Optics for a single-lens video projector with color-specific polarization channels

A high-efficiency optical system for use in a liquid-crystal light-valve video projector. High intensity, unpolarized white light is separated into primary color components by color-selective filters transmitting light containing both first and second polarization states. Light of each primary color is characterized by a wavelength passband whereby the endpoints of the passband are defined by first and second wavelengths. The second passband endpoint of the first polarization state of the first primary color overlaps the first passband endpoint of the second polarization state of the second primary color, and the second passband endpoint of the second polarization state of the second primary color overlaps the first passband endpoint of the first polarization state of the third primary color. In addition, the video projector design utilizes low f/# optics in conjunction with absorptive polarizers to remove residual light of an unwanted polarization state allowed into the system by the low f/# optics. This allows high brightness with high efficiency and without sacrificing contrast.

CROSS-REFERENCE TO RELATED APPLICATION 
Reference is made to the application entitled, THREE COLOR CHANNEL, 
TWO-ELEVATION OPTICS FOR A SINGLE LENS VIDEO PROJECTOR, Ser. No. 
08/148,933, filed on the same date as the instant invention, having one 
common inventor and a common assignee. 
BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates generally to improved brightness and contrast 
in image projection systems. Specifically, the invention relates to 
brightness and contrast improvements in liquid crystal light valve based 
video projection systems. 
The demand for large screen video is growing rapidly as VCR, Laserdisc and 
computer driven programming are used increasingly by organizations of all 
sizes for all types of applications. Large screens, as opposed to 
monitors, are needed when the application calls for more than a few 
persons to look at the same video screen. All large screens (over 35 
inches) are either rear or forward projector units. The vast majority of 
all projectors sold are based on a design that uses 3 high-power CRT's 
(one for each primary color) to both form the image and to provide the 
actual projection light. This is accomplished by merely focusing each of 
the 3 CRT's onto the viewing screen with 3 separate projection lenses. 
The basic problem with CRT based video projectors is brightness. A CRT's 
brightness is proportional to the size of the CRT's screen and to the 
power supplied to the CRT. Thus brightness can be increased by either 
increasing the size of the CRT or by increasing the power supplied to it. 
But as more power is provided, heat becomes a serious problem and image 
resolution suffers. In addition, as screen sizes increase, the optics 
become prohibitively expensive. Currently 9 inch diagonal CRT's can 
deliver approximately 300 lumens to the screen. When projectors having 
this level of brightness are used on a 60 inch screen, the ambient light 
in the screening room must be very dim for the picture to be seen. And if 
a larger screen is used, the room must be proportionally darker. But, in a 
large number of applications such as training seminars, it is an advantage 
to have a bright screening room. 
The most promising approach to solving the brightness problem comes from a 
new projector technology, light valves. This technology holds great 
promise, but is not now commercially practical (except for a few very 
expensive units sold in small quantities). The present invention is 
intended to solve some of the problems that have prevented the light valve 
projector from becoming a widely available commercial success. 
In general, light valve based video projectors work as follows. White light 
from a high-intensity source, such as a Xenon lamp, is separated into 
component primary color beams and polarized. Each polarized primary color 
light beam is relayed through a series of lenses and mirrors to a valve 
which then modulates the polarization phase of the light as it is 
reflected from the light valve. The high-intensity light pattern in each 
color channel is analyzed by polarizer-analyzer optics and then projected 
onto a viewing screen. Low power CRT's are used to address the liquid 
crystal light valves. 
Projector light output determines as a practical matter both screen size 
and how bright the screening room may be since a good picture is partly a 
function of the difference between the brightness of the on screen image 
and the level of light in the room. But image quality is also strongly 
affected by the difference between the brightest and darkest parts of an 
on screen image regardless of the level of light in the room. This 
difference is called contrast ratio. The higher the contrast ratio, 
generally speaking, the more pleasing the projected image. 
One of the salient problems standing in the way of the commercialization of 
light valve projectors is as follows. It is particularly difficult, yet 
necessary, to achieve a high contrast ratio and high brightness and at the 
same time have an efficient optical system. The only suppliers of 
commercial light valve based projectors have approached this problem by 
sacrificing efficiency. These suppliers make projectors that have a huge 
projection light source. But the optics of the system are such that only a 
small portion of the available light is used. This design yields a 
projector with high contrast. It also yields a projector with brightness 
that in spite of the inefficiency is much greater than that of CRT based 
projectors. The problems with this approach are at least twofold. First, 
the projectors are large, heavy and expensive. Second, they require at 
least 220 volt power which inherently means a custom, fixed installation. 
