June 11, 1974
`
`-F. E. AUZEL
`PREPARING FLUORESCENT MATERIALS FOR
`OPTICAL FREQUENCY CONVERSION
`Original Filed Feb. 8, 1971
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`3,816,576
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`TCL 1013, Page 1
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`United States Patent Office
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`3,816,576
`Patented June 11, 1974
`
`1.
`3,816,576
`PREPARING FLUORESCENT MATERHALS FOR
`OPTICAL FREQUENCY CONVERSION
`François E. Auzel, 39 Avenue Port-Royal des Champs,
`Le Mesnil-Saint-Denis, France
`Original application Feb. 8, 1971, Ser. No. 113,317, now
`Patent No. 3,709,827, dated Jan. 9, 1973. Divided
`and this application July 26, 1972, Ser. No. 275,435
`Int, CI, C04b 33/32
`-
`U.S. Cl, 264–56
`2 Claims
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`ABSTRACT OF THE DISCLOSURE
`Fluorescent material for the optical frequency conver
`sion of near infrared radiation from 0.85 to 1.06 um. into
`visible radiation. The constituents of the material are (i)
`vitrifying fluorides of lead, beryllium and magnesium, (ii)
`devitrifying and activating fluoride of ytterbium and (iii)
`doping fluoride of erbium for a green and red response
`and doping fluoride of thulium for a blue response. The
`content of ytterbium fluoride controls the form of the
`material, either glassy ceramic or polycristalline. Proper
`preparation conditions allow to prepare either a glass ma
`terial or a ceramic material.
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`ytterbium (Yb3+) and as the case may be with erbium
`(Er&#) (for a green response) or with thulium (TM**)
`(for a blue response), or holmium (Hoºt) (for a red
`response);
`(b) Mixed barium and yttrium fluorides (Bay'Fs),
`mixed barium and lanthanum fluorides (Balaf's), doped
`with ytterbium and either erbium (for a green response),
`or thulium (for a blue response), or holmium (for a red
`response);
`(c) Yttrium oxy-chlorides (YOCl, YsCCli), doped with
`ytterbium and erbium (for green response).
`As regards these monocrystals which are drawn from a
`fusion-bath (a, b) or obtained through evaporation (c),
`pertinent references are as follows:
`(a) Hewes and Sarver, “Bulletin of the American Physi
`cal Society,” 1968, vol. 13, p. 687, and “Physical Review,”
`June 1969, vol. 182, p. 427;
`Kingley, Fenner and Galginaitis, “Applied Physics,”
`August 15, 1969, page 115;
`(b) Guggenheim and Johnson, “Applied Physics
`Letters,” June 15, 1969, pp. 51 and 52;
`(c) Van Uitert, Singh, Levinstein, Johnson and
`Orodkiewicz, “Applied Physics Letters,” June 15, 1969,
`pp. 53 and 54.
`By concentrating on the ions erbium (Erºt) and thuli
`um (Tm3+), but by utilizing new hosts which will be
`specified below, the applicant has obtained the following
`new results:
`Notably modifying the relative intensities of the radia
`tion emitted from the ion Erºt as a result of different
`energy levels, i.e. the properties of the ion itself, thus per
`mitting the emission of at least one new color (green and
`red response instead of green response);
`Utilizing the capability of the new hosts mentioned to
`be produced in the different forms of glass, glassy ceram
`ics, or cristalline powders, hence permitting infinitely more
`sophisticated methods of manufacture, less cumbersome
`than drawing out monocrystals. This results notably in
`ceramics with superior efficiency for any given excitation
`power in comparison with mono-crystals and glass, and
`opens new devolopmental possibilities in the way of mixed
`glass-ceramic forms which will be discussed below.
`The principal object of the invention is to provide new
`solid fluorescent materials for applications in optical con
`verters from near infrared radiation into visible radiation;
`another object of the invention is to provide methods of
`manufacturing said new infrared-visible converting fluo
`rescent materials.
