R 431 Philips Res. Repts 16, 356-370, 1961

EFFICIENCIES OF PHOSPHORS FOR SHORT-WAVEULTRA-VIOLET EXCITATION

by A. BRIL and W. HOEKSTRA535.373.3

SummaryThe absolute radiant efficiency and quantum efficiency of phosphors,especially for the standard samples issued by the National Bureauof Standards (Washington), for short-wave ultra-violet excitation, aremeasured using a fast thermopile with a constant spectral power re-sponse as detector. The results of the measurements are given intables III and IV. A simple method for relative efficiency measure-ments making use of a standard phosphor (e.g., MgW04) is described.

RésuméLes rendements énergétiques et quantiques absolus, particulièrernentdes phosphores d'étalonnage émis par le National Bureau of Standards(Washington), pour l'excitation par du rayonnement ultraviolet d'ondescourtes, ont été mesurés, faisant usage d'une thermopile avec un tempsde response court et une sensibilité spectrale constante. Les résultatsdes expériments sont donnés dans les tables III et IV. Une méthodesimple pour mesurer le rendement relatif à l'aide d'un phosphord'étalonnage (p.e. MgW04) est décrite .

.."" \

ZusammenfassungEs werden die absoluten Strahlungsausbeuten und Quantenausbeutender Phosphore gemessen, besonders die der Standard-Phosphore, aus-gegeben von dem National Bureau of Standards (Washington), fürAnregung mit kurzwelligem Ultraviolett, mit Hilfe einer Thermosäulemit geringer Trägheit und konstanter spektraler Empfindlichkeit. DieResultate der Messungen sind in den Tabellen III und IV gegeben. Eineeinfache Methode für relative Ausbeutemessungen unter Verwendungder Standardphosphore (z.B. MgW04) wird gegeben.

1. Introduetion

Many measurements of the efficiencies of phosphors have already been car-ried out, using various methods 1). Tregellas-Williams reviewed the experimen-tal results some years ago 2). It may be seen from his paper that the resultsobtained by different workers often do not agree very well.The present paperdescribes a direct method of measuring the absolute value of the efficiency witha discussion of the sources of error involved. When for one single phosphor (astandard phosphor) the. absolute efficiency has been found, it is very easy todetermine the efficiencies of other phosphors by a comparison method.

First we shall discuss what is meant by the efficiency of a phosphor. Theradiant efficiency 7J is defined as the ratio of the emitted fluorescent power tothe power absorbed by the phosphor from the exciting radiation (both powers

EFFICIENCIES OF PHOSPHORS FOR SHORT-WAVE ULTRA-VIOLET EXCITATION 357

e.g. to be expressed in watts). The quantum efficiency q is the ratio of the numberof emitted fluorescent quanta to the number of the absorbed quanta. A thirdquantity is the luminous efficiency, i.e. the ratio of the emitted luminous fluxin lumens to the absorbed power in watts.

Let us assume that the phosphor is excited by monochromatic radiation ofwavelength .\0 and that the absorbed power is Pa. If the fluorescence is emittedover a wide range of wavelengths the total power involved is

. P . f p(.\)dÀ,where p(.\) is the emitted power at wavelength À, the integration being takenover the whole range of wavelengths. The radiant efficiency is then given by

'rJ = Pipa = f p(À)dÀIPa. (1)

To obtain the intrinsic (or true) radiant efficiency 'Y}tthe measured value 'Y}must be corrected for the absorption of the emitted fluorescence by the phos-phor itself. This absorption is generally small for well-prepared phosphors .. ,.~The intrinsic efficiencyis approximately given by ,

,'Y}t= 2'Y}1(1+ Reo)'

where Reo is the reflection coefficient of an "infinitely thick" layer of the phos-phor for its own emission. This 'formula which has been used by Bril andKlasens 3) for cathode-ray excitation is, also valid here.The energy of one qu~ntum is hv ="hc/.\ where h is Planck's constant and c

is the velocity of light; so the quantum efficiency is

q = f .\p(.\)d.\/ÀoPa.

