CA1156034A - Cool-white fluorescent lamp with phosphor having modified spectral energy distribution to improve luminosity thereof - Google Patents

Abstract

COOL-WHITE FLUORESCENT LAMP WITH PHOSPHOR
HAVING MODIFIED SPECTRAL ENERGY DISTRIBUTION
TO IMPROVE LUMINOSITY THEREOF

Abstract of the Disclosure An improved cool-white fluorescent lamp utilizes a particular two-component phosphor combination exhibiting a narrow "blue" emission spectrum and a broad "yellow" emission spectrum to achieve improved luminous efficacy. In one preferred embodiment, the combination is a blended mixture of europium activated strontium chlorapatite with manganese and antimony coactivated calcium fluorapatite phosphors.

Description

115~03k COOI.-I~IITE FLUO~ESCENT ~ L? WITH PH()5PHOR
HAVING MODIFIED SPECTRAL ENE~GY DISTRIBUTION
____~QI~BQ!~LU~IINOS ITY_r~r~REOF
Background of the Inven~ion The present invention relates ~o fluorescent lamps and more particularly to a novel phosphor blend having two principal phosphors, with each phosphor emltting in c'L
different region of the visible spectrum to increase the luminous efficacy o~ the ~luoresc~n~ Larnp.
It is known to ut.ilize halophosphate phosphors in a fluorescent lamp for generating one of a plurality of standard "white'l spec~ral power distributions. A ~ypical 36 watt fluorescent lamp, having a particular spectral power distribution,e.g. the so-ealled "cool-white" color, utilizes the halophosphate phosphors to generate approx~.mately 2850 l~nens, yielding a lumlnous efficacy of about 79 lumens per watt (lm/W.) Luminous ef~icacies of greater than 80 l~/W. have not been generally available with practical fluorescent lamp phosphors, although increased efficacy is highly de~irable in this age of ener~y scarcity and high cost.
I~ Ls well kno~n to those skllled in the science o~
c~olor-Lmetry that art inirlite numbar of spectral power di~ribution.q (SPD) exlst that have iden~:Lccll color coordincltes ~Hard~ andbook o~ Colorlmetry", ~-CT Pre~s (193fi)). Any black body raclLation o~ speciied tempera~ura has a known SPD ancl, ~herefore, a m:Lq-ue set of color coordinates. The Locus of ~uch sets ~or all temperatures is known as the black body locu~.A colo~ temperaLure may be computed ~or any other SPD
lying near the black body Locus. It is also well known that a .: : : : ; -. . : : . . . : . : . .

Ll~ 7~48 ~156~3~

theoret:ical L-mlinous ef:Eicacy (TL~) for a phosphor of known SPD, respGnsive to ab~orption of ultraviolet power of a ~pecified w~velength, can be calculated assuming a quantum efficLcncy (QE) of unity.Any real phosphor has a quantum S efficiency less than unity~ld an experim~ntal luminous efficacy (ELE), i.e. the product of TLE and QE, less than its TLE.
Another igure of merit 19 the color rendering index (CRI), which measures the degree to which the perceived colors of ~tandard color plaques illuminated wit:h a glven SPD conform to those of the same plaques illuminated by a black body radiation with the same colar temperature(W~szecki and Stiles, Color Science, page 470 et s~q., Wiley (1967)).
Light from hot radiators such as a tungsten lamp or su.nlight ar2 characterized by a CRI close to 100. Deluxe fluorescent lamps have a CRI on the order of 80-90. Standard fluorescent lamps, used for msst commercial and industrial lighting, typlcally have a CRI on the order of 50-70.
State of the Art It is known to utilize halophosphate phosphors activated with divalent manganese and trivalent antimony in a fluorescent lamp to generate "white" light having color coordinate~ o~ or ad~flcen~ to the black body locu~. ~o ob~ain th~ color coordinates of .standard coLors such as "cool ~hi~Q"
and l'warm whlte", the concentra~lon of the manganese activaeor ions a~ well as the ractlon o:f chlorlne and ~luorine in the phosphor can be ad~usted. Lt 19 knot~ that the an~imony ac~iva~or ~erves ~wo E~nctlon~ .nis~-ion o~ ~
relatlvely broad blue ~peccrum band a hal~ power width on the order o~ 140 nanometers (nm)~ and e~icient transfer of energy to the manganese ions present in the phosphor host lattlce.

