Photoluminescent Conversion of Laser Light for
`Black and White and Multicolor Displays. 1: Materials
`
`L. G. Van Uitert, D. A. Pinnow, and J. C. Williams
`
`A number of photoluminescent materials have been found to have properties that make them extremely
`useful for improving the quality of laser displays as well as offering simplifications for multicolor systems.
`The principal function of these materials is that of color conversion when coated onto a laser illuminated
`viewing screen. A secondary, yet significant, role is that of rendering the converted light incoherent.
`This eliminates the unpleasant granular or speckly texture associated with direct viewing of diffusely scat-
`tered coherent light. It is concluded that virtually any visible color may be achieved by photoluminescent
`conversion of a monochromatic blue or ultraviolet laser beam.
`
`1.
`
`Introduction
`In a recent review of laser display technology Baker'
`pointed out that considerable motivation in this field
`stems from the promise of producing a cathode ray tube
`type of display with essentially unlimited screen size.
`He concluded that although adequate light beam modu-
`lation and scanning techniques are presently available,
`broadly applicable equipment awaits the development
`of an efficient multicolor laser source.
`Although such a multicolor source would indeed be
`desirable, the prospects for its realization, even in the
`future, are speculative. However, it is clear
`distant
`that efficient monochromatic or limited color range laser
`sources can and will be developed.2
`The purpose of the
`laser
`present work is to show that a monochromatic
`source is satisfactory for white light and multicolor
`displays when advantage is taken of photoluminescence,
`the absorption of light of one wavelength and the sub-
`sequent emissions at another wavelength. By prop-
`erly coating a viewing screen with existing organic and
`inorganic phosphors it is possible to efficiently convert
`into
`light
`laser
`monochromatic blue or ultraviolet
`virtually any visible color including white. An addi-
`tional benefit of this conversion is the elimination of
`the unpleasant granular texture generally associated
`with direct viewing of diffusely scattered coherent
`light,3 because the converted light is incoherent.
`The basic physics of photoluminescent conversion is
`quite simple. Photons from a light beam such as a
`laser beam are absorbed in a material which is thereby
`raised to an excited state. This excitation equilibrates
`in a brief interval, typically 1O- sec to 10- sec. Equil-
`
`The authors are with Bell Telephone Laboratories, Inc., Mur-
`ray Hill, New Jersey 07974.
`Received 16 March 1970.
`
`150 APPLIED OPTICS / Vol. 10, No. 1 / January 1971
`
`ibration can proceed both radiatively by the emission of
`a photon and nonradiatively by, for example, a series of
`phonon interactions. A material is considered to be a
`phosphor if radiative emission is observed. The rela-
`tive strength of radiative transitions is specified in terms
`of the phosphor's quantum efficiency, defined as the
`ratio of emitted photons to absorbed photons. In
`general, the energy of the emitted photons is less than or
`equal to the energy of the absorbed photon (Stokes's
`law). That is, the color of the emitted light is either
`unchanged or shifted in the direction of longer wave-
`lengths. In certain limited cases anti-Stokes (shorter
`wavelength) emission is also possible when additional
`energy is supplied to the single photon excited state by
`other means such as thermal excitation or multiple
`photon absorption. 4
`In the present work we will con-
`sider only Stokes emitting phosphors which have suf-
`ficiently high quantum efficiencies to be of interest for
`Some emphasis will be
`laser display applications.
`placed on those materials that can be excited by the
`the most suitable
`argon ion laser which is presently
`source for a laser display system.
`
`II. Characterization
`There are four basic properties that characterize
`photoluminescent materials. They are (1) absorption
`(3)
`lifetime,
`(2) conversion
`and emission spectra,
`quantum efficiency, and (4) absorption cross section.
`It should be noted that the last three properties can be
`functions of the exciting wavelength. The features
`that make a phosphor desirable for application in laser
`to these properties.
`display systems can be related
`First, the phosphor must have a high absorption cross
`section for the exciting laser wavelength so that nearly
`total absorption can take place in a thin layer of mate-
`rial that is coated onto a screen. The phosphor should
`in a desired wavelength
`have an emission spectrum
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`

`
`range with a high quantum efficiency, say, 50% or
`greater, since the display screen size is limited by avail-
`able light intensity.