The ratio of quantum efficiency and radiant efficiency is therefore

(2)

(3)

The lumen efficiencyL can be of great practical importance. It can be written:

L - K f p(.\) Y(À)dÀ_ K f p(À)Y(.\)d.\- m Pa - m'Y} f p(À)dÀ '

where Y(À) denotes the sensitivity of the eye as a function of wavelength andKm is the maximum value of the luminous flux per watt of radiant power, i.e.683 lm/W at À = 555 mu.The ratio

(4)

ji = Jp(À)Y(À)dÀI Ip(.\)dÀ (5)

is called the luminosity factor and defines the lumen equivalent K = jiKm,which is the luminous flux per watt emitted radiant power.

358 A. BRIL and W. HOEKSTRA

According to (4) the lumen efficiency is then given by

L = 7JyKm = 'Y}K. (6)

We shall now discuss some of the possible reasons for the lack ofagreement between various authors.

(1) Most of the measurements have been carried out using photocells orphotomultipliers to detect the fluorescent radiation. The response of most of thesecells decreases very rapidly at longer wavelengths, so that measurements onorange and red phosphors will give difficulties. The sensitivity of these cellsat 750 mu is generally only of the order of 1% of the maximum sensitiv!ty at400-450 mu. Hence an accurate measurement of the spectral response of thesecells and of the spectral energy distribution (S.E.D.) curves of the phosphorsis required.

It is evident that the reliability of the measurements will be best when phos-phors with about the same S.E.D. curves are considered.

(2) The intensity of the exciting ultra-violet radiation is usually measuredafter reflection from a layer of magnesium oxide. The absolute reflection coef-ficient of the MgO is thus rèquired.' -

(3) The phosphors are generally excited by the exit beam of a monochromatorso that the exciting radiation is as nearly monochromatic as possible. In prin-ciple this is a good thing, but it means in practice that one measures with a Iowirradiating intensity, which may badly affect the accuracy of the measurement.Moreover comparisons are difficult in the case of phosphors for which theefficiency is intensity dependent, as is the case with e.g. some sulphide andselenide phosphors. When the excitation spectrum (i.e. quantum efficiency as afunction ofwavelength) ofsucha sulphide phosphor is measured with the aid ofthe u.v. lines of a mercury lamp, isolated with the help of a monochromator, it isfound that the efficiencies for the various exciting mercury lines vary in thesame way as the intensities of the lines, being high for strong lines and Iowfor weak ones. This explains also the low efficiencies for U.V. excitation in tableIII of reference 3), where the measurements are carried out with the aid of amonochromator .

(4) The radiant efficiency is defined as the ratio of the emitted power tothe absorbed power. In practice, however, only the power emitted in a certainrange of wavelengths is considered. Errors are made if radiation emitted out-side this region is not accounted for. The efficiency of the phosphors used influorescent lamps, for example, must be as high as possible. Parts of the emis-sions are however sometimes found in the ultra-violet or the infra-red. It thendepends on the use of suitable filters and detectors whether these parts areincluded in the particular efficiency measurement or not.

To overcome some of these difficulties we have taken the following pre-cautions.

EFFICffiNCIES OF PHOSPHORS FOR SHORT-WAVE ULTRA-VIOLET EXCITATION 359

(a) We used a thermopile as detector to obtain an essentially constant spec-tral power response over the whole wavelength region *).(b) The phosphor was excited with the aid of a high-pressure mercury lamp

from which the wavelength region 250-270 mp, was isolated with filters. Ahigh excitation density was thus obtained in a narrow wavelength region.The excitation of the phosphors is therefore not purely monochromatic. Forthose phosphors where in the region from 250-270 mu the efficiency and theabsorption are essentially constant, the results will however be the same asfor excitation with the 253·7 mp, Hg line. The companson with this mono-chromatic excitation is important as many phosphors are used in luminescentlamps where the excitation occurs mainly by this line.MgW04 and CaW04 for instance have nearly constant quantum efficiency

and absorption. From other points of view too these phosphors are excellentstandard phosphors, as has already been stated by Fonda 1).The wavelength region for the fluorescence taken into consideration is

limited at the short-wave side by the exciting wavelength and at the long-waveside it extends to about 5 fL when a quartz plate is placed before the thermopileand to about 3 fL for a glass plate. Separate measurements showed however thatfor the phosphors in consideration the infra-red emission up to 5 fL was ne-gligible. All these points will be dealt with more fully below.

