Single panel color projection video display having improved scanning

A color video projection video system in which a white light source is separated into red, green and blue bands with dark guard bands spaced therebetween. Scanning optics cause the colored bands to be sequentially scanned across a light valve, such as a transmission LCD panel. Prior to each color passing over a given row of on the light panel on the light valve, that row will be addressed, by the display electronics with the appropriate color content of that portion of the image which is being displayed. The image is projected by a projection lens onto a viewing surface, such as a screen. In order to optimize the color purity of the system when used with certain types of light valves, the intervals between each of the three colors is adjusted and scaled to the differing response times of the LCD panel for each of the colors. Additionally, color balance can be adjusted by changing the relative size of each color band with respect to the others.

BACKGROUND OF THE INVENTION 
This invention relates to color video projection systems and particularly 
to a single light valve color projection display having improved scan 
uniformity, linearity and color purity. 
Projection television (PTV) and video color display systems, especially 
rear projection display systems, are a popular way to produce large screen 
displays, i.e. picture diagonal of 40 inches or greater, as the projection 
method provides displays which are lighter, cheaper, and often, superior 
in brightness and contrast, than non-projection based displays. Direct 
view cathode ray tube (CRT) based systems still dominate non-projection 
display technology, especially for, 9 inch to 30 inch color displays. In 
unit and dollar volume, the major market for all such displays is the 
consumer market. Size, cost, brightness, contrast and to a lesser extent, 
resolution are important characteristics of consumer designs. Because 
large direct view CRT based displays are heavier, bulkier, and more 
expensive, projection consumer displays dominate in sizes over forty 
inches. 
Consumer projection technology has been dominated by a system employing 
three small monochrome type CRTs, one each for the red, green and blue 
portions of the image, and three projection lenses. These systems employ 
complex electronic circuits to distort the rasters of the images on at 
least two of the CRTs so that the composite projection image is converged. 
Effecting the proper adjustment of the electronics to obtain the converged 
image is a time consuming, tedious process. Further maintaining the 
quality of the convergence in the system after it has been set up at the 
factory remains an issue. 
With a view toward the advantages of projection systems over direct view, 
but also some disadvantages, which were just detailed, engineers have been 
seeking alternative means of designing projection display systems. 
Accordingly, patents have issued and products have been produced employing 
three, matrix addressed, small light valve panels, most commonly TFT (thin 
film transistors) array LCD panels, instead of CRTs. These systems require 
only a single projection lens, if the light from three LCD panels are 
combined via the use of dichroic filters, a.k.a. dichroic mirrors. 
Convergence of the images is obtained by precision adjustment of the 
alignment of two of the panels. Initial interest in such displays has been 
their compactness when employed for front projection, and excellent 
contrast. These LCD panels are costly components, and consequently these 
LCD projectors cost more than CRT based projectors. 
The present invention also differs from "single panel" designs which employ 
special light valve panels in which separate sub-pixels are used to 
modulate respective primary colors. Such color panel technologies have 
three limitations: firstly, the requirement for sub-pixels limits the 
effective image resolution. Secondly, white light falls on each of the 
sub-pixels, but only the color of the light for which the sub-pixel is 
designed is usable--the remainder is wasted. Thus a two-thirds loss of 
efficiency results. A further loss in efficiency is caused by the reduced 
effective aperture of the panel for a given polychrome resolution 
capability--because of the presence of the sub-pixels with attendant masks 
and traces. Thirdly, state of the art panel resolution is lower, or the 
panel cost is higher, because of using sub-pixels. 
SUMMARY OF THE INVENTION 
In the present invention, light from an intense white light source, for 
example an arc lamp, is collected, and separated using dichroic filters 
into primary colors--red, green and blue. The color separated light is 
caused to be formed into three sources, arrayed adjacently, such that each 
source appears to be narrow in the "vertical" direction and wider in the 
"horizontal" direction. Scanning optics are employed to cause three bands 
of light, one of each of the colors, to be positioned onto the rear of a 
transmissive light valve panel. This arrangement proves very effective 
when applied to twisted nematic LCD panel with TFT addressing. The 
scanning optics cause the bands of illumination to move across the LCD 
panel. As a band passes over the "top" of the active area of the panel a 
band of light of that color again appears at the "bottom" of the panel. 
Accordingly, there is a continuous sweep of three colors across the panel. 