2-. . , , . . ,, . , . . : .: ., .

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'L'he yellow emi~ion band o:E the manganese ions ~s of the order of 80 nm wide at ~lalL rnaxllrlum p~,~er ancl is therefore narrow2r than the bLue emissLon ~pectr~ of the arltimony.
The variation of the TLE with kheoretLcal phosphor SPDs comprised of two major phosphor ernission bands has been described by M~cAdatn (40 ~ c. Am. 120 (1950~), Ivey (62 J. Op. Soc. Am. 814 (1972)), ancl Walter (10 _ppl.
1108 (1971)); whiLe the lmpro~emel~t oE CRI by use of phosphors havirlg two and three major emls,~ion bands ha~ been 10 considered by Walter (supra~ and by Thornton and Ha~t ~ 2 J.
Illum. Eng. Soc. 29 (1972)).
~acAdam initially found that the TLE or a specified color Ssuch as "cool white") w{ll have a maxim~n value if a lamp could be made to emit only at a single blue and a 15 single y~llow wavelength. Subsequently, Ivey analyzed the TLE and lamp performance for several theoretical phosphors having one or two emissi.on bands and described how the TLE
decreased for increasing width o~ the emisslon bands. Walter, having a primary objective -Ln the optimi~ation of a l'quAlity 20 index" based on a combination of TLE and CRI of a theoretic,al phosphora describes various ~o component theore~ical phcsphor bLend~ havlng dii~:~e:rellt barlclwldth~ and al~o d~flnecl a brlght:nes~ index direct:ly re, Lated l~c) 'L'LE; h~ COlrle luded ~hat an op~.lmum blen~l~ considerlng bo~h lndlce~q, has a brc)~d 25 blLle~grRen band and a narrow orange~ed bancl, A phosphor 1~ des:irecl havlng a w~ef~ pectral power clistrib~t-lon cmd po~se~sLrlg an ELE. greater ~haTI that o~ the ! . . . . . . . .

, __,. .
... . . ... .... . . .. . ...... . .... .. .. . .. ... .. .. . .. ... ... _ . .... .. . .

-;1 15 6 ~ 3 4 corresponding hcllophosphate phosphor in a "cool-whitel' Pmi~ting fluorescent lamp . The deslred phosphor sh~uld possess a quan~um efficiency no less than 10% b~low ~hat of the halopho~phate pho~phor presently manufactured Eor a ~peclfic cool .
white ~luorescent lamp and,preEerably, the quantum efflciency should differ by less than 5%. ALthough CRI i~ not of primary lmportance in the desired phosphor, it i.s r~cognized that a reasonable CRI mu~t result, e.g., the MacAdam spectrum of two monochromatic lines has a CRI of -18 a~d would not be of practical intere3t; a CRI on the order of 40-60 has been judged acceptable by competent observers.
Brief Summary of the Disclosure In accordance with the invention, a novel phosphor composition for ~7se in a fluorescent lamp of ~he type having an envelope uf light-transmissive material filled with a low pressure mercury vapor, mean9 for causing the mercury vapor to emit radiation quanta, and having a coating of the phosphor composition deposited upon an interior surface of the envelope ~or emitting quanta of visible light responslve to receipt of ~o the radiation quanta emitted by the mercury vapor, is characterized by the coating comprL~ing a blend of a first phosphor having a relatLvely narruw emis~Lon band peaking in the short -vi~ib~e wflveLeng~h ~blue) region a~ a wave`length uf approximately 450 nm. and a second ~husphor havlng a rela~lveLy broad ~missiullpeaking in ~he 570 600 nl~n (yel.low) region of the visible sp~ctruM. The yellow phusphor cuntribute~
the ma~ority o~ the luminos:Lty of the lamp with the b~end ratio between ~irst and second pho~phorY being ad~usted by variation o~ t~e percentage (by weight) of the blue phosphor within a uniformly blended mixture o the - ' . : .: . . .