Most of the measurements have been carried out for the Phosphor Standardsamples available from the N~tional Bureau of St~ndards at Washington 4):'CaW04-Pb (no. 1026), MgW04 (no. 1027), Zn2Si04-Mn (no. 1021 and 1028),CaSiOa-Pb-Mn (no. 1029), (MgO)x (AS205)y-Mn (no. 1030), 3 Caa (P04)2-Ca(F,CI)2-Sb-Mn (no. 1031), BaSi205-Pb (no. 1032) and Caa(p04)2-TI (no.1033).

2, Experimental equipment and method of measurement

A schematic drawing of the' apparatus for the efficiency measurements isgiven in fig. 1, and a photograph of the equipment is given in fig. 2. A plateP with six shallow recesses can slide through the box B, so that each of therecesses can be irradiated in turn by the lamp L under identical conditions.Five of them are usually filled with fluorescent substances and the remainingone with magnesium oxide, all applied in a sufficiently thick layer (0·2 cm).The magnesium oxide was taken from a higly pure batch, obtained fromMerck, Darmstadt. Further details are given below.The excitation is provided by a high-pressure mercury lamp (Philips

HPK, 125 W), with the following filter combination in front of the lamp: achlorine filter (Cls gas, pressure 1 atm, thickness 4 cm), a filter of a nickel-sulphate solution in water (500 gil NiS04.6H20, thickness 1 cm) and a Schott

*) For a discussion of the reflectivity of the thermopile see sec. 2.

360 A. BRIL and W. HOEKSTRA

U'_L'/ ~ror M. ./ ~~~:§,quarfz lens<! th~mopile r //:: shutter

Plg¥~1t/;...?f D C0_fille

f'.= ..UG5_fJlter

chORer / / c::::=:::J..NiS0I._filterglass filter--- '/ --e

6077

plate P with'------''='''''''-._----_j- pl>osphors

Fig. 1. Schematic diagram of the apparatus for tbe efficiency measurements. The glassfilter is present only in part of the measurements.

Fig. 2. Photograph of the equipment for the efficiency measurements.

EFFIcmNCIES OF PHO~PHORS FOR SHORT-WAVE ULTRA-VIOLET EXCITATION 361

filter UG 5 (thickness 0·2 cm). The transmission of' these filters and of the com-bination, determined with the aid of a Beckmann DK2 double beam auto-matic recording spectrophotometer, is given in fig. 3.

The small transmission peak of the filter combination d at about 0·9 IJ.does not give difficulties because the radiation transmitted in' this region is

100

I

I \\

I....., u,,,

IÁ I}' b

... I/! I \b :~,

I I ', , I I

I I , I,

\

I1 11 \ ,e ,..li \ ,.-

I , , ", ,I: , I I -_ .\

I \

il !d•• tt P'" \, . \

! \ -i

, \ I \U " \\ \ \ {j. x i-..

(}2 DiJ (}S DO (}7 08 (}9 I{) I'"

Wavelength in )l 6078

Fig. 3. Transniission curves of the 'filt~rs used in fr~nt ö( the mercury discharge lamp.a. Chlorine, 1 atm, thickness 4 cm. .

- - - - b. NiSû4. 6H20, 500 gIl, thickness I cm.- - - - - - - - - c. Schott UG5 glass filter, thickness 0·2 cm .• - . - . -. d. Combination of the filters inentioned in a, band c.

'_

2600 2700 2800 2900---: Wavelength in.R6079

Fig. 4. Spectral energy distribution of the exciting radiation. (Mercury discharge HPK lamp+ filter combination d given in fig. 3.).The power emitted by the spectrallines has been given as bands with lOA. in width.

