Download - Interlaboratory Comparison of Measurements of the Spectral Irradiance from Fluorescent and Incandescent Lamps: a Report

Interlaboratory Comparison of Measurements of theSpectral Irradiance from Fluorescent andIncandescent Lamps: a Report

C. L. Sanders and C. W. Jerome

A comparison of spectroradiometric measurements has been completed for the wavelength region from300 nm to 800 nm. The measurement procedures and results obtained in eighteen laboratories are de-scribed. The results are considerably better than those in earlier comparisons. Conclusions regardingprocedures to be avoided are given. Without special precautions 450 illumination on the diffuse receivermay cause serious errors. Normal illumination on a plane or a spherical receiver is better. The power inthe spectral lines was inaccurately measured by Rossler's method.

IntroductionAt the CIE meeting in Washington in 1967 a sub-

committee on spectroradiometry was established byCommittees E1.2 Photometry, E1.3.1 Colorimetry,and E1.3.2 Color Rendering. A summary of the pur-pose of the subcommittee, compiled following aquestionnaire to interested people, was as follows:

1.1 To specify a simple but satisfactory proce-dure for spectroradiometry of continuous, line, andmixed sources of energy. The method should be ap-plicable to measurement of spectral radiance, irra-diance, and radiant flux.

1.2 To follow this method of spectroradiometry inorder to test the reproducibility of the results whenthe method is followed in different laboratories tomeasure spectral irradiance.

1.3 To use also the routine method of the partici-pating laboratory and to report the differences in re-sults and in the methods and to provide, if possible,an explanation for the differences.

1.4 To summarize all such measurements andanalyses and to estimate the error, if any, which islikely to be caused by following any specific proce-dure.

1.5 To state procedures that should not be usedand to provide the reasons why they should not beused.

C. L. Sanders is with the Radiation Optics Section, Division ofPhysics, National Research Council of Canada, Ottawa KlA OS1;C. W. Jerome is with the Lighting Products Division, SylvaniaLighting Center, Danvers, Massachusetts 01923.

Received 2 April 1973.

1.6 To establish the different accuracy require-ments of spectroradiometry when the results are tobe used for different specified applications. Theseare to include photometry, colorimetry, color ren-dering, and any other unique application.

As part of its task the subcommittee undertook in1969 and completed in 1971 an interlaboratory com-parison of measurements of the spectral irradiancefrom fluorescent and incandescent lamps.

Table I shows the spectral irradiances that were tobe compared. Columns 2 and 3 show, respectively,the values. for incandescent and fluorescent lamps.Column 4 shows the ratio that was to be measuredat wavelengths from 300 nm to 800 nm at 10-nm in-tervals. The ratio varies from 0.13 to 0.001. Thespectral response of most detectors is low at 800 nmcompared to the response at 450 nm so the measure-ments provide a severe test of linearity and of straylight in the instrument.

Eighteen laboratories compared three lamps ofeach type in order to see how well spectroradiometricmeasurements agree when stable fluorescent lampswere measured under specified conditions usingstandards of spectral irradiance that were all cali-brated against one standard.

Replies from each laboratory to a questionnaireabout procedures and equipment were studied tofind causes for discrepancies in the measurements.Discussion and correspondence has resulted in elim-ination of some of these causes. As a consequence ofthe comparison the subcommittee is in a better posi-tion to recommend the precautions to be taken andthe methods to be followed for spectroradiometrywith a given required accuracy. The preparation ofa committee document on principles of and proce-dures for spectroradiometry will take considerable

2088 APPLIED OPTICS / Vol. 12, No. 9 / September 1973

Fluorescent

pW-cmn2nm-

1

0.000070.00025'j.001590.006570.016280.02750. 03390.03850.04410.05130.05950.07010.08180.09750.11350.12880.13850. 14370.14420.14340.13920.13730.13520.13000.12080.11020.10190.09570 .09090.08630.08260 .08040.08640.10040.08630.11840.15570.07600.04450.03540.02760.02270.01850. 01530.01290. 01080.00920.00780. 00620.00520. 0044

pW.*CM- 2

0.0940 .0100.3860.7301.0001.0 0840.304

Ratio

(4) = (3)/(2)

0.0160. 0040. 0180.0570.1060.1380.1340.1230.1150.1120 .1110. 1100. 1110.1150.1180.1180. 1130.1060. 0960. 0880.0780.0710. 0640.0580.0500.0420.0370.0320.0290.0260.0240.0220. 0220.0250.0210. 0270.0350.0160. 0090. 0070.0050.0040.0030.0030.0020.0020.0020 .0010. 0010 .0010. 001

Note: The top figure in the right-hand column(0.016) should read 0.002.

time. In the interval a summary of the results isgiven below to aid those involved in spectroradiome-try in deciding how reliable their measurements maybe if certain precautions are taken or to inform thoseusing spectroradiometric measurements how reliablethe data provided are likely to be.