Prior to each color passing over a given row of pixels on the panel, that 
row will have been addressed with the appropriate information for that 
color. This means that each row of the panel will be addressed three times 
for each video field which is to be displayed. This can be accomplished by 
either using extra addressing lines to the panel array, and writing the 
horizontal rows in parallel, or by writing three separated rows 
sequentially, but at three times the field rate. The information being 
written to the separated rows must be appropriate for the color content of 
that portion of the image which is being displayed. 
The simultaneous use of a large portion of the available red, green and 
blue light through a single light valve panel is an important feature of 
the present invention. This means that projection video systems based on 
the present invention have optical efficiencies at least comparable to 
that of three panel systems employing the same panel technology. Using 
only a single panel eliminates the need to mechanically converge the 
image, and further reduces system cost. Additionally, beam combining 
dichroic filters are not needed which leads to further cost savings. 
The present invention also includes scanning optics that provide an 
extremely uniform scan of the three colors across the light valve. The 
scanning optics include three prisms, one for each color, which are 
mounted for rotation coaxially in side by side relation. Furthermore, 
using three prisms disposed side by side and adjusted in phase instead of 
a single prism to scan the color bars across the panel permits the time 
intervals between the colors to be staggered. The provision for staggered 
intervals will more efficiently accommodate certain types of light valves 
(LCD's in particular) whose rise and fall times are not the same for each 
color. Therefore, instead of equal 5 msec intervals between each of the 
colors the improved arrangement can allow, by way of example only, an 8 
msec interval for the red bar, 4 msecs for the green bar and 3 msec for 
the blue bar. This will have the effect of improved red transmission and 
thus better color purity, particularly in the critical mixed colors (i.e. 
fleshtones), and improved overall transmission of single panel projectors 
constructed in this fashion. Other apparatus for addressing the three 
colors across the panel surface in unequal time intervals may also be 
utilized in accordance with this invention such as phased rotating prisms, 
mirrors, goniometric or rotating dichroics or filters. Also electronically 
controlled devices such as interferometers which scan in unequal intervals 
may also be used. Color balance may also be adjusted by this mechanism. 
In the series of applications of which the present application is a part 
the means for scanning the color bands across the surface of the light 
valve have evolved. The first application in the series, application Ser. 
No. 634,366 filed Dec. 27, 1990 was directed to a single panel color 
projection video system in which the mechanism for scanning the bands of 
colors across the surface of the light valve is a four-sided prism which 
is used to scan all three colors. An improved scan arrangement was set 
forth in application Ser. No. 990,776 filed Dec. 9, 1992 in which improved 
scanned linearity was achieved by tie use of, rather than a single prism, 
three four-sided prisms, one for each color. The present application is 
directed to a further improvement in which the scanning of each of the 
three colors is optimized to the differing relaxation response times for 
different colors of certain light valves. This arrangement provides 
improved color purity in the video image. Mechanisms other than rotating 
prisms for providing the scanned bands are also disclosed. Finally, a 
mechanism for changing the color balance by changing the relative sizes of 
each color band is set forth.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a generalized overview of the optical system of the single panel 
color projection video display which includes a light box 10, a system of 
dichroic mirrors 12 for splitting the light into bands of red, green and 
blue, a rotating prism 14 for scanning the RGB bands, relay lenses 16, 18 
a light valve 20 upon which is impressed the video signals and a 
projection lens 22. Light box 10 includes a lamp 24 of any suitable high 
intensity type such as a xenon arc lamp and an ellipsoidal reflector 25. 
The lamp output is directed to a "cold" mirror 26 which serves to reflect 
light in the visible spectrum while passing infra red light. Mirror 26 
reflects the light from lamp 24 at a 90.degree. angle and directs it to a 
series of optical lenses (not shown) which serve to modify the beam of 
light so that it is in the form of a generally uniform rectangular beam 
which exits light box 10 through an opening 28. Light box 10 may also 
include elements for absorbing ultraviolet radiation and cooling lamp 24. 
Lamp 24 has preferably a short arc length which facilitates its imaging 
and thus increases the brightness. 
As is shown in detail in FIG. 2 the beam of light 30 emerging from opening 
28 of light box 10 is directed to dichroic mirror system 12. Dichroic 
mirror system 12 serves to split beam 30 into separate beams of red, green 
and blue. Dichroic mirror system 12 includes centrally disposed crossed 
dichroic mirrors 32, 34, which pass only the green light component of beam 
30 and reflect red upwardly and blue downwardly to mirrors 36, 38. An 
upper mirror 36 (which may also be dichroic) is constructed and arranged 
to reflect the red component of the light impinging thereon and the lower 
mirror 38 reflects only the blue component of the light impinging thereon. 