I,D 7~11e modi~le~l-emi~on~pec~r-lm ta.~o~p'rLo~pnor sy~tem, ~o compensate for the differences in quan~utn efEiciency and spectraL emisslon characteri~ics of ~he phosphQrs i.n tL-Ie vi31bl2 regiorL arlcl to adjust or ~he visible mercury-vapor-radicLt~orl escapirLg through the wall o:~ the phosphor-coa~ed envelope. The color coordinates of the lamp spectr-lrl are thus placed within the 5 tandard "cool-white" emi3slon oval. de~-_ned in a region around a black body radiation locu~ h t.he yellow phosphor capturing a high percentage of the u:Lt-La-violet radiation emi~ted by the mercury vapor, In one preferred embodiment realizing color coordinates in the "cool-white" emission oval, the uniformly blended composition comprises approximately 4-11% by weight of a blue strontium, europium chLorapatit~ (SECA~ phosphor, while approximately 89-963~3 of the composition is a yeLlow phosphor of the manganese-antimony calcium fluorapati~e type.
This noveL composition provides a 5-10% increase in l~mi.nosity relative to a standard halophosphate phosphor coating.
In other pr~erred embodlmen~q, the blue pho~phor may be europium activated bcLrium mcLgneslum alumlna~e; thP luminosity increa~e 1~ comparable tc tha~ o ~he S~CA phosphor, if equal qua-lltum e~flc:Lencie~ are ach-Leved, frhe lumen increLse pro~i.ded by a p~Lo~phor ln ~ccorcLance w:Lth the pre~ent in~rentionf as oompared with conv~ntlonal ~5 calc Lum LLalopho~phate pllo~E~hors, can he character:Lzed by an incraasc in the E~LE. For example, khe pre~Jently ~L~ecl halophQsphate Çor ~he cooL-~whlte ~ orescen~ :Lamp has arL

~ 72~
0 ~ 4 lni~ial quantum eEflcienc~ oll ~he order of 0.9 and an ELE of about 147 lm/wat~. Due to ~he gas discharge eficiency of about 50%, the overall lai~p eficacy is about BO lmlwatt.
The SPD and aLl of its specific embodiments disclosed below to be of usefuL purpose must of course po~sess an ELE greater than ~hat o the corresyonding pho~phor presently u~ed in cool-white emitting fluorescent lamps.
I'herefore, the preferred embodimeTlt of this lnvention hereina~ter disclosed for z cool-whlte color mu~t po~sess a quantum efficiency no less than 10% below that of the halopho3phatc phosphor presently manufactured for cool-white fluorescen~
lamps. Prefer~bly, the quantum efficiency should differ by less than 5%. One embodiment of this invention has been measured to have an ELE of about 160 lm/watt. I~Le preferred phosphor blend herewithin illustrated employs a mixture of a yellow phosphor and a blue phosphor which possess a Tl.E
of about 180 ltn/watt when properly blended to result in a cool white lamp.
Accordingly, it i5 one obj ec t of the present invention to provide a novel phosphor blend having a pair of phosphors respectively em:Ltting in the blue and yellow portions of th~
vLslble spe~ctr~lm.
It i~ another object oE the pr~ent lnverltion to provide novel phoAphor blends eacilltating a-lJus~rflen~ o ~he 2S emi9~ion color po~nc at incre~L~ed lt~nLrlosity.
Still arlo~her ob~ect oE ~he pre~ent i-rlverl~lon is ~o provide a novel phosptlor blend em-lttirLg withl.rl tLIe cool.~white oval at increflsed lumLno~Lty.
Yet another ob~ject o~ the present in~ention is -h .