362 s: BRIL and Wo HOEKSTRA:

less than 1% of the total transmitted u.v. radiation. The spectral' distributionof the exciting radiation transmitted through the filter combination is givenin fig. 4. It can be seen that the excitation takes place with radiation of 250-270mu with a centre of gravity at about 260 mu, 0

The are ofthe high-pressure discharge lamp is small and can thus be easilyfocussed with a quartz lens on the phosphor area. Thanks to this focussing ofthe radiation and the fact that no monochromator is used, a relatively highexcitation density is obtained.The fluorescence and the reflected u.v. radiation are detected by a fast Hilger

and Watts-Schwarz linear air thermopile (area 2 X 0·2 mm, sensitivity 4 (J.VI (J.W)type FT1 without window, denoted by T in fig. 1 (a disadvantage of a thermo-pile with window is that the transmission of the window cannot be measured).A quartz plate Q was placed in front of the thermopile to absorb the long-waveinfra-red radiation. 0

The radiation coming from the phosphors or the MgO and concentratedon the thermopile with the aid of a Perkin-Elmer elliptical mirror M is choppedwith a frequency of about 12·5cis. The thermo-emfis amplified with a sensitivetuned A.C. amplifier from the "Physikalisch Technische Werkstätten" (P.T.W.)in Wiesbaden.

Let us assume for the moment that the thermopile has a constant spectralsensitivity for equal amounts of incident power, and that the quartz windowand the mirror in front of the thermopile have constant transmission andreflection, respectively, as a function of wavelength. (This is not quite true,the. corrections will be discussed below.) No glass filter between phosphorand thermopile is present.The radiant efficiencies can then be calculated if the reflection coefficient r

of the phosphor for the exciting radiation is known. The efficiencies are thendetermined by two thermo-emf's VM and VA, where VM is due to the u.v.radiation reflected by the magnesium oxide (reflection coefficient p) and VAis the thermo-emf due to the fluorescence from the phosphor and the u.v.

o radiation reflected by the phosphor.The exciting u.v. power is CVMlp, C being a proportionality factor. The

absorbed energy is thus . .C(1- r)VMlp.

The emitted fluorescence is

So we find for the radiant efficiencyVA

p--rVM

'YJ=1- r

(7)

EFFICIENCIES OF PHOSPHORS FOR SHORT-WAVE ULTRA-VIOLET EXCITATION 363

When the reflection coefficient r of the phosphor is not known, 3 quantitiesare measured, VM, VA and VB, where VB is the thermo-emf due to the radiationfrom the phosphor when a filter, absorbing the exciting radiation, is placed infront of the thermopile so that only the fluorescence is measured. A glass filtermay be used in the case of the short-wave u.v. excitation. When the transmissionof the filter for the fluorescent radiation is 'T, we find

VA - VB/'T VA - VB/'Tr =---- = P ---- (8)

VM/p VM

and P VB7]= -.''T(l- r) VM

To find the quantum efficiency q the radiant efficiency must be multiplied byJ Àp(À)dÀ

sl» = ÀoJp(À)dÀ'

(9)

The multiplication factor for the lumen efficiency is yKm (see introduction,eqs (3) and (6».To determine quantum efficiencies and lumen efficiencies from the radiant

efficiencies, the spectral distributions must thus be known.The result given in eq. (7) is especially useful for ultra-violet fluorescing

phosphors, where the fluorescence cannot easily be separated from the excitingradiation with a suitable filter.An important check on the accuracy of the measurement is the thermo-emf

for MgO when the glass filter is placed before the thermopile. This should bezero, because no reflected u.v. radiation is transmitted through the glass filter.In all our measurements this thermo-emf was less than 1% of VM·

We shall now discuss some possible sources of error in more detail'.(1) The reflection coefficient R of the elliptical mirror as a function of wave-

length, decreases somewhat at shorter wavelengths, ·as may be seen fromtable I.

TABLE I

R = reflection .coefficient of elliptical mirror, relative to reflection at

À = 436 mu,T = transmission of the quartz window.