`According to this invention, the fluorescent materials as
`well as the optical converters applying these materials and
`to be used for the conversion of the near-infrared band
`into visible radiation, are characterized in that they are
`made up of at least one of the following mixtures:
`(a) Mixture of fluorides of lead, beryllium and magnesi
`um, having vitrifying properties, ytterbium fluoride having
`devitrifying and activating properties and of erbium fluo
`ride, a mixture which has a red and green response giving
`a yellow-orange color;
`(b) Mixture of fluorides of lead, beryllium and mag
`nesium, having vitrifying properties, ytterbium fluoride,
`having devitrifying and activating properties and of thu
`lium fluoride, a mixture which has a blue response;
`(c) Mixture of fluorides of lead, beryllium and mag
`nesium, having vitrifying properties, ytterbium fluoride
`having devitrifying and activating properties and of erbium
`and thulium fluorides, a mixture that has a red, green
`and blue response.
`The fluorescent materials of the invention can be ob
`tained in the form of glass, of more or less glassy ceramics,
`and of cristalline powder by a proper selection of the
`ytterbium fluoride content and the cooling conditions of
`the melted mixture.
`TCL 1013, Page 2
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`The present application is a division of applicant's U.S.
`patent application Ser. No. 113,317, filed Feb. 8, 1971,
`entitled, “Fluorescent Materials for Optical Frequency
`Conversion,” now U.S. Pat. No. 3,709,827 granted Jan.
`9, 1973.
`The present invention generally concerns the materials
`for optical converters and, more particularly, for optical
`converters which permit the conversion of near-infrared
`radiation (ca. 0.85 to 1.06 um.) into visible radiation. The
`invention deals equally on one hand with the methods of
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`producing these materials as well as the optical converters
`made therewith, and, on the other hand with certain ap
`plications thereof in devices which use them.
`Optical converter materials and optical converter
`screens made therewith are known from French Pat. No.
`1532609, filed June 1, 1967, by the present applicant. This
`patent discloses two materials having the following com
`positions:
`-
`(a) A mixed tungstate of an alcaline metal and of
`ytterbium, lightly doped by a mixed tungstate of an alcaline
`metal and erbium, a composition which results in a green
`response material;
`(b) A mixed tungstate of an alcaline metal and of ytter
`bium, lightly doped by a mixed tungstate of an alcaline
`metal and of thulium, a composition which results in a
`blue response material.
`The same patent gives quantitative formulas for the
`presented compositions, as well as their methods of prepa
`ration in powder form, and certain applications thereof
`for the manufacture of screens which function as optical
`converters, e.g. by packing these powders between two
`glass plates which must be smooth and parallel, or by
`settling these powders on any convenient support and hold
`ing them in place by the use of a coat of a synthetic trans
`parent resin.
`The physical and photoluminescent phenomena thus
`produced have been the subject matter of two papers given
`at the Paris Academy of Sciences, in 1966, tome 262, pp.
`1016 to 1019, and tome 263, pp. 819 to 821. The above
`cited patent presents a simple model of the phenomenon
`which consists of irradiating an ion-pair ytterbium-erbium
`or ytterbium-thulium in the near-infrared range.
`Other compositions in mono-crystal form are now
`known which differ from the ones above only by the host
`utilized. The principal ones are:
`(a) Simple lanthanum fluoride (LaFa), gadolinium
`fluorides (Gdfa), ytterium fluorides (YFA) doped with
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`whereby AB is a constant which at constant temperature
`depends only upon the proportion of the dope.
`For a mixed material responding simultaneously to
`infrared irradiation with yellow-orange and with blue
`light, one can write:
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`3,816,576
`3
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`The fluorescent materials therefore include fluorides of
`Similarly, it can be shown that the theoretical conver
`three kinds:
`sion efficiency is limited to 74% for the red light (ion
`Three fluorides with vitrifying properties constituting
`Erä4).
`the “body” of the material, no matter what the final physi
`Further efficiency restrictions may result from the ab
`cal form of the material is going to be; these fluorides
`sorption saturation of the ytterbium ion Ybºt which is
`have been selected in such a way that the practical energy
`expected to occur for an infrared radiation power of
`efficiency of the final material be the best possible;
`several tens of kilowatts per cubic centimeter. As long as
`A devitrifying and activating fluoride; it is said to be
`no such magnitudes of power are attained the conver
`devitrifying because its concentration enables the composi
`sion efficiency can principally be increased, provided that
`tion to take the form of glass or ceramic on the one hand,
`the generated heat can be eliminated as quickly as it is
`or the powdery form on the other hand; it is said to be
`formed. In reality, however, the practical conversion
`activating because its presence is necessary for the optical
`efficiency limits are due to one or the other of the two
`conversion by permitting coupling between the ytterbium
`causes mentioned above depending upon whether the
`ion Ybºt and at least one other ion of a rare earth
`conversion process follows the quadratic or the cubic
`element;
`law.