However, most users need the flexibility to wheel the projector from room 
to room and plug into widely available 110 volt office power supplies. 
These problems with light valve projectors have been obvious for years and 
many have attempted to solve or at least improve on the situation. 
This problem was addressed in U.S. Pat. Nos. 4,191,456 by Hong, and 
4,464,018 by Gagnon. Their approach was the addition of a reflective 
pre-polarizing prism of the MacNeille type to the polarization optics of 
the high-intensity projection light. Light of one polarization state is 
transmitted into the optics train while light of the second polarization 
state is discarded from the system. This approach improves the contrast 
ratio because the introduction of the prepolarizing prisms improves the 
polarizing efficiency of the system. The problem with this approach is 
that the system throughput efficiency is limited by the initial rejection 
of 50% of the input light. Thus brightness and efficiency were sacrificed. 
To enhance contrast and improve throughput efficiency in a two-color 
projection system, Gagnon, U.S. Pat. No. 4,500,172 describes the use of a 
prepolarizing prism to transmit light of a first polarization state to a 
selective color filter which reflects light of the first polarization 
state and first color to a beam combiner and hence to a second polarizing 
prism to be transmitted to a first liquid-crystal light valve. Light of 
the second polarization state is reflected from the prepolarizing prism to 
a selective color filter where light of a second color is reflected to the 
beam combiner and hence transmitted to the polarizing prism to be 
reflected to a second liquid crystal light valve. In this way, the light 
in both polarization states is preserved. However, this design requires 
that the polarizing prisms maintain polarization efficiency over the 
passbands of both the first and second colors. To accomplish this requires 
additional processing time and expense to deposit the additional 
polarizing layers. In addition, this design is limited to two colors. 
To extend the two color design of U.S. Pat. No. 4,500,172 to full color, 
Gagnon, U.S. Pat. No. 4,425,028, described a fluid-coupled optical tank 
with color selective pre-polarization. This is a complicated optical 
design. It calls for plate prepolarizing beam splitters which require as 
many as 15 thin film layer pairs. In addition, the optical system is 
immersed in a high refractive index fluid. This system selectively rejects 
light of a first color passband and first polarization state while 
transmitting light of first color and first polarization state as well as 
light of second and third colors with first and second polarization 
states. The advantage of this design is that it allows high contrast and 
is compact in size and configuration. A disadvantage is that as in the 
previous design, polarization splitting of the "S" and "P" polarization 
states by the dichroic filters and polarizers reduces the color passband 
ranges and consequently causes low throughput and loss of brightness. 
In spite of the foregoing efforts, little progress has been made in solving 
the problem of providing high contrast and brightness in a projector that 
can be run from a 110 volt power source. 
SUMMARY OF THE INVENTION 
The principal object of the invention is to provide a high efficiency 
liquid crystal light valve projector system with simultaneously improved 
brightness and contrast without reducing efficiency. 
This object as well as other objects is achieved by a video projector 
design which utilizes low f/# optics in conjunction with absorptive 
prepolarizers to remove residual light of an unwanted polarization state 
allowed into the system by the low f/# optics. This allows high brightness 
with high efficiency and without sacrificing contrast. In addition, the 
present invention provides for color-specific polarization channels such 
that light in the first and third color channel is of a first polarization 
state and light in the second color channel is of a second polarization 
state so that the projected image contains corresponding color and 
polarization states. Color wavelength passband endpoints of the second 
color have intersecting overlaps with color passbands of the first and 
third colors to enhance screen image brightness.

DETAILED DESCRIPTION OF THE INVENTION 
Generally, brightness efficiency in a video projector is a function of the 
numerical aperture of the illumination and projection optics and the 
extent of the visible light spectrum that can be used by the optical 
system. The intensity of the light falling on the viewing screen is 
related to the f-number (f/#) of an optical system as follows: 
EQU I.varies.(NA).sup.2 (1) 
where 
EQU I=intensity; 
and 
EQU NA=numerical aperture 
and 
##EQU1## 
and 
EQU NA=sin.theta. (3) 
Where .theta. is the half angle of a cone of light emanating from a point 
source and containing all of the light passing though the optics. Thus, 
light intensity varies inversely with the square of the f/# of the optical 
system. So, all else being equal, a lower f/# means more brightness. And 
the relationship is non-linear. For example, a projector with f/4.5 optics 
will have approximately 4 times the brightness of a projector with f/9.5 
optics. This being the case, a logical question is what is preventing the 
use of low f/# optics in video projectors. And the answer lies in the 
competing requirements for high contrast ratio and power efficiency. 
