Indian Journal of Pure & Applied Rhysics Vol. 37, February 1999, pp. 136-141

Qualitative characteristics of fluorescent lamps and their phosphors

B S Chakrabarty , K V R Murthy & T R Joshi

Department of Applied Physics, Faculty of Technology & Engineering, M S University of Baroda, Baroda 39000 I

Received 29 October 1998; revised 24 December 1998; accepted 5 February 1999

Study on commercial lamp phosphors reveals that the phosphors used by the industry still remains to be calcium halophosphate doped with antimony and manganese . The emission characteristics of all these samples for an exc itation wavelength of 254 run show peaks arow1d 474 and 557 nm corresponding to antimony and manganese respectively. For both these peaks, the maximum excitation intensity has been found to be between 270 and 280 nm. The average particle size was found to be around 12 microns. Lumen output of lamps made out of these phosphors varied in proportion with the quantum efficiency of the samples, although the relation was not fow1d to be linear. The variation in the lumen output of the lamps having the same phosphor san1ple indicates the dependence on fac tors like mercury vapour pressure, tube dimensions, etc. Coating thickness plays a major role . The colour po ints of the lamps also show variation. The colour points calculated theoretica lly, using the emi ssion characteristics of the sample, was found to be in agreement with the experiment values.

I Introduction

The essential aspects of fluorescent lamps arc the gas di scharge and the phosphor coating . The former is a low pressure mercury discharge which when ignited electri­cally gives rise to a substantial amount of in visible radiation in the ultrav iolet range, most of which IS at 253 7 nm . The later converts this radiation into visible This conversion should cover the entire range of visible

region, if the e ffect of whitc light is to be faithfully produced . Another important aspect is the anlOunt of visible radiation generated from a g iven amount of UV radiation . This ratio g ives the conversion facto r and it should be high. The quality of lamps is decided in

general by two factors , lum en output and colour coordi­

nates . 111e former gives a measure of the intensity of light produced by the lamp, while the later g ives thc colour quality of the lamp and its proximi ty to the ideally deS irable colour quality. An attempt has been made to co rr~ late the above pa rameters .

Six commercial lamp phosphor samples were col­lected from reputed lamp manufacturers hke PhIlips (Holland , India) , General Electric (USA(, Osram , Cona and Solichem . However the samples have been coded as P-l , P-2 , ----, P-6 to maintain confidentiality. Lamps

using these phosphors werc made at the manufacturing site of a repute company . The data on the lumen output and colour coordinates fo r fi ve lamps were studied for each category of phosphor

2 Results & Discussion

2. 1 Characterisat;ion

The six samples of commercial lamp phosphors were characterised using X-ray diffraction technique. The XRD pattem was recorded on a PHILIPS PW-1840

machine All the samples had maxima around 32° cor­responding to a d-value of around 2.79A, which clearly identifies with the calcium halophosphate structure . A typical pattern is given in Fig I . The samples have a blend of chlorofluoroapatite and agree well with the reported valuesu and ASTM card data . As in the case of XRD, which give similar pattern for all the samples, the emission spectra of the samples for eill excitation of 254 nm gave identical peaks for all the sanlples . One peak was around 474 nm while the other around 557 nm , which agrees well w ith the reported peaks fo r antimony and manganese respectivel/

2.2 Quantum efficiency

The function of the phosphor is to convert the incident ultraviolet radiation into visib le light The efficiency of this conversion is the most important criteria for selec­tion of a phosphor Quantum efficiency is defined as the ratio of nl!mber of photons emitted to that absorbed by the phosphor (Q) The QF of samples were measured by direct optical method4

.5 Sample P-4 has the maximum

quantum efficiency followed by P-2 and P-5 . Samples P-6, P-l and P-3 take the last three positions in descend­ing order The values of quantum efficiency are in arrangement witb the reported values . Quantum effi -

,.

CHAKRABARTY et al.: FLUORESCENT LAMPS & PHOSPHORS 137

ciencles of calcium halophosphates doped with anti­

mony and manganese range from 52 to 96%. The phos­

phors used commercia ll y hav e hi gh quantum

effici encies as they have to be viable , e .g . halophos­

phates and blends of halophosphates used by Philips

have quantum efficiencies ranging from 84 to 90%. The

quantum efficiency is dependent on the effective absorp­

tion of the UV radiation of 253 .7 nm , wh ich in tum

depends on the phosphor quality viz its crystallinity, the

grain size distribution (ball milling 111 excess can have a

negative effect on quantum efficiency), presence of im­

purities (leading to transfer of energy to killer centres ,

wh ich utilize the energy for non-radiative transitions)

and other factors6

!II C (lJ .... oS

25 30 35 40

2 e (deg)

f ig. I - XRD patten! or a typical sample (calcium halo­phosphate)

2.3 Excitation and emission spectra

ll1e excitation spectra was recorded on a Hitachi make fluorescence spectrophotometer (Model No . F-40 I 0), which has a flat spectral response over the entire range of wavelengths of measurement 200 to 900 nm .