`In addition, the conversion
`lifetime should be sufficiently short so that the screen
`will not be overly persistent;
`there is no perceptible
`short.5
`consequence
`if the
`lifetime is exceptionally
`Finally, the phosphor should be inexpensive, stable, and
`preferably nontoxic.
`Ill. Materials
`Rather than attempting to cover the broad field of
`phosphors, we will discuss, by way of particular ex-
`amples, those materials that we consider to be par-
`ticularly
`suitable
`for laser display systems. They
`generally fall into two categories, inorganic rare-earth
`phosphors and organic dye phosphors. To a lesser ex-
`tent we will also consider inorganic compounds that are
`activated by transition metals and other organic mate-
`rials such as the aromatic hydrocarbons.
`
`A.
`Inorganic Phosphors
`According to Pringsheim,' practically all molecules
`that are photoluminescent
`in condensed states are
`rather complex. The only exceptions are the positive
`ions of some rare-earth metals. The optical properties
`of these ions are so little perturbed by the surrounding
`medium that, even in crystals, they behave almost like
`isolated atoms, and their energy levels are well known.7
`The rare-earth Tb 3 + is a particularly useful ion since
`the wavelength for maximum absorption of its
`'D4
`manifold corresponds quite closely to the blue (4880
`A) emission of the argon ion laser. When this ion is
`embedded in a properly chosen host, such as a tungstate
`or a molybdate,
`it will emit upon excitation a strong
`greenish-yellow color which has a peak at approximately
`5440 A.7
`The
`lifetime is approximately 0.5 msec,
`and the quantum efficiency is high, approaching 100%.
`We have examined the performance of Nao.5Tbo.5W0 4
`under excitation by an argon laser (4880 ) and find
`that 1-mm thick sample absorbs approximately 50%
`of the incident beam. This absorption cross section is
`substantially
`lower than optimum since a considerable
`thickness of material would be required to coat a display
`screen.
`
`1.0
`
`Z
`
`W 0.5-
`
`ABSORPTION
`
`/Eu
`
`EuEMISSION
`
`Tb
`
`/EU\
`
`5 000
`4 880
`
`6000
`
`7000
`
`WAVE LENGTH (A)
`Fig. 1. Relative absorption spectra of Tb'+ and Eu'+ and the
`emission spectra of Eu 3
`+. When these two rare-earth ions are
`included in the same host, such as Nao.5Tbo.25Euo.25WO4 , absorp-
`tion of argon laser radiation at 4880 A is due to Tb3 +while excita-
`tion transfer to Eu + results in the characteristic Eu + emission
`spectrum which peaks in the red at 6140 A.
`
`I-
`z
`
`4In O.~
`
`J
`
`4 880A
`
`5 000
`WAVE LENGTH (A)
`
`Fig. 2. Relative absorption and emission spectra of YAG: Ce.
`The broad absorption band is due to a d-band excitation.
`
`It is possible to achieve other colors by photolumines-
`cent conversion using different rare-earth
`ions. For
`example, Sm3 + has a peak emission at 5980 A, Eu3 + at
`6140 A, Dy3+ at 5740 A, and Er'+ at 5520 A.7 How-
`ever, none of these transitions other than those for Tb3 +
`can be directly excited to any extent by the 4880-A line
`of the argon
`laser. Generally, shorter wavelength
`excitation is required. However, indirect excitation of
`Eu 3 + is possible through an intermediate Tb3 + ion.8
`For example, excitation transfer from Tb'+ to Eu3+ in
`Nac.5Tbo.25Euo0 25W04 causes this material to emit a
`strong red color under 4880-A illumination. The ab-
`sorption bands of Tb3+ and Eu3+ and the emission of
`Eu3 + are displayed as a function of wavelength in Fig.
`1. The absorption cross section, quantum efficiency,
`and lifetime of this material are similar to Nao.5Tbo.5-
`W04 discussed above.
