United States Patent
`Pinnow et al.
`
`3,699,478
`[15]
`[45] Oct. 17, 1972
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`* ...
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`-
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`- -
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`Geusic et al................ 330/4.3
`7/1969
`3,453,604
`[54] DISPLAY SYSTEM
`[72] Inventors: Douglas A. Pinnow, Berkeley ; #: A.- - - - - - - - - - - :*:
`*: * * *... 3,474,248 10/1969 Brown et al................. 250/71
`b º of N * p, Morri
`* 3,488,503
`1/1970
`Reich et al................. 250/199
`O
`** *
`3,513,346
`5/1970 Awazu et al............... 313/108
`[73] Assignee: Bell Telephone Laboratories, Incor-
`3,524,011
`8/1970
`Korpel....................... 250/199
`-
`porated, Murray Hill, N.J.
`FOREIGN PATENTS OR APPLICATIONS
`[22] Filed:
`May 26, 1969
`1,564,271
`3/1969
`France.................... 252/301.4
`[21] Appl. No.: 827,644
`Primary Examiner—Benjamin A. Borchelt
`Assistant Examiner—N. Moskowitz
`-
`-
`[52] U.S. Cl.................... 332/7.51, 250/199, 178/6.8
`[51] Int. Cl................................................. tº Attorney–R. J. Guenther and Edwin B. cave
`[58] Field of Search ......... 332/7.5 l; 250/71 3. 80, 199;
`[57]
`ABSTRACT
`330/334, 108; 252/301.4; 340/324, 173;
`- *
`•
`178/6.8
`A black and white display is produced by projection
`using a scanning argon laser beam operating at 4,880
`A and a phosphorescent screen of cerium-doped yttri
`um aluminum garnet which emits a broad range of
`frequencies centering about 5,500 A. The yellowish
`cast of the phosphor output is compensated by a small
`amount of reflected blue argon light.
`8 Claims, 2 Drawing Figures
`
`[5 s
`
`References Cited
`UNITED STATES PATENTS
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`: ;: ? ; º • * * * * sº e s - - - - e. :::::::
`3.341.825
`9/1967
`Schrieffer.................. 340/173
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`LASER 10
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`A. MODULATOR 12
`* ~ `s y DEFLECTOR |4
`Š()
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`§§ #.OR
`SCREEN 15
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`PATENTEDDCI 17 (972
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`3,699.478
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`AT/G. ?.
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`EXCITATION
`SPECTRUM
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`A
`|
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`!
`—EMISSION
`SPECTRUM
`P
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`| 00
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`80
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`6 O
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`4 O : 2 O
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`44164
`(CADM10M ION
`LASER LINE)
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`:488
`(ARGóN ION
`LASER LINE)
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`LASER 10
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`/*/G. 2
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`####!of
`sº 2. MODULATOR 2
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`13
`DEFLECTOR 4 º
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`1
`DISPLAY SYSTEM
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The invention is concerned with projection display
`systems and is primarily concerned with those produc
`ing black and white images.
`2. Description of the Prior Art
`Interest in laser display systems is based on the
`premise of screens of essentially unlimited size. Many
`10
`of the elements necessary for such systems are
`presently available. High intensity lasers operating at a
`variety of frequencies within the visible spectrum have
`been demonstrated as having modulation and scanning
`techniques of sufficient capacity for most projected
`UlS?S.
`One popular approach, production of images by
`direct reflection of visible emission is subject to two
`drawbacks. First, images are monochromatic of a par
`20
`ticular well-defined wavelength so that images
`produced by use of an argon-ion laser, for example,
`may be blue and black; and, second, reflection of
`coherent laser output produces a speckled image due
`to periodic reinforcement of the scattered beam. See
`25
`Vol. 46, Bell System Technical Journal, p. 1,479, Sept.
`1967.
`So far as is known, there is no demonstrated or
`proposed laser visual display system producing black
`and white images free from speckle problems.