For taking the excitation spectra, equal quantities of phosphor were taken for each sample and the emission

spectra of eaeh of the sample was recorded for an exci ­tation wavelength of254 nm . The excitation spectra was

seanned for the two val ues of monitoring emissi on lines i .e . 474 and 557 nm . The peak excitation as shown in

Fig 2 is not exaetly at 254 nm but between 270 to 280 nm . Peak excitation values lower than these i .e . about 254 nm are desirable for maximum absorption of the 254

nm radiation emitted by the low pressure mercury dis ­charge . For the 557 nm monitoring line, there are addi ­

tional peaks around 350 and 408 nm having much lower intensity .

The incident UV ene rgy is absorbed by the crystal

lattice of the halophosphate . A part of this energy is consumed as lattice vibration energy while a substantial

amount is transferred to antimony atoms . The antimony gets exeited by undergoing a transition from I s ground

state to exeited I p and 3p states . The excited antimony

:~ !II C (lJ +­c .....

500

200

Monito-

,-I , , \

I \ ( ,

300

" (nm)

Peak ring line (nm)

(nm)

474 280

559 280

400

Peak lntensi ty (Relative

Units)

466.3

320.5

Fig . 2 - Excitation spectra for sample P-I

138 INDIAN J PURE APPL PHYS VOL 37, FEBRUARY 1999

atoms transfer fraction of the energy of exc itation to the manganese IOns, which undergo transitions within the 3d levels to give the green emission 7

The maximum emission intensity was recorded for sample P-4 followed by P-2, P-S , P-6, P-I and P-3 in that order Here again , the order observed in case of quantum efficiency and intensity of excitation has been main­tained

There are two main indicators of the quality oflamp Intensity of light given in terms of lumen and colour rendering property given by colour coordinates or CRt The properties mentioned above are taken under stand­ard electrical conditions .

2A Lumen Output

The light output in lumens is reeorded after burning the lamp for 100 hours . The light intensity is measured in te rms of lum en, using an Integrator LMT-2000 Lu­men output values of fi ve lamps in each category, se­leeted on a random basis a re available The maximum average lumen output is for the sample P-4 fo llowed by P- 2, P-S , P-6, _ I and P-3 in that order This order is the same as that of quantum efficiency of the samples Although this can be expected, the unusual thing about these resu lts is the extent of variation in lumen output for vanous lamps made from the same phosphor This aspect calls for the consideration of factors re lated to the manufacturing process as a ll other factors are identical for the samples . Theoretically, the maximum output of around 80 lumens/watt ie . about 3200 lumens for a 40 watt lamps can be obtained It can be seen however that the ou tput of the lamps arc much belo\\' this theoretical maximum . TIle values prescribed by lSI is 24S0 lumens . The e lectrical and geometrical parameters of lanlps also playa signi ficant role

2.5 Colour coordinates

The measurement of the colour points \\'as done usmg a deVice of Minolta make, speciall~' meant for the pur­pose . It given direct reading of the x and y coordinates These colour coordinates obviously depend upon the emiss ion characteristics o f the phosphor to a large ex­tent. Companng with speCI fi cations of Phil ips , the col ­our point comes nearest to the cool da~' li ght vanety. for \\hich the colour coordinates are x = 0309 andy = 037.7 corresponding to a colour temperature of 675K

2A Grain size

Grain size analysis us ing Micro sizer -E particle size analyse r shows that the phosphor samples have g rain size range. predominantl~. bet\\ecn 7 to 17 microns . However in all the cases . the g rain size is almost en tire ly

below 2S microns , as the amount of particles be yond that size is insignificant For the better performing phos­phors , this is below I % . It is common knowledge in lamp processing that too much ball milling, required for smaller grain size brings down the effiCiency . On the other hand, bigger grain size leaves interstices between the grains, through which the UV radiation passes into

the glass surface, leading to a loss of excitation energy Hence an optimum size has to be considered . An opti­mum range of grain size has been proposed here between 8 to 17 microns approximately . This is because the average particle size is between 12 to 13 microns and maximum number of particles are covered by the pro­

posed range (ie 12 .S± 4.S microns) The maximum

pe rcentage of g rains in this range has been fou nd for sample P-4 , followed by P-2, P-S , P-6, P-I and P-3 Lower grain size can be obtained by chemical methods 8

