This application is based on application No. H11-008674 filed in Japan on Jan. 18, 1999, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an optical system for a projector, and particularly to such an optical system for a projector as incorporates a reflection-type spatial light modulator, such as a digital mirror device(trademark) (or DMD(trademark) for short, manufactured by Texas Instruments Incorporated; hereinafter referred to simply as a digital mirror device or DMD), that is provided with a large number of variable-reflection-angle mirror elements that can vary the reflection angle of the light incident thereon in accordance with a video signal so that only the light corresponding to the video signal will be reflected toward a projection optical system.
2. Description of the Prior Art
In recent years, as higher and higher resolution is desired in images in general, also in the field of projectors, development of techniques has been sought that achieve a substantial increase in the number of pixels without making the optical system larger. One attempt to meet such needs is the development of a projector employing a DMD.
A DMD is produced by forming a large number of minute rectangular high-reflectance mirror elements, of which the inclination is variable in accordance with a video signal, on a silicon memory chip by a process as used to manufacture a semiconductor device. A projector employing such a DMD, by varying the inclination of those mirror elements, controls the direction in which it reflects the illumination light incident thereon in such a way as to converge only the desired parts of the reflected light on a screen and thereby project a desired image thereon.
In accordance with a video signal, the individual mirror elements of a DMD, when in an on state, reflect light toward a projection optical system and, when in an off state, change their inclination to reflect light in a different direction so that the light will not enter the projection optical system. However, owing to restrictions imposed on the design of a projector, the light reflected from the mirror elements when they are in an off state is shone on a side-wall portion of a prism constituting a prism system disposed between the DMD and the projection optical system, and therefore there is a possibility of this light being further reflected therefrom so as to enter the projection optical system.
In particular, when the prism is surrounded by a medium, such as air, having a small refractive index, most of the above-mentioned light shone on the side-wall portion of the prism is reflected from the inner surface of the side-wall portion of the prism, and this greatly increases the amount of such secondary-reflection light. This secondary-reflection light (unnecessary, or stray, light), when it enters the projection optical system, may cause another image (a ghost) separate from the normal image to appear on the screen.
One way to prevent such entry of secondary-reflection light into the projection optical system is to form the side-wall portion of the prism into a diffusive surface that diffuses the light incident thereon. Another way is to paint the outer surface of the side-wall portion of the prism black, or to vapor-deposit on that surface a light-absorbing dielectric film, or to affix to that surface a member having a bottom surface so shaped as to absorb light so that this bottom surface will absorb light and convert it into heat.
The method of absorbing light and converting it into heat by the use of a member having a bottom surface so shaped as to absorb light is adopted, for example, in the projector-oriented optical system disclosed in Japanese Laid-Open Patent Application H9-96867. In this optical system, a heat-dissipating member having a comb-tooth-shaped bottom surface is arranged so as to face a side wall of a prism, with a shock-absorbing pad in between, in such a way that this bottom surface is kept in close contact with the shock-absorbing pad. In this optical system, the recessed portions of the comb-tooth-shaped bottom surface absorb light and convert it into heat, and the heat-dissipating member as a whole dissipates the resulting heat.
However, even if light is diffused, it is inevitable that part of the diffused light will enter the projection optical system. It is practically impossible to form a black thin film that completely absorbs the light incident thereon, and therefore, even if a black thin film is applied, some light, left unabsorbed, may enter the projection optical system. It is difficult to produce a member having so intricate a shape as to absorb completely the light incident thereon, and therefore it is inevitable that part of the incident light will be reflected or diffused in the vicinity of the protruding portions of the comb-tooth-shaped bottom surface of this member.
An object of the present invention is to provide a projector-oriented optical system in which entry of unnecessary light into a projection optical system is prevented more strictly than ever.
To achieve the above object, according to one aspect of the present invention, an optical apparatus is provided with: a reflection-type spatial light modulator having a plurality of minute variable-reflection-angle mirrors that individually deflect the light incident thereon in one of two different directions, namely in a first direction or in a second direction, in accordance with a signal fed in; an optical system to which the light deflected in the first direction by the modulator is directed; a prism disposed between the modulator and the optical system so as to direct the light deflected in the first direction by the modulator to the optical system and direct the light deflected in the second direction by the modulator to a side face of the prism; and a light-absorbing member shaped like a plane-parallel plate and disposed with an entrance face thereof kept in close contact with the side face of the prism. The light-absorbing member is so designed that the light deflected in the second direction by the modulator enters the light-absorbing member through the entrance face thereof but does not exit from the light-absorbing member through the entrance face back into the prism.
