Projection display apparatus

A projection display apparatus which is designed to separate a light from the light source into color lights of three primary colors by a color separator, irradiate the respective color lights to corresponding three light valves, combine the optical images on the light valves by a light combiner formed by intersecting dichroic reflection surfaces in X-letter form, followed by projecting the combined image by enlargement on a screen with a projection lens. The color separator is made by intersecting in X-letter form a first and second flat plate type dichroic mirrors. The second dichroic mirror is composed of two small dichroic mirrors made by separating into two parts of the intersecting part. The two small type dichroic mirrors are disposed on mutually different places so that, when viewed from the output side of the light which advances straightly through the color separator, the two edge surfaces which form the intersecting part of the two small dichroic mirrors are seen as being in overlapped state. Due to the existence of the two places of the intersecting parts of the dichroic mirrors, it is possible to make the produced bright lines on the projected image lens conspicuous.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a projection display apparatus for 
producing on a screen an enlargement of an optical image formed on light 
valves by modulating lights with the light valves and projecting the 
modulated lights on the screen by a projection lens. 
2. Description of the Prior Art 
In order to effect display of images in a large picture size, there has 
been well known a method of forming on a relatively small light valve an 
optical image corresponding to an image signal as a variation of optical 
characteristic, modulating a light with the optical image, and projecting 
the modulated light on a screen by means of a projection lens. In the 
projection display apparatus of this type, the resolution of the projected 
image is nearly determined by the resolution of the light valve, and when 
the light source is intensified, a larger optical output is obtainable, so 
that when a light valve of high resolution is used, it becomes possible to 
realize a projection display apparatus having high resolution and large 
optical output even if its display area is small. Besides, recently notice 
has been made on a method of using a liquid crystal panel as a light 
valve. For example, Morozumi, et. al. proposed in "LCD Full-Color Video 
Projector", SID 86 Digest, p. 375, a method of obtaining a color 
projection image using three liquid crystal panels. A basic construction 
of the optical system proposed by Morozumi, et al. is shown in FIG. 6. 
Lamp 1 radiates a light including the color components of red, green, and 
blue. The light radiated from the lamp 1 is converted into parallel rays 
by a condenser lens 2 and a concave mirror 3, passed through a heat 
absorbing filter 4, and then put into a component color separator 5. The 
component color separator 5 is disposed by placing a flat plate type red 
reflection dichroic mirror 6 and flat plate type blue reflection dichroic 
mirrors 7, 8 in X-form intersecting manner, by which the dichroic mirror 
surfaces 9, 10 lie on the same plane. The light incident on the component 
color separator 5 is divided into the lights of red, green, and blue. The 
red light is reflected by the plane mirrors 11, 12 and incident on the red 
liquid crystal panel 15. The green light is straightly passed and incident 
on the green liquid crystal panel 16. The blue light is reflected by the 
plane mirrors 13, 14 and incident on the blue liquid crystal panel 17. On 
the liquid crystal panels 15, 16, 17 there are formed the optical images 
of red, green, and blue as the variation of the transmissivity in 
proportion to the image signals, respectively. The output lights from the 
liquid crystal panels 15, 16, 17 are combined into a one-piece form by a 
light combiner 18, and a color image is formed substantially at the 
position of the green liquid crystal panel, and the color image is 
projected by enlargement on a screen 80 by means of a single projection 
lens 19. The light combiner 18 is a prism type dichroic mirror made by 
joining four rectangular prisms 20, 21, 22, 23, in which the red 
reflection dichroic multi-layered film is vacuum deposited on the joining 
surfaces 24, 25 and the blue reflection dichroic multi-layered film on the 
joining surfaces 26, 27. 
The projection display apparatus shown in FIG. 6 has a characteristic to 
make it possible to change readily the picture size or the distance from 
the projection lens 19 to the screen 80, because of the use of a single 
projection lens. Further, since the component color separator 5 and the 
light combiner 18 cause to cross the dichroic reflection surface in 
X-letter form, there is a characteristic to reduce the space necessary for 
the optical system. The construction to intersect a pair of dichroic 
mirrors in X-letter form as a component color separator or a light 
combiner has already been disclosed by Schroeder in U.S. Pat. No. 