Accordingly, the system of mirrors 32, 34, 36 and 38 serves to split beam 
30 into its red, green and blue components which are arranged in the form 
of a vertical array. A vertical aperture plate 40 includes 3 vertically 
disposed rectangular apertures 42, 44, 46 which also serve to 
rectangularize the three RGB light beams exiting the apertures with the 
red beam on top, the green beam in the middle and the blue beam on the 
bottom. 
After leaving aperture plate 40 the red, green and blue beams impinge upon 
an optical scanning mechanism in the form of a rotating prism assembly 14. 
Prism assembly 14 includes a prism member 50 which has four equal flat 
sides (i.e. its cross section is square) and is rotated about its central 
longitudinal axis by a motor (not shown) which is driven in synchronicity 
with the video signals to light valve 20. The action of rotating prism 
member 50 is to cause the red, green and blue band of colors to be scanned 
downwardly (or upwardly) in a sequential manner by refraction. The 
sequentially scanned RGB bands are directed towards light valve 20 by 
relay lenses 16, 18. Lenses 16 and 18 constitute an anamorphic imaging 
system (of a 4.times.1 ratio) which images the light from apertures 42, 
44, 46 onto light valve 20. As such, the rectangular active surface of 
light valve 20, which is a transmission LCD, receives sequential scanning 
of red, green and blue rectangular color bands. LCD panel 52 modulates the 
light impinging thereon in accordance with the desired input video 
information for the colors impinging on its various portions thereon. 
Thereafter, the video modulated sequential bands of light are projected 
upon a suitable viewing surface, such as a projection screen, by means of 
projection lens assembly 22. 
The scan linearity of the optical system can be improved to a significant 
degree by making the surfaces of the revolving prism cylindrically concave 
as shown in the dotted surface 62 in FIG. 2. The preferred radius of 
curvature is on the order of 10 inches when the length between adjacent 
optical facets of the prism is 2.4 inches. For maximum projector 
performance the use of concave faces is preferred. Negative cylindrical 
faces can be achieved by direct fabrication (grinding), or by cementing 
plano-concave cylindrical lenses onto the four faces of a conventional 
prism. The refractive index of such facing lenses need not be unusually 
high, but the refractive index of the bulk of the prism should be high 
(N&gt;1.6). If the refractive index is too low, then rays that otherwise 
would pass into one facet would exit through an adjacent facet. If this 
occurs, the phenomenon of total internal reflection (TIR) happens, and the 
final direction of the existing ray will not be in the proper direction 
for the light to be useful. 
In the electronics for the device, separate red (R), green (G) and blue (B) 
signals are derived from the appropriate input source (broadcast, cable, 
direct) as is well known to those skilled in the art. However, in order to 
drive light valve 20 in accordance with the sequential color bands certain 
video signal processing is necessary. The parallel RGB signals must be 
serialized to a serial stream with, for example, the green signal delayed 
one third of a video field behind the red signal and the blue signal 
delayed one third of a video field behind the green signal. Thereafter, 
this serial stream must be processed to conform to the column driver and 
geometrical arrangement of light valve 20. For example if there are four 
column drivers there must be four parallel video signals. This signal 
processing utilizes the drivers of the light valve in a different manner 
than usually utilized for driving LCD displays. However, the same number 
and type of drivers are used so that the topology of the light valve need 
not be radically changed from that used with conventional video displays. 
FIG. 3 is a generalized representation of the row and column drivers on a 
thin film transistor (TFT) LCD array which may be used in accordance with 
the invention. As is known in this art in such displays the rows are 
addressed sequentially with all of the TFTs in one row being turned on 
simultaneously through a common gate line by one of the row drivers R1, 
R2, R3. The individual pixels in a row are driven by a series of column 
drivers which may be arranged as illustrated in FIG. 3. The LCD array is 
laid out such that drivers 1 and 3 are connected to the pixels in 
odd-numbered columns while drivers 2 and 4 are connected to the pixels in 
even-numbered columns. The column drivers, which are basically memory 
devices, sample the incoming video signal and store the sampled value in 
the respective memory cell. 