`The Ce 3+ and EuS+ ions are exceptions to the general
`observation
`that the fluorescence of rare-earth
`ions is
`little affected by the host. This is so because their
`emissions are from d-bands which strongly interact with
`the crystal field.9 A rather unusual but useful material
`is made by adding cerium to Y3AlO 5 (YAG). The
`lower excited states of the crystal field components of
`the 5d configuration of the YAG:Ce composition are
`sufficiently low in energy that absorption of 4880-A
`light becomes appreciable. 10 Figure 2 shows the ab-
`sorption and emission spectra in detail. Note that the
`peak of the emission spectrum occurs at 5500 A, the
`wavlength at which the eye is most sensitive. We
`have found that at 4880 A the absorption cross section
`is approximately 30 dB/mm/wt % of Ce added to the
`YAG host. In addition to this relatively large absorp-
`tion cross section, this compound has a very short life-
`time of approximately 0.07 usec (Ref. 11) and a quan-
`tum efficiency of approximately 70%. '0 These proper-
`ties make YAG: Ce very attractive
`for display screen
`applications. Furthermore,
`this material may be
`tuned for a particular use. By replacing some Y with
`Gd the peaks of the absorption and emission spectra
`shift to somewhat longer wavelengths, while replacing
`Alwith Ga causes the opposite effect.' 0 Other Ce3 + and
`Eu 2 + compositions
`investigated9 " 0
`that have been
`require excitation at wavelengths substantially shorter
`than 4880 A, generally in the ultraviolet. 4
`In addition to the rare-earth compounds, there are
`large classes of inorganic phosphors which have II-VI
`hosts such as ZnS and activators such as Bi, Mn, Cu,
`
`January 1971 / Vol. 10, No. 1 / APPLIED OPTICS 151
`
`VIZIO 1006
`
`

`
`has found considerable use in
`dayglow phenomenon
`to strong absorp-
`is due
`display
`and
`advertising
`tion of blue and green light with subsequent emission
`at the various longer wavelengths.' 3
`and dyes are em-
`The fluorescing hydrocarbons
`ployed in very dilute form in order to achieve optimum
`quantum efficiency. Usually pigments (solid particles
`2900-4600 violet
`that carry dye) are formed by dissolving the dye in an
`(3200)
`strong
`400-5200 51090 0 yellow-green organic resin solution which is subsequently condensed
`to an insoluble state by the application of heat.' 4
`In
`very strong
`(4940)
`(5180)
`44
`if the dye is
`certain cases the efficiency is enhanced
`500-5600 5200-6000 yellow
`adsorbed on colloids such as fibers or gel particles of
`the
`high polymers. 14 Representative materials and
`(blue),
`color which they fluoresce include pyrelene
`fluorescein (yellow-green), eosin (yellow), Rhodamine-B
`(yellow), acridine (blue), acri-
`(red), Rhodamine-6G
`flavine (yellow-green), naphthalene
`red (red), Auro-
`mine-O (yellow-green), and 7-diethylamino-4 methyl
`(blue) as well as other xanthene, azine, oxa-
`coumarin
`and
`acridine, flavin, naphthalimide,
`zine,
`thioazine,
`Additional data on the absorp-
`coumarin derivatives.
`tion and emission of selected dyes are given in Table I. 1
`We have examined in considerable detail a naph-
`4-amino-1,8-naphthal-p-xenylimide
`dye,
`thalimide
`(yellow-green), and two Rhodamine dyes (orange and
`red) that have been cast into an adhesive coated plastic
`film for commercial use (Minnesota Mining and Manu-
`facturing Company, Scotchcal Fluorescent Film, types
`3483
`,34S4, and 3485). Their emission spectra for
`4880-A excitation, which are shown in Fig. 3, exhibit
`peaks at 5200 A (yellow-green), 6000 A (orange), and
`6200 A (red). We have determined that their lifetimes
`are all considerably less than 1 sec, and their absorp-
`tion cross sections are so large that the entire laser beam
`is absorbed within the thin films which are approxi-
`mately 0.1 mm thick. Their absorption bands are
`quite broad, including essentially all the violet and blue
`and a portion of the green. We have estimated from
`analysis of reflection spectra reported by Ward' 6 that
`
`blue-violet
`medium
`4750-6400 green
`5050-6700
`greenish-
`(5850)
`yellow
`medium
`green weak
`4700-6500
`4850-6600 yellowish-
`green
`strong
`L000-6000 5500-7000 red strong
`41
`6000)0-7000redstron
`(0)
`(6000)
`(5240)
`vP11rnw-red
`(53Q0
`uvvs
`I------ ___
`
`av-5200
`.iv-5400
`
`1 1
`
`iv-5100
`uv-5000
`
`1 1
`
`000-4500
`3(
`
`4000-4800
`
`"3
`
`8
`
`3484
`
`3485
`
`1.0
`
`z 0-
`
`'WW
`; 0.5
`
`0
`
`5 000
`
`6 000
`5 500
`WAVE LENGTH (A)
`
`6 500
`
`Fig. 3. Relative emission spectra of naphthalimide dye (3485)
`and Rhodamine dyes (3483 and 3484) due to excitation by the
`4880-A line of an argon laser.