À R T À R T

436m(J. (100) 92% 300m(J. 99 91·5

405 100 92 280 99 91

366 100 92 270 97 91

334 lQO 92 260 95 90

320 100 92 250 93 87·5

364 A. BRIL and W. HOEKSTRA

The reflection coefficient relative to the value for visible radiation was deter-mined in the following way. For À > 330 mp. the mirror was irradiated by anHg source and for À < 330 mu by a hydrogen source. The reflected radiationwas again reflected by a plaque of MgO onto the entrance slit of a monochro-mator. A photomultiplier behind the exit slit recorded the photocurrents forthe various wavelengths. The measurements were then repeated without themirror. The ratio of the two corresponding values of the photocurrents foreach À gives the relative reflection coefficient. The plaque of MgO was usedto give homogeneous illumination on the entrance slit of the monochromator.It may be seen from the table that, in the region 250-270 mp, R is about 5%

lower than in the visible region. The measured efficiencies must thus be in-creased by 5% when the fluorescence is in the visible or near infra-red.

(2) The transmission of the quartz window in front of the thermopile is alsogiven in table I.There is a small decrease in transmission at short wavelengths,which leads to a correction of +2 % for visible or near infra-red fluorescence.

(3) The spectral response of the thermopile. When a thermopile is blackenedwith a layer of constant reflectivity, the spectral response will be constant forequal incident power. When one wants to make thermopiles with very fastresponse, however, - which is necessary in view of the chopping of the radia-tion - it follows that all parts must be as small and thin as possible. It maythen happen that e.g. too thin a blackeninglayer is used so that the reflectionof the thermopile changes with wavelength.Measurements of the u.v. response of thermopilès are very scarce. Dr

Gillham 5) of the National Physical Laboratory (Teddington, England) toldus that his preliminary results for Hilger-Schwarz thermopiles ofthe 9 X 0·5mmêvacuum type with fluorite window indicated that the sensitivity for u.v. radia-tion was not likely to differ by more than 5 per cent from the value for visibleradiation.We checked the u.v. sensitivity in the following way. A few fast modern

thermopiles, namely a Hilger and Watts FT! and a P.T.W. vacuum thermo-couple (quartz window) were compared with a large air thermopile of an oldertype (Kipp en Zonen, Delft) which we blackened ourselves with a relativelythick layer of camphor soot. The ratio of the response at 254 mp, to the res-ponse at 436 mu was determined for each thermopile. Both radiations wereobtained from a low-pressure mercury lamp, the 254 mu line being isolatedwith the aid of the filter combination Cle +NiS04 + UG5 (described above),and the' 436 mp, line with a Schott-glass-filter combination. We found that theresponse ratio ofthe Hilger FT! was equal to that ofthe intentionally blackenedKipp thermopile to within 3%, while that of the P.T.W. thermoelement wasabout !2% lower.After this paper was written, an article was published by Christensen and

EFFICIENCIES OF PHOSPHORS FOR SHORT-WAVE ULTRA-VIOLET, EXCITATION 365

Ames 8), in which it is a!so shown that thermopiles ~o not always have a con-stant response.In the experiments described in the next section, the windowless air thermo-

pile Hilger FT1 was always used and no correction was applied.(4) The reflection coefficient of the magnesium oxide. The most reproducible

results for the reflection of MgO are obtained with a smoked layer of suffi-cient thickness. Investigations have been carried out by e.g. Taylor, Luckieshand Taylor, and Middleton and Sanders 6).Their results for various wavelengths are given in table 11.It is seen that the

reflection varies only slightly with wavelength. Middleton and Sanders foundthat the reflection coefficient of MgO increased a few per cent during irradia-tion with ultra-violet. We had the same experience especially with short u.v.Because in our experiments the irradiation is rather high, a constant level wasreached in a few minutes. Itmay be seen from the table that the reflection coef-ficient for 254 mu is only 1-3% lower than the value for visible radiation. Weconfirmed this with a relative method, comparing the reflection coefficients at254, 365 and 436 mu,We preferred to carry out our experiments with gently pressed powder layers

(thickness 0·2 cm) rather than with a smoked layer: Therefore some MgOand MgC03 powders were compared with a smoked layer of MgO for the250-270 mIL radiation used for the efficiency measurements. A batch of verypure MgO (pro analysi) from Merck, Darmstadt, was finally chosen for themeasurements.When we assume a value of 96% for a smoked MgO layer in the mentioned

region, we find for the Merck powder a reflection of 91%. This value hasfinally been used for the calculation of the ef:ficienciesfrom our measurements.