In standard monochrome operation the column drivers would be loaded in a 
sequential fashion: During the first half of the video line driver 1 
receives all odd pixel values while driver 2 receives all even pixel 
values. Drivers 3 and 4 store the respective values during the second half 
of the line. After the video line has been completely written, the outputs 
of the driver are enabled while at the same time the according row is 
activated, resulting in a "dump" of the video information onto a specific 
pixel row on the panel. The whole LCD array is "reprogrammed" in this 
fashion once per video frame in the sequence video line 1, 2, 3, 4 . . . 
478, 479, 480. 
In the presented invention a different sequence is required with which the 
LCD array has to be programmed. The three color bands red, green, and blue 
are scanning vertically over the panel. During one video frame each row is 
illuminated by, in this realization, first passing red, then a green and 
finally a blue lightband. The programming of a particular row has to be 
performed in a way that e.g. the green values are loaded before the green 
lightband reaches this row but after the red band has passed by. Since all 
three color bands are illuminating the panel at any one time three rows 
have to be programmed during the time of one regular video line. Since the 
column driver arrangement does not allow independent programming of more 
than one row at a time this operation has to be performed sequentially. 
In case of equally spaced color bands which scan in a strictly linear 
fashion with no overscan present and 450 rows (video lines) per frame the 
programming of the LCD panel would be performed in the following sequence 
(R=red, G=green, B=blue, (xx)=row number): 
R(1), G(151), B (301), R(2), G(152), B(302), R(3) . . . R(150), G(300), 
B(450), R(151), G(301), B(1), . . . 
The programming would track the color bands as they move over the panel. 
The numbers also indicate that the red video information lags 150 lines or 
1/3 of a frame behind green which in turn lags 1/3 of a frame behind blue. 
In case the rotation of the prism 14 causes non-linear scanning of the 
color bands, and/or overscan is introduced the timing of the two video 
signals and sequence will be modified to accommodate the changing scan 
speed and spatial separation of the color bands. This can be achieved by 
e.g. varying the system clock for each color according to the respective 
position on the panel (for the present row-driver arrangement), 
introducing a varying "blanking" time for the video or changing the line 
sequence to account for the non-linear behavior (which will require random 
access programming of the LCD panel rows). 
FIG. 4 illustrates the signal processing for the RGB signals in a 
diagrammatic manner. Each of the signals is input to A/D converters 62, 64 
and 66 so that signal processing takes place in digital form. Thereafter 
the R signal is input to a first delay line 68 which will delay the red 
signal for a time .tau..sub.1. The G signal is input to a delay line 70 
which will delay it for a time .tau..sub.2 and the blue signal is input to 
a delay line 72 to delay it a time .tau..sub.3. The times .tau.1, .tau.2, 
and .tau.3 are selected according to the position and scan speed of the 
respective color band on the panel. Unless the scanning operation is 
performed completely linearly these delay times will vary during the 
course of one video frame, both absolutely and relative to each other. 
The signals then pass to a switch 74 which selects each of the outputs of 
the delay circuits 68, 70, 72 sequentially so that the output of switch 74 
is a serial stream with, for example, the pixels of the video lines in the 
aforementioned sequence. Thereafter as described below the signals are 
input to switching mechanism for applying the serialized delayed stream to 
the light valve. 
The effective threefold increase of the field rate exceeds the speed 
capabilities of present column drivers. Additional demultiplexing and 
buffering is used to program the column drivers with four independent and 
parallel signals, each of which exhibits a data rate of only one quarter 
of the total rate. 
The video stream passes to a switch 76 to separate the video stream into 
first and second streams 78, 80. Switch 76 is operated at a speed so as to 
divide the video stream into halves corresponding to the first and second 
half of each line. Thereafter the output of switch 76 is connected by a 
line 78 to a switch 82 which is operated at a speed so as to separate the 
odd and even pixels. The odd pixels are directed to a buffer memory 84 
which will hold in this example 120 pixels (one quarter of one line), 
thereafter the output of buffer memory 84 is output to a D/A converter 86 
whose output is in turn directed to column driver 1 as shown in FIG. 3. 
The even pixel stream is directed to a buffer memory 88 and D/A converter 
90 and thereafter to column driver 2 of FIG. 3. The second halves of the 
video lines carried by line 80 are similarly processed by odd/even switch 
92 with the odd pixels directed to buffer 94 and D/A converter 96 to 
column driver 3. Even pixels are directed through buffer 98 and D/A 
converter 100 to column driver 4. 