Organizational Arrangements

The National Research Council aged and made ir-radiance measurements on all tungsten-halogen

lamps and on the fluorescent lamps for operation at60 Hz. The Sylvania Lighting Center made mea-surements on all fluorescent lamps sent to the labo-ratories using 50 Hz. All lamps were measured in.these coordinating laboratories before and after thefirst measurements in the participating laboratories.The persons and laboratories participating in thiscomparison were the authors, as coordinators of the60- and 50-Hz measurements, respectively, and thefollowing:G. Bauer and K. Bischoff,Physikalisch-Technische

Bundesanstalt,Bundesallee 100,33 Braunschweig, Germany.

H. D. Einhorn,Dept. of Elect. Engineering,University of Cape Town,Private Bag, Rondebosch,Cape Town, South Africa.

W. Heaps,Macbeth Corporation,P.O. Box 950,Newburgh, New York 12550,U.S.A.

B. Jewess and M. Halstead,Thorn Lighting Limited,Research & Engineering Labs.,Cambridge House,Great Cambridge Road,Enfield, Middlesex,England.

C. J. Kok and M. C. Boshoff,Precise Physical Meas. Div.,National Physical Res. Lab.,P.O. Box 395,Pretoria, South Africa.

J. R. Moore,National Physical Laboratory,Teddington, Middlesex,England.

Leo Mori,Toshiba Research and

Development Centre,Tokyo Shibaura Elec. Co. Ltd.,Komuka, Kawasaki 210,Japan.

L. Morren,Laboratoire Central

d'Electricit6,1640 Rhode St-Genese,

Belgium.

M. Nonaka,Central Research Laboratory,Hitachi Limited,Kokubunji, Tokyo, Japan.

Dr. Nundel and B. Fisher,Deutsches Amt fur Messwesen

und Warenprufung.,108 Berlin,Niederwallstr. 18-20,Germany.

J. L. Parascandola,Durotest,2321 Kennedy Boulevard,North Bergen, New Jersey 07047,U.S.A.

Magda K. Poppe,United Incandescent Lamp &

Electrical Co. Ltd.,Budapest, IV,Vaci ut 77,Hungary.

F. Rotter,Bundesant fur Eich und

Vermessungswesen,Arltgasse 35,A-1160 Vienna, Austria.

R. Saunders,Room B 308-Met,Photometry Section,Institute for Basic Standards,Washington, D.C. 20234,U.S.A.

J. Schanda,Research Institute for Technical

Physics of the HungarianAcademy of Sciences,

Ujpest 1, PF 76,Budapest, Hungary.

J. Williams,Opcalite Inc.,2110 South Anne St.,Santa Anna, California 92704,U.S.A.

Three lamps of each type were sent to the labora-tories the first time but one out of three of the tun-sten lamps was retained at NRC after the repeatmeasurements. The final measurements were madeat the participating laboratories. The tungsten-

September 1973 / Vol. 12, No. 9 / APPLIED OPTICS 2089

Table I. Representative Spectral Irradiance of Incandescentand Fluorescent Lamps Used in Comparison, and Ratio of

Fluorescent to Incandescent Lamps

Incandescent

VW-cn2n- 1

0.0450.0620.0860.1160.1540.2000.2520.3140.3830.4590.5430.6350.7340.8470.9641.0921.2221.3561.4951.6391.7881.942.102.262.432.602.782.963.143.323.503.683.854.024.194.364.504.654.794.925.055.205.365.515.665.795.885.966.036.106.15

nm

3003103203303403503603703803904004104204304404504604704804905005105205305405505605705805906006106206306406506606706806907007;10720730740750760770780790800

313334365405436546578

halogen lamps retained at NRC will be used to alignthe spectral irradiance scale used in this comparisonwith the mean scale that will result from an inter-comparison of national irradiance scales that is beingcoordinated by Yoshie of the Electrotechnical Labo-ratory, Tokyo, Japan.

Experimental Procedure and Lamps UsedThe fluorescent lamps, donated by Macbeth Cor-

poration, were of high color-rendering index (about92) and had a correlated color temperature of ap-proximately 8500 K. A magnesium germanate phos-phor produced a pair of peaks in the red part of thespectrum. The lamps were 40 W, 1200 mm long by38 mm diameter.