2,642,487 as "component color separator". 
By the way, when a pair of dichroic mirrors are intersected in X-letter 
form, an invalid area is necessarily formed on the intersecting part, and 
the obscure image of the intersecting part may appear on the screen. 
Accordingly, the light combiner 18 of the construction as shown in FIG. 6 
makes the invalid area of the intersecting part 28 very fine. The 
intersecting part 29 of the component color separator 5 is also projected 
in obscure form on the screen, which however does not practically provide 
a problem if the F-number of the projection lens 19 is small, or if the 
diameter of the iris of the projection lens 29 is sufficiently large. 
The prism type dichroic mirrors to be used as a light combiner 18 for the 
construction as shown in FIG. 6 are relatively expensive in comparison 
with those of a flat type, and become more expensive when the invalid area 
of the intersecting part 28 is made extremely fine. Accordingly, in order 
to reduce the cost of the apparatus, it is considered to use a flat type 
dichroic mirror made by intersecting with a light combiner 18 of a 
construction as shown in FIG. 5 in X-letter form, or to use a prism type 
dichroic mirror having no fine invalid zone of the intersecting part 28. 
However, if there are the intersecting parts 28, 29 in front and back of 
the liquid crystal panel, a bright line is formed at the center of the 
projected image. This bright line causes a significant problem in 
deteriorating the image quality. 
In view of the fact that no bright line is seen when there is an 
intersecting part only on the component color separator 5 or the light 
combiner 18, the mechanism for generating the bright line is distinctly 
different from that in which the obscure images at the intersecting parts 
28, 29 appear on the projected image. As a result of extensive study, the 
inventors have found out that the cause of this bright line could be 
explained, as follows: 
A state of the case where parallel rays are incident on the component color 
separator 5 as shown in FIG. 6 is shown in FIG. 7. In this case, it is 
assumed that the thicknesses of the dichoic mirrors 6, 7, 8 are all equal. 
The inputted light 30 is dissolved into the rays 31, 32, 33 which advance 
in division into three directions by the dichroic mirrors 6, 7, 8. Due to 
the edge surfaces 34, 35 with which the small type dichroic mirrors 7, 8 
are in close contact, there are produced the invalid areas 36, 37, 38 on 
the three output rays 31, 32, 33. These invalid areas 36, 37, 38 are seen 
as dark zone when observed through the projection lens 19. It is worth 
noticing that, when the thicknesses of the dichroic mirrors 6, 7, 8 are 
all equal, the width d.sub.G of the invalid area 37 of the light emitted 
in the direction of straight advance becomes two-fold the widths d.sub.R, 
d.sub.B of the invalid areas 36, 38 of the light emitting in the 
width-wise direction. 
Then, when the inside of the projection lens 19 is observed from the 
position near the center 81 of the screen 80, there are seen the two dark 
zones 40, 41 corresponding to the two intersecting parts 28, 29 in the 
iris 39, as shown in FIG. 8. The two dark zones 40, 41 are seen in 
overlapped state when observed from the center 81 of the screen 80, and 
they shift in the opposite directions to each other when the position of 
the eye is moved. Under the state where the two dark zones 40, 41 do not 
overlap, the total area of the dark zones becomes approximately the same 
without respect to the distance thereof, but under the state of overlap 
with the condition of contact between the two dark zones 40, 41, there are 
great differences in the total areas of the dark zone. When the eye 
position is moved in the vicinity of the center 81 of the screen 80, the 
size of the iris 39 is approximately the same, and only the total range of 
the dark zones sharply varies. Since the illuminance on the screen 80 is 
proportionate to the amount obtained by substracting the total area of the 
dark zone from the area of the iris 39, there appears sharp change in the 
illuminance near the center of the screen, which is seen as a bright line. 
