Polarization illumination apparatus and projector using the apparatus

A polarization illumination apparatus is constituted by only a light source, light condensing means having a reflection mirror arranged behind the light source, polarization beam splitting means for splitting light emerging from the light condensing means into first and second polarized light components having different directions of polarization, and return means for returning the first polarized light component to the light condensing means. The direction of polarization of the first polarized light component is changed in such a manner that the first polarized light component from the return means is obliquely incident on and reflected by a mirror surface of the reflection mirror. Therefore, a polarization illumination apparatus which can modulate the vibration direction of polarized light without using an optical phase plate, and has high light utilization efficiency, and a projector using the apparatus can be realized.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a polarization illumination apparatus and 
a projector having the polarization illumination apparatus. 
2. Related Background Art 
FIG. 1 is a view showing an arrangement of main part of a conventional 
projector using a polarization illumination apparatus. The projector 
includes a light source 1, a reflection mirror 2, a polarization beam 
splitter 6, a polarization beam splitting film 20, a liquid crystal light 
valve 7, and a quarter-wave optical phase plate 13. In FIG. 1, the end 
portions of two polarization beam splitting films 20.sub.1 and 20.sub.2 
are in contact with each other to form an angle of about 90.degree. 
therebetween. Random light Ao emitted from the light source 1 is converted 
into substantially parallel light (i.e., light substantially parallel to 
the optical axis) by the reflection mirror 2, and the parallel light is 
incident on the first polarization beam splitting film 20.sub.1. In this 
case, p-polarized light Ap.sub.1 is transmitted through the film 20.sub.1, 
and s-polarized light As is reflected by the film 20.sub.1. The 
S-polarized light As is further reflected by the second polarization beam 
splitting film 20.sub.2 arranged along the optical path, and is then 
converted into circularly polarized light Ar via the quarter-wave optical 
phase plate 13 whose optical axis is set in a desired direction. The 
circularly polarized light Ar is transmitted through the quarter-wave 
optical phase plate 13 again via the light source 1 and the reflection 
mirror 2, and is converted into light Ap.sub.2 including p-polarized 
light. The light Ap.sub.2 is transmitted through the polarization beam 
splitting film 20.sub.1, and is then incident on the liquid crystal light 
valve 7. 
In this projector, the two polarized light components, i.e., the 
p-polarized light Ap and the s-polarized light split by the polarization 
beam splitting film 20.sub.1 or 20.sub.2 are converted to polarized light 
components having the same direction of polarization so as to illuminate 
the liquid crystal light valve 7. With this projector, light utilization 
efficiency can be improved as compared to a projector using no 
polarization illumination apparatus. 
However, since the conventional projector uses the optical phase plate, the 
light amount is undesirably decreased due to absorption or reflection when 
light is transmitted through the plate. A conventional plastic optical 
phase plate has a light transmittance of about 90%. In particular, since a 
light beam, which returns to the light source 1, and then emerges 
therefrom, is transmitted through the optical phase plate a total of three 
times, the light amount is considerably decreased. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a polarization 
illumination apparatus, which can modulate a vibration direction, i.e., a 
direction of polarization of polarized light without using an optical 
phase plate, and has high light utilization efficiency, and a projector 
using the polarization illumination apparatus. 
According to the present invention, the polarization illumination apparatus 
comprises a light source, light condensing means including a reflection 
mirror arranged behind the light source, polarization beam splitting means 
for splitting light from the light condensing means into first and second 
polarized light components having different directions of polarization, 
and return means for returning the first polarized light component to the 
light condensing means. The first polarized light component returning from 
the return means is obliquely incident on and reflected by the reflection 
mirror of the light condensing means, thereby modulating the direction of 
polarization of the first polarized light component, and outputting the 
modulated first polarized light component.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 shows an embodiment of the present invention. 
