Two lamp, single light valve projection system

A projection system includes first and second light sources which are disposed at an angle with respect to each other and are activated during respective different phases of an illumination cycle to illuminate a spot on different sides of a segmented rotary filter wheel. The filter wheel is alternately used to transmit light from one of the light sources, and to reflect light from the other light source, to a light valve, the output of which is projected onto a screen. The wheel segments may be alternately clear and mirrored or may alternately transmit and reflect colors to produce color sequential illumination of the light valve. In addition to filter wheels producing sequences of red, blue, green illumination, including the possibility of splitting a color phase into non-adjacent smaller phases to reduce color artifacts, a simple filter wheel which in two rotations produces a sequence of red, blue, green, cyan, magenta, yellow color illumination, resulting in an expanded color gamut, is also disclosed. Blanking intervals during which both lamps are turned off span each passage of a segment boundary through the spot.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to projection systems employing a single 
light valve. 
2. Description of the Related Art 
Light valves operating in a transmission or reflection mode, using liquid 
crystal, moving mirror, oil film, or other technologies are known for 
modulating a cross section of a light beam in two dimensions in response 
to an image control signal. A key use of such light valves is in systems 
employing rear or front projection of light to display video images, in 
particular color video images. Prior art single lamp, single light valve 
systems provide full color through a variety of methods, such as employing 
red, green and blue sub-pixels in the light valve with microfilters, color 
sequential addressing or falling raster addressing. The choice of lamps 
suitable for light valve projection is very limited, especially if long 
life is desired. One suitable lamp from the point of view of long life and 
high lumens per watt is the 100 W UHP lamp available from Philips 
Lighting, or similar lamps available from other manufacturers. The main 
problem with using these long life, short arc lamps is that, they often 
cannot be scaled up to higher power levels. In general, higher power lamps 
such as xenon or metal halide have other undesirable properties such as 
short life or large arc size. Therefore, if a brighter system is desired, 
multiple lamps must be used. 
A two lamp, one light valve system is disclosed in commonly-owned U.S. Pat. 
No. 5,386,250. This patent describes a two lamp projection system where a 
deformable micro-mirrored device (DMD) is used as a combiner of the lamp 
outputs. This system suffers from two main problems: high cost and low 
efficiency of the DMD, such that even with improvements in efficiency 
expected in the near future, there would be far less than a doubling of 
brightness of the light illuminating the light valve relative to 
illumination from a single lamp. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to allow the light from two lamps 
to be efficiently multiplexed onto a single light valve, thereby 
substantially doubling the system brightness. 
The foregoing and other objects are fulfilled by providing a projection 
system comprising a segmented rotatable wheel, first and second light 
sources disposed at an angle with respect to each other and with respect 
to the wheel such as that each illuminates a focussed spot on a different 
side of the wheel at the same location. The light sources are activatable 
during respective different phases of a light source illumination cycle. A 
light valve is positioned to receive light from the spot illuminated by 
the first light source after transmission through the wheel and light from 
the spot illuminated by the second light source after reflection from the 
wheel, for modulating an output light beam formed from the received light. 
In accordance with a first embodiment of the wheel, it has an even number 
of segments, one half the number of segments being used in transmission 
alternating with the other half of the segments being used in reflection. 
A wheel useable in systems which are not color sequential preferably has 
two segments, one transparent and one reflective. 
In accordance with a second embodiment of a filter wheel for the system of 
the present invention for use in a color sequential system, the wheel has 
six color filtering segments, three segments which are used in 
transmission for transmitting different ones of a set of three colors from 
the first light source toward the light valve and three segments which are 
used in reflection for reflecting different ones of the three colors from 
the second light source toward said light valve. The segments are arranged 
and the wheel is synchronized in rotation with the activation of the first 
and second light sources such that the path portion from the first light 
source to said one side of the wheel and the path portion from said other 
side of the wheel to the light valve are coupled by a segment used in 
transmission when the first lamp is activated and said path portion from 
the second light source to said other side of the wheel and said path 
portion from said other side of the wheel to the light valve are coupled 
by a segment used in reflection when the second lamp is activated. 
Preferably, the three segments used in transmission are consecutive as are 
the three segments used in reflection. This allows the period of 
illumination cycle of the light sources to be twice the period of the 
video field (i.e. the light sources are active during alternate video 
fields) and the wheel to be rotated at one half rotation per video field. 