The bright line is prominently seen with the green light, but it scarely 
matters with the rays of red and blue. This is attributed to the fact that 
the optical path length from the intersecting part 29 of the component 
color separator 5 to each of the liquid crystal panels 15, 16, 17 is short 
in the green light, and long in the red and blue lights. That is to say, 
it is because of the fact that, when the dark zones of red and blue lights 
are observed from the area near the center 81 of the screen 80, the dark 
zones of the red and blue lights are seen at a distance farther than the 
dark zone of the green light, and for that part the dark zone of the red 
and blue lights are seen finer. 
It is seen from the above that, in order to make the bright line less 
prominent, it is better to make at least one dark zone finer than the 
other dark zone. Thus, it becomes impossible to use a flat plate type 
dichroic mirror made by intersecting in X-letter form with both the 
component color separator 5 and the light combiner 18, or a prism type 
dichroic mirror in which the invalid area of the intersecting part is not 
fine. In order to make one of the two dark zones 40, 41 finer, it is 
conceivable to make at least one of the component color separator 5 or a 
light combiner 18 into extremely thin flat type dichroic mirror, but such 
dichroic mirror involves many problems from the points of mechanical 
strength and processability. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a compact and low-priced 
projection display apparatus by settling the problems of bright line by a 
low-priced method. 
A projection display apparatus of the present invention comprises a light 
source for irradiating a light containing three primary color components, 
a color separating means for separating the light from said light source 
into three primary color lights, three light valves for modulating each 
color light from said color separating means, a light combiner means for 
combining the outputted lights from the respective light valves into one 
by dichroic reflection surfaces intersecting in X-letter form, a 
projection lens for projecting on a screen an optical image of the 
outputted light from said light combiner means, and a driving circuit for 
driving each of the light valves according to an image signal, wherein 
said component color separator means is made by intersecting in X-letter 
form a first flat plate type dichroic mirror with a second flat plate type 
dichroic mirror, and a light path length from the intersecting part to 
each of the light valves is so made that a color light which advances 
straightly through said color separator means is the shortest, said second 
dichroic mirror being constructed by two small dichroic mirrors separated 
at said intersecting part so that the dichroic reflecting surfaces of said 
two small type dichroic mirrors are placed on mutually different planes to 
allow the two edge surfaces of said two small type dichroic mirrors 
forming said intersecting part to be seen at least partly overlapping. 
When the two small type dichroic mirrors of the color separator means are 
constructed as mentioned above, the width of the invalid area of the color 
light which advances straightly through the color separator means becomes 
finer than the case where the reflecting surfaces of the two small type 
dichroic mirrors are each on the same plane. Accordingly, with respect to 
the color light which advances straightly through the color separator, the 
dark zone corresponding to the intersecting part of the color separator 
becomes finer when the iris of the projection lens is observed, and the 
steepness of the variation of the total area of the dark line is relaxed, 
so that the problem of bright line is improved. 
As the light of the color reflected by the first dichroic mirror of the 
color separator means is free from the effect of the other small type 
dichroic mirror due to the use of one of the two small type dichroic 
mirrors, the problem of the bright line may be neglected as heretofore. 
In the light of the color reflected by the second dichroic mirror of the 
color separator means, due to the fact that each of the dichroic 
rerflection surfaces of the two small type dichroic mirrors is not on the 
same plane, the width of the invalid area becomes coarse to about two-fold 
that of the conventional one, but because of the long distance from the 
intersecting part of the color separating means to the projection lens, 
the problem of the bright line may be neglected as heretofore. 
In the above manner, the problems of bright line are improved. 
These features and effects of the present invention will be more clarified 
by reference to the later-describd embodiments and the appended drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a construction of the optical system in one embodiment of the 
present invention, wherein the parts denoted by numerals are respectively, 
as follows: 47--component color separator, 48, 51, 54--light valves, 
55--light combiner, 56--projection lens, 62, 63, 64--dichroic mirrors, 
90--driving circuit, and 93--screen. 