In FIG. 2, an apparatus of this embodiment includes a metal halide lamp 1 
serving as a light source, a reflection mirror 2 serving as a light 
condensing means, and having a shape of a paraboloid of revolution, a 
polarization beam splitting (to be abbreviated as PBS hereinafter) film 
20, rectangular prisms 21a and 21b, and a plane reflection mirror 22. A 
polarization beam splitting means is constituted by the PBS film 20, the 
rectangular prisms 21a and 21b, and the plane reflection mirror 22. The 
light source 1 is arranged at a focal point position 2c of the parabolic 
mirror 2, thus obtaining a substantially parallel beam. The substantially 
parallel beam is split into two linearly polarized light components by a 
polarization beam splitter. One polarized light component is 
perpendicularly reflected by the plane reflection mirror 22 arranged on 
one outgoing face of the polarization beam splitter, and is converted into 
light (to be referred to as return light hereinafter) returning to the 
parabolic mirror 2. 
A process for obtaining linearly polarized light from substantially 
parallel light as indefinitely polarized light will be described in detail 
below. 
A ray 1.sub.10 emitted from the center of the light source arranged at the 
focal point 2c of the parabolic mirror 2 is converted into a parallel ray 
1.sub.11 (i.e., a ray parallel to the optical axis) by the parabolic 
mirror 2, and the parallel ray 1.sub.11 is incident on the prism 21b. At 
this time, the ray 1.sub.11 is natural light whose direction of 
polarization is indefinite. The ray 1.sub.11 which has reached the PBS 
film is subjected to the polarization beam splitting effect, and is split 
into a pair of linearly polarized light components 1.sub.11p and 1.sub.11s 
having different directions of polarization. The light component 1.sub.11p 
is normally called p-polarized light since its direction of polarization, 
i.e., the vibration direction of polarization is parallel to the plane of 
drawing. The light component 1.sub.11p is transmitted through the PBS film 
20, propagates through the prism 21a, and emerges as polarized 
illumination light from the outgoing face at the side opposite to the 
incident face of the ray 1.sub.11. 
On the other hand, the light component 1.sub.11s is normally called 
s-polarized light since its vibration direction of polarization is 
perpendicular to the plane of drawing. The light component 1.sub.11s is 
perpendicularly reflected by the PBS film 20, and propagates through the 
prism 21b toward the plane reflection mirror 22. Since the plane 
reflection mirror 22 is arranged perpendicularly to the propagation 
direction of the s-polarized light 1.sub.11s, the s-polarized light 
1.sub.11s changes its propagation direction through 180.degree., and then 
propagates toward the PBS film 20 again. Since the light component 
1.sub.11s is s-polarized light, it is reflected by the PBS film 20 again, 
and returns as return light 1.sub.11s ' along the same optical path (FIG. 
2 illustrates distinguishing optical paths before and after reflection for 
the sake of easy understanding) in the opposite direction. Since the 
return light 1.sub.11s ' is parallel light, it is reflected at a point 2a 
on the parabolic mirror 2, and returns to the light source 1 located at 
the focal point 2c. 
The return light 1.sub.11s ' propagates toward the parabolic mirror 2 again 
as if it were light emitted from the light source 1, and is reflected at a 
point 2b to be converted into a parallelray 1.sub.12. The parallel ray 
1.sub.12 is re-incident on the prism 21b. The polarization state of the 
ray 1.sub.12 is considerably disturbed for a reason to be described later, 
and the ray 1.sub.12 is split into a pair of linearly polarized light 
components 1.sub.12p and 1.sub.12s by the PBS film 20. Since the light 
component 1.sub.12p is p-polarized light as in the light component 
1.sub.11p, it is transmitted through the PBS film, propagates through the 
prism 21a, and emerges from the prism as polarized illumination light. On 
the other hand, the light component 1.sub.12s behaves as return light 
which returns to the light source via the plane reflection mirror 22 as in 
the light component 1.sub.11s. Upon repetition of the above-mentioned 
process, all natural light components can be converted into polarized 
light components in principle. 
A change in polarization state of linearly polarized light after it is 
split by the polarization beam splitting means and returns to the light 
condensing means will be described below with reference to FIGS. 3 and 4. 
In FIG. 3, return light 1.sub.21 is reflected at the two points 2a and 2b 
on the parabolic mirror 2, and emerges as rays 1.sub.22 and 1.sub.23. In 
FIG. 3, 2c represents the focal point of the parabolic mirror 2, Ga and Gb 
represent the tangent planes of the points 2a and 2b, and Ha and Hb 
represent the normals to the points 2a and 2b. 