It should be kept in mind that many other components may be substituted for 
the above described optical system. Other arrangements of components which 
provide sequential red, green and blue bands across the surface of a light 
valve may be utilized in conjunction with the present invention. For 
example, rather than a single source of white light, three sources of 
appropriately colored red, green and blue light may be utilized in 
conjunction with a scanning mechanism. Similarly, dichroic mirror system 
12 and rotating prism 50 could be replaced by, for example, a rotating 
wheel of colored filters or a rotating drum of colored filters. Dichroic 
mirror system 12 could be replaced by a refractory prism and rotating 
prism 50 could be replaced by a polygonal mirror system. The scan 
direction need not be vertical but could also be horizontal or diagonal 
(with suitable light valve signal processing). 
It is also noted that this invention is utilizable with any type of known 
electronic light valves such as transmission or reflection LCDs, 
ferroelectric devices, deformable mirrors and the like. Additionally, the 
light path could be straight as illustrated or folded in a more compact 
arrangement. The light valve could also be utilized in a direct view 
system. In certain applications a two color band rather than three band 
system could be used. A requirement for the light valve is that it have 
sufficient switching speed to be switched at about three times normal 
video rates as each pixel of the LCD is at various points in time a red, a 
green and a blue pixel. Techniques to speed the response time on a LCD 
include: heating the panel, low viscosity liquid crystal material, highly 
anisotropic material and/or making the liquid crystal layer thinner. Any 
combination of these techniques may be used. 
The color band scanning system described above uses a four-sided prism of 
relatively high refractive index glass. However, a glass prism of 
sufficient size for this application is relatively heavy, and has a large 
amount of rotational inertia, thus requiring a relatively powerful motor 
to rotate it. Recently optical plastics such as PMMA have become 
available. These plastics are lighter than glass and are moldable which 
permits inexpensive mass production. Furthermore, since such plastics may 
be molded more complex shapes can be made than by the traditional grinding 
and polishing methodology used for optical glass prisms. A color band 
scanning system having improved scan uniformity is described below. 
As is illustrated in FIG. 2 the three colors emerge from aperture plate 40, 
through apertures 42, 44, 46 which are arranged vertically. However, as is 
seen in FIG. 2 only the middle color band (green) is located on the 
rotational axis of scanning prism 14 with the upper (red) and lower (blue) 
bands off-axis. Due to the vertical arrangement the different color bands 
change their position relative to the prism. This action may cause color 
overlap or too large a gap between adjacent colors. This may impede color 
purity since it is difficult to compensate for this non-uniformity and 
address the light valve appropriately. The scanning system illustrated in 
FIGS. 5 through 7 provides an identical scan for each of the colors across 
the light valve, thus maintaining color purity. 
As is seen in FIGS. 5 and 6, the single four-sided rotating prism 14 has 
been replaced by three narrower prisms 110, 112 and 114 which are disposed 
in side by side relationship. Each prism 110, 112, 114 acts only on a 
single color and each is coaxially mounted for rotation along rotational 
axis 113 and shifted 30 degrees in phase with respect to the next prism. 
As shown in FIG. 7 the input white light from the projection lamp is split 
into three colors by three dichroic mirrors 116, 118 and 120. The incoming 
white light first impinges upon mirror 116 which is mounted in front of 
prism 110. Mirror 116 reflects blue light to prism 110 and passes light of 
other colors. The light passed by mirror 116 next impinges upon mirror 118 
which reflects only green light to prism 112. Finally, the light passed by 
dichroic mirror 118 impinges upon dichroic mirror 120 which reflects only 
red light to prism 114. Because each prism 110, 112, 114 is 30 degrees 
ahead of rotational phase with the preceding prism the output light is as 
is shown in FIG. 6 which consists of an upper band of blue, a middle band 
of green and a lower band of red in a continuing sweep. The arrangement of 
prisms provides that each of the scans of each of the colors is uniform. 
Prisms 110, 112, and 114 may be either manufactured individually, made of 
glass and cemented together or, if made of optical plastic such as PMMA 
may be molded as a single unit. 
As is seen in FIG. 6 each of the three color bands is in a proper "stacked" 
vertical position but are offset horizontally from each other. In order to 
maximize the use of the light output each of the color bands should be 
aligned horizontally as well. Horizontal alignment is accomplished with 
the aid of correction lenses 124, 126 and 128 positioned at the output of 
prisms 110, 112 and 114 respectively. As is shown in FIG. 7 the correction 
lenses 124 and 126 serve to deflect inwardly the outermost beams towards a 
LCD panel 130 (or transfer optics) with the centermost beam left 
undeflected (but focused by lens 128). Thus horizontal alignment as well 
as uniform vertical scanning is accomplished by this arrangement. This 
maximizes the use of the light and increases the optical efficiency of the 
system. 