`SUMMARY OF THE INVENTION
`A laser display system results in a black and white
`image with a minimum of speckling. The system de
`pends upon the use of a phosphorescent screen of ceri
`35
`um-activated garnet energized by a laser emitting in the
`visible at a somewhat shorter wavelength than the bulk
`of the emission from the screen. In a preferred arrange
`ment yttrium aluminum garnet containing cerium is
`used. The characteristically yellowish cast of the emis
`40
`sion from this phosphor as seen by the eye is adjusted to
`appear more nearly white by deliberate reflection of a
`portion of the laser emission.
`From the compositional standpoint, a preferred em
`bodiment of the invention utilizes a screen coated with
`45
`cerium-doped yttrium aluminum garnet (YAG) ener
`gized by an argon-ion laser arranged so as to emit at
`4,880 A. The cerium-activated phosphor emits over a
`broad range of wavelengths centering about 5,500 A.
`Variations include other laser sources, such as a cad
`50
`mium-ion laser which may emit at 4,416 A, as well as
`variations in the phosphor composition. All such com
`positions are cerium-activated and utilize a host of the
`garnet structure (i.e. the structure of YsAl:01a) since
`this is the only known combination to produce reemis
`55
`sion of appropriate color and brightness. The absorp
`tion peak for the phosphor may, however, be shifted to
`more closely match a particular energizing source; and
`to this end, aluminum may be partially replaced by gal
`lium to shift the absorption to shorter wavelength, or
`yttrium may be replaced, in whole or in part, by
`gadolinium to shift the absorption to longer
`wavelength. Since a shift in absorption generally
`produces a corresponding shift in emission in the same
`direction, color adjustment (for example, to produce a
`white image) by reflection of a portion of the laser
`beam continues to be feasible. Other variants are
`discussed.
`
`2
`BRIEF DESCRIPTION OF THE DRAWING
`FIG. 1, on coordinates of relative intensity based on
`a maximum scale value of 100, and wavelength in
`microns, is a plot of the emission and associated excita
`tion spectra for unmodified cerium-doped YAG; and
`FIG. 2 is a perspective view of a system in ac
`cordance with the invention.
`DETAILED DESCRIPTION
`1. Drawing
`Referring again to FIG. 1, the data presented are the
`emission and related excitation spectra for cerium
`doped YAG. The emission spectrum is in broken out
`line with the broad peak of concern having its max
`imum value at a wavelength of about 0.55 micron. The
`associated excitation spectrum shown in solid outline is
`a measure of the intensity of the emission noted for
`various pump frequencies. The most pronounced ex
`citation peak coincides with a pump wavelength of
`about 0.46 micron. The emission wavelengths for two
`prominent laser lines are also indicated. The first, for
`the argon-ion laser, is at 0.488 micron. The second, for
`the cadmium-ion laser, is at 0.4416 micron. The laser
`lines are shown as solid vertical lines.
`FIG. 2 is a perspective view of a simple system in ac
`cordance with the invention. Energizing light is
`produced by laser 10 which may, for example, be an
`argon-ion laser or a cadmium-ion laser. The emerging
`30
`beam 11 first enters modulator 12 which is provided
`with a modulating signal by means, not shown, for am
`plitude modulating the beam. Modulation may be ac
`complished, for example, by electro-optic, acousto-op
`tic, or magneto-optic techniques.
`A description of suitable acousto-optic devices is
`contained in Vol. 46, BSTJ, p. 367, February, 1967. A
`description of suitable electro-optic devices is
`described in Vol. 38, Journal of Applied Physics, pp.
`1,611–1,617, March, 1967. In any event, modulation
`may be accomplished by altering the total amount of
`light of a particular polarization sense which is passed
`by an analyzer incorporated in the modulator, or alter
`natively by controlling the amount of light which is
`deflected acousto-optically.
`Upon emerging from modulator 12, the beam, now
`denoted 13, enters deflector 14 which produces the ap
`propriate horizontal and vertical deflection so as to fill
`screen 15. Deflector 14 may advantageously operate
`on an acousto-optic principle, see, for example, Vol. 57
`Proceedings of the IEEE, pg. 160, Feb., 1969. The
`deflector 14 may also perform the modulation function
`eliminating the need for a separate modulator 12. Earli
`er deflector systems utilize mechanical, sometimes
`motor driven, scanners.