`
`4800-6300
`orange
`medium
`(5800)
`red medium
`5500-7000
`in A; peaks of bands
`
`in
`
`limits of bands
`
`tungstates, molybdates,
`and Ag, and many activated
`vanadates, phosphates, germanates, and silicates that
`also have strong band fluorescence.12 We have not
`to be particularly useful since
`found these materials
`most are not readily excited by argon radiation or are
`of inadequate quantum efficiency.
`
`B. Organic Phosphors
`There are a number of aromatic hydrocarbons and
`organic dyes that when properly treated exhibit strong
`fluorescence over a broad portion of the visible spec-
`trum. Under white light illumination several of these
`to glow with a particular color ranging from
`appear
`greenish-yellow through orange to bright red. This
`
`152 APPLIED OPTICS / Vol. 10, No. 1 / January 1971
`
`Fluorescence Bands of Dyes in
`Table I. Absorption and
`olutions, after Pringsheimlsa
`Aqueous or Alcoholic S
`
`First
`sorption
`aC
`band
`
`UV
`
`Fluorescence
`Color
`Band
`
`4(
`
`strong
`(5400)
`(5170)
`5180-5880 yellow
`60-5560
`.5165
`weak
`5375
`(5438) 5500-6700 orange
`very wea
`(6000)
`
`4(cid:1)
`
`4(cid:1)
`
`800-6000 5500-7000 red
`(6050)
`strong
`(5500)
`300-5900 5360-6020 yellow
`(5550)
`strong
`(5260)
`orange
`5600-6800
`550-6000
`4,'
`medium
`orange
`medium
`
`54
`
`00-5900
`
`5600-6500
`
`Compounds
`
`I. Xanthene
`Fluoran
`
`Fluorescein
`(Dihydroxyfluoran)
`Eosin
`(Tetrabromofluo-
`rescein)
`Erythrosin
`(Tetraiodofluo-
`rescein)
`Rose bengale
`(Tetraiodotetra-
`chlorofluorescein)
`Rhodamine B extra
`
`Rhodamine 6G
`
`Acridine red
`
`Pyronine B
`
`II. Acridine
`Acridine
`
`Acridine yellow
`Euchrysine
`
`Rheonine A
`Acriflavine
`(Trypaflavine)
`
`III. Azine
`Magdala red
`
`Safranine
`IV. Thiazine
`Thionine
`
`Methylene blue
`a Approximate
`parentheses.
`
`VIZIO 1006
`
`

`
`their quantum
`efficiencies are above 50%. Thus,
`these materials are almost perfectly suited for laser dis-
`play systems.
`The colors of these fluorescing dyes may be modified
`somewhat by varying the type of carrier which is used
`to form pigments and, to a lesser extent, by varying
`the type of vehicle, or binder, into which the pigment is
`incorporated.
`It is also possible to modify colors by
`combining fluorescent dyes with nonfluorescent dyes
`that selectively absorb a portion of the emission spec-
`trum. For example, the emission spectrum of the
`naphthalimide dye (type 3485) show in Fig. 3 peaks at
`5300 A, in the green. Normally this fluorescence ap-
`pears to have a vellowish-green cast due to the broad
`tail of the emission spectrum which extends into the
`yellow and red. However, this tail can be substan-
`tially reduced by the addition of a nonfluorescing green
`toner such as phthalocyanine," which absorbs in the
`yellow and red.
