Projection type display device having a mask for cutting off unnecessary light parts of displayed picture

A display device of projection type capable of projecting an image with well defined image frame is disclosed. The device comprises 3 spatial light modulators on which color image information is written in 3 primary colors respectively and is read out as a combined optical color image in a form of light beam, the light beam is then projected out of the display device through lens groups for forming a projected image on a screen, the device further comprises a mask disposed in a proximity of an imaginary image formation plane arranged to locate in a vicinity of one of the lens groups, the light beam is caused to form an optical image on the imaginary image formation plane. The mask has a variable light shielding configuration controlled either electronically or electro-mechanically for cutting off unnecessary light portions of the light beam carrying the optical image so that an image of high contrast and well defined clear fringe is projected on the screen.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
Particularly, the present invention relates to a device capable of 
projecting an image, a frame of which is made variable on the screen 
depending on an aspect ratio of the image to be displayed. 
2. Description of the Related Art: 
FIG. 1 shows a construction of an image projector disclosed in the U.S. 
patent application Ser. No. 913,445. The image projector shown in FIG. 1 
is constituted mainly with a spatial light modulator 1 for forming a red 
image, a spatial light modulator 2 for forming a green image, a spatial 
light modulator 3 for forming a blue image, a color combining optical 
system 4, a first lens group 5, a second lens group 6, a third lens group 
7 and a screen 8. 
In the reproduction of image by this prior art display device, a reading 
light from a light source which is not shown is separated through a 
polarizing beam splitter and dichroic mirrors, etc., to monochromatic red, 
green and blue lights and supplied to the respective spatial light 
modulators 1, 2 and 3. Then, images formed on the respective spatial light 
modulators 1, 2 and 3 are directed to the color combining optical system 4 
and combined thereby and the combined image is focused through the first 
lens group 5 on the second lens group 6. The focused image on the second 
lens group 6, is passed to the third lens group 7 through which it is 
projected onto the screen 8 as an image of a predetermined magnifying 
power. 
According to this image projector, it is possible to make a back focal 
length larger to thereby minimize the thickness or depth of image 
projector and to obtain an image of high resolution. 
It has been known that, with the use of such spatial light modulators of 
image (light) writing type, it is possible to wright an input image while 
arbitrarily varying its frame size, configuration or aspect ratio of 
writing image according to the original form of the input image. 
However, since reading light from a light source irradiates uniformly and 
fully an effective area (area available for writing in or reading out) 
inherent to the spatial light modulator, unnecessary areas outside the 
formed image on the spatial light modulator are also irradiated by the 
reading light resulting in too bright presentation of such unnecessary 
areas on the screen, this deficiency is aggravated by the reading light 
reflected from a substrate surface of each spatial light modulator and 
reaching to the screen so that a frame or fringe of the projected image 
becomes unclear. 
In such a case, it is conceivable that a contrast of an image area to a 
periphery thereof may be improved by cutting unnecessary light by means of 
an aperture member composed of a light shielding plate having a 
predetermined aperture formed therein. 
When such aperture member is provided as a fixed structure, there is a 
problem that it can not respond to an input image whose frame is, for 
example, larger than a size of the predetermined aperture, resulting in 
that even necessary area of the image is cut off. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a display 
device of projection type which includes a mask for cutting off only 
unnecessary bright periphery of a projected image on a screen. 
In order to achieve the above object, the display device of the present 
invention comprises spatial light modulators on which color image 
information is written in primary colors respectively and is read out as a 
combined optical color image in a form of light beam, the light beam is 
then projected out of the display device through lens groups for forming a 
projected image on a screen, the device further comprises an imaginary 
image formation plane arranged in a vicinity of one of the lens groups, 
the light beam forming an image on the imaginary image formation plane, a 
mask disposed in a proximity of the imaginary image formation plane. The 
mask has a variable light shielding configuration controlled either 
electronically or electro-mechanically for cutting off unnecessary light 
portions of the light beam carrying the optical image so that an image of 
high contrast and well defined clear fringe is projected on the screen.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be described with reference to 
FIG. 2 which shows a construction of a display device of projection type 
(hereinafter referred to as "image projector") according to a first 
embodiment of the present invention. In FIG. 2, a signal source 10 
produces a time-series television signal of, for example, an NTSC 
television system, which is supplied to a series-parallel converter 
circuit 11, such as a shift register, in which the time series video 
signal is converted into a simultaneously present video signal 
(simultaneous video signal). 
The simultaneous video signal is supplied to light emitting device 12. The 
light emitting device 12 is composed of a linear array including a 
plurality of light emitting elements and emits beams of light being 
intensity modulated correspondingly to the simultaneous video signal. The 
beams of light emitted from the respective light emitting elements are 
deflected by a deflector 13 and directed to a spatial light modulator 15 
through a lens 14 and thus an image of one of the 3 primary colors is 
formed on the basis of the incident beams of light. 