A third embodiment of a color filtering wheel has first and second light 
filtering segments which are used in one of transmission or reflection and 
a third segment which is used in the other of transmission or reflection. 
In such an embodiment, the light sources may be different, the one 
supplying the first and second colors being relatively richer in these two 
colors and the one supplying the third color being relatively richer in 
that color. 
A fourth embodiment of a color filtering wheel useable in the present 
invention has four segments, two non-adjacent segments of which transmit 
or reflect the same color. These two segments are each half the angular 
width of the other segments. 
In a fifth embodiment of a color filtering wheel, it has at least three 
light filtering segments for producing a set of first, second and third 
colors, including a first segment for transmitting the first color of said 
set from the first light source toward the light valve, a second segment 
for reflecting a second color of the set from the second light source 
toward the light valve and a third segment for reflecting the third color 
of the set from the second light source to the light valve. A similar 
embodiment is also possible where the segments of the three segment wheel 
are chosen such that one is in transmission and two in reflection. 
Lastly, in a sixth embodiment, a wheel having three light filtering 
segments is used and the first and second light sources are alternately 
activatable during alternate revolutions of the wheel such that the 
illumination cycle comprises two revolutions of the wheel. Each segment is 
used in reflection and in transmission in alternate revolutions of the 
wheel. This last embodiment allows for an apparent expanded color gamut.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first to FIG. 1 of the drawing, the system 10 of the present 
invention comprises two lamps L1 and L2 which are oriented at 90.degree. 
to each other and direct converging input light beams, IB1 and IB2, 
respectively, to a focus at the same spot 12 on opposite sides of a filter 
wheel W oriented at 45.degree.. Each of the lamps L1 and L2 comprise a 
bulb 14, reflector 16 and focusing lens 18. An annular portion 20 of 
filter wheel W is rotated about its axis via a hub motor 22. The beam 
annular portion 20 is alternately used for transmission of light from lamp 
L1, and reflection of input light from lamp L2, to form a multiplexed 
light beam MB illuminating the light valve LV. The multiplexed light beam 
MB exiting the wheel W may pass through an optional integrator (not shown) 
before reaching the light valve LV. The light valve LV modulates the 
multiplexed light beam MB and the image is projected by a lens 24 onto the 
projection screen 26. The diameter of the focussed spot is typically no 
smaller than one cm. and will subtend a not insignificant angular extent 
of the wheel, for example, about 7.5.degree. when the radius from the 
center of the wheel W to the spot 12 is three inches. 
Video input is received at a light valve control circuit 28 which provides 
a control signal via lines 30 to set the state of the pixels of the light 
valve LV in response to the video and thereby produces 2D modulation of 
the light exiting the light valve and projected onto screen 26. The hub 
control circuit controls the rotational speed and phase of the wheel W. It 
may include a suitable angular position sensor (not shown) incorporated in 
a phase-locked loop. Lamps L1 and L2 are alternately energized from a 
power supply 36, via lines 38 and 40, respectively, in accordance with an 
illumination cycle, which typically is synchronized with the video field 
rate in response to a signal provided from light valve control circuit 28 
on line 42. 
The lamps L1 and L2 may be at angles other than 90.degree. to each other. 
The requirement is that the filter wheel be oriented such that the 
reflected beam and transmitted beam after the wheel be coincident with 
each other so as to form the multiplexed beam MB. Further, while only a 
single light valve LV is shown, it should be understood that the two lamp 
and filter wheel arrangement may be used to illuminate any type of light 
valve projector that would normally be illuminated with a single lamp, 
including two and three light valve systems and reflection type light 
valves. 