~03~ Llr~ ~.?-'~ J

~o provide a novel phosphor compo.3i~ion facil1tating adjustmen~ of th~ eml~s~lorl color poLn~ to compensate for eseap iIlg Vi S ib le mercury rad:ia~ iQll .
These and Ot her object~ of the present :Lnv~ntio will becomP apparent to those sl~ille~ in ~he art upon a consideration of the follo~ing letailed descriptiGn and the drawings.
D crlption o the Drawln~
Figure l 1s a ~sectlona1 ~Lde vi.ew of a fluore~cent larnp and a schematic representation of one possible ci.rcuit in which the lamp is used;
Figure 2 is a CIE(X,Y)chromaticity diagram containing information useful in ~derstanding the principles of t:he present invention;
Figure 3 is an enlarged portion of the chromaticity diagram of Figure 2;
_ Figure 4 is a spectral power distrlbu~ion for the preferred blend of the present invention relative to one conventional cool-white halophosphate phosphor.
Detailed Description of the Invention -Re~erring to Figure L, a fluorescen~ Larnp lO compri~es, in one eMbodirnent, a cyllndrical en~elope :Ll of light ~ransparent rnaterlal, StlCh ag gla~s and the like, having a co~ting L2 o~ a pho~phor cornpos1tlon deposlted upon its lnLerior sur~ace. An end cap 14 enclo~ed and :~orms a ga~s~type sea1 15 at each opposite end o cylinder ll. A fllalnent 16 ls po~:Lt:Lonecl ad~acent to each end ~ap ~ithln the borc o~ tube 1l. and includes a pair of leads 17a, 17b passing through and supported by ~he associa.ted end cap l4. A quantity o~ n~ercury vapor 1~ ls initially dep~slted, at ma~lufacture, within the cylindrlcal volume bo~mded by phosphor layer 12 and end caps 14.
A source 22 of alternating-current electrical energy, it3 3 ~ LD 724~3 a ~witch ~3 and ball~st: me~rls 2~; are in electrical series connection betweerl a first lead 17a of each opposite ilaMen~:
16. Starting means 25 is coupl~d between the ret~lalnlng leads 17b of each of the pair of ilaments 16. I-t shoulcl be understood that other known :~luorescent l~p embodiments may be equally well u~ilized and may alLow certain of the circuit components (such as startin~ meanc; 25) to be dispensed with.
A~ i~s wel.l known, in operation 9 tartlng means 25 c~uses a flow o E current through each of iLaments 16 responsi~e to the closure o~ switch 23. Starting means 25 thereafter causes a sudden cessation of current flow to cause ballast means 24 to generate a relat,vely high voltage between filaments 16 to cause a flow of current to be initiated 1$ to es~abllsh a conventional mercury arc discharge. The phosphor is caused to fluoresce and re-emit a signiEicant portlon of the impinging energy as quanta 28 o visible light having spectral characteristics determin~d by the composi~ion of the specific blend of the fluorescent materlals utilized for the phosphor coating in ~he individual tube.
Referring now to the chromaticity diagrams of Figures 2 and 3, any color oE the vi~ibLe ~qpectrurn is loc~table wi.t:hin the area boun~ed by a ~pec~rllm locus 30 and identif:ied by the corre~pondln~ value~ oE ~he X arld Y trichromatlcity coord.Lrlates as~ocla~ecl ~herewl.th. A spec~ral:ly pure color9 l.e. a vislb:Le color cotnprised sole:Ly o~ a sillgle wavelerlg~h~ iY locatecl upon ~pectrum loc-ls 30, whereon the Ypectra11y pure color wavlen~th is indicQted ln nanometers o wavelength. Chroma, includillg white light, coTnprlsing a hlend o~ two or ~lo.re ~pectrally pure color~ has X ancl Y vaLu~s withln area 31 bownded hy spectrum Locu~ 30. Real phosphors emit ~uanta a~ an ln~inlte .

?-~ (3 6 1) ~ ~

n~nber of waveLengtil~s. 'I'he SP:D of a real phosphor ls, -~herefore, a continuous curve from which a set o color coordinates are calculated by perEorrni.rl~ appropriate ma~hemat:ical cal.culatior ~as shownl e.g., in ~he above--mentioned Wyszecki and Stiles text).
Curve 32 i5 the b1ack body locus of co.Lor coordinates of a black body light ~o~fce eml~ing at a de~ignated kemperaturc.
The "cool-whi~e" color has been standardi~ed to a range o~
values situated ln an oval 33 generallyabout a polnt 35 at X~0.372, Y=0.375 and corresponding to a black body temperature of approximately 4200K. A typical cool-white lamp of kno~m design has a color point 36 lying wlthin oval 33 at X~0.377, Y=0.382.
In a 3Ç watt fluorescent lamp, approximately one watt.
of radiant energy is con~erted to produce approximately 175 lumens of visible mercury light 27. The relative magllitude of the blue mercury monochromatic emlssion 37, the green mercury monochromatic emission 38 and the yellow mercury monochromatic emission 39 may be comblned to yield a single mercury color point 40 typically ha~irlg trichroma~icity coordinate~ substall~laLl.y at X~0.~.2 3 ~-0, ~ L, SllbstaIItially a1L (.L8 watts) o the remainlng energy emlt~ed ~rorn mercury vapor 1~ tlt ~:he mercuroy resonance w~avelength~ of ~pproximate.L~ 5 and ~5~ nrll., altho-ugh a flmall amourl~ (appro~imat~ly one-half w~tt) I.s emLt-ted a~ the near-ultravioLet waveLengchs of 2~)5~00 nrn. 'I'h~ resoncmce e~ergy is k~own to stimulate re~emi.ssion of visible li~h~
;Erorn a wide var-.Let:y ot pho~phor~. In a nov~l phosphor t~, 115~