Generally, contrast ratio in a polarization based video projector is a 
measure of the effectiveness of a polarizing prism to analyze light 
containing both S- and P-polarization states. 
The usable optical passband and the system optical aperture (f/#) are 
dictated primarily by the wavelength and angle sensitivity of the 
polarizing prism to angle of incidence (i.e. .theta.). This phenomenon is 
illustrated in conjunction with FIG. 1. 
FIG. 1(a) is a graph of the transfer function of a typical high quality 
polarizing prism used in a light valve projector where all of the incident 
light hits the refracting surface of the prism at exactly 45.degree.. That 
is, the rays of all entering light are parallel (collimated) and 
orthogonal to the prism. The transfer function graphically illustrates the 
efficiency of the prism. Referring to FIG. 1(a), curve 1 shows the percent 
of R.sub.s that is reflected by the prism. Curve 2 shows the percent of 
R.sub.p that is reflected by the prism. As can be seen from FIG. 1(a); the 
prism reflects nearly 100 percent of R.sub.s over the meaningful optical 
spectrum until about 580 nanometers. The prism effectively blocks all 
R.sub.p except for a tiny amount around 510 nanometers. 
FIG. 1(b) is a graph of the transfer function of a typical high quality 
polarizing prism used in a light valve projector where all of the incident 
light hits the reflecting surface at an angle of 45.degree..+-.3.degree.. 
That is, the rays of all entering light are within a cone having a half 
angle of 3.degree. to a center ray that is at 45.degree. to-the prism's 
refracting surface. The transfer function graphically illustrates the 
efficiency of the prism. Referring to FIG. 1(b), curve 3 shows the percent 
of R.sub.s that is reflected by the prism. Curve 4 shows the percent of 
R.sub.p that is reflected by the prism. As can be seen from FIG. 1(b), the 
prism reflects nearly 100 percent of R.sub.s over the meaningful optical 
spectrum until about 550 nanometers. The prism is much less effective at 
blocking R.sub.p. Indeed around 450 nanometers, the percent reflection of 
R.sub.p reaches about 25%. Thus, the passband is narrower and the 
polarizing efficiency is diminished relative to a system in which all 
incident light is collimated. A .theta.=3.degree. corresponds to f/9.5. 
FIG. 1(c) is a graph of the transfer function of a typical high quality 
polarizing prism used in a light valve projector where all of the incident 
light hits the reflecting surface at an angle of 
45.degree..+-.6.3.degree.. That is f/4.5 optics are used. The transfer 
function graphically illustrates the efficiency of the prism. Referring to 
FIG. 1(c), curve 5 shows the percent of R.sub.s that is reflected by the 
prism. Curve 6 shows the percent of R.sub.p that is reflected by the 
prism. As can be seen from FIG. 1(c), the prism reflects nearly 100 
percent of R.sub.s over the meaningful optical spectrum until about 550 
nanometers. The prism is even less effective at blocking Rp than was the 
case with a 3 degree half angle. Indeed around 450 nanometers, the percent 
reflection of R.sub.p reaches about 40% and around 525 nanometers it is 
about 25%. Thus while the passband is about the same as that for a 3 
degree half angle system, the polarizing efficiency is diminished even 
more than in a 3 degree half angle system. 
FIGS. 1(a), (b) and (c) taken together show that as the half angle of the 
optical system increases, the ability of a high quality polarizing prism 
to analyze polarized light decreases. 
As shown in FIG. 1(a), (b) and (c), the transmission and reflection of S- 
and P-polarization is largely determined by the angle of incidence and the 
wavelength passband. The incidence angle of 45.degree. is high contrast 
for all wavelengths in the passband, with .+-.3.degree. (f/9.5) delivering 
lower contrast and .+-.6.3.degree. (f/4.5) delivering still lower 
contrast. Therefore, light valve projection systems utilizing polarizing 
prisms only to achieve high contrast will be limited to color pass bands 
that are narrower than the width of the wavelength passband available for 
each of the three colors, and illumination and projection optics with high 
f/#. Hence, the amount of light (brightness) in the passband will be 
limited. All prior art in this field describes the use of polarizing 
prisms exclusively to achieve high contrast. 