`The result
`then is a tradeoff of
`brightness for the ability to limit the spectral content.
`In contrast to the many yellow and red emitting dyes,
`blue emitting dyes are less common. However, ex-
`amination of pyrelene in dilute alcoholic solutions in-
`dicates that it is blue fluorescing when excited by short
`wavelength blue light such as 4579-A emission of an
`argon laser or the 4416-A emission of a cadmium laser,
`while it becomes green fluorescing under longer wave-
`length blue excitation such as the 4880-A line of an ar-
`gon laser.
`In addition, pigments of coumarin which
`fluoresces blue under near ultraviolet excitation are
`commercially available.' 3
`
`IV. Conclusion
`There exists a sufficiently wide range of efficient or-
`ganic and inorganic phosphors that can be excited by
`laser wavelengths ranging from the ultraviolet to green
`and that have emission spectra covering the entire
`visible. For example, a suitable combination of blue,
`green, and red emitting phosphors can be made to
`fluoresce any color desired including white when illumi-
`nated by a blue or ultraviolet
`laser source.
`If the
`
`source itself is blue, a portion of its intensity may be
`directly scattered and blended with the emission from
`green and red phosphors
`to achieve a similar color
`gamut. These concepts have been verified using an
`argon ion laser source which emitted only blue 4880-A
`radiation.
`In conclusion, photoluminescent conver-
`sion of a monochromatic laser source provides an attrac-
`tive means for achieving white and light and multicolor
`for display system applications.
`The authors acknowledge with thanks helpful dis-
`cussions with E. A. Chandross and C. V. Lundberg
`concerning organic dyes and pigments. The authors
`also thank H. W. Grodkiewicz, A. G. Dentai, and C. J.
`Schmidt for the growth and preparation of test samples
`and S. R. Williamson for assistance in experimental
`measurements.
`References
`1. C. E. Baker, IEEE Spectrum 5, 39 (December 1968).
`2. J. E. Geusic, H. J. Levinstein, S. Singh, R. G. Smith, and L.
`G. Van Uitert, Appl. Phys. Lett. 12, 306 (1968).
`3. L. H. Enloe, Bell System Tech. J. 46,1479 (1967).
`4. P. Pringsheim, Fluorescence and Phosphorescence (Intersci-
`ence, New York, 1949), pp. 1-10.
`5. J. D. Gould and W. L. Makous, Information Display 5,
`25 (November/December
`1968).
`6. Ref. 4, pp. 285-289.
`7. L. G. Van Uitert, J. Electrochem. Soc. 114, 1048 (1967).
`8. L. G. Van Uitert and R. R. Soden, J. Chem. Phys. 36, 1289
`(1962).
`9. G. Blasse, W. L. Wanmaker,
`J. W. ter Vrugt, and A. Bril,
`Philips Res. Rep. 23, 189 (1968).
`10. G. Blasse and A. Bril, J. Chem. Phys. 47, 5139 (1967).
`11. G. Blasse and A. Bril, Appl. Phys. Lett. 11, 53 (1967).
`12. Ref. 4, pp. 594-645.
`13. Day-Glo Color Corporation, Cleveland, Ohio, Tech. Booklet
`No. 1170-A.
`14. E. G. Bobalek and W. von Fisher, Organic Protective Coatings
`(Reinhold, New York, 1953), pp. 115-118.
`15. Ref. 4, p. 423.
`16. R. A. Ward, "The Day-Glo Daylight Fluorescent Color
`Specification System (Day-Glo Color Corp., Cleveland,
`Ohio).
`
`COVER
`
`This month's cover shows constant-hue and saturation loci derived from geodesic chroma-
`ticity diagram. The radii of the constant-saturation circles on the
`,7 diagram were 20n;
`the angular separations of points were 180 /n. The constant-hue loci shown correspond to
`radii separated by 180 in the
`,X diagram.
`
`January 1971 / Vol. 10, No. 1 / APPLIED OPTICS 153
`
`VIZIO 1006