compos~tion ~o be described hereinbelow, additional luminous OUtp-lt is obtained by the proper choice of one phosphor component of ~ dual phosptlor blend to also partially absorb the near ultraviolet mercury emissions for additional production of vi~ible light.
~ divalent-europi~ activated phosphor, i.e., a phosphor having europium as the activator ion supported in a host lattice, will partiall.y absorb the near ultraviolet mercur~ errllssions substantialLy over the ent:Lre range of waveLengths from 297 nm. to 400 nm.
We have achieved additional luminous ou~put by modification of the lamp output spectrum to have a relatively narrow "blue" emission band in conjunction wil:h a relatively broader "yellow" emission band. Preferably, 15 the activator ion emitting the relatively narrow blue spectrum is divalent europium, in contrast to a standard fluorescent tube which does not use rare-earth-doped halophosphates. A useful divalent-europium activated phosphor will generally have a narrow emission band peak ~ at a wavelength oE approximately 450 nm.
(and will thus be a "blue" emitter) and a ~uantum efficiency oE at least 80%, and preEerabl.y about 90%, with the quantum e~ficiency being dependent upon processing considera~ic)rls. Pre-~erabLy, the acti.vator -lon em-ltting the broacler yelLow b~nd -i~ cllva:l.ent man~ane~e. A phosphor doml.natecl by mang~nese in a ha:lopho~pha~e Lat~ice ~ill g~nerall~J ha~e ~he colorpoln~
o:~ it~ eml~slon at coorclinate~ above the bLack bod~ locus. Thus~
an important Eunctlon o~ the "bl.ue'l p~sphor i9 to pull ~he cc~osite color coordinates on~o the b~Lack body :Locus; a narrow band near the peak oE the Z trlstilnulus va:Lue ef~iciently per~orm~ this I~UllCtit~ while ~acl.litatL-ng the use o:l~ a ~reater proportlon of -L~-115~3~ 77,lS~, the incldent radiation in the more highLy luminous m~ng~nese emlssion.
In particular, we have found that a fluorescent lamp having lts color point situated within the cool-white oval 33 and utilizing a dlvaleIIt-Puropium activated phosphor requires at least one additional phosphor component with the compositepeak wavelen~th ~hereof lying in the range between 570 nrn. and 600 nm. Processing costs are minimal lf a single additional phosphor ls utilized. Additionally, use of a two-phosphor blend al'Low~ simplified placement of the blend point within oval 33. By restricting the light below a wavelength of 520 nm. substantially to the narrow blue band cssentially peaked at about 450 nm., an accept,ble color rendition index can be achieved with high luminous eficacy, if the second phosphor broadly emits over a band centered in the "yellow" porticon of the visible spectr~n.
A suita'ble broad yellow emission spactrum is obtained from a second phosphor having a divalent manganese activ~tor ion supported within its host lattice.
A stoichiometric strontillm, europlum c'hlorapatite (SÆCA~ phosphor having the cheTnical fo~n~lla Sr10 zEuz(PG~)6Cl2, where 0.02 ~z ~0,2, may~ be ukillæecl ~or t'he blue ernlttar, In partlc~lar, Eor achieving t'he cool~w'hite oval 33, x ~ 0.l4 ~ 0.~5 1~ a pre~er~ecl utllizatloTI range. 'rhe blue-~5 emittlng SECA compound ha~ ~ color point 4l haYing trichromatic coordina~e~ X ~ 0.152, Y - 0.027. Alt~rnatlvely, a s~oichlometrlc europi~ actlva~ed bar-lum ~agnesium alumina~e having ~he chemlcal fo~lrula Ba2~æEuzMg2Al2~037, where 0.1 ~x ~0.~, and havln~

:: . .. .