This invention interposes absorptive film prepolarizers in such a manner as 
to attenuate reflected P-polarization. Since the polarizing prisms have 
maximum polarizing efficiency at a unique angle of the incident light and 
also have maximum polarizing efficiency at a single color wavelength, any 
expansion of the color passband to include multiple wavelengths or any 
increase in the range of incident angles will degrade the effectiveness of 
the polarizing prism to polarize and analyze light containing both S- and 
P-polarization. This shows up in the image as a loss of contrast. However, 
the absorptive sheet polarizer is less sensitive to a broad spread of 
passband wavelengths and angles of incidence. This allows the use of high 
cone angles of incident light and wide color passbands to pass more light 
through the optical train and hence increase screen brightness without 
loss of contrast. 
FIG. 1(d) is a graph of the transfer function of the same polarizing prism 
used in FIGS. 1(a)-1(c) receiving light with a cone half angle is 
6.3.degree.. Referring to FIG. 1(d), curve 7 shows the percent of R.sub.s 
that is reflected by the prism. Curve 8 shows the percent of R.sub.p that 
is reflected by the prism. As can be seen, R.sub.p has been nearly 
eliminated by the addition of an absorptive film polarizer. 
FIG. 2 illustrates a preferred embodiment of that aspect of the present 
invention that utilizes sheet polarizers in cooperation with polarizing 
prisms to lower the f/# of the optics without sacrificing contrast. 
Referring now to FIG. 2, lamp 10 is a high-intensity Xenon or metal-halide 
lamp. An Osram HTI 400 is typical of the metal-halide type and an Osram 
XBO 1000 is typical of the Xenon type. Reflector 12 is elliptical and 
focuses the image of arc 14 of lamp 10 at point 16. Condensing lens 18 
focusses the image of arc 14 at the plane of liquid crystal light valves 
20, 30 and 40. The focal distance from lens 18 to the plane of light valve 
20, 30 and 40 is equidistant. The cone half-angle of light .theta. as 
measured from the axis of the principal ray is approximately 6.3.degree., 
corresponding to f/4.5. Field lens 22, 32, and 42 cause the light passing 
through polarizing prisms 26, 36 and 46 to be telocentric so that all 
light incident at all points on the plane of light valves 20, 30 and 40 
will have a .theta.= 6.3.degree.. Field lenses 28, 38 and 47 are part of 
projection lens 62. 
White light from lamp 10 is separated by dichroic filter 50 into light of a 
first color channel 52 containing the blue band of the visible spectrum 
and a second color channel 54 containing the red and green bands of the 
visible spectrum. At this point, the light in both channels 52 and 54 
contain both S- and P-polarization states. No optical energy has been 
thrown away. 
Light 52 is transmitted through polymer film polarizer 24 where P-polarized 
light is absorbed and S-polarized light is transmitted. The S-polarized 
light emerging from sheet polarizer 24 is transmitted through field lens 
22 to polarizing prism 26 where any residual P-polarized light is passed 
through the prism and out of the optical system. S-polarized light is 
reflected by prism 26 to light valve 20 where it is phase modulated 
between the S- and P-polarization states in accordance with the image 
presented to the input of the light valve. 
The light valves utilized in this invention are multi-layered, planar 
structures consisting of a liquid crystal layer, a reflective dielectric 
mirror, a light-blocking layer, and a photoconductive layer all disposed 
between transparent, conductive layers on glass substrates. When a 
spatially variable, amplitude modulated pattern of light from an 
addressing source such as a CRT is focused on the photoconductive layer, 
it is transformed into a nearly identical pattern in the liquid crystal 
layer. The liquid crystal modulates the polarization state of a 
high-intensity projection light to form a replica of the addressing light 
pattern. This light-valve is described by Boswell in U.S. Pat. No. 
4,019,807. 
The modulated light from light valve 20 is analyzed by polarizing prism 26. 
That is, S-polarized light is reflected back to illumination source 10, 
and the P-polarized light is transmitted through. The P-polarized light is 
passed by field lens 28 through dichroic combining filter 60 and focussed 
by projection lens 62 onto viewing surface 64 to form a light and dark 
pattern of blue light. 