In this optical apparatus, preferably, the light-absorbing member is so designed that the distance t between the entrance face and a second flat face opposite to the entrance face fulfills the following condition:
txe2x89xa7[1xe2x88x92{(n1/n2)sin xcex11}2]xc2xd/2K
where n1 represents the refractive index of the prism, n2 represents the refractive index of the light-absorbing member, xcex11 represents the minimum angle of incidence of light incident on the entrance face, and K represents the absorption coefficient of the light-absorbing member.
According to another aspect of the present invention, a projector is provided with: an illumination optical system that includes a light source and that emits the illumination light generated by the light source; a reflection-type spatial light modulator having a plurality of minute variable-reflection-angle mirrors that individually deflect the illumination light emitted from the illumination optical system in one of two different directions, namely in a first direction or in a second direction, in accordance with a signal fed in; a projection optical system to which the light deflected in the first direction by the modulator is directed; a prism disposed between the modulator and the optical system so as to direct the illumination light emitted from the illumination optical system to the modulator, direct the light deflected in the first direction by the modulator to the optical system, and direct the light deflected in the second direction by the modulator to a side face of the prism; and a light-absorbing member shaped like a plane-parallel plate and disposed with an entrance face thereof kept in close contact with the side face of the prism. The light-absorbing member is so designed that the light deflected in the second direction by the modulator enters the light-absorbing member through the entrance face thereof but does not exit from the light-absorbing member through the entrance face back into the prism.
In this projector, preferably, the light-absorbing member is so designed that the distance t between the entrance face and a second flat face opposite to the entrance face fulfills the following condition:
txe2x89xa7[1xe2x88x92{(n1/n2)sin xcex11}2]xc2xd/2K
where n1 represents the refractive index of the prism, n2 represents the refractive index of the light-absorbing member, xcex11 represents the minimum angle of incidence of light incident on the entrance face, and K represents the absorption coefficient of the light-absorbing member.
In the optical apparatus and the projector described above, the light shone on the side face of the prism enters the light-absorbing member. The light having entered the member is either absorbed by the member and thereby converted into heat or, if absorbed incompletely, driven out of the member. Here, the light driven out of the member is never driven out of the member through the face that is kept in close contact with the prism. Specifically, for example, the light-absorbing member is so designed that the light having entered the member is completely absorbed within the member before striking again the face that is kept in close contact with the prism. In this way, it is possible to prevent unnecessary light from returning to the prism (and thus entering the (projection) optical system).
Assume that FIG. 1 is a sectional view of the light-absorbing member used in the optical apparatus or projector described above along the plane that is perpendicular to both the first and second faces (what is called respectively the entrance face and the second flat face above) of the member. In FIG. 1, consider, for example, a case where light, coming from inside the prism 22, enters the light-absorbing member 5 through the bottom face 5a thereof (i.e. the first face that is kept in close contact with the prism 22), at a position P1 thereon, is then reflected from the top face (the second face) 5b, at a position P2 thereon, and then travels toward the bottom face 5a, toward a position P3 thereon. Here, assume that the angle of incidence and the angle of emergence at the position P1 are xcex11 and xcex12, respectively, the distance between the bottom and top faces 5a and 5b (i.e. the thickness of the light-absorbing member 5) is t, the absorption coefficient of the light-absorbing member 5 is K, and the distance between P1 and P2, which equals the distance between P2 and P3, is L.
If the light that has entered the light-absorbing member 5 at the position P1 is absorbed before reaching the point P3 (i.e. while traveling an optical path length 2L), then
1xe2x89xa62LKxe2x80x83xe2x80x83(1)
Here, according to the trigonometric function,
xe2x80x83L=t/cos xcex12xe2x80x83xe2x80x83(2)
According to Snell""s law,
n1 sin xcex11=n2 sin xcex12
sin xcex12=(n1/n2)sin xcex11xe2x80x83xe2x80x83(3)
Combining Expressions (2) and (3) gives
L=t/[1xe2x88x92{(n1/n2)sin xcex11}2]xc2xdxe2x80x83xe2x80x83(4)
Substituting Expression (4) in Expression (1) gives
1xe2x89xa62Kt/[1xe2x88x92{(n1/n2)sin xcex11}2]xc2xd
txe2x89xa7[1xe2x88x92{(n1/n2)sin xcex11}2]xc2xd/2Kxe2x80x83xe2x80x83(5)
As described above, for the light that has entered the light-absorbing member 5 at the position P1 to be absorbed before reaching the point P3 (i.e. while traveling an optical path length 2L), Expression (5) needs to be fulfilled. Expression (5) defines the lower limit of the thickness t of the light-absorbing member 5. Expression (5) shows that, the smaller the angle of incidence xcex11, the greater the lower limit of the thickness t. Accordingly, as long as the light-absorbing member 5 is so formed as to have a thickness t that fulfills Expression (5), assuming that the minimum angle of incidence of the light entering the member 5 through the bottom face 5a thereof equals xcex11, light never returns to the prism through the bottom face 5a.