1 1 5603k subs~an~lally the same ~richromatic coordlnates as the SECA
compoun~ may be utill~ed.
As previously mention~, most of the excLtation energy is utilized to stim~llate the relatively broad yellow emission band of the divalent manganese,which broadband emisslon ha~
a considerably higher total luminous flux r~lative to the total luminous flux contalned -ln the short:er wavelength "blue"
emission band. The increased luminous flux output o the yellow-emitting phosphor provides the improvement in total luminous flux, while the relatlvely low luminous 1ux of the blue-emitting phosphor both adds to the total luminous flux and more importantly serves to "pull" the yellow spectrum to the desired lamp emission coordinates, as hereinbelow more fully explained.
We have found that a suitable broad "yellow" emitting phosphor ls a stoi~.hiometric dlvalent-manganese-actlvated compound having ~he chemical fonmula Ca10_w_x ~d~ nxSb~(PO4)6F2 0 where 0.0 _w 40.2,0.2S ~x ~0.5, and 0,02 ~y ~0,2, In particular, for achleving the cool-whi~e oval 33 J
20 , w ~ 0.10 t 0.03, x ~ 0.32 ~ 0.03, and y ~ 0.07 :t 0.02 are ~' preferred ranges of w, x, and y, respectively. I~e particular man~anQseqactivated calc.lum 1uorapatite phosphor ~et or~h above is advantageous 1~ ~ha~ its peak wavqlerlgt.h 1~ ln ~he r~gion o~ S70 600 ~m.; the utilizati~n o a subs~antial mol ~r~ctiorl o~ mangane~e in ~he phosphor no~ o~ly se~v~s to quench ~h~ an~imony emi~:Lon, but also serve~ to provide a luminosity ou~pu~ whlch, when blended with ~he SECA phosphor (a~ described herainbelow), provides approximately 90~/O o~
the ~o~al luminous output/ whereby ~he relatively low luminance LD ~24~
l 1~603~
of the narrow blue emisslo1-lspec~m not only increases the available Lumlnosi~ but al~o provlde~ remarkably good fleshtone rendit-Lon with ~he broad yellow emis~;ion phosphor even though the color rend-Ltion index o the compo~itlon is approximately 5C and therefore approximately 15 points lower than the standard cool whlte halopho~phate fluore~cent phosphor blend.
The use of ~ palr o phosphors, each containing a di~ferent activator ion withln its lattice, is conveni~nt from the standpoint of processing considerations, whereby the mol fraction o~ each activator atom (preferably being divalellt europlwn for the narrow blue emitter and divalent manganese for the yellow emitter) can be adjusted.
We have found that the mol fraction of manganese must be adjusted, typically towards the pure manganese emission level, to obtain the desired ~olor poLnt in cool-white oval 33, to sihift the operating slope of the bLue-yellow blend line of the phosphor coating. Illustratively, to match the cool-white oval 33, as the mol fraction o~ manganes~ is increased, the trichromatic coefficients (for the yellow manganese-~'activated calcium fLuorapatL~e) lie along line 44 (FIGURE 3)and inc~e~e ln the direction o~ arrow X, ~oward~ the pure mang~ne~a color poln~ ~$. l'he trLchrom~tic co~f-lcients along line 44 lnclude tha e~ect o~ the vlsible-~llercury emission lineis thereon. rhus, blend line 46 is es~abllshed ~or ~he visible ou~put oE a two~phosphor i~ys~em utllizi.n~ the blue SECA
phosphor (havin~ color point 4L~ ~o provide ~he ~iri~t activator ion and the yelLow calcium phosphor to provide the second activa~Qr ion. In par~lcular, A yellow phosphox h~ving ~ mangAnese content of 3.00/0, ~nd having trichromatlc .

