Note: Descriptions are shown in the official language in which they were submitted.
1330~88
The use of lasers in the production of images, and in particular, the
production of a sequential set of electrical signals which represents an original
picture for direct display through the use of lasers ("video imaging") is also known
in the art. See, for example, U.S. Patents to Baker, supra; 3,737,573, issued June 5,
1973 to Kessler; 3,818,129, issued June 18, 1974 to Yamamoto; 3,958,863, issued May
25, 1976, to Isaacs, et al; 3,977,770, issued August 31, 1976 to Isaacs, et al; 3,994,569,
issued November 30, 1976, to Isaacs, et al; U.S. Patent 3,636,251, issued January 18,
1976, to Daly~ et al. See also, "High-Quality Laser Color Television Display", by
Taneda, et al, reprinted from the Journal of the Society of Motion Pictures and
Television Engineers, June 1973, Volume 82, No. 6; "A 1125 Scanning-Line Laser
Color TV Display" by Taneda, et al, published and presented at the 1973 SID
International Symposium and Expedition; and "Laser Displays" by Yamamoto,
reprinted from Advances and Image Pick-up and Display, Volume 2 of the Academic
Press, Inc. in 1975. For general references to video imaging, see U.S. Patent
3,507,984, issued April 21, 1970, to Stavis; 3,727,001, issued April 10, 1973, to
Gottlieb; and 3,636,251, issued January 18, 1972, to Daly, et al.
It is also known in the art to use isotropic Bragg cells and acoustic-
optical modulation with video imaging systems as discussed in the patent to
Yamamoto, supra, the patents to Isaacs, supra, and the articles supra.
Because of the short pulse duration and high average power, Nd:Yag
Q-switched lasers were chosen by Yamamoto. The infra red light emission, which
was converted into second harmonic waves by using an appropriate non-linear
optical crystal, provided the necessary visible light. For example, the 1.06-micron
spectral line emission is converted into green light which has a wave length of
approximately .534 microns. The 1.318-micron spectral line emission is convertedinto red light which has a wave length of approximately .660 microns. Because ofthe impractical operation, the .946-micron spectral line emission was not used to
obtain the blue line. Instead, optical mixing of 1.32 microns and .660 microns was
converted into an additional wave length of approximately .439 microns.
Although the theory of second harmonic generation and parametric
mixing in non-linear crystals appears to be a realistic solution, the practical
limitations are too severe. The prior art examples show a situation that uses a
luminous flux of one thousand lumens, with two watts of green and approximately
ten watts each of red and blue. The conversion efficiency of the non-linear crystals
is poorj being less than twenty percent in the best of cases. Ten watts of red
couldn't be obtained; ten watts of blue could hardly be achieved. The amount of
radiation incising on the crystal would have to be of significant energy level to
2 ~
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133~8~
achieve the required light; achieving this would lead to the destruction of the non-
linear crystals. The luminous efficiency of the red and blue lines, when taking the
spectral response curve of the human retina into consideration, is so low that the
amount of light needed is in excess of the limits imposed by the Nd:Yag laser
family, although the reproducible color spectrum is excellent for low luminosity.
Because of the low luminous efficiency of red .660 microns and blue
.440 microns, dye laser systems were proposed in the prior art. The green .532-
micron emission was used as the pumping source and a solution of a fluorescent dye
was used as the active medium, to produce the red laser light which is tunable,
allowing the selection of a spectral line having relatively high luminosity. This does
not really solve the problem because twice as much green is now needed, and the
relatively poor efficiency incurred by the addition of pumping a dye cell is a
disadvantage. Similarly, blue light which is tunable requires ultra-violet light, which
has a wave length of approximately .350 microns, to pump a dye laser. The ultra-violet light was achieved by parametric mixing of 1.06 microns and the second
harmonic .534 microns, the sum frequency being .350 microns. This requires,
however, additional green light in order to produce the blue. Accordingly, prior art
systems make an impractical projector. Although the Nd:Yag has relatively good
overall energy to light e~ficiency, the applicable mixing hinders the practical
application.
For general references to Bragg cells, both anisotropic and isotropic,
including shear cut and shear wave propagation of radio frequencies, sometimes
referred to as shear Bragg defraction, see U.S. Patents 3,644,015, issued February 22,
1972, to Hearn; 3,653,765, issued April 4, 1972, to Hearn; 3,701,583, issued October
21, 1972, to Hammond; 4,052,121, issued October 4, 1977, to Chang; 4,110,016,
issued August 29, 1978, to Derg, et al; 4,126,834, issued November 21, 1978, to
Coppock; 4,201,455, issued May 6, 1980, to Vadasz, et al; 4,339,821, issued July 13,
1982, to Coppock, et al; 4,342,502, issued August 3, 1982, to Chang; 3,828,276,
issued August 6, 1974, to Cohen; 4,083,976, issued June 6, 1978, to Das; 3,485,559,
issued December 23, 1969 to De Maria; 4,371,964, issued February 1, 1983; to
Podmaniczky et al; 3,389,348, issued June 18, 1968, to De Maria; 3,749,476, issued
July 31, 1973, to Daly, et al; 4,000,493, issued December 28, 1976, to Spaulding, et
al; and 4,016,563, issued April 5, 1977, to Pedinoff. For Bragg cells, perhaps used in
video imaging systems, see U.S. Patent 3,935,566, issued January 27, 1976, to
Seopko.
