
Mitsubishi Digital Electronics America Inc.
Orrick Herrington & Sutcliffe LLP
Yenke Brian
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