115~03k ~ 72~

coordlnates X - 0.40g, Y ~ 0.432, when solely present without the SECA compound but lncluding ~he visible mercury ~nissions effect, establis~es blend llne ~6 whlle blend line 47 is establlshed for the same compounds, but wi~h mang~nese present in the yellow calci~m phosphor in the amount of 3.2S%, the yellow pho~phor then having trichromatlc coo.rdinates 49 with X ~ 0.443~ Y - 0.466, when the yellow calc~n phvsphor is solely considerecl ~wlthout the SECA pho~phor) but combined with the vislble mercury radiatlon.
As can be seen froM FIGURE 3, a range of blend ~ralues can be found along either blend llne 46 or 47 whereby the color points of a blend ~all within the desired cool-w~ite oval 33. Typically, the proper blend requires an amount of the blue SEC~ compound of approximately 6% by weight of the total phosphor composition~ which compositlon hence contains approximately 94% by weight of the yellow c~lcium fluorapatite compound. The 'tblue" phosphor can represent 4-11% by weight of the total phosphor composition, to attain a selected one of a plurality of different color points. The illustrated cool-white oval may be attained by adjusting the blending proportion along either blend line to obtain the ratio of blue-to-yellow lumino~l~y ou~puts which colnpensa~e ~or the dif~erence ln ~he ral~tiv~ qua~t~n e~iciencle~ bet~een the two pha~phor~ and or thel~ dif~erlng spectral emis~ion characteristics~ In ~ener~ he d~lred l~inasity ou~pu~l w:L~h impro~ed l~mi~ou~ e~icacy and adequate color rendltion, nay be ob~aLned, Eor a palr o "blue" and "yellow~' p~osp~ors ancl ~or the cool-white ob~ectlve point~ when the "yellow"
pho~phor c~p~ures approximately 91~/o of the rnonochromatic radiation emit~ed at the approximately 254 ~n. wa~elength : .. . - . . . .. . .. . .. , ,, , . :

I 1 560~
of mercury vapor 18. As hereinabove implied, the mol fraction of the dlvalent manganese in the yellow phosphor must be adjusted, within the preferred limits of 2.9-

3.5%, to color-com~ensate the broad yello~ emission of of the phosphor composition into cool-white oval 33 ~or the differing levels of visible mercury radiation escaping through phosphor coating 12 and the transparent material 11 of the fluorescent tube; a greater magnitude of escaping visible mercur~ radiation will require that the mol fraction of manganese be greater along phosphor line 44 in the direction of the arrow X and toward the pure manganese emission at point 45, while reduction of the visible mercury radiation magnitude excaping through the wall of fluorescent lamp 10 will require that the phosphor host lattic be activated by a somewhat lower mol fraction of divalent manganese.
Further understanding of the increased luminous efficacy which is gained with the present phosphor combination as compared with conventional cool-white emission is provided with Figura 4. The relat:ive emission curves ~or both phosphor materials are shown wherein -the present phosphor combination (curve A) being illus-tra-ted constitutes a blend having apprQximately 6~
be weight Q.~ the pre~errecl europium-activated strontium chloro~patite phosphor above described mixed with approxima~ely 94% hy weight o~ ~he above described pre-Eerred aalcium Eluoxoapatite phosphor activated with mangan~se and antimony. As can be noted ~rom the emission curves, the ~pectra]. dl~.~erences ~or the present blend are increased emisson in the 530 610 ~n. wavelength region accompanied by decreased emission in the 470~530 nm.
region as well as in the 350~-430 nm. wavelength region 1 :~5603~

relative to -the conventional phosphor (curve B). Such emission transfer to the more luminous 530-610 nm. wave-length region produces the desired increase in luminous efficacy compared with the cool-white emission obtained with conventional calcium halophosphate phosphor. It will also be apparent from the foregoirlg considerations that the narrow blue emission peaking at approximately 450 nm. wave-length which is attributable to the blue phosphor component in the present blend concentrates additional power near the peak o~ the Z tristimulus coordinate. Thus, the necessary value of Z is attained whith less total blue power in the spectrum by using a narrow blue band. The excess power can then be used in the yellow band to further enhance the overall luminosity of the spectral power distribution. The desired lumen gain is achieved in this manner when the phosphor blend contains approximately 4-11 weight percent of the blue phosphor component and with the desired color point of composite emission being maintained in the cool-white oval.
In practice, several ~luorescent lamps 10, identical in construction with a known 36 watt fluorescent lamp utilizing known halophosphate phosphors, which produce, as herelnabove mentioned, an output o~ 2850 lumens at a luminou~ e~icacy oE 791m/W., were fabricated and coated with the above-de3cxibed blend o~ the eu.ropium-activated strontium chlorapatite and khe manganese-activated calcium ~lu~rapatite~ A ~ir~t lamp utili~es the yellow calcium phQ~phor havin~ a 3~00~ manganese content, while s~cond and third lamps utilize the yellow calcium phosphor with 3.25~ manganese content; all three compo~.i-tions having 6~ or 7%, by weight, o~ the blue europium-activated stron-tium chlorapatite compound in a total phosphor bl.end.