Generally, see also U.S. Patent 4,115,747, issued September 19, 1978, to
Sato, et al; 4,229,079, issued October 21, 1980, to Wayne, et al; 4,308,506, issued
.
1330588
December 29, 1981, to Ellis; 4,337,442, issued June 29, 1982, to Mauck; 3,633,995,
issued January 11, 1972, to Lean; and 3,711,791, issued January 16, 1973, to
Erickson; and 4,130,834, issued December 19, 1978, to Mender, et al.
However, none of the prior art set out above discloses a system of a
video projector which can equal the present system in such parameters as maximumlight output capacity and energy-to-light conversion efficiency.
DISCLOSIJRUE OF THE IN~ENTION
A video imaging system is disclosed using monochromatic light source,
or sources, modulated by signals within an acoustic-optical cell or cells using an
isotropic or anisotropic medium for modulation of the light from the light source or
sources. Preferably, metal vapor lasers are used for the monochromatic light source.
Further, shear wave propagation of the sound waves in an anisotropic acoustic-
optical cell is preferred to reduce crystal size of the acoustic-optical cell. A data
compression system for use with the acoustic-optical cell, to reduce the time period
required to propagate sound waves which have been modulated to correspond to theelectrical signals which correspond to a line of image, is also disclosed to minimize
the size of the crystal in the acoustic-optical or Bragg cell. Such video imaging
system is used to project monochromatic or mixed chromatic video images onto a
projection surface by sequential line writing (plane projection) with sufficientresolution and brightness to substitute for and/or exceed the capabilities of
conventional large and small screen imaging systems.
BR~EF DESCR~PTION OF THE D~I~WINGS
For a further understanding of the nature and the objects of the present
invention, reference is made to the following drawings in which~ like parts are given
like reference numerals, and wherein:
Figure lA is a schematic drawing of the imaging system of the
preferred embodiment of the present invention using a single monochromatic
source;
Figure lB is a schematic drawing of the imaging system of Figure lA
rotated ninety degrees;
Figure 2A is a schematic drawing of the imaging system of the
preferred embodiment of the present invention using a multicolor set of
monochromatic sources;
Figure 2B is a schematic drawing of the imaging system of Figure lA
rotated ninety degrees;
Figure 3 is a schematic drawing of an alternate embodiment of a
multicolor imaging system of the present invention, with a single color source
. .
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. s~
-- 1330~88
:
producing two monochromatic signals;
Figure 4 is a schematic drawing of an alternate embodiment of the
present invention of a multicolor imaging system showing two separate
monochromatic sources;
Figure 5 is a schematic drawing of an alternate embodiment of the
present invention utilizing multicolor imaging through multiple monochromatic
sources;
Figure 6 is a set of three drawings showing a sequence of the acoustical .
modulation in the crystal of the preferred embodiment of the present invention,
using a monochromatic source;
Figure 7 is a block diagram showing the construction of the video
compressor used in the imaging system of the preferred embodiment of the presentinvention;
Figure 8 is a timing diagram showing the timing signals related to the
compressor and the laser pulse time of the preferred embodiment of the present
invention;
Figure 9A is a schematic drawing of an alternate embodimen~ of the
present invention of the imaging system utilizing simultaneous, multiple line
writing; and
Figure 9B is a schematic drawing of the imaging system of Figure 9A
rotated ninety degrees.
DESCRIPTION Oli THE INVENTION
1. Introduction
As used in the following description and the previous discussion, the
following definitions shall apply:
(1) Video imaging - A sequential set of electrical signals which represent
1 an original picture.
(2) Pulsed Laser - Any laser source whose radiation is emitted in a non-
,.51 continuous-mode burst which lasts for a finite period of time.
! (3) Metal Vapor Pulsed Laser - A laser system that uses one or more
i pure metals or metal halides as a lasing medium, a means for vaporizing the metal or
~ metals to the necessary vapor pressure, and a device to switch the operating current
`1 on and off at a particular repetition or "rep" rate.
(4) Bragg Cell - A device that changes the intensity of a light beam by
' the interaction between sound waves and the light in a solid medium.
(5) Anisotropic - That property of a material which determines the
velocity of propagation of sound within the material, such that when tested along
;~ 5
1330r~l8~
axes in different directions, the velocity of propagation is different in each direction.