The vibration direction (polarizing direction) of the return light 1.sub.21 
is inclined by .alpha..sub.i with respect to the plane of drawing. 
.alpha..sub.i is called an azimuth, and assumes a positive value when the 
vibration direction rotates clockwise with respect to the propagation 
direction of light. FIG. 4 is a view for explaining the azimuth 
.alpha..sub.i when the opening of the reflection mirror 2 shown in FIG. 2 
is viewed from the direction of the PBS film. In FIG. 4, a plane including 
m.sub.1 is parallel to the plane of drawing of FIG. 2, and a plane 
including m.sub.2 is perpendicular to the plane of drawing of FIG. 2. In 
FIG. 4, a double-headed arrow indicates-the vibration direction of the 
return light 1.sub.21. FIG. 3 is a sectional view taken along m.sub.3 
rotated from this vibration direction by .alpha..sub.i. 
As described above, the return light 1.sub.21 is linearly polarized light. 
When linearly polarized light is reflected by a given boundary surface 
(e.g., reflected at the point 2a in FIG. 3), we have: 
EQU tan.alpha..sub.r =-tan.alpha..sub.i {cos(.theta..sub.i 
-x)/cos(.theta..sub.i +x)} (1) 
EQU for sinx=sin.theta..sub.i /n (2) 
where .alpha..sub.i is the azimuth (an angle defined between the vibration 
direction and the plane m.sub.3, as shown in FIG. 4, will be referred to 
as an azimuth hereinafter) upon incidence of the light 1.sub.21, 
.alpha..sub.r is the azimuth upon reflection of the light 1.sub.21, n is 
the refractive index of the parabolic mirror 2, and .theta..sub.i is the 
incident angle with respect to the planes Ga and Gb ("Principle of Optics, 
Tokai Univ. Press"). 
The refractive index n assumes a real number when the parabolic mirror 2 is 
a dielectric, and a complex number when it is a conductor. The way of the 
change in polarization state after reflection depends on the refractive 
index n. 
A case will be described below wherein the parabolic mirror 2 is a 
dielectric, i.e., the refractive index n assumes a real number. n is the 
refractive index of a medium of the parabolic mirror 2 when viewed from a 
medium (air in FIG. 3) where the light 1.sub.21 propagates, and satisfies 
n&gt;1. For this reason, x is also expressed by a real number from formula 
(2), and .alpha..sub.r is expressed by a real number, too, from formula 
(1). Therefore, from formula (1), the vibration direction rotates in a 
direction to separate from the plane m.sub.3 ("Principle of Optics", p. 
71). As can be understood from formulas (1) and (2), .alpha..sub.r at that 
time varies depending on .theta..sub.i. Also, as can be seen from FIG. 3, 
since .theta..sub.i satisfies 0&lt;.theta..sub.i &lt;.theta..sub.imax 
(.theta..sub.imax is the angle when the point 2a is located at the end 
portion of the parabolic mirror 2), and assumes every possible value, the 
azimuth of the light 1.sub.21 also assumes every possible value. 
In reflection at the point 2b, the incident angle assumes every possible 
value while satisfying: 
EQU .theta..sub.ib =.pi./2-.theta..sub.ia (3) 
Consequently, the azimuth of the light 1.sub.23 can assume every possible 
value. Therefore, the vibration directions of outgoing light components 
including the light 1.sub.23 propagating parallel to the plane of drawing 
of FIG. 3 are various, and the outgoing light components are in a 
non-polarized state as a whole. Since the mirror 2 has a shape of a 
paraboloid of revolution, the azimuth .alpha..sub.i of the light 1.sub.21 
naturally assumes every possible value within a range of 
-.pi./2&lt;.alpha..sub.i&lt;.pi./ 2. From this viewpoint, the polarization state 
of the outgoing light from the parabolic mirror 2 is disturbed. 