0 3 ~
The lO0 hour burniny results obtained with our novel two-~hosphor blend, particularly synthesized to emit within the standard cool-white oval, are summarized in the following table and illus-trate the increased luminous output at an increased value of luminous efficacy (in lumens per watt) with the percentage gain of our novel two-phosphor blend being particularly set forth with reference to a standard cool-white halophosphate blend~

_ Lamp (RAP 17) (R~P 36) (RAP 40) %Mn= .3.00~ 3.25~ 3.~5%

lm/W 84.9 83.5 85.3 Gain(% over 79 lm/W) 7.4% 5.4% 7.9%

Trichromatic Coordinates, 0.372 0.375 0.379 (X,Y) 0.381 0.381 0.383 % SECA 6~ 7~ 6%
There has just been described a novel phosphor blend having one phosphor emitting a relatively broad band of visible light in the yellow portion of the visible light spectrum and the other phosphor emitting a relatively narrow band of visible liqht in the blue portion of the visible light spectrum to realize a phosphor coating for a fluorescent lamp havi.ng an improved luminous e~:ficacy.
In particular, a novel two-phosphor blend Eor use in a aool-white ~rnitting fluorescent lamp comprises ~pproxi-mately ~ by weight o:~ a :~lrst phosphor having its narrow emission band in the "bluq" region of th~3 visible spectrum peaking around 4$0 nm. with the remainder (39-96% by weigh~) of the blend comprising ~ second phosphor having its relatively broad emi~ssion spectrum in the "yellow" region of -the visible spectrum and peaking l156034 LD 7248 at about 570-600 r~., with the two phosphors yielding a 5-10~ lncrease in luminous efficacy compared to presently used lamp phosphors.
While several preferred embodiments of the two-activator-ion system have been described, many variations and modifications will now occur to those skilled in the art. It is our intention, therefore, to be limited not by the present disclosure, but solely hy the appended claims.

Claims (8)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. In a fluorescent lamp of the type having a sealed envelope which contains means to generate a low-pressure mercury discharge within said envelope, and a coating within said envelope for conversion of at least a portion of radiation emitted from said discharge to visible light having a white color, the improved coating comprising:
a first phosphor having a relatively broad emission spectrum with a mean wavelength in a yellow portion of the visible spectrum and having the formula Ca10-w-x-yCdwMnxSby(PO4)6F2-y0y where w is in the approximate range 0.0-0.2, x is in the approximate range 0.25-0.50 and y is in the approximate range 0.02-0.2; and a second phosphor having a relatively narrow emission spectrum in a blue portion of the visible spectrum;
said first and second phosphors being uniformly blended together in a proportional relationship preselected to provide increased luminous efficacy.

2. The improved coating of claim 1, wherein said second phosphor has the formula Sr10-zEuz(PO4)6Cl2 where z is in the approximate range 0.02-0.2.

3. The improved coating of claim 2, wherein said coating contains a quantity of said second phosphor sufficient to match the emission spectrum of said first phosphor to a cool-white color point having trichromatic coordinates of approximately X=0.377, Y=0.382.

4. The improved coating of claim 3, wherein the mol fraction of manganese in said first phosphor is varied to achieve said cool-white color point.

5. The improved coating of claim 3, wherein said second phosphor is in the range of 4-11% by weight of said coating, said first phosphor being in the range of 89-96% by weight of said coating.

6. The improved coating of claim 5, wherein said second phosphor is approximately 6% by weight of said coating, said first phosphor being approximately 94% by weight of said coating.

7. The improved coating of claim 3, wherein w = 0.10 + 0.03, x = 0.32 ? 0.03, y = 0.07 + 0.02 and z = 0.14 + 0.05.

8. The improved coating of claim 1 , wherein said second phosphor has the formula Ba1-zEuzMg2Al22O37 where Z is in the approximate range 0.1-0.4, said second phosphor having a peak emission at a wavelength of approximately 450 nanometers.