`At least one doping fluoride, namely that of erbium
`In the case of the green conversion an efficiency of
`and (or) that of thulium, the corresponding rare earth
`7.10-0 was measured for a large band excitation power
`ions Erºt and (or) Tm3+ cooperating by coupling with
`density of 250 mw./cm.8 which by way of a linear extra
`that of ytterbium in order to assure the conversion of
`polation (quadratic law) suggests for an irradiation limit
`infrared radiation into visible radiation the spectral com
`of 20 kw./cm.8 a practical maximal efficiency of 56%,
`20
`position of which will depend upon the nature of the
`an efficiency therefore that is below the theoretical value.
`doping ions.
`In the case of the blue emission an efficiency of 3.10-7
`According to another aspect of the invention the
`was obtained for the same excitation power density of
`finished compositions contain on one hand 20 to 45% by
`250 mw./cm.8 which by way of a quadratic extrapolation
`weight of lead fluoride, 20 to 40% of beryllium fluoride
`(cubic law) indicates an efficiency of 68%, equal to the
`and 5 to 20% of magnesium fluoride and less or more
`theoretical yield up to an excitation power density of 365
`than 20% of ytterbium fluoride, depending upon whether
`W./cm.8 at which point saturation starts to occur.
`the solid form to be obtained is a glass or a more or less
`The method of preparing the fluorescent materials of
`glassy ceramic on the one hand or a crystalline powder
`the invention and the optical converters including the
`same comprises the following steps:
`on the other hand. Selection between a glass material or a
`ceramic material depend upon the cooling conditions of
`(a) The vitrifying fluorides are mixed first in the cold
`the melted material.
`and as powders, then ytterbium fluoride is added to this
`According to another aspect of the invention the com
`mix, followed by the addition of erbium and (or) thulium
`positions are such that the proportion of doping fluorides
`fluoride and mixed;
`amounts to 1 to 4 atom-grams of erbium fluoride and
`(b) The thus obtained mixture is heated to about
`(or) 0.25 to 1 atom-gram of thulium fluoride per liter of
`1200° C. for about 6 minutes in a muffle oven in order
`the finished material.
`to melt it;
`It can be demonstrated that at a macroscopic scale,
`(c) The product is allowed to cool slowly at room
`taking into account the statistical behaviour of all the
`temperature to about 500° C.;
`various ions, the radiation intensity IGR of the green and
`(d) The product is then given the desirable shape.
`red light (seen as a yellow-orange light) emitted by the
`The last step of the process can be varied in order to
`fluorescent material varies with the square of the incident
`obtain the desired form of the finished material:
`infrared radiation intensity IIR hence according to the
`(e1) Glass: the product is poured at 500° C. into a
`formula:
`mold with a jacket as a steel, uniformly heated to about
`IGR=AGRIIR”
`100° C. and maintained there until the mixture has solid
`ified whereupon it is allow to cool further to room tem
`whereby AGR is a constant which at constant tempera
`perature;
`ture depends only upon the dope concentration, and the
`(ez) Ceramic, more or less glassy; one proceeds as with
`radiation intensity IB of the emitted blue light varies with
`the glass, but with one essential difference, the mold is
`the cube of the incident infrared intensity IIR, hence ac
`uniformly preheated to 250° C. and is maintained at this
`cording to the formula:
`50
`temperature until the mixture has solidified;
`(eg) Cake with a glass mantle and a ceramic center
`piece: again one proceeds as with the glass, but one
`chooses a relatively deep mold, so that the liquid in con
`tact with the mold wall will vitrify whereas the center
`portion will transform into ceramic;
`(eA) Crystalline powder: the last step of the general
`process is simplified to the point that the product is left
`to cool to room temperature in the fusion crucible itself
`and it is then treated so it can serve at least for one
`of the methods of a group that comprises (a) packing be
`tween two thin smooth parallel, transparent glass plates,
`(b) sedimentation and coating by a transparent, synthetic
`resin, (c) suspension in a transparent gel; in this instance
`there is no vitrification, not even partial vitrification;
`(es) Cake with a glass coating which is lenticular on
`one side and with a ceramic body inside. The glass lenses
`can be utilized to concentrate the infrared excitation radia
`tion. One proceeds as with the previous cake (es) using
`a honeycomb mold instead. Taking into account the re
`fraction index of the finished glass of the order of 1.37
`and the calculated distance of the infrared source of radia
`tion, the honeycomb cavities will have the desired curva
`ture radius in order for the radiation to focus in the inner
`bordering ceramic behind the lenses.