Light in the second color channel 54 is transmitted through half wave 
retarding waveplate 55 where the polarization state of the light polarized 
S is converted to P-polarization and the light polarized P is converted to 
S. Waveplate 55 is a half wave retarder for the red and green wavelength 
light and is of a type manufactured by Meadowlark Industries. 
The light from wave plate 55 is separated by dichroic filter 70 to reflect 
green light 56 and transmit red light 58. Light 56 is transmitted through 
polymer film polarizer 34 where P-polarized light is absorbed and 
S-polarized light is transmitted. The S-polarized light is transmitted 
through field lens 32 to polarizing prism 36 where the residual 
P-polarized light is passed through the prism and out of the optical 
system. S-polarized light is reflected to light valve 30 where it is phase 
modulated between the S- and P-states in accordance with the image 
presented to the input of the light valve phase modulated between the S- 
and P-polarization states. The image pattern is analyzed by polarizing 
prism 36 by reflecting S-polarized light back into illumination source 10 
and passing P-polarized light through. The P-polarized light from prism 36 
is transmitted through field lens 38 to waveplate 39 where the state of 
the light polarized P is changed to S. The resulting green light is then 
first reflected from dichroic filter 90 and then reflected from dichroic 
filter 60, and then passed through projection lens 62 onto viewing screen 
64. The result is a pattern of varying intensity of the primary color 
green. 
Red light 58 is transmitted through field lens 42 to polarizing prism 46 
where the P-polarized component is transmitted through to light valve 40 
and S-polarized light is reflected out of the optical system. Light valve 
40 phase modulates the incoming P-polarized light between the S- and 
P-polarized states. The image pattern is analyzed by polarizing prism 46 
by passing P-polarized light back to illumination source 10 and reflecting 
S-polarized light further down the red optical channel. 
The modulated S-polarized red light from polarizing prism 46 is transmitted 
through field lens 47 to polymer film polarizer 48 where any residual 
P-polarized light is absorbed. From film polarizer 48, the red light is 
passed through 1/2 wave retarder 49 where the S-polarized light is 
converted to P-polarized light. 
Modulated red, P-polarized light is combined with modulated green, 
S-polarized light at dichroic filter 90. The combined red and green 
polarized light is combined with the modulated blue, P-polarized light at 
dichroic filter 60 and focussed onto viewing screen 64 by projection lens 
62. The result is a full color video image. 
According to a second aspect of the invention, each of the primary colors 
is composed entirely of light of a single polarization state with the 
green band being opposite to red and blue and the cutoff frequencies of 
each band chosen to provide overlap of blue and green and of red and 
green. 
FIG. 3 and the associated curves 3(a) through 3(n) are useful in explaining 
that aspect of the invention that optimizes brightness by allowing the 
overlap of the high end of the blue bandwidth with the low end of the 
green and the high end of the green with the low end of the red band. Each 
of the graphs is a plot of light intensity as a function of wavelength in 
nanometers. The cutoff points of the curves are idealized to simplify 
explanation. In reality, each of the curves is continuous. The reference 
numerals of FIG. 2 and 3 refer to the same elements. 
Referring now to FIG. 3, high intensity light from lamp 10 is separated by 
dichroic filter 50 into light of first and second color channels 52 and 
54. Color channel 52 contains the blue wavelengths as shown in FIG. 3(a). 
As can be seen, the pass band extends from the shortest wavelengths up to 
approximately 480 nanometers for P-polarized light and up to approximately 
520 nanometers for S- polarized light. The difference in the pass band for 
the S- and P-polarized light is a fundamental characteristic of the way 
optical elements work on light containing both S- and P-polarization 
states. In general, S-polarized light has a broader reflected pass band 
than P-polarized light. 
The spectrum of light 54 is illustrated by FIG. 3(b). As can be seen from 
FIG. 3(b) the red and green light in the P-polarization state commences at 
a frequency of approximately 480 nanometers and in the S-polarization 
state at a frequency of approximately 520 nanometers. 
Light 52 is transmitted through polymer film polarizer 24 where the 
P-polarized light is absorbed and the S-polarized light is transmitted as 
shown in FIG. 3(c). The light leaving film polarizer 24 passes through 
field lens 22 to polarizing prism 26 where any residual P-polarization is 
rejected. S-polarized light reflected to light valve 20 is modulated as 
previously described. 