(6) Isotropic - That property of a material that determines the velocity
of propagation of sound within the material, such that when tested along axes indifferent directions, the velocity of propagation is the same in all directions.(7) Shear Cut - A physical cut of the crystal in order to launch the
sound waves at a shear angle; or, a physical mounting of the transducer to
accomplish same.
(8) Shear Cut Propagation of RF - The propagation of the sound
waves in an anisotropic medium, the direction of which is non colinear with a beam
of light. Transverse sound waves may be launched with a specified transducer.
(9) Compression - A means for producing information corresponding
to an original signal but whose time is shorter in comparison to the original length
of time of the original signal.
(10) Shear Bragg Defraction - In a longitudinal mode, the output
defraction angle is small (e.g., typically measured in milliradians). In a shear mode
the output defraction angle is high (e.g., typically measured in degrees). Shear Bragg
defraction is useful for deflectors and filters because it provides relatively high
separation.
The use of sequential plane projection for video imaging systems
through the use of metal vapor lasers, such as gold at 627 nm for red, copper at 510
nm for green, and bismuth at 472 nm for blue is disclosed below. It has been -
discovered that metal vapor lasers have typically much higher wattage output
efficiency than do continuous or non-metal pulsed lasers in the visible spectrum.
Light from the metal vapor lasers is introduced to anistropic acoustic-
optical crystals or Bragg cells for modulation. By the use of an anisotropic property ~ !
jJI or paratellurite crystals, with an acoustic wave travelling in a shear mode through
the crystal, the length of crystal necessary to accommodate a full line of signal
within the acoustical part of the system in order to modulate emitted light from the
metal vapor laser, is significantly reduced. It is reduced to the point where crystals
can be manufactured of a reasonable length to permit appropriate signal modulation
in an economical manner. The shear waves also allow a higher resolution having abetter defraction efficiency in a relatively small crystal. This is compared with the
crystal of Yamamoto, supra.
~ However, there is an alternate or additional way to minimize the
i length of the crystal through data compression as discussed infra. The compression
i may be used in conjunction with the anisotropic crystals, as well as isotropic
'`!
crystals. For understanding this, one must recognize that the length of a standard
:, .
~ .
~' ~ .
133~gNTSC video raster scan line, for example, is 63.5 micro-seconds including a blanking
sync interval. Because the light source (metal vapor laser) is pulsed, and the time
length of the pulse may be less than or equal to the time required to write a pixel of
information on the projection surface in real time ("pixel of time"), the light which
is to be modulated by means of acoustic-optical modulation need be introduced only
after the entire line is compressed and fully propagated along the crystal; i.e., the
modulated line of an RF signal is fully within and still travelling through the
acoustic-optical crystal when the pulse of light occurs, the pulse occurring in less
than a pixel of time. In the NTSC example, by limiting the light pulse time to
approximately thirty nano-seconds or less per pulse, the pulse of light may be
modulated to correspond to one full line, thus making available a significant amount
of blanking time for the signal to die out in the crystal and for a new signal to be
introduced by acoustic optical modulation.
Compression can be accomplished through the use of electrical signals,
which corresponding to the original video source signals, and which are written into
RAM at the video pixel input rate and read out of RAM at any faster rate, e.g. twice
or more that rate. By buffering the video signal information and increasing its
frequency, the length of the crystal needed to hold a horizontal line would be
shortened, such as approximately one-half of the length that would be required for a
non-compressed signal when the RAM read out rate is increased by a factor of two.
Accordingly, for example, for fifty percent compression of data comprising videoinformation, when the horizontal line starts to enter the crystal, there is an elapsed
time of approximately thirty-one micro seconds (in the case of NTSC raster line)between the time the "horizontal line" of signal corresponding to a horizontal line
~j .
of display starts to enter the crystal and when the last part of the full line of signal
has entered the crystal (and the light pulse can thus commence). The compressor is
preferably a double-buffered memory with a line of video data being written intoone of the line buffers at the video real time frequency while the other line buffer
waits the appropriate time and then reads said data out of RAM at the increased
rate, such as twice as fast, to the crystal.
The modulated, pulsed light signal is then projected onto a projection
surface.
2. Detailed Description of the Invention
-Monochromatic System-
Referring to Figure 1, there is shown a pulse-operated metal vapor laser
11 emitting monochromatic light 12 to optical system 21. Optical system 21
includes two cylindrical, anamorphic lenses 22, 23 which compress the
133~88 - ~
monochromatic light from beam 12 into a narrow set of parallel rays 24 which areintroduced into Bragg cell 31, or other suitable acoustic-optical light modulating
device. Preferably, Bragg cell 31 uses an anisotropic acoustic-optical cr,vstal 32 and
has a transducer 42 mounted on a side non collinear ~o the path of beam 24. A
wave absorber 44 is mounted on another side of anisotropic crystal 32 facing theside of anisotropic crystal 32 upon which transducer 42 is mounted, with acoustic
waves 43 propagated between transducer 42 and absorber 44. Acoustic waves 43 aregenerated by transducer 42 and response to signals from a radio frequency source 45
connected to transducer 42 by cable 47. Radio frequency source 45 may be
stimulated directly by video signal receive 51 or, as preferably shown in Figure 1,
may be driven by a data compression system 53 connected by cable 55 to RF source45 with cable 57 connecting compressor 53 and video signal receiver 51.