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`indicating that the yellow-orange light is more intensive
`than the blue light when the intensity of the infrared beam
`is such that:
`IIR3AGR/AB
`and the opposite is true, i.e. the blue light is more inten
`sive than the yellow-orange light, when the intensity of
`the infrared beam is such that
`IIR-AGR/AB
`Therefore, the resulting light color depends upon the
`excitation power.
`The theoretical conversion efficiency, i.e. the ratio of
`emitted visible radiation power to infrared radiation power
`is limited by the production of phonons to:
`90% for the green light (ion Erºt)
`68% for the blue light (ion Tmºl.)
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`TCL 1013, Page 3
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`The invention will be more easily understood by read
`The optical converters according to this invention are
`ing the detailed description below of several embodiments
`parts that are apt to be used in numerous optical systems,
`together with the accompanying drawings in which:
`such as, among others:
`FIGS. 1 to 3 are views in perspective of partially cut
`Luminous marks for illumination or for signalling, for
`molds which are used to cast optical converters which
`instance in telephone switchboards, automobile and air
`respectively have the shape of glass plates with parallel
`plane dashboards, instrument panels of satellites, data
`faces, lenses, screens with a lenticular face of glass and
`visualization consoles for calculators;
`an inner ceramic body;
`Screens for the detection of infra-red radiation;
`FIG. 4 represents a cut through a screen in accordance
`Low definition television receivers with flat screens.
`with the invention as it was produced by means of the
`As already discussed, the conversion efficiency increases
`mold in FIG. 3; and
`with the infra-red radiation intensity which irradiates an
`FIG. 5 represents a schematic view of a cut through a
`optical converter. There is therefore an inducement to
`luminous mark in accordance with the invention, involv
`spectrally concentrate said irradiation.
`ing an infrared source, an optical converter of ceramic
`A photodiode with an emission band as narrow as pos
`material and an optical filter.
`sible, centered upon a wavelength close to 0.97 p.m must
`be selected since the value of the wavelength of optimal
`EXAMPLE NO. 1
`sensibility depends largely upon the ytterbium ion's
`(Yb2+) own properties and the maximal excitation of the
`Fluorescent material with a green and red response
`optical converters of the invention is obtained with a radi
`Composition of the material:
`ation of 0.97 p.m. wavelength. Nevertheless a satisfactory
`Grams
`excitation can be obtained by a radiation of wavelength
`Beryllium fluoride in form of a mixture of ammo
`of between 0.92 and 1 p.m. Thus, the classical diodes of
`nium fluoride and beryllium fluoride NH4BeF4 -- 3,633
`the gallium arsenide type are not convenient as excitation
`Magnesium fluoride ------------------------- 0,683
`sources since their radiation has a wavelength of 0.90 p.m.
`Lead fluoride ------------------------------- 9,808
`It is possible though to use gallium arsenide diodes doped
`Ytterbium fluoride -------------------------- 4,598
`with silicium, since the radiation it emits has a wavelength
`Erbium fluoride ----------------------------- 1,192
`that falls within the lower bracket of the range mentioned
`above. For instance, it is possible to use:
`EXAMPLE NO. 2
`Fluorescent material with a blue response
`the photodiode PEX 1206 of the Texas Instrument Corp.
`Composition of the material:
`(0.93 um)
`Grams
`the photodiode SSL 15 of the General Electric Corp.
`Beryllium fluoride in form of a mixture of ammo
`(0.94 p.m.)