The spectrum and polarization of light leaving polarizing prism 26 is shown 
in FIG. 3(d). From polarizing prism 26 the optical path for blue passes 
through field lens 28 to dichroic filter 60. The spectral characteristics 
of dichroic filter 60 are such that it passes wavelengths below 
approximately 550 nanometers. Thus, all of the spectral energy from light 
valve 20 is passed through dichroic filter 60 where it is focused by 
projection lens 62 to form a bright and dark pattern of blue on viewing 
screen 64. 
Light in the second color channel 54 as shown in FIG. 3(b) contains all 
wavelengths from 480 nanometers up, that is red and green. The cutoff 
wavelength is 480 nanometers for P-polarization and 520 for 
S-polarization. Waveplate 55 is positioned in color channel 54 and causes 
the polarization state of the light polarized S to be converted to the 
P-polarization and the light polarized P to be converted to the 
S-polarization. This conversion is shown in FIG. 3(e). 
Dichroic filter 70 has a spectral characteristic that causes it to reflect 
S-polarized light below approximately 620 nanometers and to reflect 
P-polarized light below approximately 580 nanometers. Thus the green 
spectrum is reflected from dichroic filter 70 as shown in FIG. 3(f). 
Dichroic filter 70 passes wavelengths of light in the S-polarization state 
from about 620 nanometers and higher and those in the P-polarization state 
from approximately 580 nanometers and higher. 
Light from waveplate 55 is separated by dichroic filter 70 into green light 
56 and red light 58. Light 56 is transmitted through polymer film 
polarizer 34 where P-polarized light is absorbed and S-polarized light is 
transmitted as shown by FIG. 3(g). The S-polarized light is transmitted 
through field lens 32 and polarizing prism 36 where the residual 
P-polarized light is rejected. S-polarized light is transmitted to light 
valve 30 where it is modulated as previously described. The spectrum and 
polarization characteristics of the light emerging from polarizing prism 
36 is shown in FIG. 3(h). From polarizing prism 36 the optical path for 
green passes through field lens 38 to waveplate 39 where the polarization 
state of the light is changed from P to S as shown in FIG. 3(i). 
Red light 58 is transmitted through field lens 42 to polarizing prism 46 
where the P-polarized component is transmitted through to light valve 40 
where it is in turn modulated as previously described. The spectrums and 
polarization of light leaving prism 46 is shown by FIG. 3(j). 
S-polarized red light emerging from polarizing prism 46 is transmitted 
through field lens 47 to polymer film polarizer 48 where the residual 
reflected P is absorbed as shown in FIG. 3(k). Light emerging from film 
polarizer 48 is transmitted to halfwave retarder 49 where the S-polarized 
light is converted to P-polarized light as shown in FIG. 3(l). 
Dichroic filter 90 has the spectral property of reflecting all S-polarized 
light below 620 nanometers and transmitting all P-polarized light above 
580 nanometers. Accordingly, all light in the green channel is reflected 
and light in the red channel is transmitted by dichroic filter 90. This 
dichroic filter effectively combines the images from the green and red 
channels as shown by FIG. 3(m) . 
The red and green channels are combined with the blue channel by dichroic 
filter 60 which yields a spectrum and polarization state as shown in FIG. 
3(n). As can be seen, the red and blue channels on either end of the 
spectrum are polarized P and the green channel is polarized S. Also, there 
is a substantial overlap in the blue-green and red-green. 
As mentioned previously, the waveforms as shown in the various figures of 3 
are idealized. In reality there is a gradual falloff in intensity as the 
cutoff frequencies are reached. A spectrum more accurately reflecting the 
actual overlap is shown in FIG. 4. Referring now to FIG. 4, the tail of 
the blue spectrum 110 overlaps into the green. And the tail of the red 112 
overlaps into the green at the other end of the frequency spectrum. This 
overlapping is made possible by insuring that all light in the red and 
blue channels are of a different polarity than the green channel. Those 
skilled in the art will recognize that it would have been equally 
practical to have the red and blue channels be S-polarized and the green 
channel be P-polarized. The effect of this approach is to increase the 
intensity of the output light by the amount of the spectrum overlap as 
shown by the hatched areas 114 and 116. In applications where the 
maximization of light output is critical this is a considerable advantage. 
Rather than the 50% loss of efficiency associated with typical prior art 
projectors using prepolarizing prisms, this invention allows white light 
efficiencies in excess of 50% due to the additive effect of the overlap of 
polarization specific passbands.