As shown in the preferred embodiment of Figure 1, the video
compressor 53 serves to decrease the time required for sound waves 43 which
correspond to a video line, to fully into the Bragg cell 31. The modulated output -
beam 61 is transmitted to cylindrical anamorphic lens 63 which is used for output `
projection. Lens 63 focuses the modulated light beam into a beam 65 which is
projected onto optical slit 67. Op~ical slit 67 is positioned to block undiffracted
light 64 exiting from the Bragg cell 31. The light beam 69 emitting from slit 67 is
projected on a frame scanning mirror 71, such as a frame scanning galvanometer, -
and is thence positioned appropriately on a projection screen 81.
As seen in Figure 1, beam 64 includes the undiffracted light exiting
from crystal 32. This beam 64 is blocked by the optical slit 67 which keeps
undiffracted light from reaching the projection screen 81. This is indicated by the
lower dashed lines in Figure 1. The dashed lines in Figure 1, representing beam 65,
indicate diffracted rays which are reflected by the mirror 71, such as a vibrating
galvanometer, to the projection screen 81 at the particular line position which
corresponds to the ordinal position of the corresponding line of video data of the
original video signal received by the video signal receiver 51.
Preferably, the light modulation system or Bragg cell 31, uses
anisotropic Bragg defraction by shear waves travelling in a paratellurite crystal such
as tellurium oxide (TeO~ as a principle of modulation.
-Compression-
The details of compressor 53 are shown in Figure 7. The dot clock 100is set to a frequency which represents the pixels along the compressed horizontal
scan line. As in the example set out above, a scan line in an NTSC format is equal
to approximately 63.5 micro-seconds, which includes approximately 10 micro-
.
: 133~58~
seconds for sync and blanking interval. As an example, if 512 pixels are included inthe scanning time and the visual line equals approximately 53.5 micro-seconds, then,
each pixel will have an update rate of approximately 100 nano-seconds.
As shown in Figure 7, the dot clock signal 100 is divided by the
compression valve "S", which is, for purposes of this example, equal to two, by
divider 102 and produces a signal 104. Signal 104 is used as a clock for the "N only"
generator 106. The other input for "N only" generator 106 is the horizontal syncinput 108. Every time a horizontal sync 108 arrives, the "N only" generator 106
will produce a series of N pulses 110. For a system as described above having 512
pixels per horizontal line, N would preferably be 512, with each pulse being
approximately 100 nano-seconds long.
The N pulses 110 are used to clock the input video RAM address
counter 112. Address counter 112 supplies the address codes 119 needed to address
the random access memories 114, 116 described below. As discussed below, at the
same time the input video is being sampled and the data is being loaded into one of
the RAM arrays 114, 116, the other RAM array 116, 114 is being read out at a
compressed rate represented by pulses 118 originating from a second "N only"
generator 120. "N only" generator 120 is driven by dot clock 100 as well but has no
divider. Therefore it will run at the higher clock rate needed for compression.
Preferably pulses from "N only" generator 120 are at fikty nano-seconds, if the
compression rate is to be twice as fast as the input rate, and the input clock is fiky
nano-seconds. Accordingly, the horizontal sync 108 is delayed by delay circuit 122
before it triggers "N only" generator 120 in order to maintain synchronization. The
delay is equal to the horizontal scan line width divided by the compression ratio
"S". Expressed as an equation, the horizontal scan line "H" is divided by the
compression ratio "S" to equal the compressed scan line "CH", or H/S = CH.
Accordingly, (H - CH~ is the delay time for delay circuit 122. For the example set
out above, with "S" equal to two and "H" equal to the NTSC standard of 63.5
micro-seconds, the delay time would be approximately equal to 31.5 micro-seconds.
It should be noted that delay described here is prior to filling the
crystal. Alternately, the crystal could be filled immediately and then the delaytaken.
Aker the appropriate delay time, the "N only" generator 120 produces
512 compressed rate pulses 118 at a rate of 50 nano-seconds. Signal 118 is used to
clock the output compressed video RAM address counter 24 which supplies the
necessary code 121 to change the address of either of the RAM arrays 114, 116 for
output. The output data 126 is sent to the RF source 45.
.