TABLE 11Reflection of MgO in %

I author12~4

wavelength in mIL

297 313 366 400 I 436 I 500 I 600 I 700'Taylor 93 95 96 96 97Luckiesh andTaylor 93 93 94 95-97 95-97Middletonand Sanders 93-96 94-97 94-98 95-98 96-98 97-98 97 97

(5) The angular distribution of the radiation. It is assumed that the angulardistribution of the reflected ultra-violet radiation reflected from the MgO isthe same as that ofthe fluorescence ofthephosphor. We confirmed thatthis wasindeed the case for MgW04.

366 A. BRIL and W. HOEKSTRA

The 'greatest error ill the efficiency measurements will therefore be due tothe uncertainty in the reflection coefficient of the magnesium oxide.

3. Absolute measurements

As has already been mentioned in the introduction, MgW04 and CaW04.are good standard phosphors. The preparation of these phosphors is fairlyeasy to standardize, as they contain no activators. We used samples of MgW04from the General Electric Company (New York), the Osram Gesellschaft,(Berlin), the National Bureau of Standards (Washington) (Standard no. 1027)and our own factory. The results are given in table III and show that all thevarious samples have the same efficiency to within 2%.The measurements were carried out as described in the previous section,

determining VM, VA and VB and calculating the reflection coefficient andefficienéy from eqs (7), (8) and (9). The results with eqs (7) and (9) alwaysgave the same value, also for the N.B.S. standard phosphors no. 1026:1030,showing that the glass filter used for the determination of VB was adequate forseparating the exciting radiation and the fluorescence. While our work was inprogress? we were informed that Dr Studer of G.E.C. was also carrying outmeasurements for MgW04 with a similar method, also using a thermocoupleas detector"). Dr Studer told us that he found somèwhat higher values than wedid. The difference between our results may be partly due to the fact that wehave applied a correction for the transmission of the quartz plate in front ofthe thermopile.

TABLE III

efficiency of MgW04 in %'T} and q are measured efficiencies, 'T}t and qt are intrinsic efficiencies

. ., reflec-

tion radiant efficiency quantum.coeffi- efficiencycientr 'T} I 'T}t q I qt

General Electric Co no.PL 19BW 8 42·5 44·5 82· 87Osram, no. 220a 7·5 43·5 46 84 89N.B.S. no. 1027 6·5 43·5 46 84 89Philips, no. U l00b 6·5 42·5 44·5 &2 87

;

The results of our measurements on the standard phosphors of the NationalBureau of Standards are given in table IV.

EFFICIENCIES OF PHOSPHORS FOR SHORT-WAVE ULTRA-VIOLET EXCITATION 367

The reproducibility of the measurementsis very good: for sixteen measure-ments ofthe MgW04 sample U 100b the deviation in both reflection coefficientand efficiency is about 1%. This good reproducibility is partly due to thehigh excitation density which ensured that all readings of VM, VA and VBon the built-in meter of the thermopile amplifier were obtained with moderateamplification and we~e very stable. The accuracy of the absolute values willbe about 5%, mainly due to the uncertainty in the reflection coefficient ofMgO.The reflection coefficients for all phosphors were also measured separately

with a monochromator. The differences between these values and the valuesfound with eq. (8) were generally about 1% (e.g. for no. 1029 CaSi03-Pb-Mn,r = (17±1)%). For the phosphors no. 1021, 1032 and no. 1033, the error isabout 3% owing to the fact that the reflection coefficient varies appreciablywith the wavelength in the region 250-270 mu.The spectral energy distributions of the fluorescence emitted by the phos-

phors are shown in fig. 5. These curves were obtained with a Leiss (Berlin)

TABLE IV

Standard samples NBS

excitation

wavelength regionÀ = 254m(J.