Referring to the filter wheel W as illustrated in FIG. 2A and also to the 
corresponding timing diagram in FIG. 3A, it is seen that the annular 
portion 20 of wheel W is composed of two segments of 180.degree. angular 
extent, one segment TRANS being clear so as to transmit the illumination 
from lamp L1 and the other segment REFL being silvered or mirrored so as 
to reflect the illumination from lamp L2. There results an illumination 
cycle having the period T.sub.i corresponding to one rotation of wheel W 
during a rotation period T.sub.r, in which lamps L1 and L2 are alternately 
energized. When lamp L1 is on, the wheel in FIG. 2A is synchronized such 
that the clear segment TRANS is in the illumination path. The light from 
lamp L1 passes through the wheel and illuminates the light valve LV. When 
the wheel rotates to the point where the mirrored segment REFL enters the 
optical path, lamp L1 is switched off and lamp L2 is switched on. The 
light from lamp L2 now illuminates the light valve LV in the same manner 
as lamp L1 did earlier. To improve output light efficiency, there are 
preferably blanking intervals BL between alternate activations of lamps L1 
and L2, which correspond to the time when the angular extent of spot 12 
traverses the boundaries between the clear and mirrored segments of wheel 
W. Assuming each blanking interval BL corresponds to a 9.degree. angular 
extent of wheel W, each of lamps L1 and L2 is run at full average power, 
but at approximately a 47.5% duty cycle, which is synchronized to the 
rotation of the filter wheel W. As appears from FIG. 3A, in each 
illumination cycle of lamps L1 and L2 spanning the illumination period 
T.sub.i, the light valve LV sees an output light O from the wheel W which 
goes through two cycles of illumination, each having the period T.sub.o. 
Period T.sub.0 may conveniently be set equal to the field period T.sub.f 
of the input video, so that the wheel W need only rotate at, and the 
illumination cycle of lamps L1, L2 need only repeat at, one half the field 
rate. 
In a system using a wheel W as illustrated in FIG. 2A where the wheel W 
does not produce a multiplexed beam MB which is color sequential, the 
synchronization signals from the light valve control circuit 28 on lines 
34 and 42 may be eliminated if desired and instead a synchronization 
signal can be supplied from one of the hub motor control circuit 32 lamp 
power supply 36 to the other on lines 44. In such case, the illumination 
cycle of lamps L1, L2, and the corresponding rotation rate of wheel W, may 
be chosen independently of the field rate. In this mode of operation, the 
blanking intervals BL at the segment transitions are not utilized and the 
multiplexed light beam produces essentially steady illumination of the 
light valve LV. 
If the projection system is a color sequential system, the rotating annular 
portion 20 of wheel W which is alternately used in transmission and in 
reflection can be a color filter wheel as shown in FIG. 2B. When seen in 
transmission, the filters are, for example, in the order: red, green, 
blue, cyan, magenta and yellow. 
As is evident from FIG. 3B, during the red/green/blue half of the wheel, 
lamp L1 is turned on during each segment, with the blanking intervals BL 
located such that when the boundaries between segments pass through the 
optical path, during which lamp L1 is off. The wheel W transmits red, 
green and blue light in succession with the blanking intervals BL between 
each color, illuminating the color sequential light valve LV with 
multiplexed beam MB exiting wheel W. Of course, in this system the wheel 
and lamps must be synchronized with the video signal so that half of the 
illumination cycle T.sub.i equals the video field period T.sub.f. 
Consequently, the duration of each red, green or blue segment is 1/3 the 
video field period T.sub.f. The segment duration is 5.6 mS for a 60 Hz 
NTSC system, and 6.7 mS for a 50 Hz field rate system. 
As the wheel continues to rotate, and the half of the wheel with the cyan, 
magenta and yellow segments comes into the optical path, lamp L1 is turned 
off and lamp L2 is turned on during each segment, again with the blanking 
intervals BL in between segments. The cyan segment reflects red light to 
illuminate the light valve LV. The magenta segment reflects green light 
and the yellow segment reflects blue light onto the light valve LV. 
For one rotation of the wheel during a rotation period T.sub.r equal to the 
illumination cycle T.sub.i, the light valve LV sees two sequences of red, 
green, and blue illumination, first by transmission of light from lamp L1, 
second by reflection of light from lamp L2. Therefore, for a 60 Hz video 
field rate, the wheel only needs to rotate at 30 revolutions per second 
(RPS), with the illumination cycle of lamps L1, L2 being 30 Hz. With 
activation of each of lamps L1 and L2 being interrupted by three blanking 
intervals corresponding to the time necessary for wheel W to rotate 
through about 9.degree., each lamp is operated at full average power but 
at about a 42.5% duty cycle. 
The six segments shown in FIG. 2B are of equal width. The widths may be 
made unequal for color correction purposes. 