: 9
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133Q588
Multiplexers 128, 130 and control flip flop 132 control the sequence of
use of ~he RAM arrays 114, 116. The control flip flop 132 determines which inputsignals are to be supplied to the RAM array 114, 116. For example, in the first
cycle, RAM array 114 may read at the compressed rate through output data bus 126,
while RAM array 116 writes at the video rate from the video signal 134. The cycle
is triggered or changed by horizontal sync pulse 108. When the next sync pulse 108
arrives, the control signals 136, 138 from flip flop 132 to multiplexers 128, 130,
respectively, switch and cause the RAM arrays 114, 116, respectively to reverse roles.
The data from signal 134 which has been written into RAM array 116 will then be
read at the compressed clock rate 118 while RAM array 114 inputs the next video
scan line 134 at the rate set by clock pulses 110. The timing of these pulses isshown in the upper portion of the Timing Diagram of Figure 8.
-Acoustic-Optical Crystal-
The length of the crystal 32, measured from the acoustic transducer 42to the wave absorber 44, is set to be equal to the distance the acoustic wave 43 must
propagate to accommodate one horizontal scan line, divided by the compression
ratio "S". In this regard, the standard NTSC horizontal scan line "H" is equal to
approximately 63.5 micro-seconds including blanking which takes about 10 micro-
seconds, as discussed above.
For use of the acoustic wave 43 in anisotropic crystal 32, one should
remember that because the output of the radio frequency generator 45 is amplitude
modulated by the compressed video signal system 53, the acoustic waves to crystal
32 are also amplitude modulated. In this regard, the laser beam shaping optics 21
are used to compress the laser beam into narrow parallel rays suitable to the size of
a crystal 32. The incident pulsed beam 24 is diffracted by the acoustic wave 43
which propagates through crystal 32. In Figure 6A, there is illustrated one of the
ii horizontal video scan line signals within Bragg cell 31. In Figure 6A, a horizontal
line designated by the designator "A" is impressed across length "X" of crystal 32.
In this case, "A" represents the time the video horizontal sync (modulated acoustic
wave) first enters the Bragg cell 31. The signal (shown exaggerated for clarity) is
propagating from the front left to the back right of Figure 6A. Figure 6B shows the
acoustic wave after it has propagated approximately half-way through crystal 32. As
shown in Figure 6C, the modulated acoustic wave for a single horizontal line haspropagated through the crystal 32. During the time the wave bearing the sync at
"A" has been propagating through the crystal 32 as illustrated in Figures 6A-6C, the
system is in a blanking mode. No light will illuminate the modulated wave 43 in
the crystal 32. During this blanking period, the information contained in one
: ,
133~88
horizontal scan line is preferably, but not necessarily, compressed, preferably by
compressor 53, to the appropriate size to fit within length "X" (l?ig. 6A) of crystal
32. The propagation time is equal to H/S. At the time the "A" sync is being
absorbed and "A + 1" (the next sync pulse) has entered the crystal 32, the laser 11 is
pulsed for a time not longer than the pixel time which is (H/S) . N, where H is
one horizontal line time, "S" is a compression ratio and "N" equals the number of
pixels. It should be noted that if the light pulse is longer than the required time set
out above, then the acoustic waves would be observed in motion which would blur
the image. By having the length of time of the light pulse being synchronized with
the propagation of one full horizontal scan line to modulate the light pulse, the
objective is to, in effect, stop the motion of the acoustic wave after there has been
enough time for the information of one horizontal scan line to have propagated into
the crystal 32. This effect is achieved by strobing or pulsing the light source 11.
The acoustic-optical crystal 32 is made, preferably, of tellurium oxide
TeO2. The usual sound velocity of tellurium oxide is 4.22 x 105 cm/sec. When a
tellurium oxide crystal is cut in a sgear mode, the velocity of sound propogatedcolinearly with the shear cut is approximately .617 x 105 cm/sec. The compressedscan line is equal to the NTSC standard scan line "H" (H=63.5 x 10~ seconds)
divided by the compression ratio "S" (S=2) which results in a compressed scan line
equal to 31.5 x 10-6 seconds. Accordingly, the length of the crystal 32 is equal to the
sound velocity times the compressed scan line or approximately 19.44 millimetresfor the illustration given. Because of the relative length of the modulator and the
relatively low radio frequency (approximately fifty megahertz) required, acoustic
i~ attenuation is minimized. Also, because of the shear waves, a high resolution can be
obtained within a much smaller crystal than the same resolution in an elongated
crystal such as the prior art of Yamamoto. The acoustic radio frequency is also
lower in the shear mode as discussed above, of, for example, fifty megahertz. The
same resolutions for an elongated crystal would require an acoustic wave frequency
about twice as much or approximately one hundred megahertz which would
contribute significantly tO acoustic attenuation.
-Multicolor Projector-
A several color projector, such as three colors, is shown in Figure 2.