In addition to scan uniformity, linearization of the scan may also be 
accomplished by the use of scanning prisms having more than four sides, 
such prisms are shown in FIGS. 8 and 9. These may have six sides (FIG. 8) 
or eight (FIG. 9) sides. As is shown in FIG. 10 multiple sided prisms 
reduce the scanning error which begins to approach maximum linearity 
(perfect linearity would be shown by a completely flat line). In FIG. 10 
the vertical axis represents the amount of deviation from linearity, with 
the horizontal axis representing rotation from the middle (0) to the top 
of the scan of each prism face. The light valve is reprogrammed to 
accommodate the additional color bands generated by these prisms. 
Furthermore, these multiple scanning prisms may also be used in a three 
prism side by side arrangement for extremely precise scanning of the color 
bands across the light valve. 
FIG. 11 illustrates the scan of the color bands across the surface of a 
light valve 210 by the three prism scanning mechanism shown in FIGS. 5-7. 
As is shown in FIG. 11, all three of the primary colors (red, green and 
blue), are scanned in the form of color bands across the face of light 
valve 210. However, any single line of pixels of the light valve 210 has 
only a single color applied to it at once. A band of red 212 is swept 
downwardly as shown in FIG. 11. Following red band 212 is a dark "guard" 
band 214 or space between red band 212 and a green band 216. Similarly, a 
guard band 218 follows green band 216 and precedes a blue band 220. When 
red band 212 has been swept downwardly past the bottom of light valve 210, 
another red band will appear at the top of the light valve and the 
sequence of color bands and guard bands is repeated. As shown in FIG. 11, 
in the usual mode, the guard bands 211, 214, 218 between adjacent colors 
are equal. At normal video rates the cycle time for scanning all the 
colors and guard bands across the light valve is 15 msec. Thus only 5 msec 
is allotted for any color band and adjacent guard band to pass any single 
line in the light valve. 
Any of the scanning mechanisms described above may be used to create the 
scan of FIG. 11. In the arrangement of FIGS. 5-7 the scanning device 
consists of three four-sided prisms mounted coaxially in side-by-side 
relationship. Prism 114 acts on only the red colored light; prism 112 acts 
on green colored light; and prism 110 acts on the blue colored light. As 
is seen, each prism is shifted in phase 30.degree. with respect to the 
other prisms. The color bars thus emitted from each prism are spaced 
equidistantly (d.sub.r =d.sub.g =d.sub.b). Thus, when prisms 110, 112 and 
114 rotate, the color bands pass the same spot on the panel at equal time 
intervals. However, such equal spacing of the color bands does not 
accommodate the relaxation response characteristics of many types of 
liquid crystal light valves. 
When using liquid crystal display panels, more efficient transmission of 
the light and improved color purity is obtained by scaling the time 
intervals between the colors to the relaxation response time of the light 
valve for that particular wavelength (color). For example with respect to 
twisted nematic liquid crystal light valves it can be shown that the 
relaxation times (t) (i.e. the time it takes for the LCD to go from 10% to 
90% transmission between crossed polarizers) for each of the three primary 
colors differ such that t(red)&gt;t(green)&gt;t(blue). Accordingly, performance 
is enhanced if the scanning mechanism is designed to be able to adapt to 
the differing relaxation times of the LCD for different colors of 
illumination. 
To implement the scaled color intervals and compensate for the unequal 
response of the liquid crystal to the differing wavelengths (colors) it is 
only necessary to change the phase angles between the faces of the three 
prisms used to scan the three colors. As shown in FIGS. 12a and 12b three 
four-sided prisms are again used, a prism 222 acts only on the red light, 
a prism 224 acts only on the green light and a prism 226 acts only on the 
blue light. Red prism 222 is rotated 30.degree. in phase from green prism 
224, but blue prism 226 is rotated only 20.degree. in phase from green 
prism 224. This, in turn, orients blue prism 226 at 40.degree. from red 
prism 222 such that the emitted color bars are no longer spaced 
equidistantly. This phase adjustment may also be used with prisms having 
more than four sides such as the prisms illustrated in FIGS. 8 and 9. 