Therefore, the light 1.sub.23 reflected by the parabolic mirror 2 is split 
into a pair of different linearly polarized light components again by the 
PBS film 20 shown in FIG. 2. 
On the other hand, when the parabolic mirror 2 is a conductor, i.e., when 
the refractive index n assumes a complex number, the case is different. 
Since n assumes a complex number, x also assumes a complex number from 
formula (2). Therefore, since .alpha..sub.r assumes a complex number from 
formula (1), a phase shift occurs, and the reflected light 1.sub.22 is 
generally converted to elliptically polarized light ("Principle of 
Optics", p. 911). 
One of necessary conditions that the light 1.sub.22 converted into 
elliptically polarized light is converted into linearly polarized light 
again upon reflection at the next reflection point 2b is given by: 
EQU .theta..sub.ia =.theta..sub.ra =.theta..sub.ib (4) 
where .theta..sub.ra is the reflection angle at the point 2a. This is 
because when linearly polarized light which is incident on a given 
interface at an incident angle .theta..sub.i is reflected at a reflection 
angle .theta..sub.r (=.theta..sub.i), and is converted into elliptically 
polarized light to have a phase difference .delta., elliptically polarized 
light having a phase difference -.delta. which is incident on a similar 
interface at the incident angle .theta..sub.r (=.theta..sub.i) is 
reflected onto the interface, and is converted into linearly polarized 
light according to the principle of retrogradation of light. That is, in 
order that linearly polarized light and elliptically polarized light 
having a phase difference .delta. therewith retrograde and substitute each 
other upon reflection, they must satisfy at least formula (4). 
Now, since .theta..sub.ia +.theta..sub.ib =.pi./2, formula (4) is not 
satisfied in general. Even though formula (4) is satisfied, the light 
1.sub.23 cannot be converted into linearly polarized light having the same 
vibration direction as that of the light 1.sub.21 unless a phase 
difference of .pi./2 is generated at each of the reflection points 2a and 
2b of the parabolic mirror 2 shown in FIG. 3. Therefore, since it can be 
considered that the light 1.sub.23 can never be linearly polarized light, 
it is obvious that the light 1.sub.23 is split into a pair of linearly 
polarized light components by the PBS film 20 shown in FIG. 2. 
When the azimuth .alpha..sub.i =0 or .pi./2, the vibration directions of 
the light 1.sub.21 and the light 1.sub.23 coincide with each other. In 
other words, the vibration direction is not rotated even via the parabolic 
mirror 2, and the light is always incident on the PBS film as s-polarized 
film. Therefore, the light reciprocally propagates between the plane 
reflection mirror 22 and the parabolic mirror 2 via the PBS film 20, and 
does not serve as illumination light. 
Upon reflection on a dielectric reflection surface, the rotation (change in 
.alpha.) of the vibration direction is small, and the number of 
p-polarized light components of the light 1.sub.23 with respect to the PBS 
film 20 may become smaller than the number of s-polarized light component. 
Similarly, upon reflection on a conductor reflection surface, a small 
phase difference is generated, and the light 1.sub.23 is converted into 
elliptically polarized light close to linearly polarized light. As a 
result, the number of p-polarized light components with respect to the PBS 
film 20 may become smaller than the number of s-polarized light 
components. In such cases, in order to positively disturb the vibration 
direction of the return light 1.sub.21, the light source lamp preferably 
has a diffusion surface. 
The diffusion surface is normally considered as a state wherein a large 
number of very small prisms cover a surface. Since a prism normally has a 
very small capacity for changing the vibration direction of polarized 
light, the diffusion surface also has a very small capacity for changing 
the vibration direction of polarized light. However, the diffusion surface 
can change the propagation direction of light. For example, as shown in 
FIG. 5, polarized light 1.sub.22 having a given vibration direction is 
diffused by the surface of the light source 1, and is split into polarized 
light components 1.sub.221, 1.sub.222, 1.sub.223, and 1.sub.224. When 
these light components are incident on various points on the parabolic 
mirror 2 at various incident angles and are reflected at various azimuths, 
different polarized light components 1.sub.231, 1.sub.232, and 1.sub.233 
can be obtained in addition to polarized light 1.sub.23, thus further 
disturbing the vibration direction of polarized light. 