. 10

The light output increases with increase 111 quantum efficiency Although it can be inferred that the light output of lamp increases \vith increase in the quantum efficiency of the phosphor used , the variation has not been found to be strictly linear. It would be easier to predict the lumen output for a given efficiency of ph os­phor, if the trend is linear. However the linearity here

can be visualized in two directions given by segments A and B (broken lines) in Fig 3 . Although the linear variation from P-3 to P-2 suggests the segment A to a logical consequence, this trend is quite unlikel y. -fbi s is because, as per the sequence given by segment A. a lamp with a phosphor of quantum efficienH around 89 .5% (P-4) would give a lumen output of 2700 lumens ap­proximatel y. On the other hand, the sequence followed by segment B is more likely According to this sequence, the lamp output corresponding to a qlJantum effiCiency of around 895 % (P-4) would be 2S60 lumens . This looks more practical , as per the lSI stcilldard of 2450 lumens!40 W Considenng this sequence of segment B. the non-linearity becomes more evident Hence it can be concluded that other factors related to the phosphor and lanlp manufacturing process plays a role .

It can be seen from the Table I that the range of variation is maximum for sample P-3 foll owed by P-l , P-6, P-S , P-2 and P-4 in the descending order. Relating this to the quantum efficiency of the samples and lumen output of the corresponding lamps. it becomcs evident that better phosphor has a low range of variation It can be again seen that the decrease in the range of variation of lumen output from P-3 to P-4 docs not correspond to

.~

CHAKRABARTY et al.: FLUORESCENT LAMPS & PHOSPHORS 139

Vl C III E . :J -.J

2500

:; 2450 0.. ... :J o

2400

eo 85 90 Quantum Efficiency (010)

Fig. 3 - Variation of light output with quantum efficiency

Table I - Variation of lumen output with quantum efficiency

Sample Quantum Average Range of efficiency lumen output variation (%) (Lumens) (Lumens)

P-I 83.02 2399 108

P-2 8609 2503 39

P-3 82.81 2391 109

P-4 89.45 2520 36

P-5 85.86 2487 46

P-6 84.43 2442 93

a linear increase in either the quantum efficiency or

average lumen output Hence one should look to the other factors Onc of the parameters that is likely to be

responsible for the variation is the coating thickness , as

it varies fr(;m region to region inside the tube .

2.7 Coating thickness

Coming back to the wide range of variation in lumen

output and its probable relation with coating thickness ,

one can calculate the absorption of radiation for different

coating thicknesses. The intensity of absorption de­

creases according to the formula .

This formula would also hold true for the absorption of ultraviolet radiation in a phosphor coating . In case of halophosphates, the absorption coefficient has been found to be around 1500 cm -I, with the assumption that

the particle size would be between 1-50 !J., which is indeed the case here . For a coating thickness i.e . pene­

tration depth d = 20!J.m, the amount of unabsorbed radiation is about 5%, which reduces to 2.35% for d =

25 !J.m and to about 1 % for d = 30 !J.m Thus the unabsorbed radiation , which is ultimately lost in the glass wall will increasc with decrease in coating thick­ness, whi le for higher coating thickness, the visible light generated may be absorbed by the phosphor layer Thus on both sides of the optimum coating thickness , there would be a marginal fall in the light output. However the range of variation is quite substantial and so other factors like Mercury vapour pressure, tube dimensions, electri­cal characteristics, ctc . also come into play.