I-<250-270 mu

ti)

§ ~XoD sample ..... §~S ~ ;X .....;::l s:l 'i;~ s:l~ .S :a s:l «I

._ s:l«I d«I ._ ::s o 0 ~ .~ ;:I ._..

... '" O'~ o d ... '" 0' t:><d ti "d » "d 0

s:l ._ -e » "d ».g .~ '"0 ~ d o .. '"0 . '"03 ~ ::s ._ .~d 3 ~ 3 ~gE CIl ._

~ t:>< ~ ·0CIl ._

«I 0 '" '" «I 0'ij 8 S~ "'!ti

lI::l ._

S~ S~'"0...0 S '" ... <I'l

1026 CaW04-Pb 5 42 75 5 41 761027 MgW04 7 44 84 6 43 851028 Zn2Si04-Mn 8 33 68 6 33 691021 Zn2Si04-Mn 36 35 70 301029 CaSi03-Pb-Mn 17 29 68 111030 (MgO)x(As2Os)y-

Mn 5 29 73 4 30 781031 3Ca3(P04)2.

Ca(F,Cl)2-Sb-Mn . 23 34 ·71 141032 BaSi20s-Pb 35 55 75 ,141033 Ca3(P04)2-TI s::::!15 49 56. 4

368 A. BRIL and W. HOEKSTRA

71/

1/, 1/V ,-

v , _ti- .~, - - "/--OF:O{ - :;

_-:

/ ~-~~ ;

1/ 1

6lOlj- 1\ '".~

./ I

r".. ,I.l L / -..... 1 1

I~, Ï' I I.

'- , t-.. /v, .- .' ><t...:)';.:.,... -,_.,.. ~ I

~F.k' ;~ \r----~ . "_ ( II "7r-,:rr-. .

. L L ,I

.... .......: "WJlr ! ,\:,I \ , iI "

9l01'",

\\ '.. , i\, '.. ~.. ,

" , \ ;.. ,... ' '~

". 7,... .J,

l~ ._ ._ -. ,......... ...... . .

~ ,~~ r-r-I 'I r-r-

t

Fig. 5. Spectral energy distributions of the N.B.S. (Washington) standard phosphors.

1021 Zn2Si04-Mn1026 CaW04-Pb1027 MgW041028 Zn2Si04-Mn1029 CaSiOa-Pb-Mn

1030 (MgO),,(As20S)y-Mn1031 3CAa(p04)2.Ca(F,Cl)2-Sb-Mn.1032 BaShOs-Pb1033 Caa(P04)2-TI.

EFFICIENCIES OF PHOSPHORS FOR SHORT-WAVE ULTRA-VIOLET EXCITATION 369

mirror double monochromator with NaCI prisms. As detector was used aselected RCA IP28 photomultiplier up to 650 mu. For longer wavelengths aPhiIips 150 CVP (with Cs-O-Ag photocathode) was used. The spectra wereautomatically rerecorded.For the u.v. emitting phosphor no. 1033, Ca3(P04)2-TI with a maximum

emission at 3000 À, the glass filter before the thermopile cannot be used toisolate the fluorescence. Equation (7) was therefore used to calculate the effi-

• ciency for this phosphor using te value of the reflection coefficient determinedseparately with the monochromator.In the case of phosphor no. 1032, BaSi20s-Pb, part of the fluorescence is

absorbed by the glass filter. Equations (8) and (9) are therefore replaced by

P ( f p(iI)dil )r = VM VA - VB f p(iI)T(iI)dil '

P VB f p(iI)dil7]=----

1- r VM Ip(iI)T(iI)dil'

(8a)

(9a)

where p(iI) and T(iI) are the emission of the phosphor and the transmission 'ofthe filter respectively.The difference in reflection coefficient between the two willemite samples

no. 1021 and no. 1028 is due to a difference in Mn content. Sample no. 1028has a higher Mn content giving a much lower reflection. This type of willemiteis used in luminescent lamps while for cathode-ray excitation the other typegives somewhat higher efficiency. As has been pointed out before, a differencebetween the excitation used here (region 250-270 mu) and monochromatic254 mp excitation will be present for the efficiencies and reflections of somephosphors.

