Rear-projection screen and rear-projection image display

Details Rear-projection screen and rear-projection image display Rear-projection screen and rear-projection image display

2001-08-28

2003-12-16

Mack, Ricky (Department: 2873)

Optical: systems and elements

Projection screen

Embedded particles

C359S455000, C359S460000, C359S599000, C353S074000, C353S077000

Reexamination Certificate

active

06665118

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
Both of Aspects I and II of the present invention relates to a rear-projection screen and a rear-projection display (rear-projection image display device) in which the rear-projection screen is used.
2. Related Background Art
Aspect I of the Present Invention
Needs for large screens have grown, mainly in the field of television picture tubes recently, and rear-projection displays have gained a spotlight as suitable for such a large screen. Generally, a CRT is used as an image source for the rear-projection display. Further, a type in which a spatial modulation element such as a liquid crystal element is used for advantages of lightness and compactness has been proposed and drawn attention.
First of all, the following description will depict a type in which a CRT is used as an image source.
FIG. 6
is a view schematically illustrating the basic configuration of the same.
In this display, images are formed by single-color CRTs
1
(
1
R,
1
G, and
1
B) for the three main colors, respectively, and are enlarged and projected by projection lenses
2
(
2
R,
2
G, and
2
B) corresponding to the same, respectively, so as to be superimposed on a screen
3
. Here, the reference codes R, G, and B correspond to red, green, and blue, respectively. As shown in the figure, light that is divergent from the center to the periphery and that partially has a sharp directivity is incident on the screen
3
disposed at the image formation plane. Besides, red, green, and blue lights incident on respective parts have angles differing from each other, respectively. The screen
3
is required to arrange such projected lights appropriately so as to allow good image recognition.
Minimum image observation is enabled by using a simple light diffusing sheet as the screen
3
. Since the projected light is incident thereto divergently as described above, however, light at the peripheral part has outward directivity since the projected light is incident divergently thereon. Therefore, the brightness of the screen is remarkably uneven. For instance, the screen has an extremely low luminance at the periphery as compared with a luminance at the center when observed from the front, and has a high luminance at an end closer to the observer and a low luminance at an end farther from the observer when observed diagonally.
To avoid such unevenness, generally a Fresnel lens sheet
31
is provided on a light-projected side of a diffusing sheet. The Fresnel lens sheet
31
functions to convert the projected light divergently incident from the projection lenses
2
on the screen
3
into substantially parallel rays. By this function, green projected light is converted into parallel rays perpendicular to the screen surface, while blue and red projected lights are converted into parallel rays that are vertically parallel with each other and that have certain set angles, respectively, with respect to the normal line of the screen surface in any horizontal plane. In the case where the projected light simply is diffused in this state, the green projected light leaves the screen symmetrically with respect to the normal direction of the screen surface, while the red and blue projected lights leave the screen asymmetrically, thereby causing colors of the screen to change depending on the viewing direction. This phenomenon is called “color shade” and degrades the image quality.
To cope with this, a lenticular lens sheet
32
that has a special configuration having black stripes (BS) and pairs of lenticular lenses (this configuration is hereinafter referred to as “BS Paired-Lenticular-Lens Structure”) is used so as to diffuse projected light with a sharp directivity so as to make the same observable at various angles, and to suppress color shift. The function thereof is depicted with reference to FIG.
7
.
FIG. 7
illustrates a cross section of the lenticular lens sheet
32
in the horizontal direction, and ray trajectories of green projected light and red projected light are indicated with a solid line (G) and a broken line (R), respectively. As shown in the figure, light-incident-side lenticular lenses
321
and light-exiting side lenticular lenses
322
that are paired are provided so that the lenses of each pair share the same optical axis. By doing so, an exiting angle of the red light that has been incident diagonally is corrected so that diffusion symmetric to the normal direction of the screen is realized, as is with the green light, whereby the color shift is suppressed. Furthermore, because light passes through limited portions of the light-exiting surface due to the light collecting function of the light-incident-side lenticular lens
321
, it is possible to provide light absorbing layers
323
at light non-transmission portions of the light-exiting surface. Since the light absorbing layers are black in color and are provided in a stripe form, they are called black stripes, abbreviated as BS, and function significantly to reduce the diffusing reflection of external light incident on the screen in a bright environment, thereby improving the contrast.
It should be noted that generally the lenticular lens is formed so that its lengthwise direction is directed in the vertical direction, and the refraction by the lenticular lens affects only in the horizontal direction, and does not contribute to diffusion in the vertical direction. Therefore, light diffusing microparticles made of a material having a refractive index different from that of a base are dispersed inside the lenticular lens sheet so that light is diffused in the vertical direction. At interfaces between the base and the light diffusing microparticles, light rays are refracted depending on a refractive index difference &Dgr;n according to the Snell's law, thereby being diffused isotropically. This refracting function is more intense as the difference between the refractive index of the base and that of the light diffusing microparticles is greater, which means that light is diffused more as the difference between the refractive index of the base and that of the light diffusing microparticles is greater.
Generally, a material tends to have a greater refractive index at a shorter wavelength, and this is called the wavelength dispersion of the refractive index, which is represented by an Abbe constant &ngr;d. The dispersion increases and the Abbe constant &ngr;d decreases as a material has a higher refractive index. The relationship between the refractive index nd of a typical material as an optical resin material and the Abbe constant &ngr;d is shown in Table 1 and FIG.
10
.
TABLE 1
POPULAR NAME,
REFRACTIVE
ABBE
MATERIAL
TRADE NAME
INDEX nd
CONSTANT vd
PMMA
Acryl
1.492
57.6
Polystyrene
Styrol
1.590
30.9
Polycarbonate
PC
1.585
29.9
Allyl Glycol
CR39
1.504
57.8
Carbonate
Copolymer
Zeron ®
1.533
42.4
Styrene
Methacrylate
Copolymer
Lustran ®
1.569
35.7
Styrene
Acrylonitryle
Polymethyl
TPS ®
1.466
56.4
Pentane
Note: ® means Registered Trademark.
Thus, in order that a base and light diffusing microparticles are made of materials selected from generally-used transparent resin materials so that they have a refractive index difference &Dgr;n therebetween, unavoidably a high-refractive-index high-dispersion material and a low-refractive-index low-dispersion material are combined. Consequently, the refractive index difference &Dgr;n also is made wavelength-dependent, and hence, the refractive index difference &Dgr;n tends to increase as the wavelength is shorter.
In the case where the combination of the light diffusing microparticles and the base is such a combination of general materials, the refractive index difference &Dgr;n increases as the wavelength is shorter, thereby leading to significant diffusion. As a result, the diffusion exhibits a wavelength-dependency such that the diffusion of blue light having a shorter wavelength exceeds the diffusion of red light having a longer wavelength.
As the base material of the lenticular lens sheets, a transparent resin is used, for instance, poly

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