The light source 42 is constructed by a lamp 43, a condenser lens 44, a 
concave mirror 45, and a heat absorbing filter 46. The lamp 43 radiates a 
light including the color components of three primary colors of red, 
green, and blue. The light radiated from the lamp 43 is converted into 
nearly parallel rays by means of the condenser lens 44 and the concave 
mirror 45, and outputted through the heat absorbing filter 46. The light 
outputted from the light source 42 is incident on the component color 
separator 47 and separated into the rays of red, green, and blue. The blue 
light as such advances straightly and is incident on the green light valve 
48. The red light is subjected to bend of its light path by means of the 
two plane mirrors 49, 50, and then incident on the red light valve 51. The 
blue light is subjected to bend of its light path by means of the two 
plane mirrors 52, 53, and then incident on the blue light valve 54. The 
light valves 48, 51, 54 are the passing type liquid crystal panels each 
having matrix electrodes. The driving circuit 90 outputs electric signals 
R, G, B for controlling the transmittances of image elements of respective 
light valves 48, 51, 54, respectively, according to the image signal Y, 
and the light valves 48, 51, and 54 modulate the respective incident 
lights. The output lights from the light valves 48, 51, 54 are combined 
into a single form by means of the light combiner 55, and substantially a 
color image is combined at the position of the light valve 48. This color 
image is projected onto the screen 93 by means of the projection lens 56. 
The light combiner 55 is disposed by rectangularly intersecting in X-letter 
form the flat plate type red reflection dichroic mirror 57 and the flat 
plate type blue reflection dichroic mirrors 58, 59 which are split into 
two parts. The blue reflection dichroic mirrors 58, 59 have the same 
optical characteristics, and the dichroic reflection surfaces 60, 61 are 
on the same plane. 
The component color separator 47 is, as shown by enlargement in FIG. 2, 
disposed by rectangularly intersecting in X-letter form the flat plate 
type red reflection dichroic mirror 62 and the small sized flat plate type 
blue reflection dichroic mirrors 63, 64 which are separated into two 
parts. The small sized dichroic mirrors 63, 64 have the same thicknesses 
and the same optical characteristics. When the output light 94 from the 
light source 42 is inputted into the component color separator 47, the red 
light 95, green light 96, and blue light 97 are outputted in division into 
three directions. What should be noted here is that the two dichroic 
reflection surfaces 65, 66 are on the planes which are mutually different 
from each other. This is a great characteristic of the construction of the 
present invention. And, it is so made that, viewed from the component 
color separator 47 from the direction of emission of the green light 96 as 
shown in FIG. 2, the edge surface 67 and the edge surface 68 are seen as 
if they are completely overlapped. Further, the conventional component 
color separator 5 as shown in FIG. 6 is seen, as shown in FIG. 3 (b), that 
the edge surface 34 and the edge surface 35 are in adjacent relations. 
Hereinafter the activity of the component color separator 47 as shown in 
FIG. 2 is explained. FIG. 4 shows the state where parallel lights are 
allowed to be incident on the component color separator 47 which has been 
constructed into the desirable condition. To simplify explanation, it is 
assumed that the thicknesses of each of the dichroic mirrors 62, 63, 64 
are all equal. In the same manner as in the case of the conventional 
component color separator as shown in FIG. 7, there are produced the 
invalid areas 73, 74, 75 on respective output lights by means of the edge 
surfaces 67, 68. Assuming the width of the invalid area 73 of the red 
light to be d.sub.R, that of the invalid area 74 of the green light to be 
d.sub.G, and that of the invalid area 75 of the blue light to be d.sub.B, 
the following equation is obtained: 
EQU d.sub.R :d.sub.G :d.sub.B =a:a:2a (1) 
Further, assuming the thicknesses of the dichroic mirrors 62, 63, 64 to be 
t each, and the refractive index of their glass substrates to be 1.5, then 
the relation will be: 
EQU a.apprxeq.t/3 (2) 
In the case of the conventional color separator means as shown in FIG. 7, 
the relation will be: 
EQU d.sub.R :d.sub.G :d.sub.B =a:2a:a (3) 
Assuming that the thicknesses of the dichroic mirrors are to be equal, and 
comparing the widths of the invalid areas of each color light between FIG. 