The general experience with halophosphates show that phosphor compositions are preferably non­stoichiometric l l

. Maximum emission occurs at an over­all mctal/phosphorus ratio of5/3 . The attaining of proper crystallinity by the material depends on the firing tem­perature. The energy transfer and thus the emission characteristics as the excitation characteristics being

140 INDIAN J PURE APPL PHYS. VOL 37, FEBRUARY 1999

0 . 350

0.340 Y

0 .340 0.310

• p-s

• P-4

P-6. • P-2

• P-3 P-1·

0.315 0.320

Fig . 4 - Section of chromaticity diagram showing average colour points of the lamp samples

structure sensitive, this factor assumes importance . To maintain a proper atmosphere during the firing process IS imperative In certain cases, reducing atmosphere is needed and so phosphors fired in air give a lower effi­cicncy . This is because of the presence of air facilities in the formation of undesirable phases. The influencc of H20 and gases like NH3 is also unfavourable l 2 Trace Impurities like aluminium , radium , iron etc . also have an adverse effect on the efficiency of phosphors

2.8 Chromaticity diagram

The colour coordinates of the phosphor samples i.e the average value of x and y coordinates for the five lamps move in a narrow range and the range of variation is also identical for all the samples. In general , it has been observed that for higher lumens, the colour coordinates also shift slightly to the higher side . As the colour points move in a narrow range , a small section of the chro­maticity diagram has been considered in Fig . 4 . The values ofxcoordinate move in a narrow range compared to the y coordinate which arc more spread out. llle san1ples with higher lumen output have higher y coordi­nates .

It is also possible to find out the colour coordinatcs from the emission spectrum of the phosphor using the data on spectral sensitivity of human eye The emission spectra i.e the spectra characteristics of the sample was

superposed on the eye sensitivity curves for the three colour regions . "[be area under the curve was calculated for each of them This is given in Fig . 5. On doing so it

was found that the area under the curves x (A), y(A) and

Z(A) was around 32%, 39% and 29% respectively . ll1Us the x andy coordinates would be around 0.320 and 0390 respectively . However, the emission of mercury gas has to be also taken into consideration , which accounts for about 10% of the light on an average . Most of this is in the violet-blue region (about 50%) while the rest is spread out in the green and yellow regions evenly . Taking these into account, the revised value of x and y coordinates would be around 0.315 and 0375 respec­tively . In calculating the above, the em.iss ion charac­teristics of sample P-4 has been taken . The experim ... ntal and theoretical values of x and y coordinates are given below: Experimental Theoretical

0.3146 03150

03466 0.3750

The theoretical values arc a bit higher than the experi ­mental values, the difference being more in case of y coordinate compared to the x coordinate . A precise value of colour coordinates is difficult to predict theoretically . Data relating the colour coordinates to CRl and the corresponding colour temperature assigns a colour tem­perature of about 6350 K and a CRI value 72 . The value

,

CHAKRABARTY et al.: FLUORESCENT LAMPS & PHOSPHORS 141

2.0

(

Emission spectra of '\ calcium halophosphat e

\ -\ y(~) /X(~) \ / \ 1.0 ,

500 600 700 Wavelength (nm)

Fig . 5 - The CIE colour matching functions for standard observer (bold lines). Broken line shows the spectral characteristics of the phosphor sample i.e . emission spectra of ha lo phosphate sample with antimony and manganese as activators. The peaks correspond to emission by antimony (474 nm) and manganese (557mn).

of CRI is a bit low and this is expected in view of the fact that the emission is concentrated more in the blue and green region and lacks in red component

References

2

3

4

Fukuda Y, Mizugadi K, Yokota S & Takenchi N, Radiat Prot Dosim, 70 (1987) 93 .

Seshagiri T K, NataIjanV & Sastry M D, Pramana-} Phys, 39, (1992) 131.

Kapoor J K, in Thennoluminescence and its applications, edited by K V R Murthy et al. (Tata McGraw-Hill, New Delhi), 1992, p 19.

Butler K H, Proceedings of the International Conference of Luminescence, Brussels, 1966, pp. 1313-1329.

5

6

7

8

Alexander G, Ramakrishan P & Mukherjee T K, Indian J Pure

& Appl Phys, 31 (1993) 531.

PalillaF C & O 'Reilly B E,Electrochem Soc, 11 5 ( 1968) 1076.

Blasse G & Brill A,} Electrochem Soc, 115 (1968) 1068.

Kottaisamy M, Jeyakumar D, Jagannathan R & Mohan Rao

M, Mater Res Bull, 31 (1996) 10\3.

9 Ekambaram S & Patil K C, Bull Mater Sci, 18 (1995 ) 921 .

10 Kottaisamy M, Jagannathn R, Jeyagopal P, Rao et aI. , J Phys

D : Appl Phys, 27 (1994) 2210.

II Rabatin JG, Gilloly G R& Hunter J W,J Electrochem Soc ,114

(1967) 956 .

12 Watchtel A,} Electrochem Soc , 113 (1966) 128.