United States Patent
`Pinnow et al.
`
`3,699,478
`[15]
`£451 Oct. 17, 1972
`
`£541 DISPLAY SYSTEM
`Pinnow, Berkeley
`(72]
`Inventors: Douglas A.
`Heights; Le Grand G. Van Uitert,
`Morris Township, Morris County,
`both of N.J.
`[ 7 3] Assignee: Bell Telephone Laboratories, Incor(cid:173)
`porated, Murray Hill, N.J.
`May 26, 1969
`[22] Filed:
`[21] Appl. No.: 827,644
`
`[52] U.S. Cl .................... 332/7.51, 250/199, 178/6.8
`Int. Cl ................................................. HO Is 3/00
`[51 ]
`[58] Field of Search ......... 332/7.51; 250/71,80, 199;
`330/334, l 08; 252/301.4; 340/324, 173;
`178/6.8
`
`[56]
`
`References Cited
`
`3,541,542
`3,495,034
`3,341,825
`
`UNITED STATES PATENTS
`11/1970
`2/1970
`9/1967
`
`Duguay et al... ........... 340/324
`Areno et al... ............. 350/l60
`Schrieffer .................. 340/ 173
`
`3,453,604
`3,322,682
`3,374,381
`3,474,248
`3,488,503
`3,513,346
`3,524,011
`
`7/1969 Geusic et al. ............... 330/4.3
`5/1967 Thompson .............. 252/30 1.4
`3/1968 Albinak et al ............. 313/1 08
`10/1969 Brown et al. ................ 250/71
`1/1970 Reich et al... .............. 250/199
`5/1970 Awazu et al. .............. 313/1 08
`8/1970 Korpel... .................... 250/l99
`
`FOREIGN PATENTS OR APPLICATIONS
`France .................... 252/30l.4
`1,564,271
`· 3/1969
`
`Primary Examiner-Benjamin A. Borchelt
`Assistant Examiner-N. Moskowitz
`Attorney-R. J. Guenther and Edwin B. Cave
`
`[57]
`
`ABSTRACT
`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(cid:173)
`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
`
`LASER 10
`
`CERIUM DOPED
`
`[_ II MODULATOR 12
`:~l DEFlfCJOR 14
`~~~~~~OR
`'·~.-~~---
`~~- W'.
`I
`\
`;/;A
`
`.
`
`Vizio EX1006 Page 0001
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`PATENTED ocr 1 7 1972
`
`3.699.478
`
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`FIG. I
`
`EXCITATION
`SPECTRUM
`
`' \
`\
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`
`.4416p
`(CADMIUM ION
`LASER LINE)
`
`FIG. 2
`/II MODULATOR 12
`-~ /
`. ...._~~3 DEFLECiOR 14
`
`CERIUM DOPED
`GARNET PHOSPHOR
`SCREEN 15
`
`.... id, -----(cid:173)
`~---r;l'.
`·\_~'
`
`D. A. PINNOW
`INVENTORS L. G. VAN UIT£RT
`
`BV /.~ ~ ATTORNEV
`
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`

`

`1
`DISPLAY SYSTEM
`
`3,699,478
`
`2
`BRIEF DESCRIPTION OF THE ORA WING
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The invention is concerned with projection display
`systems and is primarily concerned with those produc(cid:173)
`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 IS
`uses.
`One popular approach, production of images by
`direct reflection of visible emission is subject to two
`drawbacks. First, images are monochromatic of a par(cid:173)
`that
`images 20
`ticular well-defined wavelength
`so
`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
`Vol. 46, Bell System Technical Journal, p. 1,479, Sept. 25
`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(cid:173)
`pends upon the use of a phosphorescent screen of ceri- 35
`urn-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(cid:173)
`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(cid:173)
`bodiment of the invention utilizes a screen coated with 45
`cerium-doped yttrium aluminum garnet (YAG) ener(cid:173)
`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(cid:173)
`positions are cerium-activated and utilize a host of the
`garnet structure (i.e. the structure of Y 3AI50 1t) since
`this is the only known combination to produce reemis- 55
`sion· of appropriate color and brightness. The absorp(cid:173)
`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(cid:173)
`lium to shift the absorption to shorter wavelength, or 60
`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 65
`white image) by reflection of a portion of the laser
`beam continues to be feasible. Other variants are
`discussed.
`
`FIG. 1, on coordinates of relative intensity based on
`a maximum scale value of 100, and wavelength in
`5 microns, is a plot of the emission and associated excita(cid:173)
`tion spectra for unmodified cerium-doped Y AG; and
`FIG. 2 is a perspective view of a system in ac(cid:173)
`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(cid:173)
`doped Y AG. The emission spectrum is in broken out(cid:173)
`line with the broad peak of concern having its max(cid:173)
`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(cid:173)
`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(cid:173)
`cordance with
`the
`invention. Energizing
`light
`is
`produced by laser 10 which may, for example, be an
`30 argon-ion laser or a cadmium-ion laser. The emerging
`beam 11 first enters modulator 12 which is provided
`with a modulating signal by means, not shown, for am(cid:173)
`plitude modulating the beam. Modulation may be ac-
`complished, for example, by electro-optic, acousto-op(cid:173)
`tic, or magneto-optic techniques.
`A description of suitable acousto-optic devices is
`contained in Vol. 46, BSTJ, p. 367, February, 1967. A
`is
`description of suitable electro-optic devices
`described in Vol. 38, Journal of Applied Physics, pp.
`1 ,611-1 ,617, March, 196 7. 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(cid:173)
`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(cid:173)
`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(cid:173)
`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 Ce3+ is generally in the near ul-
`
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`