4 and FIG. 7, it is seen that the red light is the same, the green light 
is half, and the blue light is two-fold. Although the blue light shows 
double as much width of the invalid area 75, because the blue light has a 
long optical path from the intersecting part 76 to the projection lens 56, 
the problem of bright line may be neglected as in the conventional case. 
From the above, it is seen that the width of the invalid area of the color 
light which advances straightly in the component color separator 47 which 
has a dominant position on the problem of bright line is made half the 
value of that of the conventional ones. And, it is seen that, when the 
iris of the projection lens 56 is observed, the dark zone corresponding to 
the intersecting part 76 of the component color separator 47 becomes half 
the conventional case, and so much the conspicuousness of the bright line 
is improved. The effect of the present invention can be explained from the 
comparison between FIG. 3 (a) with FIG. 3 (b). Even by comparing the 
widths of the dark zones actually observable through the projection lens, 
the width of the dark zone of the component color separator 47 as shown in 
FIG. 4 is seen as being about half the width of the dark zone of the 
conventional component color separator 5 as shown in FIG. 7. 
In order to obtain the maximum effect with the present invention, attention 
may be paid to the following points: That is to say, the edge surfaces 67, 
68 are respectively in planes at right angles to the dichroic reflection 
surfaces 65, 66, and the ridges 69, 70, 71, 72 of the edge surfaces 67, 68 
have sharp edges. The edge surfaces 67, 68 are brought into contact with 
the red reflecting dichroic mirror 62. These steps are taken in order to 
make the widths of the invalid areas by the edge surfaces 67, 68 as fine 
as possible. In order to prevent the color lights reflected by the blue 
color reflecting small type dichroic mirrors 63, 64 from spreading before 
arriving at the light valve 54, it is recommended to keep the dichroic 
reflection surfaces 65, 66 of the small type dichroic mirrors 63, 64 
parallel to each other. 
Whether or not the two small type dichroic mirrors 63, 64 are disposed most 
optimally may be examined in the following manner: A white light source is 
disposed on the input light 94 side of the component color separator 47 
and the component color separator 47 is observed from the direction of 
emission of the green light 96. Alternatively, using an optical system in 
which the light combining means 55 is excluded from the optical system as 
shown in FIG. 1, the intersecting part 76 of the component color separator 
47 is observed through the projection lens 56. And, when the intersecting 
part 76 is observed while moving the eyes at the position corresponding to 
the optical axis 91 of the projection lens 56, the coarseness of the dark 
zone may become the smallest. Alternatively, the ridge lines 69, 71 at the 
ends of the dichroic reflecting surfaces 65, 66 of the small type dichroic 
mirrors 63, 64 may be allowed to coincide with each other. When the 
intersecting part 76 of the component color separator 47 is observed in a 
bright room, if the two small type dichroic mirrors 63, 64 are not 
optimally disposed, there can be seen the band of the color different from 
the effective area at the end of the invalid area of the intersecting part 
76. Accordingly, it may be so practised that the intersecting part 76 of 
the component color separator 47 is observed from the position 
corresponding to the optical axis 91 of the projection lens 56 to examine 
whether or not the color band is seen at the end of the invalid area or 
the width of the color band is sufficiently fine in comparison with the 
width of the dark zone. 
Next, explanation will be given using the example of substantial numerical 
values. 