Concurrently, similar images of other 2 primary colors are also formed on 
other spatial light modulators 16 and 17, respectively. Electric signal 
and/or optical systems from the signal source 10 to the spatial light 
modulators 16 and 17 are identical to that for the spatial light modulator 
15, thus details thereof are omitted here. 
The spatial light modulators 15, 16 and 17 themselves are also identical to 
each other and therefore only the spatial light modulator 15 will be 
described in detail with reference to FIG. 3. 
In FIG. 3, the spatial light modulator 15 is composed of a glass substrate 
15a, a transparent electrode E1 formed on one surface of the glass 
substrate 15a, a photoconductive member 15b, a dielectric mirror 15c, a 
photo-modulation member 15d which may be of TN (twisted nematic type) 
liquid crystal or vertically oriented liquid crystal utilizing 
birefringence effect and a glass substrate 15e having a transparent 
electrode E2 formed on its surface opposing to the photo-modulation member 
15d, all of which are laminated in the described order. 
In writing an image information, a voltage is applied between the 
transparent electrodes E1 and E2 to produce an electric field across the 
photoconductive member 15b and an image light or writing light WL is 
directed through the transparent electrode E1. The writing light WL passed 
through the transparent electrode E1 is incident on the photoconductive 
member 15b. Electric resistance of the photoconductive member 15b is 
varied correspondingly with an intensity distribution of the writing light 
WL over the surface of the photoconductive member 15b. 
In reading the image information, a reading light RL is directed to the 
spatial light modulator 15 from the opposite direction to the writing 
light WL. The reading light RL passed through the glass substrate 15e 
reaches the photo-modulation member 15d in which reading light RL is 
modulated correspondingly with the intensity distribution (written image) 
of the writing light WL, which may vary when it is representing such as 
moving pictures, and then the modulated reading light RL reaches the 
dielectric mirror member 15c and is reflected thereby and emitted out of 
the spatial light modulator 15 from the side of the glass substrate 15e 
having the transparent electrode E2. 
Returning to FIG. 2, the images formed on the spatial light modulators 15, 
16 and 17 are read out by the reading light RL in real time. 
The reading light RL is supplied from a light source 27 through a beam 
splitter 28, where a linearly polarized light (S wave) of the incident 
light is reflected by a vapor-deposited surface 28a of the beam splitter 
28 to a color decomposing/combining optical system 29. 
Green light (component) of the linearly polarized light (S wave) is passed 
through dichroic filters 29a and 29b to the spatial light modulator 16. 
Red light (component) thereof is passed through the dichroic filter 29a 
and reflected at a right angle by the dichroic filter 29b to the spatial 
light modulator 17. Further, blue light (component) thereof is reflected 
at a right angle by the dichroic filter 29a to the spatial light modulator 
15. 
As a result, the images formed on the respective spatial light modulators 
15, 16 and 17 are read out by these incident reading lights and combined 
by the color decomposing/combining optical system 29 to produce a beam of 
three color combined image light. 
The beam of combined image light is directed to the polarizing beam 
splitter 28 and only a P wave component thereof passes therethrough. The 
beam of image light passed through the polarizing beam splitter 28 is 
passed through a first lens group 30 having an iris 31 by which a quantity 
of the image light is restricted, and is focused on an imaginary image 
formation plane caused by the lens group 30 and arranged to locate in a 
vicinity of second lens group 32. 
An aperture member 33 is provided as a mask device behind and in the 
vicinity of the second lens group 32 i.e. in a proximity of the imaginary 
image formation plane. The aperture member 33 serves to cut out a 
peripheral unnecessary area in cross section of the image light passing 
therethrough. 
FIG. 4(A) is a perspective view of the aperture member 33. As shown in FIG. 
4(A), the aperture member 33 takes a form of lamination of a glass 
substrate 33a, a polarizing plate 33b capable of passing only P wave 
component, an aperture portion 33c, a TN liquid crystal 33d, a transparent 
electrode E3, a polarizing plate 33e capable of passing only S wave 
component and a glass substrate 33f, in this order. 
FIG. 4(B) is a plan view of the aperture portion 33c of the aperture member 
33 shown in FIG. 4(A). In FIG. 4(B), the aperture portion 33c includes a 
hatched area "a" as a shielding portion, dotted areas "b", "c", "d" and 
"e" as transparent electrode portions and an open area "f" as an open 
portion. A rectangular area formed by the open portion "f" and the 
transparent electrode portions "b" and "d" has an aspect ratio of 3:4. A 
rectangular area formed by the open portion "f" and the transparent 
electrode portions "c" and "e" has an aspect ratio of 9:16. 
Assuming that the image light incident on the aperture member 33 is of NTSC 
system in which the aspect ratio is 3:4, a voltage is applied to the 
transparent electrode portions "b" and "d" with respect to a potential of 
the transparent electrode E3 leaving the transparent electrodes "c" and 
"e" unbiased. Thus, portions of the TN liquid crystal 33d corresponding to 
the areas of the transparent electrode portions "b" and "d" allow light to 
pass through while portions thereof corresponding to the areas of the 
transparent electrode portions "c" and "e" do not. 