The reason why the diffusion surface is arranged on or near the lamp 
surface is that light diffused by the diffusion surface is caused to 
behave as if it were emitted from the light source 1. If the diffusion 
surface is arranged at another place, substantially parallel light cannot 
be obtained by the light condensing means. 
The PBS film of this embodiment is normally formed of a dielectric 
multi-layered film, and is adhered sandwiching between the two rectangular 
prisms 21a and 21b, thus constituting a so-called polarization beam 
splitter. The PBS film is designed to split light having an incident angle 
of 45.degree. into p- and s-polarized light components. A polarization 
beam splitter corresponding to another incident angle can be manufactured 
as long as the incident angle is not considerably shifted from 45.degree.. 
As the plane reflection mirror 22, a mirror prepared by depositing aluminum 
on a flat glass plate is normally used. In order to increase the 
reflectance, a coating for increasing the reflectance may be formed on the 
mirror. The mirror 22 must be arranged, so that its reflection surface 
extends perpendicular to the propagation direction of s-polarized light 
split splitted by the polarization beam splitter. 
The parabolic mirror 2 is preferably formed by a cold mirror to prevent a 
temperature rise of an object to be illuminated since the light source 1 
emits a large amount of infrared light. In this embodiment, the reflection 
mirror having a shape of a paraboloid of revolution, whose sectional shape 
can be normally expressed by y=ax.sup.2, is used. Alternatively, a mirror 
having a substantially paraboloidal shape whose sectional shape can be 
expressed by y=ax.sup.2 +bx.sup.4 +cx.sup.6 (a&lt;&lt;b, c) may be used to 
further improve light condensing characteristics in correspondence with 
the light emission characteristics of the light source. Also, a light 
condensing means as a combination of an elliptic reflection mirror and a 
refraction element may be used. In addition, any other light condensing 
means such as a light condensing means constituted by a plurality of 
mirror surfaces to improve the light condensing rate, a light condensing 
means for adjusting the sectional shape of a beam emerging from the light 
condensing means using an auxiliary mirror, a light condensing means 
adopting an aspherical surface, and the like may be adopted as long as 
substantially parallel light is obtained. In this case, if the reflection 
loss is small, the number of times of reflection of light by the 
reflection mirror is preferably as large as possible since the vibration 
direction of polarized light can be more disturbed. 
In the polarization illumination apparatus of the present invention, since 
return light propagating toward the reflection mirror of the light 
condensing means is not transmitted through a quarter-wave optical phase 
plate unlike in the prior art, it remains linearly polarized light. 
Therefore, the return light can be split into p- and s-polarized light 
components by the PBS film 20 again owing to the following two effects: 
1. rotation of the vibration direction of linearly polarized light upon 
reflection; and 
2. modulation from linearly polarized light into elliptically polarized 
light due to a phase shift upon reflection when the reflection mirror is 
formed of a conductor. 
Other embodiments of a polarization illumination apparatus according to the 
present invention will described below. 
In an embodiment shown in FIG. 6A, p-polarized light split by the 
polarization beam splitter formed by the prisms 21a and 21b and the PBS 
film 20 is caused to return to the light condensing means, i.e., the 
parabolic mirror 2 by the reflection mirror 22 unlike in the first 
embodiment wherein s-polarized light returns to the light condensing 
means. 
In embodiments shown in FIGS. 6B and 6C, the PBS film 20 is adhered between 
triangular prisms 21c and 21d each having a right-angled triangular shape. 
In FIG. 6B, light incident on the prism 21c via the light condensing means 
(parabolic mirror 2) reaches the PBS film 20 or a total reflection surface 
c. A light component which has reached the PBS film 20 is split into s- 
and p-polarized light components by the PBS film 20. The p-polarized light 
component emerges from the outgoing face of the prism 21d, and the 
s-polarized light component is reflected by the PBS film and reaches the 
total reflection surface c. The s-polarized light component is reflected 
by the total reflection surface c to be converted into return light, which 
returns to the light condensing means. 