FIG. 2B shows a six segment wheel, with each segment 60.degree. in angular 
extent. One or more of the segments may be split, in order to reduce the 
color sequential artifacts. For example, green may be split on a wheel 
with the following segment size and order: red-60.degree., 
green-30.degree., blue-60.degree., green-30.degree., cyan-60.degree., 
magenta-30.degree., yellow-60.degree. and magenta-30.degree., producing an 
eight segment wheel. 
By implication, the lamps in FIG. 1 are identical. However, they may be 
different and tailored to different colors. For example, if lamp L1 is a 
Philips UHP lamp (rich in blue and green) and lamp L2 is a metal halide 
lamp (rich in red), the wheel in FIG. 2C could be used. In this example, 
as illustrated in FIG. 3C, lamp L1 has about a 61.7% duty cycle and is on 
while the green and blue segments are in the optical path, and lamp L2 has 
about a 30.8% duty cycle and is on while the cyan segment is in the 
optical path. Both lamps are off during the blanking intervals BL of about 
9.degree. each when the segment boundaries pass through the optical path. 
This wheel must rotate at 60 RPS since there is only one sequence of 
colors around the wheel. 
FIG. 2D shows a split green wheel for use with two different lamps. 
Operation is similar to that with the wheel of FIG. 2C except that the 
lamp power supply 36 produces an additional blanking interval BL in which 
lamp L1 is turned off, reducing the duty cycle of lamp L1 to about 59.2%, 
and the color sequential control signals produced by light valve control 
circuit 28 must accommodate the resultant split green phases in the color 
sequential illumination O exiting the wheel video as shown in FIG. 3D. The 
advantage of this embodiment is the reduction of color artifacts. 
Other arrangements of filters may be used on the color wheel. For example, 
the segments to produce a single color may be collected together in the 
order red (L1), cyan (L2 red), green (L1), magenta (L2 green), blue (L1), 
yellow (L2 blue). This requires switching each lamp on and off at 3 times 
the video field rate. This higher frequency may in fact be the preferred 
mode of operation for some lamps. 
Another arrangement is to do the green or other color field splitting with 
the green/magenta segments. This would lead to a filter order of red, 
cyan, green, yellow, blue, magenta. The apparent reversal of yellow and 
blue is to keep the pulse width on both lamps equal for all colors. 
Other segment orders will readily occur to one skilled in the art. 
The same filters (red, green and blue) can be used for both the 
transmissive and reflective filters, replacing the cyan, magenta and 
yellow filters. This can be done by either duplicate sets of filters or by 
rotating a wheel W with only three segments, as shown in FIG. 2E, twice as 
fast. As appears from FIG. 3E, lamp L1 is the source of a sequence of red, 
green and blue colors during one revolution of wheel W and lamp L2 is the 
source of a sequence of their complimentary colors, cyan, magenta and 
yellow during the next revolution of wheel W. As before, the blanking 
intervals BL appear both in the illumination by lamps L1, L2 and between 
the successive colors of light exiting wheel W. Since the actuation of 
each of lamps L1 and L2 is interrupted by three blanking intervals BL and 
an illumination cycle of lamps L1, L2 spans 720.degree. rotation of the 
wheel, each lamp is operated at full average power but at a duty cycle of 
about 46.25%. FIG. 4 illustrates that the color gamut of the display is 
expanded from a triangle having the sides T into a hexagon having the 
sides H. 
When using the filter wheel of FIG. 2E, light valve control circuit 28 is 
configured to a standard RGB video signal into a non-standard RGB-CMY 
video signal. By assigning decoding angles to cyan, magenta and yellow, 
the RGB-CMY is easily derived directly from NTSC composite video. This 
system might have particular utility when used as a display for printing 
systems, because printer inks are normally cyan, magenta and yellow, 
rather than red, green and blue. An additional advantage of this system is 
that the white brightness would be 50% higher than the standard two lamp 
RGB system. 
The rotating wheel system with good coatings on the wheel should be 
approximately 98% as efficient as a single lamp system. Therefore, the 
total light from a two lamp system would be 2.times.0.98=1.96 times, or 
nearly double, the light from a single lamp system. 
While the present invention has been described in particular detail, it 
should also be appreciated that numerous modifications are possible within 
the intended spirit and scope of the invention.