Therefore, any unnecessary peripheral portions of the image light incident 
on the aperture member 33 is shielded, allowing only the necessary image 
light having aspect ratio of 3:4 to pass through. 
The necessary image light passed through the aperture member 33 is incident 
on a third lens group 34 and restricted in area by an iris 35 thereof. 
After enlarged by the third lens group 34, the optical image is projected 
onto a screen 36. 
Accordingly, the image projected on the screen 36 is well defined at its 
fringe having the aspect ratio of 3:4 due to removal of unnecessary 
portions by the aperture member 33. 
When the video signal from the signal source 10 is not the NTSC television 
signal but another television signal having an aspect ratio of 9:16, a 
switch (not shown) for changing the aspect ratio is operated causing that 
the voltage applied to the transparent electrode portions "b" and "d" is 
removed with respect to the potential of the transparent electrode E3, and 
concurrently a voltage is applied to the transparent electrode portions 
"c" and "e" with respect to the transparent electrode E3. 
In this case, the portions of the TN liquid crystal 33d corresponding to 
the transparent electrode portions "c" and "e" allow light to pass through 
while portions thereof corresponding to the transparent electrode portions 
"b" and "d" do not. 
Therefore, as in the previous case, the image having a well defined fringe 
area or frame of aspect ratio of 9:16 is projected onto the screen 36. 
Instead of the TN liquid crystal 33d, a polymer containing liquid crystal 
in dispersed state (PDLC) can be used for this purpose. Further, a 
transparent electrode may be formed to cover a full area of the open 
portion "f" of the aperture portion 33c so that it functions as a 
projection shutter capable of blocking a projected light operating in 
conjunction with the other portions "b" "c" "d" "e". 
FIG. 5 shows another embodiment of the aperture member 33. In FIG. 5, an 
open portion "g" is formed by four shielding plates "h", "i", "j" and "k" 
which are movable in directions shown by double head arrows as such that, 
in a normal state before moving, the open portion "g" has an aspect ratio 
of 3:4 and, at a completion of the movement of the shielding plates, its 
aspect ratio becomes 9:16. 
This embodiment can be realized without using any special structure. For 
example, it may be realized by a conventional system using stepping 
motors. 
FIG. 6 shows a further embodiment of the aperture member 33. In FIG. 6, the 
aperture member 33 takes the form of a slidable shielding plate "m" having 
an opening "n" having an aspect ratio of 3:4 and another opening "o" 
having an aspect ratio of 9:16. The aperture member 33 shown in FIG. 6 is 
moved as shown by a double head arrow according to an input signal. This 
embodiment can be realized easily without using any special structural 
components as in the case shown in FIG. 5. 
It should be noted that, although the configuration of the aperture portion 
has been described as rectangular having aspect ratio of 3:4 or 9:16, the 
present invention is not limited thereto. The present invention can be 
applied to various image information by changing the size and/or 
configuration of the open portion (light passing portion) of the light 
shielding plate. 
Further, in the described embodiments, the kind of signal information from 
the signal source 10 is visually monitored by an operator and the present 
aperture member is operated manually by turning the switch ON or OFF, 
however, it is possible to operate the aperture member of the present 
invention automatically. That is, connectors are provided correspondingly 
with the respective signal sources so that the switch is turned ON when a 
connector corresponding to a specific signal source is connected to apply 
a voltage to the transparent electrode portions "b" and "d" or "c" and "e" 
with respect to the transparent electrode E3, or to move any of the 
slidable shielding plates. 
Further, it is also possible to detect electrically or optically a type of 
the signal, and upon the detection the light shield configuration is 
changed automatically according to the aspect ratio required for the type 
of the signal. By doing above mentioned alternatives, any manual 
operational error or inconvenience in the manually operating switch, or 
the like can be removed. 
As described in the foregoing, according to the projection type display 
device of the present invention, it is possible to trim out unnecessary 
portion of an image light correspondingly with different frame forms 
(aspect ratio) of a video signal and therefore it is possible to project 
an image onto a screen with high contrast and with well defined frame. 
Accordingly, the projection type display device of the present invention, 
in which an image information written in spatial light modulators is read 
out as an optical image by means of a light source and the optical image 
is projected through lens groups, is featured by comprising a mask 
provided in the vicinity of one of the lens groups for cutting off 
unnecessary light contained in a projecting light beam carrying the 
optical image and wherein portions of the light beam to be cut off by the 
mask is variable, it is possible to trim out unnecessary portion of the 
light beam which corresponds to a video signal even if its frame 
configuration is different depending on a type of the video signal and 
therefore it is possible to project an image onto a screen with high 
contrast and with well defined fringe.