On the other hand, of light components incident on the prism 21c, a light 
component which has reached the total reflection surface c is reflected by 
the surface c, and propagates toward the PBS film 20. The light component 
is split into s- and p-polarized light components by the PBS film 20, and 
the s-polarized light component becomes return light, which returns to the 
light condensing means. The p-polarized light component is reflected by a 
total reflection surface d, and emerges from the outgoing face. 
In FIG. 6C, light incident on the prism 21c via the light condensing means 
(parabolic mirror 2) reaches the PBS film 20 or the total reflection 
surface c. The light which has reached the PBS film 20 is split into s- 
and p-polarized light components by the PBS film 20. The p-polarized light 
component is reflected by the total reflection surface d, and then emerges 
from the outgoing face. The s-polarized light component is reflected by 
the PBS film, and reaches the total reflection surface c. The s-polarized 
light component is further reflected by the surface c, and becomes return 
light, which returns to the light condensing means. 
On the other hand, of light components incident on the prism 21c, a light 
component which has reached the total reflection surface c is reflected by 
the surface c, and propagates toward the PBS film 20. The light component 
is split into s- and p-polarized light components by the PBS film. The 
s-polarized light component is reflected, and becomes return light, which 
returns to the light condensing means. The p-polarized light component 
emerges from the outgoing face of the prism 21d. 
In an embodiment shown in FIG. 7, three triangular prisms 21e, 21f, and 21g 
each having a right-angled triangular section are combined, as shown in 
FIG. 7, and PBS films 20a and 20b are arranged on the boundary surfaces of 
these prisms. 
Light incident through the parabolic mirror 2 is transmitted through the 
prism 21e, and is split into pand s-polarized light components by the PBS 
film 20a or 20b. The p-polarized light component directly emerges from the 
outgoing face. The s-polarized light component is reflected by the other 
PBS film, and becomes return light, which returns to the parabolic mirror 
2. 
In the embodiments shown in FIGS. 6B, 6C, and 7, since a portion formed by 
the prisms has a volume about half that of the embodiments shown in FIGS. 
2 and 6A, a compact, low-cost polarization illumination apparatus can be 
realized. 
In embodiments shown in FIGS. 8A and 8B, a glass plate layer 23 is used in 
place of the prisms and the PBS film used in the above-mentioned 
embodiments. Since a glass plate has polarization beam splitting 
characteristics as long as a Brewster angle .theta..sub.i is maintained, a 
polarization beam splitting means can be constituted by stacking a 
plurality of glass plates without forming a PBS film. As the number of 
glass plates to be stacked is increased, the polarization beam splitting 
characteristics can be improved but the transmittance may be decreased. 
Thus, a PBS film formed of a dielectric multi-layered film may be arranged 
between the glass plates, as needed. The plane reflection mirror 22 is 
arranged perpendicularly to light reflected by the glass plate layer so 
that light reflected by the glass plate layer 23 returns along the same 
optical path. 
FIG. 8B shows an application illustration of FIG. 8A. In FIG. 8B, two sets 
of the glass plate layers 23 and the plane reflection mirrors 22 are 
arranged to have the optical axis of the light condensing means as an axis 
of symmetry. 
In the embodiments shown in FIGS. 8A and 8B, a lightweight, low-cost 
apparatus can be realized as compared to the embodiments using the prisms. 
In the embodiment shown in FIG. 8B, although the number of components is 
larger than that of the embodiment shown in FIG. 8A, the apparatus can be 
rendered compact. 
In the embodiments shown in FIGS. 6A to 8B, the polarization state of the 
return light returning to the light condensing means is disturbed since 
the light is reflected by the parabolic mirror 2 as the light condensing 
means, and the light emerges from the light condensing means again. In 
these embodiments, the light source 1 may also have a diffusion surface. 
FIG. 9 shows still another embodiment, which has two sets of light sources 
and light condensing means. Of light emerging from one light condensing 
means, s-polarized light reflected by the PBS film 20 propagates toward 
the other light condensing means. This embodiment also comprises a prism 
21h having a reflection surface 22 for directing p-polarized light 
components, transmitted through the PBS film 20, of light components from 
the two light condensing means in the same direction. 