`nium fluoride and beryllium fluoride NHABeF4 – 3,633
`But it is still preferable for the sake of the optical con
`Magnesium fluoride ------------------------- 0,683
`verters of this invention to make diodes the radiation of
`Lead fluoride ------------------------------- 9,808
`which centers upon the wavelenth of 0.97 p.m.
`Ytterbium fluoride -------------------------- 4,598
`It is now known that the wavelength of the radiation
`Thulium fluoride ---------------------------- 0,149
`emitted by the semiconductor diodes made up of alloys
`EXAMPLE NO. 3
`of ternary compounds of III–V elements (III and V desig
`nate the groups in the periodic table of the elements)
`Fluorescent material with a red, green and blue response
`can be varied simply by varying the relative composition
`Composition of the material:
`of each alloy. The energy gap of the alloy varies substan
`Grams
`tially linearly with its mole composition between the
`Beryllium fluoride in form of a mixture of ammo
`energy gaps of its individual constituents.
`nium fluoride and beryllium fluoride NHABeF4 – 3,633
`Alloys which are convenient for making diodes whose
`Magnesium fluoride ------------------------- 0,683
`radiation centers reasonably close upon the 0.97 p.m.
`Lead fluoride ------------------------------- 9,808
`wavelength are:
`Ytterbium fluoride -------------------------- 4,598
`Erbium fluoride ---------------------------- 1,192
`(1°)—the indium-phosphor-arsenic alloy of the formula
`Thulium fluoride ---------------------------- 0,149
`In (P,Asi_x)
`We now refer to FIGS. 1 through 3 which give examples
`of molds that can be used to cast the mixtures for form
`ing the optical converters.
`:
`Mold 1 of FIG. 1 is formed with rectangular cells
`11-13; however, these sections might as well assume other
`geometric shapes; here, the rectangular section permits
`the casting of the plates with parallel faces of glass or
`ceramic. Mold 2 of FIG. 2 is formed with cavities 21–23
`in form of a portion of a sphere which are used to pro
`duce glass plano-convex lenses; the curvature radius of
`the cavities and hence of the convex faces of the lenses
`determines the convergence-value of the lenses, taking
`into account that the refractive index of the finished glass
`product is of the order of 1.37.
`The mold 3 of the FIG. 3 has cavities 31–33 which are
`analogous to those of the mold in FIG. 2, but it is deeper
`and is intended to be used for the production of a “mixed
`cake” 4 (see FIG. 4) having an inner ceramic body 40
`enrobed in glass, whereby the glass envelop has on one
`of its faces an array of regularly distributed lenses 41–4s.
`Again taking into account the refractive index of the
`finished glass of about 1.37 and the anticipated position
`of the infrared source on the axis of each lens, the curva
`ture radius of the mold cavities and hence of the finished
`TCL 1013, Page 4
`
`where 0.983-1
`On this subject one can refer to the paper of F. B.
`Alexander, “Applied Physics Letters,” tome 4, 1964, p. 13.
`(2°)—the arsenic-indium-gallium alloy of the formula
`As (InzGa1-x)
`where 0.0053.<0.15
`60
`On this subject one can refer to the paper by I. Mel
`engailis, A. I. Strauss, R. H. Rediker, “Proceeding of the
`I.E.E.E.,” August 1963, p. 1154.
`(3°)—The antimony-aluminum-gallium alloy of the for
`mula
`Sb (AlxGa1-x)
`where 0.373.<0.74
`On this subject one can refer to the paper by I. I. Bardi
`yan, “Fizika Tverdogo Tela,” vol. 1, No. 9, Moscow, Sep
`70
`tember 1959, p. 1360.
`Since a semi-conductor diode can attain a luminous effi
`ciency of 10% the optical converters of this invention
`asymptotically permit the attainment of a total electro
`luminous efficiency of 5 to 7%.
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`the gel can contain a suitable dye permitting it to play the
`lenses of the cake can be so chosen as to allow focussing
`of the infrared irradiation in the subjacent ceramic
`role of optical filter.
`layer 40.
`For the sake of the appended claims, it is to be noticed
`FIG. 5 relates to a luminous mark for signal boards.
`that a concentration of x atom-grams per liter correspond
`It essentially comprises a solid state source of infrared
`to a concentration of xXN/108 par cm.8 where N is the
`radiation, preferably, a photodiode of the infrared semi
`Avogadro's number équal to 6× 1033.