Laser light in beam 212 is caused by lasers 211 to incide on crystal 32. Optic system
;,
j 21 narrows the beam through a series of lenses 22, 23 as discussed above, so that a
!J narrow beam 24 incides on crystal 32 of Bragg cell 31. A series of transducers 240,
241, 242 are driven by a series of RF 245, 246, 247, respectively with transducers
240, 241, 242 adhered to crystal 32. RF sources 245, 246, 247 are modulated one
.
11
,
," ~,
`' 13305g8
horizontal line at a time by compressed video signals from compression systems 253,
254, 255, respectively. These cause acoustic compression systems 256, 257, 258,
respectively, to propagate across crystal 32 to absorber 44, while the laser provides a
light pulse when a full horizontal line has propagated into crystal 32 for each color.
The width of the light pulse is less than or equal to a pixel of time.
The configuration of Figure 2 allows the modulation of more than one
light beam, each of the light beams being monochromatic and each usually of a
different wave length from the others, in a single modulator or Bragg cell 31. Amuki-color picture can be displayed if the light beam colors are, for example, three
diverse wave lengths, such as wave lengths W1, W2, W3 representing the red, green,
and blue primaries. For this purpose, the three radio frequency sources 245, 246,
247 must simultaneously cause the three acoustic waves 256, 257, 258, respectively,
to exist simultaneously in crystal 32 for modulating each horizontal line. For this
purpose, the acoustic waves 256, 257, 258 are presumed to be constant-frequency,amplitude-modulated signals, having waves lengths T1, T2, T3 respectively. The
modulation of the video signals corresponding to radio frequency sources 245, 246,
247 is supplied by the video signal compression systems 253, 254, 255, respectively,
after being received from, for example, the air waves or cable by the video systems
3 51.
' If the wave lengths of light 212 and the wave lengths of the amplitude-
modulated acoustic waves 256, 257, 258 are selected to satisfy the equation 2 sin
Fl=W1/T1; 2 Sin F2=W2/T2; 2 Sin F3 = W3/T3, independently, with "Fi" (i =
1,2,3) being equal to the angle of incidence, then the light beams of differing colors
from laser light sources ~12 will be modulated by the respective acoustic waves
separately. In this manner, a full-color display can be projected by utilizing only
, one modulator or Bragg cell 31. Any combination of laser light sources 211 other
than the primary colors may be used to provide other combinations of colors which
can provide for multi-color display so long as they satisfy the above relationship.
- Multicolor or Monochromatic Lasers -
A laser family which meets the needs necessary to obtain the high
energy to light efficiency, average power and the appropriate red, green and blue
lines to produce a multi-color display, and which is accordingly preferable, is the
metal vapor family.
In general, the neutral atoms are excited to the first residence level by
electron impact. Although these levels are strongly coupled to the ground state,radiation trapping at densities above about 10l2/CM3 increases the upper state
lifetime. Laser action occurs between levels which are metastable. Build-up of
~ .
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12
,.~
13313~8~
atoms in the lower levels (both by stimulated emission and by direct electron
pumping) causes the inversion to cease. After the return of the metastable atoms to
the ground state, the laser may be pulsed again. With neon as a buffer gas and
appropriate pressure and field strengths, deactivation times are estimated to be less
than 25 micro-seconds. Because of the short deactivation times, the laser may bepulsed at high "rep" rates, such as greater than 30,000 Hertz. For an NTSC- ~-
standard television format, the required "rep" rate of approximately 15~750 Hertz is
easily obtained. The pulse width is approximately 30 tO 40 nano-seconds. Anotherimportant feature of the metal vapor family of lasers is the energy to light efficiency.
This efficiency is at least as good as Nd:Yag lasers and may be better. When
producing wave lengths required for full color video displays, the overall output
efficiency is undiminished by any need for using wave length conversion optics such
as dye cells. Because of average power scaling, the metal vapor laser is capable of
very high average power. Fifty watts in single bores and higher, greater than one
hundred watts in an amplifier condition, may be achieved.
With regard to obtaining green light for the laser, one should note that
NTSC green is centred approximately at .540 microns. Green light of
approximately .510 microns may be produced by using waste heat from the gas
discharge which provides the power necessary to vaporize copper present within the
plasma tube. Also .578 microns yellow light is produced which can be used for a
limiting dichroic display or increased luminance.
Other metals that my be suitable to provide the necessary green color
besides copper could be manganese which has a wave length of approximately .534
microns, lead which has a wave length of approximately .537 microns and iron
which has a wave length of approximately .540 microns. It should be noted that
not all of these may lase at high average powers. However, the lower average
powers may be used for smaller displays that may be under a total projection
luminosity of, or example, one thousand lumens. Because of the technical advances
of copper vapor lasers, they will be acceptable for producing the green although any
of the other me~als could be used.