FIGS. 10A and 10B show other embodiments, each of which has two sets of 
light sources, light condensing means, and PBS films. In FIG. 10A, of 
light emerging from one light condensing means, s-polarized light 
reflected by one PBS film propagates toward the other light condensing 
means via the other PBS film, and of light components emerging from the 
two light condensing means, p-polarized light components transmitted 
through the PBS films emerge in the same direction. 
In FIG. 10B, of light emerging from one light condensing means, p-polarized 
light transmitted through one PBS film propagates toward the other light 
condensing means through the other PBS film, and of light components 
emerging from the two light condensing means, s-polarized light components 
reflected by the PBS films emerge in the same direction. 
In the embodiments shown in FIGS. 9 to 10B, since the two light sources are 
used, the light amount can be greatly increased. Normally, in order to 
increase the light amount, the output of the light source may be simply 
increased. However, when the output of the light source is increased, the 
size of the light-emitting portion of the light source is inevitably 
increased, and parallelness of light via the light condensing means is 
impaired. When an object to be illuminated having angle dependency such as 
a liquid crystal light valve is illuminated, it is a necessary condition 
that illumination light be approximate to parallel light. For this reason, 
it is very preferable if the light amount can be increased without 
increasing the size of the light-emitting portion like in the embodiments 
shown in FIGS. 9 to 10B. 
FIG. 11 shows still another embodiment. In this embodiment, a section 
constituted by prisms in the embodiment shown in FIG. 6B or 7 is rendered 
compact, and a plurality of sections are aligned on the same plane. In 
FIG. 11, these sections include PBS films 20 and PBS films or reflection 
films 20'. 
An embodiment for minimizing a decrease in parallelness of return light 
will be described below. When an apparatus is arranged to scatter return 
light by the surface of a lamp bulb portion, since the return light 
behaves to have the surface of the lamp bulb portion as a secondary light 
source, this is equivalent to an increase in diameter of the lamp, and 
parallelness is decreased. Therefore, in order to minimize a decrease in 
parallelness, a light source, which does not scatter light by the surface 
of the lamp bulb portion, can be used. A xenon lamp can form a smooth bulb 
portion as compared to that of a metal halide lamp. For this reason, the 
xenon lamp allows easy formation of a lamp, in which light incident on the 
lamp bulb portion is not easily scattered. 
FIG. 12 shows an embodiment for preventing return light from being absorbed 
when it passes through the lamp bulb portion and the light-emitting 
portion, and from becoming loss light. 
This embodiment is substantially the same as the embodiment shown in FIG. 
6B, except that the prism 21c is replaced with a prism 21c'. 
In this embodiment, a rectangular prism 21d having a triangular prism shape 
whose section has a right-angled triangular shape, and an acute-angle 
(close to a right angle) prism 21c' (a section including a broken line in 
FIG. 12 has a right-angled triangular shape) having a surface which has 
the same shape and the same area as those of surfaces sandwiching the 
right angle of the rectangular prism 21d are adhered to each other, as 
shown in FIG. 12. The PBS film 20 is provided between the adhered 
surfaces. Note that the PBS film 20 as a multi-layered film may be 
provided to one of the rectangular prism 21d and the acute-angle prism 
21c, and thereafter, these prisms may be adhered to each other, or the PBS 
film 20 may be provided to both the prisms, and thereafter, the prisms may 
be adhered to each other. In addition, aluminum may be deposited on total 
reflection surfaces c and d, as needed. 
In FIG. 12, of parallel light 1.sub.11 converted through the parabolic 
mirror 2, p-polarized light 1.sub.11p is transmitted through the PBS film 
20, and emerges from the rectangular prism 21d. On the other hand, 
s-polarized light 1.sub.11s reflected by the PBS film 20 is reflected by 
the total reflection surface c, and becomes return light 1.sub.11s '. 
Since the PBS film 20 and the total reflection surface c do not form a 
right angle the return light 1.sub.11s ' is not parallel to the parallel 
light 1.sub.11, and propagates toward the parabolic mirror 2. The return 
light 1.sub.11s ' propagates toward the light source 1 arranged at the 
focal point position of the parabolic mirror 2 via the parabolic mirror 2. 