Compare FIG. 13, which illustrates the spacing between the color bands as 
caused by the scanning arrangement of FIGS. 12a and 12b, with FIG. 11 
which illustrates the scan of the arrangement of FIGS. 5-7. As seen in 
FIG. 13, the distance (separation) (d.sub.B) between blue band 220 and 
green color band 216 is less than that (d.sub.G) between green band 216 
and red color band 212 and again less than the separation between the red 
and blue color bands (d.sub.R) (i.e. d.sub.r &gt;d.sub.g &gt;d.sub.b). The 
duration (size) of each of the color bands 212, 216, 220 remains the same 
as in FIG. 11, it is the intervals (guard bands 211', 214', 218') 
therebetween that have changed. When the prisms of FIGS. 12a and 12b 
rotate, the color bands do not pass the same spot on the panel at equal 
time intervals. Proper adjustment of the phase angles (the angles between 
the faces of the prisms) to compensate for the light valve response time 
for each wavelength (color) optimizes the transmission of each color and 
improves color purity. This is because the larger spacing d.sub.r between 
the red and blue bars accommodates the longer fall time t(red) of the LCD. 
Similarly, the fact that the spacing d.sub.b between green and blue is 
shorter is no detriment to color purity because the fall time t(blue) is 
nearly twice as fast as t(red). 
FIG. 14 illustrates another system for implementing scaled color intervals. 
This system consists of a source of white light 230 and a light valve 232. 
The light emitted by source 230 impinges upon three sets of pivotable 
dichroic mirrors that act to separate the white light of source 230 into 
three colors (red, green and blue), and scan same across light valve 232 
with varying color intervals. The scanning of blue light is done by a 
dichroic mirror 238 which is pivotally mounted on a piezoelectric actuator 
240. Dichroic mirror 238 reflects only blue light. The pivoting of mirror 
238 by the action of piezoelectric actuator 240 sweeps the band of blue 
light downwardly across the light valve 232. Similarly, only green light 
is reflected by a dichroic mirror 244 which is pivoted by a piezoelectric 
actuator 246. Finally, only red light is reflected by a dichroic mirror 
250, controlled by a piezoelectric actuator 252. 
Driving waveforms for piezoelectric actuators 240, 246 and 252 are shown in 
FIG. 15. As the voltage for each actuator ramps up, the piezoelectric 
actuator's crystals deform linearly causing mirrors 238, 244 and 250 to 
pivot and scan their respective color bands across the panel. When the 
voltage returns and drops (vertical lines in FIG. 15), the mirrors quickly 
return to the reset position, the top of light valve 232. As shown in FIG. 
15, the scan of the red band begins at an elapsed time of 8 msec with the 
green scan beginning at 12 msec for a spacing (d.sub.G) of 4 msec. The 
blue scan begins at 15 msec for a spacing (d.sub.B) of 3 msec from the 
green scan and the red scan begins again at 23 msec for a spacing 
(d.sub.R) of 8 msec (d.sub.R &gt;d.sub.G &gt;d.sub.B). The rate of return can be 
made fast enough so that very little light of that color transmits to 
light valve 232 during the return. If the rate of return is too slow, a 
shutter wheel (not shown) can be added below or in front of each mirror to 
block the light during the return phase. Piezoelectrically controlled 
actuators are readily commercially available for this application. 
Additionally, the mirrors 238, 244 and 250 may also have optical power to 
focus the light on the panels. An adjustment of the phase of each waveform 
in FIG. 15 shown will adjust the time between the color bars in which 
light valve 232 relaxes. 
The description of the means for scanning by the use of pivoting mirrors 
has been simplified for the purposes of clarity. In the system as 
described, efficiency is reduced by using dichroic mirrors with the 
piezoelectric actuators since light not reflected by each dichroic mirrors 
is lost. The efficiency can be improved by using parallel sets of dichroic 
mirrors to recover light not reflected by the first set of dichroic 
mirrors. 
In the examples shown thus far the color bands all had the same dimensions. 
This reflects the situation where the light source radiates "white" light, 
i.e. the red, green and blue colors have equal energy. In general the 
light source will not be white and will radiate unequal energies in the 
three color bands. Consequently the colors in the picture could be wrong. 
This can be corrected by placing attenuating filters in the path of those 
beams that have "excess" energy. A better way is to increase the light 
throughput of the color that is "weak" and simultaneously decrease the 
light throughput of the color that is "strong". This can be achieved by 
increasing the width (height) of the band for the weak color while 
decreasing the width (height) of the strong color. Next to compensating 
for light source characteristics this procedure can also be used for 
giving the user control of picture color (tint) without reduction in 
overall brightness. 