`conductor type, and an optical transformer in accordance
`What I claim is:
`with the invention.
`1. A method for preparing fluorescent material for
`More precisely, a photodiode 11 is placed on a metallic
`optical conversion of near infrared from 0.85 to 1.06
`base 12 with two terminals 13 and 15. The photodiode is
`millimicrons into visible radiation comprising the steps
`soldered onto the base. A gold wire 14 is attached to the
`of
`upper surface of the diode and to terminal 15 and is iso
`(a) mixing at room temperature finely divided solid
`lated from base 12 by means of a glass pearl 16. The
`fluorides of lead, magnesium, berryllium, as vitrifying
`diode 11 and its base 12 are covered by a metallic hood
`agents with ytterbium fluoride, which has devitrifying
`17. The top of the hood 17 consists of an optical con
`properties and with a powdered doping agent selected
`verter in accordance with the invention, in this case, a
`from the group consisting of erbium fluoride for
`ceramic plate with parallel faces 18. A metallic lid 19
`red and green at a concentration of 6× 1020 to
`whose bottom has a circular opening 194 fits into the
`24× 1020 atoms per cm.8 and thulium fluoride at a
`upper end of the hood 17. This lid holds an optical filter
`concentration of 1.5× 1020 to 6×1020 atoms per cm3
`20 known in the prior art in case it should be required.
`for blue, the weight percentages being
`Such a luminous mark can emit visible light of five differ
`lead fluoride: 20–35%
`ent colors:
`beryllium fluoride: 20–40%
`magnesium fluoride: 5–20%
`1.—glue: plate 18 thulium-doped, only; no filter 20
`ytterbium fluoride: balance to 100%
`2.—green: plate 18 erbium-doped, only; filter 20 green
`except for doping amounts,
`3.—yellow-orange: plate 18 erbium-doped, only; no filter
`(b) heating the mixture to about 1200° C. in a
`20
`crucible, the mixture containing a sufficient amount
`4.—red: plate 18 erbium-doped, only; filter 20 red
`of devitrifying fluoride to prevent glass formation,
`5.—white or blue
`plate 18 erbium and thulium-doped
`and
`ot
`depending upon intensity; no
`(c) cooling the powder mixture from the heating step
`in (b) to room temperature whereby a powdered
`yellow/orange
`filter 20
`product is obtained adapted to be formed as an elec
`Several changes can be made in the structure of the
`troluminescent self-supporting layer when dispersed
`luminous mark of FIG. 5:
`in a binder.
`1.—The homogeneous plate 18 can be replaced by a
`2. A method as claimed in claim 1 wherein said
`plate of the mixed glass-ceramic type (of FIG. 4) with
`powder is packed between parallel transparent glass
`plano-convex glass lenses facing the diode; the luminosity
`plates to provide an electro-luminescent converter of
`of such a mark will be notably increased for reasons
`rectangular shape.
`already discussed;
`2.—While using a homogeneous ceramic plate 18 with
`parallel faces the diode 11 can be replaced by a prior art
`diode, a so-called dome-diode, such as the PEX 1206 of
`the Texas Instrument Corp.; this diode has its own source
`of radiation in the center of a portion of sphere of trans
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`parent semiconductor material; by giving the hood con
`venient dimensions, the infrared radiation emitted by the
`diode can be focussed in the plate 18.
`3.—The interior of the hood 17 can be filled entirely
`with a translucid silicone gel which opens up two pos
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`sibilities:
`the gel can contain grains of a crystalline powder accord
`ing to the invention in suspension and thus play the role
`of an optical converter;
`
`References Cited
`UNITED STATES PATENTS
`3,541,018 11/1970 Hewes et al. ---- 252–301.4 H
`3,203,899
`8/1965 Fisher -------- 252–301.4 H
`3,533,956 10/1970 Snitzer -------- 252—301.4 R
`3,667,921
`6/1972 Grodkiewicz et al.
`252–301.4 H
`3/1972 Robinson ------ 252–301.4 H
`3,649,552
`1/1973 Auzel ------------- 106–47 R
`3,709,827
`DONALD J. ARNOLD, Primary Examiner
`U.S. Cl. X.R.
`106–47 R; 252–301.4 R; 264—332
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`TCL 1013, Page 5
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