In this case, since the principle of disturbing the vibration direction of 
polarized light upon reflection on the parabolic mirror 2 is the same as 
that in the first embodiment, a detailed description thereof will be 
omitted. As described above, since the return light 1.sub.11s ' is not 
parallel to the parallel light 1.sub.11, it does not accurately propagate 
toward the light source 1 even after it is reflected by the parabolic 
mirror 2, and passes by the light source 1 while avoiding the 
light-emitting portion. The return light 1.sub.11s ', which has passed 
while avoiding the light-emitting portion, i.e., is not absorbed by the 
light-emitting portion, is re-incident on the prism 21c' as light 
1.sub.12 via the parabolic mirror 2, and is split into light components 
1.sub.12p and 1.sub.12s by the PBS film 20 again. Since the essence of 
this embodiment is that at least some of light components returning to the 
parabolic mirror 2 and the light source 1 via the PBS film 20 do not pass 
through the light-emitting portion of the light source, the total 
reflection surface c may form 45.degree. with the parallel light 1.sub.11, 
and the PBS film 20 may form an angle other than 45.degree. with the 
parallel light 1.sub.11. One or both of the PBS film 20 and the total 
reflection film c may have a curvature, so that the optical path of the 
return light does not pass through the light-emitting portion. In this 
case, it is to be noted that the incident angle of the light 1.sub.12 
which is not parallel light should not considerably exceed an allowable 
angle of the PBS film 20 having angle dependency. 
This embodiment can be considered as an application of FIG. 6B. However, in 
other embodiments, the same effect as in this embodiment can be provided 
by changing the inclination of the plane reflection mirror or the total 
reflection surface for returning light toward the light source or by 
giving a curvature thereto. When a curvature is given, the reflection 
mirror or surface is preferably formed as a concave mirror having 
converging characteristics since light does not diverge and become loss 
light. 
FIG. 13 shows an embodiment of a projector according to the present 
invention. A polarization illumination apparatus 24 adopts one of the 
embodiments described above. 
When polarized light from the polarization illumination apparatus 24 is 
transmitted through a liquid crystal light valve 7, the light is converted 
into a beam including image information, and only image light is 
transmitted through a polarization plate 8. The image light is projected 
onto a screen (not shown) via a projection lens 10. 
As illustrated in FIG. 14, a color separation optical system for separating 
white light into color light components, i.e., red, blue, and green light 
components may be arranged between the polarization illumination apparatus 
24 and the liquid crystal light valve 7, a color mixing optical system 40 
may be arranged between the liquid crystal light valve 7 and the 
projection light 10, and liquid crystal light valves may be arranged in 
correspondence with optical paths of respective color components. Some or 
all components of the color separation optical system may be arranged 
between the light condensing means and the polarization beam splitting 
means of the polarization illumination apparatus. In this case, a 
plurality of polarization beam splitting means are required. In general, 
since the PBS film has wavelength dependency, a better design which 
achieves an increase in efficiency and satisfactory color reproduction can 
be attained if PBS films suitable for color-separated light components are 
prepared. 
A plurality of image light beams may be projected by a plurality of 
projection lenses without using the color mixing optical system, and may 
be mixed on the screen. Alternatively, a plurality of polarization 
illumination apparatuses may be prepared without using the color 
separation optical system so as to illuminate corresponding liquid crystal 
light valves. 
In the embodiment shown in FIG. 13, when a polarization plate as an 
analyzer is arranged before the liquid crystal light valve 7, the 
polarized proportion of polarized light incident on the liquid crystal 
light valve 7 can be further increased. 
The present invention is not limited to the above embodiments, and various 
changes and modifications may be made without departing from the scope of 
the invention. 
As described above, according to the present invention, a polarization 
illumination apparatus and a projector with a small light loss can be 
realized. A quarter-wave optical phase plate normally has wavelength 
dependency. Since the vibration direction of polarized light can be 
changed without using the quarter-wave optical phase plate, the present 
invention is also effective for preventing color nonuniformity.