The scanning mechanism shown in FIGS. 14-15 can, when driven by a different 
set of driving signals, be used to vary the color balance of the projected 
image as well, This is accomplished by varying the length of each color 
relative to the other colors. Varying the length implies controlling the 
time between the beginning and end of the individual color band and is the 
same as changing the size of the particular color band. If, for example, a 
shift to a more red color balance is desired the driving waveforms to the 
piezoelectric actuators are adjusted so that the scan of red band is 
slower than that of the blue and green bands, (i.e. the distance between 
the vertical lines in the red scan of FIG. 15 is increased) thus allowing 
more red light to fall on the panel during each frame time. In a system 
utilizing a scanning prism a shift in color balance may also be 
accomplished by increasing the relative size (length in the scanning 
direction) of one band, such as by increasing the height of one of the 
apertures 42, 44, 46 (in FIG. 2) through which the various colors emanate. 
Another system for providing phase adjusted scanning of the light valve is 
shown in FIG. 16 which shows a scanning system 250 in which the prisms 
acting upon each color are non-coaxially mounted. In FIG. 16 the optical 
elements for shaping the light bands have been omitted for the purpose of 
clarity. Scanning system 250 is particularly useful in systems where high 
illumination levels are required, such as projectors used for theatrical 
presentations. This system is also suitable for use where the light valve 
used has a narrow angle of acceptance. Scanning system 250 includes a 
light source 252 in the form of a reflector lamp which emits white light. 
The light beam 254 emitted by lamp 252 first passes to a first dichroic 
splitting mirror 256 which passes red light and reflects light of other 
colors. A red light beam 257 which exits mirror 256 then impinges on a 
first rotating prism 258 which operates on red light beam 257 to scan the 
beam in a vertical direction. Light beam 257 then exits prism 258 and 
impinges upon a mirror 260 which reflects light beam 257 through a 
dichroic mirror 274 (which passes red light) to a dichroic mirror 262 
which reflects red beam 257 to a light valve 264 which modulates the red 
light in accordance with the video information and passes same to a 
projection lens 266. 
The green and blue light reflected from dichroic mirror 256 impinge upon a 
dichroic mirror 268 which acts to reflect green light and pass light of 
other colors. Accordingly, a "green" beam 270 is reflected by mirror 268 
and impinges upon a rotating prism 272 which serves to scan green beam 270 
in a vertical direction as shown in FIG. 16. The green beam 270 then 
passes to dichroic mirror 274 which reflects the green beam 270 to 
re-combining mirror 262 which reflects green beam 270 onto light valve 
264. 
The light beam which passes through dichroic mirror 268 forms a blue beam 
276 as the red and green components have been subtracted from white beam 
254 by the actions of mirrors 256 and 268 respectively. Blue beam 276 is 
thereafter reflected by mirror 278 to a rotating prism 280 which scans the 
blue beam and passes blue beam 276 to mirror 262 which passes blue beam 
276 to light valve 264. By the action of rotating prism 280 blue beam 276 
is also scanned in a vertical direction. 
The action of prisms 258, 272 and 280 results in a scanning of bands of 
red, green and blue light across the light valve 264 which can be the same 
as the scans depicted in FIG. 11 and 13. If each of the prisms are 
disposed at equal phase angles 
(30.degree..backslash.30.degree..backslash.30.degree.) with respect to 
each other, the scan will be that shown in FIG. 11. If on the other hand, 
each of prisms 258, 272, 280 are phased adjusted (i.e. 
20.degree..backslash.30.degree..backslash.40.degree.), the scan will be 
that of FIG. 13. Due to the fact that prisms 258, 272, 280 are separate, 
it is relatively easy to adjust their phase relationship. Prisms 258, 272 
and 280 may be driven by a single motor or by three phase locked motors. 
The use of three separate motors permits the phasing (and thus the spacing 
between the various light bands) to be adjusted by automatic means. 
Furthermore, the fact that the three prisms are disposed in different 
locations in this system permits each of the prisms to be much wider than 
those shown in FIGS. 12a and 12b so that the light collection may be made 
more efficient without the need for light spreading elements. 
Although the present invention has been described in conjunction with 
preferred embodiments, it is to be understood that modifications and 
variations may be resorted to without departing from the spirit and scope 
of the invention, as those skilled in the art will readily understand. 
Such modification and variations are considered to be within the purview 
and scope of the invention and the appended claims.