`Inventive novelty is premised largely on the nature of
`phosphor screen 15 as incorporated in the overall
`system. Laser display systems of the general nature of
`that of FIG. 2 are described in some detail in the exist
`60
`ing scientific literature. See, for example, IEEE Spec
`trum for December 1948 at page 49, et seq.
`The chemical nature of this screen is described in
`some detail in the section which follows.
`2. Composition
`The inventive system depends upon a phosphor
`screen containing trivalent cerium in an appropriate
`host. Emission of Ce” is generally in the near ul
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`minimum concentration required for a reemission
`image discernible in ordinary room lighting, and the
`upper limit is occasioned by the fact that further in
`crease results in little improvement. Its preference is
`based largely on economics (as compared with the
`broad maximum above).
`In view of the above considerations, the overall
`phosphor limits may be set forth as:
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`traviolet. However, it is known, probably due to the
`large crystal field splittings in garnet such as YAG, that
`emission may be shifted to the visible. As seen from
`FIG. 1, the emission for YAG:Ce” is quite broad with a
`peak at about 0.55 micron (yellowish white). The peak
`absorption in that lattice centers about 0.46 micron
`and this absorption spectrum is suitable for use either
`with the argon (0.4884p1) or cadmium (0.4416A) laser.
`As discussed in a subsequent section, there may be no
`particular advantage gained by shifting the absorption
`to exactly coincide with the laser emission.
`While reference has been made to “absorption spec
`trum,” only the absorbed energy which is converted to
`visible emission as discussed, is of consequence for the
`purposes of the invention. Absorbed energy usefully
`converted in this fashion may be represented in terms
`of an “excitation” spectrum, and it is in these terms
`that the data of FIG. 1 is represented.
`The excitation spectrum in the cerium-doped garnet
`may be shifted to accommodate the lasers discussed or
`to more effectively utilize other laser sources. To this
`end, the prototypical composition, Ya Also,2, may be
`modified by partial or total substitution of gallium for
`aluminum and/or gadolinium for yttrium. The former
`has the effect of moving the excitation peak to shorter
`wavelength while the latter has the opposite effect. The
`peak of the excitation spectrum may be tailored in this
`manner within the range of from about 0.33 micron to
`about 0.48 micron; however, useful excitation may be
`accomplished over the broader range of about 0.30
`micron to 0.53 micron.
`A shift in the excitation spectrum produces an ac
`companying shift in the emission spectrum with the
`range of emission peaks being from about 0.51 micron
`to about 0.61 microns. For the preferred embodiment
`designed to produce a white or near-white image, the
`emission peak should not be at wavelengths less than
`about 0.52 micron (corresponding with an excitation
`40
`peak of about 0.43 micron which results in a YAG
`composition modified by substitution of about 45 atom
`percent gallium for aluminum). From the same stand
`point for this preferred embodiment, the phosphor
`should not be modified so as to result in an excitation
`peak at wavelengths greater than about 0.58pm (or,
`more properly energization should not exceed this
`limit) since even ineffective conversion will result in
`addition of some longer wavelength light and, there
`fore, will impart a yellowish tinge to the reflected emis
`sion. YAG in which 70 atom percent yttrium is
`replaced by gadolinium corresponds with this condition
`and, therefore, this represents the maximum uncom
`pensated partial substitution of gadolinium for the
`preferred embodiment.
`55
`Phosphor compositions suitable for use in ac
`cordance with the invention invariably depend upon
`cerium activation. A suitable cerium range is from
`about 0.001 to about 0.15 atom performula unit of gar
`net (based on the stoichiometry YsAlso,2). (Ce” sub
`60
`stitutes for yttrium and therefore reduces the amount
`of this action by an equal amount). The lower limit on
`cerium content represents the minimum concentration
`resulting in a readily discernible reemission image,
`while the maximum approximately coincides with the
`solubility limit in the garnet. A preferred cerium range
`is from 0.005 to 0.10. The lower limit is based on
`
`( !)
`Ya-r-,Ce:Gd,Als-,Ga,OI2
`in which
`x is from 0.001 to 0.15 or in the preferred range, is
`from 0.005 to 0.01,
`Y is from 0 to 2.999 and
`z is from 0 to 3.0.