Because the monochromatic 254 mp excitation is important for the low-pressure mercury fluorescent lamps the efficiencies of some phosphors havealso been determined for this excitation. This has been carried out by com-paringthe emittedfluorescent power from the phosphors with that from MgW04with the aid of a photomultiplier as detector.Thesevaluesforphosphors no.1026, 1027,1028 and 1030are given in tableIV.

Accurate results could not be obtained with the other phosphors because thereflection coefficients varied t06 much with wavelength.

4. Relative measurements

The efficiencies given in table IV for the excitation in the region 250-270 muare all absolute values. It would be a nuisance, however, if absolute measure-ments had to be carried out always for every phosphor, because of all thefactors which must be controlled. Fortunately this is not necessary. We have

A. BRIL and W. HOEKSTRA

seen that MgW04 and CaW04 are very suitable as 'standard phosphors. Theefficiencies of other phosphors can easily be determined by comparingthem with these.The principle of the determination is the same as that described in sec. 2.

When VM, VA and VB are measured, the reflection coefficient r ofthe phosphor.is found in the normal way with eq. (8): r = p(VA'- VB)/VM and the efficiency"lP of the phosphor is then given by

I-r' VB ,n» = -1-- V ,,7],

-r B

where 7]' and r' are the radiant efficiency and reflection coefficient of thestandard respectively (thus e.g. for MgW04, 7]' = 0·435 and r' = 0·065). Whenthe reflection r ofthe phosphor is already known, only VB has to be measured.The efficiency can then be calculated directly from eq. (9).The decay times, temperature dependences, reflection and excitation spectra

of the standard phosphors will be given in a second article.

Eindhoven, May 1961

REFERENCES

1) G. R. Fonda, J. phys, Chem. 43, 561-577, 1939.R. N. Thayer and B. T. Barnes, J. opt. Soc. Amer. 29, 131-134, 1939.G. R. Fonda, J. phys, Chem. 48, 303-307, 1944.F. A. Kröger, Some aspects of the luminescence of solids, Elsevier Publishing Co., Am-sterdam, 1948.Th. P. J. Botden and F. A. Kr ö ge r, Physica, 's-Grav. 14, 747-768, 1949.V. V. Antonov-Ramanovsky and M. T. Epstein, C.R. Acad. Sci., U.R.S.S. 64,483-486, 1949.Th. P. J. Botden, Philips Res. Repts 6, 425-473, 1951 and 7, 197-235, 1950.C. W. J erome, J. electrochem. Soc. 100, 586-587, 1953..z. Bodo, Acta phys. acad. sci. hung. 3, 23-30, 1953.G. R. Fonda, Brit. J. appl. Phys, Suppl. 4, 569-573, 1954.R. J. Ginther, J. electrochem. Soc. lOl, 248-257, 1954.Gy Gergely, J. Phys, Radium 17, 679-683, 1956.Y. Uehara, J. electrochem. Soc. 107, 1-8, 1960.

2) .!, Tregellas-Williams, J. electrochem. Soc. 105, 173~178, 1958.3) A. Bril and H. A. Klasens, Philips Res. Repts 7, 401-420, 1952.4) Nat. Bur. of Stand. Tech. News Bull. 42, 145-146, 1958.5) E. J. Gillham, National Physical Lab., Teddington England, private communication.6) A. N. Taylor, J. opt. Soc. Amer. 21, 776-784, 1931.

M. Luckiesh and A. N. Taylor, J. opt. Soc. Amer. 36, 227-234, 1946.W. E. K. Middleton and C. L. Sanders, J. opt. Soc. Amer. 41, 419-424, 1951.

7) F. J. Studer, General Electric Company, Schenectady, N.Y., private communication.S) R. L. Christensen and J. Ames, J. opt. Soc. Amer. 51, 224-236, 1961.