In case of the indicated sizes of the respective light valves 48, 51, 54 
being 40 mm.times.60 mm, the projection lens 56 having a focal length of 
150 mm and F-number of 2.5, the thickness of each of the dichroic mirrors 
62, 63, 64 of the component color separator 47 being 0.55 mm, and the 
thickness of each of the dichroic mirrors 57, 58, 59 of the light combiner 
means 55 being 1.1 mm, the bright line was conspicuous under the 
conventional construction as shown in FIG. 6, but the bright line was 
improved to a practically acceptable degree when the construction as shown 
in FIG. 1 was adopted. In case of the thickness of the dichroic mirror 
being 0.55 mm, there was no problem with mechanical stength or 
processability. The appearance of the bright line in this case was well 
resembling to the condition where the thickness of each of the dichroic 
mirrors 6, 7, 8 was 0.3 mm in the conventional construction as shown in 
FIG. 6. However, when the thickness of the dichroic mirror was 0.3 mm, 
there were problems with mechanical strength and processability, which 
could not be adopted. As such, according to the present invention, it is 
possible to make the dichroic mirror to have such a thickness as is free 
from the apprehension of mechanical strength and processability and yet to 
make the bright line less conspicuous. While the thicknesses of the 
dichroic mirrors 63, 64 are made thinner than those of the dichroic 
mirrors 57, 58, 59 of the light combiner 55, it is in order to make the 
width of the dark zone corresponding to the component color separator 47 
finer than the dark zone corresponding to the light combiner 55 so as to 
make the bright line less conspicuous. 
Next, explanation will be made on other embodiment of the present 
invention. 
While it was stated in the foregoing embodiment that the small type 
dichroic mirrors 63, 64 of the component color separator were made to have 
the edge surface 67 and the edge surface 68 to be completely overlapped 
when the component color separator 47 is viewed from the output side in 
the direction along the inputted light, the condition may be such that the 
edge surface 67 and the edge surface 68 overlap at least partially, by 
which the bright line can be made less conspicuous to a practically 
negligible degree in comparison with the conventional component color 
separator 5 shown in FIG. 7. 
With the construction as shown in FIG. 1, it may be possible to make the 
thickness of the undivided dichroic mirror 62 of the component color 
separator 47 thicker than the thicknesses of the small type dichroic 
mirrors 63, 64. For instance, even when, in the aforementioned examples of 
numerical values, the thickness of the red reflection dichroic mirror 62 
only was altered to 1.1 mm, twice as large thickness, the bright line was 
scarcely conspicuous. The state of the case where parallel rays are 
incident on such component color separator is shown in FIG. 5. Compared 
with the aforementioned example of numerical values, the widths of the 
invalid ranges of the red and green lights are the same, and the width of 
the invalid area of the blue light is coarse as being twofold, but, 
because of the long light path from the intersecting part 76 to the light 
valve 54 and the low visual sensitivity of blue light in comparison with 
the light of other colors, the bright line is less conspicuous. As it is 
possible to make the red reflection dichroic mirror 62 of large area 
thick, this method is advantageous in the points of mechanical strength 
and processability. 
In the embodiment shown in FIG. 1, liquid crystal panels were used as the 
light valves 48, 51, 54. But, there may be used all light valves for 
forming the optical images proportionate to the image signal as a 
variation of optical characteristic such as a light valve using 
electro-optical crystal. 
As a light combiner 55 of a construction as shown in FIG. 1, there may be 
used a prism type dichroic mirror in which the dichroic reflection 
surfaces intersect in X-letter form. For example, in the projection 
display apparatus of high resolution, the planeness of the dichroic 
reflection surface is made severer, so that there may be cases where the 
prism type dichroic mirror is preferred to the flat plate type dichroic 
mirror. In this case, due to the finer invalid range of the intersecting 
part 76 of the component color separator 47, there arises no problem of 
bright line even if the invalid area of the intersecting part of the light 
combiner 55 is coarse. As the prism type dichroic mirror having no fine 
invalid area of the intersecting part can be manufactured at relatively 
low cost, it is possible to expect lowering of the cost of the apparatus. 
In the construction as shown in FIG. 1, the light of the color which 
advances straightly through the component color separator 47 was selected 
as green light, but it may be red or blue light. In this case, the 
characteristics of the dichroic multi-layered film to be used for the 
component color separator 47 and the light combiner 55 may be modified. 
The undivided dichroic mirror 62 of the component color separator 47 in 
FIG. 1 may be divided into two parts at the intersecting part 76. It is 
necessary for the dichroic reflecting surface separated into two parts to 
be on the same plane, and the two edge surfaces formed by division into 
two parts to be on the position not to interfere with the effective light. 
In either case, there can be obtained the action and effect similar to 
those of the above embodiments.