Optical: systems and elements – Lens – With multipart element
Reexamination Certificate
1999-12-01
2001-08-28
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With multipart element
C359S456000, C359S566000
Reexamination Certificate
active
06282034
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical device and a Fresnel lens sheet comprising it.
2. Description of the Related Art
FIG. 10
is a schematic view showing the outline of the constitution of a conventional rear projection display. As illustrated, the conventional rear projection display comprises a projection tube
11
which projects an optical image, a projection lens
12
which magnifies the optical image from the projection tube
11
, and a rear projection screen
13
which forms thereon the optical image having been magnified through the projection lens
12
. A person P views the optical image having been projected, magnified and formed on the rear projection screen
13
.
In many cases of the rear projection display of that type, the screen
13
has a two-sheet structure composed of a Fresnel lens sheet
14
which gathers the beams of light from the projection tube
11
toward the viewer, and a lenticular lens sheet
15
which disperses the light having passed through the Fresnel lens sheet
14
in the direction horizontal to the screen (that is, in the direction of the width of the screen) and in the direction vertical thereto (that is, in the direction of the height of the screen) at a predetermined angle in a suitable ratio to thereby enlarge the angle of visibility to be in a predetermined range.
In that type of rear projection display equipped with a rear projection screen, the projection lens
12
and the screen
13
are desired to be so disposed that the distance therebetween is shorter so as to reduce the thickness of the display. It is also desired to increase the luminance around the peripheral area of the screen. To meet the requirements, the Fresnel angle in the peripheral area of the Fresnel lens sheet
14
must be enlarged. On the other hand, in general, the refractive index of the material that forms the Fresnel lens surface of the Fresnel lens sheet has wavelength dependency. Therefore, the angle of light passing through the Fresnel lens shall vary, depending on the wavelength of the light. With the Fresnel angle increasing, the difference in the angle of light passing through the Fresnel lens shall increase, depending on the difference in the wavelength of the light. As a result, the light having reached the screen is colored to degrade the quality of the image formed on the screen. Accordingly, it is difficult to solve the problem with thinned rear projection displays only by enlarging the Fresnel angle in the peripheral area of the Fresnel lens sheet.
SUMMARY OF THE INVENTION
Given that situation, we, the present inventors have completed the present invention with its object to provide an optical device through which the angle of light passing and going out has little wavelength dependency and which is well applicable to Fresnel lenses with little coloration of light passing through it.
The optical device of the invention to solve the problem noted above has a diffraction grating formed on one surface and capable of diffracting rays of light incident thereon, and has a refracting member formed on the other surface opposite to the diffraction grating and capable of refracting the diffracted rays of light from the diffraction grating to make the refracted rays of light go out of it, in which the wavelength dependency of the diffractive angle of the main rays diffracted by the diffraction grating compensates for the wavelength dependency of the refractive angle thereof refracted by the refracting member, or that is, the former wavelength dependency is opposite to and cancels out the latter one. Other optical device of the invention to solve the problem noted above has a refracting member formed on one surface and capable of refracting rays of light incident thereon, and has a diffraction grating formed on the other surface opposite to the refracting member and capable of diffracting the refracted rays of light from the refracting member to make the diffracted rays of light go out of it, in which the wavelength dependency of the diffractive angle of the main rays diffracted by the diffraction grating compensates for the wavelength dependency of the refractive angle thereof refracted by the refracting member, or that is, the former wavelength dependency is opposite to and cancels out the latter one. In the optical devices, the refracting member may be of a Fresnel lens to give a Fresnel lens sheet.
In the invention, the diffraction grating may be formed on the refracting member. Specifically, the optical device of this embodiment of the invention has a refracting member formed on one surface and capable of refracting rays of light incident thereon to give refracted rays of light going out of it, and has a diffraction grating formed on the refracting member, in which the wavelength dependency of the diffractive angle of the main rays diffracted by the diffraction grating compensates for the wavelength dependency of the refractive angle thereof refracted by the refracting member. In the optical device of this embodiment, the refracting member may be also of a Fresnel lens to give a Fresnel lens sheet. In another embodiment of the invention, the diffraction grating and the refracting member of, for example, a Fresnel lens may be formed in different sheets. Specifically, in this embodiment, the diffraction grating sheet with a diffraction grating capable of diffracting rays of light incident thereon may be combined with the Fresnel lens sheet with a Fresnel lens capable of refracting the diffracted rays of light from the diffraction grating sheet to give refracted rays of light going out of it, in such a manner that the wavelength dependency of the diffractive angle of the main rays diffracted by the diffraction grating compensates for the wavelength dependency of the refractive angle thereof refracted by the Fresnel lens sheet.
As a rule, light is refracted through the interface between air and a substance, satisfying the following equation for which the refractive index of air is 1 (one):
sin(&thgr;out)=n·sin(&thgr;in) (1)
wherein &thgr;in indicates the angle of the light coming in the substance; &thgr;out indicates the angle of the light going out of the substance; and n indicates the refractive index of the material forming the substance.
The refractive indices nR, nG and nB of an ordinary Fresnel lens to red light (R), green light (G) and blue light (B), respectively, from a light source are generally in the following order:
nR<nG<nB (2).
From the formula (1) above, therefore, the angles &thgr;R, &thgr;G and &thgr;B of the red light, the green light and the blue light, respectively, going out of the Fresnel lens shall be in the following order:
&thgr;R<&thgr;G<&thgr;B (3).
Accordingly, as in
FIG. 9A
showing one model of an ordinary Fresnel lens, the angles of different rays of light going out of the Fresnel lens vary, depending on the wavelength of each ray. The wavelength dependency of rays of light passing through the Fresnel lens causes coloration of the projection screen combined with the Fresnel lens sheet and greatly degrades the quality of images formed on the screen.
As a rule, light passes through a diffraction grating, satisfying the following equation:
sin(&thgr;out)=sin(&thgr;in)+m&lgr;/P (4)
wherein P indicates the pitch of the diffraction grating; &lgr; indicates the wavelength of the light coming in the diffraction grating; &thgr;in indicates the angle of the incoming light; &thgr;out indicates the outgoing light; and m indicates the grading order of the diffraction grating.
In that condition, the wavelengths of the incoming red light, green light and blue light represented by &lgr;R, &lgr;G and &lgr;B, respectively, are in the following order:
&lgr;R>&lgr;G>&lgr;B (5).
Therefore, when m>0,
&phgr;R>&phgr;G>&phgr;B (6)
wherein &phgr;R, &phgr;C and &phgr;B indicate the diffractive angles of the red light, the green light and the blue light, respectively.
Accordingly,
Kumagai Yoshihiro
Matsuzaki Ichiro
Onishi Ikuo
Epps Georgia
Kuraray Co. Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Thompson Timothy J
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