With regard to the production of red ~NTSC red being centred at
approximately .610 microns) suitable for monochrome, dichroic or multicolor
displays, a laser type similar to that described above for the green laser is acceptable
but using gold as the vaporized medium. Other metals besides gold that could be
used would be calcium which produces wave lengths of approximately .610 microns,.612 microns and .616 microns. However, gold at .628 microns is preferable for
providing a better color gamut.
r ~
~3, 13
. r~
~33~58~
With regard to blue light ~NTSC blue being centred approximately at
.470 microns), the metal vapor lasers used could be similar to those for copper and ~ -
gold except bismuth could possibly be developed as a suitable vaporizing medium.Other possible alternatives would be cadmium which has a wave length of
approximately .488 microns, beryllium which has a wave length of approximately
.467 microns, cesium which has a wave length of approximately .455 microns, ironwhich has a wave length of approximately .452 microns and indium which has a
wave length of approximately .451 microns. In this regard, cesium, iron and indium
appear to be low in the luminosity efficiency. At this time, no metal vapor laser
pulsed laser capable of producing the same output wattage !evels as the red and ~;
green metal vapor lasers has been developed. Presently, continuous wave lasers,
cavity dumped to produce a pulsed output, can be used in combination with the
preferred red and green metal vapor lasers to comprise a multicolor system of
acceptable wattage output of all colors. Also, although not preferably, a continuous
wave laser of one color used in a continuous mode with a polygonal rotating mirror
may be used, in combination with metal vapor lasers of other colors, to produce a
multicolor output.
- Alternate Dichroic and Multicolor Systems -
As shown in Figure 3, a single laser 311 is provided. Laser 311 would
preferably be a metal vapor laser which has a vaporizing metal of, for example,
copper having a simultaneous differing wave length emissions, such as, for example,
.510 microns and .578 microns for the copper. The wave lengths are separated by a
dichroic beam splitter 313 which transmits one wave length, such as, for example,
.578 microns and reflects the other wave length, such as .510 microns. The firstwa~e length 315 which is transmitted by beam splitter 313 is further reflected by a
mirror 317 into a beam 319 parallel to beam 321 reflected by beam splitter 313.
Beams 319,321 are processed in parallel. Each beam passes through beam shaping
optics 21, and then each beam incides on a separate acoustic-optical modulator or
Bragg cell 31. Bragg cells 31 are modulated, as discussed above, by a compressed or
noncompressed video signal to modulate the light beams 319,321, said signal passing
through each Bragg cell 31, respectively, one horizontal line at a time. The resultant
light beams 323,325, respectively, incide on projection optics 327,329, respectively.
Projection optics 327,329, are comprised of lenses 63 and optical slits 67. The
resulting beam 331 is reflected by a reflector 333 to a second dichroic beam splitter
335 where it is combined with the beam 337 from projector 327 through dichroic
beam splitter 335. The combined beam 339 is reflected by frame scanning mirror 71
and projected onto screen 81.
: !
14
133G~
Referring to Figure 4, an imaging system using two separate lasers 341,
343 is shown rather than the single laser system of Figure 3. Laser 341, for example,
could be a metal vapor laser which has a vaporizing metal of copper with a wave
length of approximately .510 microns. Laser 343, could for example, be a metal
laser vapor which vaporizes gold and has a wave length of approximately .628
microns. Both beams would be used as the primaries of a dichroic display. Beams
319, 321 would be passed through shapers 21 and Bragg cells 31 as discussed above
and projectors 327, 329, respectively, to be combined by reflector 333, and dichroic
beam splitter 335 to impinge on frame scanning mirror 71 and thus be reflected
onto projection surface 81.
Referring to Figure 5, a set of three monochromatic laser light sources
345, 347, 349 are shown in parallel. Laser 349 may be a metal vapor laser which has
a vaporizing metal of copper with a wave length of approximately .510 microns.
Laser 347 may be a metal vapor laser which vaporizes gold and has a wave length of
approximately .628 microns. Laser 345 may be a metal vapor laser which vaporizesbismuth and has a wave length of approximately .472 microns. The operation of
this system is substantially similar to the operation of the system of Figure 4. The
beams from each laser 345, 347, 349 are shaped by shaper optics 21 and modulatedby Bragg cells 31. The resultant modulated light beams 323, 325, 326, respectively,
incide on projector optics 327, 329, 330, respectively, and the resultant beams are
combined by reflector 333, and two dichroic beam splitters 335, 336, with beam
splitter 336 permitting the passage of the wave lengths from lasers 347, 349 andreflecting the wave length from laser 345. The combined beam is reflected by frame
scanning mirror 71 and is projected on the projection surface 81.
It should be noted that frame scanning mirror 71 may be either a
rotating polygonal mirror or be driven by a galvanometer. Either would be
synchronized to the vertical field pulse of the video signal which would cause a scan
line picture to be offset slightly on the projection surface 81 to produce the picture.