`Certain other substitutions are possible. So, for ex
`ample, lutecium or lanthanum may be substituted for
`yttrium, and indium or scandium may replace alu
`minum in part. However, since suitable excitation and
`emission spectra may be obtained in the more common
`and more economical YAG or substituted YAG
`system, it is not expected that further modifications will
`go into commercial use.
`3. Design criteria
`While occasions may arise in which it is desired to
`produce colored or off-white images, the more signifi
`cant aspect of the invention is concerned with white or
`near-white images. In the unmodified YAG:Ce system
`using an argon or cadmium laser, white images may
`result by compensation of the secondary yellow cast
`emission by some reflection of the shorter wavelength
`laser emission. Under these circumstances it is desired
`to design layer thicknesses and compositions or provide
`for some reflection such that total absorption does not
`result.
`Modification of the YAG:Ce system within the com
`positional range described may shift the emission so
`that it needs no compensation. This may be accom
`plished, for example, by partial substitution of gallium
`aluminum in the 20 to 60 percent range per formula
`unit. Under such circumstances, the phosphor layer is
`designed so as to result in little or no reflection. This
`may be accomplished by providing for essentially
`complete absorption and minimal reflection.
`In one experimental arrangement, apparently white
`images resulted from use of the composition Y2.9°Ceo.o.
`Also,2. It was found that approximately 50 percent of
`the energy of a one watt 0.488p, argon beam was ab
`sorbed in a layer thickness of about 0.4 millimeters.
`The image could be further intensified by providing a
`mirror backing thereby resulting in total absorption
`(within the excitation band) of about 75 percent of the
`laser energy. The approximate 25 percent of the con
`verter laser energy suffices to compensate for the yel
`lowish cast of the reemission.
`It is apparent that final design of a phosphor screen
`depends upon power levels, laser wavelength, phosphor
`absorption level and emission wavelength. Reflection
`of unconverted laser emission may be enhanced by
`using thinner coatings, by reflective backings (although
`this also results in additional secondary emission during
`retraversal) and by incorporation of “inert” material
`such as talc.
`In the main, inventive novelty is premised on
`phosphor composition and the chromaticity balance
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`2. Apparatus of claim 1 in which the said
`achieved between the laser wavelength and the emis
`sion wavelength. Display systems have been discussed
`phosphorescent composition and screen design are
`in terms of one exemplary arrangement. Variations
`such that a portion of the laser emission is unconverted
`may utilize a laser source which is behind rather than in
`so that the combination of reflected laser emission and
`front of a screen and a variety of other arrangements
`the pi emission from the screen appears approximately
`for folding beams, for modulation, for deflection, etc.
`white.
`What is claimed is:
`3. Apparatus of claim 1 in which the laser is an
`1. Visual display apparatus comprising a laser for
`argon-ion laser.
`emitting at a wavelength in the visible spectrum, first
`4. Apparatus of claim 1 in which the laser is a cadmi
`means for amplitude modulating the output of such
`um-ion laser.
`laser, second means for deflecting said beam, and a
`5. Apparatus of claim 1 in which the phosphor com
`screen, characterized in that said screen comprises a
`position consists essentially of Ya-2 Ce, Algola.
`layer of a phosphorescent composition consisting es
`6. Apparatus of claim 1 in which the said first means
`sentially of a material which may be represented by the
`is an electro-optic modulator and in which the said
`formula Ya-r-,Ce,Gd,Als—,Ga,OI2 in which x is from
`second means is an acousto-optic deflector.
`0.001 to 0.15, y is from 0 to 2.999 and z is from 0 to
`7. Apparatus of claim 1 in which the said first and
`3.0, in which apparatus the said laser emits at a
`second means depend upon an acousto-optic interac
`wavelength between 0.3 and 0.53 u and the said
`tion.
`phosphorescent composition appears to the eye to
`8. Apparatus of claim 7 in which said first and second
`fluoresce essentially white, it being a characteristic of 20
`means constitute a single unit.
`such apparatus that a visual display resulting from use
`is essentially free from speckle.
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