`

`3,699,478
`
`4
`minimum concentration required for a reemiSSion
`image discernible in ordinary room lighting, and the
`upper limit is occasioned by the fact that further in(cid:173)
`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:
`
`15
`
`3
`traviolet. However, it is known, probably due to the
`large crystal field splittings in garnet such as Y AG, that
`emission may be shifted to the visible. As seen from
`FIG. 1, the emission for Y AG:Ce3+ is quite broad with a
`peak at about 0.55 micron (yellowish white). The peak 5
`absorption in that lattice centers about 0.46 micron
`and this absorption spectrum is suitable for use either
`with the argon (0.4884J.£) or cadmium (0.44l6J.£) laser.
`As discussed in a subsequent section, there may be no
`particular advantage gained by shifting the absorption 10
`to exactly coincide with the laser emission.
`While reference has been made to "absorption spec(cid:173)
`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, Y 3Al50 12 , may be
`modified by partial or total substitution of gallium for
`aluminum and/or gadolinium for yttrium. The former 25
`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(cid:173)
`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
`peak of about 0.43 micron which results in a Y AG
`composition modified by substitution of about 45 atom
`percent gallium for aluminum). From the same stand(cid:173)
`point for this preferred embodiment, the phosphor
`should not be modified so as to result in an excitation
`peak at wavelengths greater than about 0.58J.£ (or,
`more properly energization should not exceed this
`limit) since even ineffective conversion will result in
`addition of some longer wavelength light and, there(cid:173)
`fore, will impart a yellowish tinge to the reflected emis(cid:173)
`sion. Y AG in which 70 atom percent yttrium is
`replaced by gadolinium corresponds with this condition
`and, therefore, this represents the maximum uncom(cid:173)
`pensated partial substitution of gadolinium for the
`preferred embodiment.
`in ac(cid:173)
`Phosphor compositions suitable for use
`cordance with the invention invariably depend upon
`cerium activation. A suitable cerium range is from
`about 0.00 I to about 0.15 atom per formula unit of gar(cid:173)
`net (based on the stoichiometry Y3Al50 12 ). (Ce3+ sub(cid:173)
`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 0.1 0. The lower limit is based on
`
`(I)
`
`in which
`x is from 0.00 I to 0.15 or in the preferred range, is
`from 0.005 to 0.0 I,
`Y is from 0 to 2. 999 and
`z is from 0 to 3.0.
`Certain other substitutions are possible. So, for ex(cid:173)
`ample, lutecium or lanthanum may be substituted for
`yttrium, and indium or scandium may replace alu-
`20 minum in part. However, since suitable excitation and
`emission spectra may be obtained in the more common
`and more economical Y AG or substituted Y AG
`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(cid:173)
`cant aspect of the invention is concerned with white or
`near-white images. In the unmodified Y AG:Ce system
`30 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
`35 for some reflection such that total absorption does not
`result.
`Modification of the YAG:Ce system within the com(cid:173)
`positional range described may shift the emission so
`that it needs no compensation. This may be accom-
`40 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
`45 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 Y 2.99Ce 0m
`Als0 12 • It was found that approximately 50 percent of
`50 the energy of a one watt 0.488J.£ argon beam was ab(cid:173)
`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
`55 laser energy. The approximate 25 percent of the con(cid:173)
`verter laser energy suffices to compensate for the yel(cid:173)
`lowish cast of the reemission.
`It is apparent that final design of a phosphor screen
`depends upon power levels, laser wavelength, phosphor
`60 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
`65 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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`3,699,478
`
`s
`achieved between the laser wavelength and the emis(cid:173)
`sion wavelength. Display systems have been discussed
`in terms of one exemplary arrangement. Variations
`may utilize a laser source which is behind rather than in
`front of a screen and a variety of other arrangements 5
`for folding beams, for modulation, for deflection, etc.
`What is claimed is:
`1. Visual display apparatus comprising a laser for
`emitting at a wavelength in the visible spectrum, first
`means for amplitude modulating the output of such
`laser, second means for deflecting said beam, and a
`screen, characterized in that said screen comprises a
`layer of a phosphorescent composition consisting es(cid:173)
`sentially of a material which may be represented by the
`formula Y a-...-11Ce..,Gd.,Ais-zGaz012 in which x is from 15
`0.001 to 0.15, y is from 0 to 2.999 and z is from 0 to
`3.0, in which apparatus the said laser emits at a
`wavelength between 0.3 and 0.531£ and the said
`phosphorescent composition appears to the eye to
`fluoresce essentially white, it being a characteristic of 20
`such apparatus that a visual display resulting from use
`is essentially free from speckle.
`
`6
`2. Apparatus of claim 1
`the said
`in which
`phosphorescent composition and screen design are
`such that a portion of the laser emission is unconverted
`so that the combination of reflected laser emission and
`the #£ emission from the screen appears approximately
`white.
`3. Apparatus of claim 1 in which the laser is an
`argon-ion laser.
`4. Apparatus of claim 1 in which the laser is a cadmi-
`10 urn-ion laser.
`5. Apparatus of claim 1 in which the phosphor com(cid:173)
`position consists essentially of Y a-..,Ce..,AI50 12•
`6. Apparatus of claim 1 in which the said first means
`is an electro-optic modulator and in which the said
`second means is an acousto-optic deflector.
`7. Apparatus of claim 1 in which the said first and
`second means depend upon an acousto-optic interac(cid:173)
`tion.
`8. Apparatus of claim 7 in which said first and second
`means constitute a single unit.
`* * * * *
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