- Alternate Writing System -
Referring to Figure 9, as discussed supra, laser light beam 12 emitted bylaser 11 incides on acoustic-optical cryslal 32. The inciding rays 24 form a narrow
beam. Prior to inciding on crystal 32, rays 24 are divided by a beam splitter 26 to
form beam 24' which is parallel to beam 24. A mirror 27 is used to cause beam 24'
to incide onto crystal 32 of Bragg cell 31. A set of transducers 42, 42' are mounted
at different positions on the same side of crystal 32, with acoustic waves 43, 43',
respectively, introduced between transducers 42, 42' and absorber 44. The acoustic
signals 43, 43' are generated by transducers 42, 42' in response to signals from radio
:
~; 15
. .
-~" 133~88 ::
frequency sources 45,45' connected by cables 47,47', respective1y. Radio frequency
sources 45, 45' are modulated, two horizontal lines at a time, by compressed video
signals from compression systems 53,53', respectively. For this purpose, a videocollator 91 supplies the compression systems 53,53' with one horizontal line signal
92 and the next horizontal line signal 93, respectively. For this purpose, the input
signal 51 is sampled by the video collator 91 one horizontal line at a time. Forexample, when the first line is sampled, it is stored by the video collator 91 until the
next line is sampled. After the second line is sampled, the video collator sendssignals 92,93, which represent two lines of complete video at the same time.
Accordingly, signals 92,93 cause acoustic waves 43,43' to propagate across crystal
32 simultaneously to absorber 44, while laser 11 provides a light pulse 12 when the
full horizontal lines have propagated completely into crystal 32. As in the previous
embodiments, the width of the light pulse is less than or equal to a pixel of time.
Accordingly, two modulated output beams 61,61' exit from crystal 32 to cylindrical
lenses 54, 56, respectively. Lenses 54,56 serve to focus the output beams 61, 61' for
display on projection surface 81. The focused light beams 61, 61' exit lenses 54, 56,
respectively, and are transmitted to cylindrical lens 58. Lens 58 is used to further
focus the light beam set 61, 61' prior to introduction to lens 63. The output from
lens 58 is then
- 16
- l33à~s
transmitted to cylindrical lense 63 which is used for output projection.
Lense 63 focuses the mcdulated light beams into light beams 65, 65'
which are projected onto optical slit 67. As discussed supra, optical
slit 67 is positioned to block undiffracted light 64 exiting from the
Bragg cell 31. The light beam set 69, 69' emitting from slit 67 is
projected on a frame scanning mirror 71. Frame scanning murror 71
positions the light signal appropriately onto projection surface or
screen 81. For this purpose, after frame scanning mlrror 71 reflects
signals 69, 69', these signals are transmitted to cylindrical lense 100
which is used to adjust the distan oe between the parallel output beam
set 69, 69' when projected onto projection surfa oe 81.
The video signals 92, 93 may also be supplied by a parallel souroe ,
such as a separate video souroe for each signal respectively, instead of
using the video collator 91. This would be used for parallel Lnput. -
It should also be noted that the multi-l;ne writing is not llmited to
I two lines as described above. This description serves as an
illustrative purpose. Any reasonable numbex of lines oould be used,
such as three or more. ~
The ~lti-line writing allows an increase in the vertical
resolution of the projection without increasing band width of the
inooming video. For example, if double line writing were used, and the
requested output resolution was limited to 525 lines, or NISC standard,
j then the light source wculd only have to pulse on oe for every two
1 horizontal lines, which would be appxoximately 8000 HZ. Accordingly,
~ because the laser source 11 would only be requixed to pulse half as much
j t~me, as discussed suPra, the operational life of the laser source 11
would be substantially longer.
~l Also, if the lasex source 11 is pulsed at appxoximately sixteen
thousand Hertz which is the rep rate used for displaying a 525-line
(NTSC-standard) video picture, and triple line writing is used, then the
output vertical resolution could exceed 1500 lines. As discussed above,
- the input signal scan lines could be from parallel sources rather than a
single sour oe .
:
......
t33~58~
In addition to the laser sour oe 11 lasting longer, if the
repetition rate is reduced, the average laser power can also be
increased because a lower repetition rate generally increases laser
energy to light efficiency.
m e abave system of multi-line writ mg can also be used with
multi-color projectors in the same manner as set out in the description
of Figure 2.
Although the system described in detail above is most satisfactory
and preferred, many variations in structure and method are possible.
Many of these variations have been set out above and are examples of
possible changes or variations. Also, for example, the sour oe for any
color could consist of more than one laser having substantially the
same, or similar, frequency. They are not to be considered exhaustive.
Because many varying and different embocinents may be made within
the soope of the inventive con oept herein taught and be.cause
modification may be made in accordan oe with the descriptive requirements
of the law, it should be understood that the details herein are to be
interpreted as illustrative and not in a limiting sense.
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