The fluorescent lamps were aged for 550 h in ordi-nary open-grid lighting fixtures using rapid start bal-lasts. They were measured after 500 h and 550 h toconfirm their stability. The final measurements atall laboratories were made using reference ballastsmeeting the specifications in the InternationalElectrotechnical Commission Publication IEC 82.The irradiance was to be measured in a plane at 66cm from a 25.4-cm by 5-cm aperture placed 5.1 cmfrom the center of the fluorescent lamp. The lampswere operated at a fixed current of 0.430 A. Thetotal harmonic distortion was less than 3% from thepower supplies used.

The standard lamps used were tunsten-halogenlamps of the type GE 6.6A/T4Q/1CL. These are200 W with coiled-coil filaments and have bare wireconnectors. They were operated vertically and posi-tioned 43 cm from the plane where the irradiancewas to be measured. The lamps were aged and se-lected in the factory. They were purchased by NRCand were aged at NRC for a further 25 h. Afterbeing measured some were aged a further 25 h toconfirm that they would not change during the com-parison. Ageing and subsequent operation was at acurrent of 6.5 A. Operation on ac or dc was option-al.

The method of receiving the irradiance in the irra-diated plane was left up to the individual laboratory.Two methods were used: (1) a plane diffuser ofpressed MgO, MgCO3, BaSO4, or ground quartz,and (2) an integrating sphere with its entrance aper-ture located in the measurement plane. In five labo-ratories, the plane diffuser was illuminated at 00 andviewed at 450 by the spectroradiometer, i.e., 0/45°.In two cases the inverse system, 450/00, was used.The arrangement 450/450 was used by five laborato-ries. A sphere used by five laboratories was viewedso that no directly reflected radiation entered themonochromator, i.e., 0d. One laboratory used aquartz diffuser in the transmitting mode 00/00.

The temperature 25 i 0.50C was specified for op-eration of the fluorescent lamps. Some laboratoriescould not meet this condition and were forced tomeasure at temperatures in the range from 2C to360C. The temperature was to be measured at 15

2

wad

0crIj.z2

0

wF

16

12

8

4

0

-4

-8

-12

-16

0 1 2 3 4 5 6 7 8 9 10 11 12 13 1415 16 17

HBW AT 660nm18 19 20

Fig. 1. Percent error at 660 nm vs half-bandwidth (HBW) ofmonochromators for individual laboratories' average values. Av-erage of seven national laboratories was taken as standard.

HBW was estimated from replies to questionnaire.

cm from the lamp center in the same horizontalplane but not all laboratories measured in this man-ner. Continuum measurements were limited to300-800 nm at 5-nm or 10-nm intervals. The sevenHg lines at 313, 334, 365, 404.6-407.7, 435.8, 546,and 377-579 nm were measured. The method andequipment actually used was reported by each labo-ratory. A summary was made of the reports andwas used in drawing conclusions about the causes ofdiscrepancies.

The monochromators used were calibrated byusing at least Hg lines, but mostly by using linesfrom a number of elements. Some of the laborato-ries checked for wavelength calibration in each mea-surement session using one or more Hg lines. Singleor double monochromators with glass or quartzprisms, gratings, or grating-prism combinations wereused. In spite of the care taken in wavelength cali-bration there was evidence of wavelength errors inmeasurement of the red phosphor peaks. Thesemeasurements may also be seriously affected by thebandwidth of the monochromators used, which var-ied with wavelength for most prism instruments. Asshown in Fig. 1, the bandwidths used at 660 nm werein the range from 1.1 nm to over 10 nm, the meanbeing about 6 nm. The number near each point isthat of a laboratory in the comparison. Laboratories5, 9, and 19 made measurements with narrower band-widths but adjusted the data to give a 10-nm band-width at 660 nm.

All laboratories used a photomultiplier as the de-tector. One used PbS above 700 nm. Most PMT'shad S20 surfaces. Most laboratories checked thelinearity of their systems. One reported significantstray light of several percent in the red above 700nm. One laboratory reported that hysteresis causedthe far red part of the spectrum to be high in its firstset of measurements.


I 1 * 2 P l I I I I I I I I I I I I

21\_R * *18QP

8GP

10PG. ..7QPOP*7GR

*13GP

0 I -9GR

*3GR I GP .5QP

.4QPI I I I I I I I I N I I I I I I

. . .. . . . . . l. l. l . .l .l l

Ro cv

Analysis

It should be noted at the outset that the data asfirst received from the laboratories contained somemeasurements that were in error because of mistakesin calculation that were later corrected. The stan-dard deviations noted in this report are therefore lessthan would be obtained if no opportunity were givenfor correction of data.

As results were received from the participatinglaboratories the following two assumptions appearedjustified and were used in the analysis of the data.

(1) There was an insignificant difference betweenthe relative spectral irradiance of the individual fluo-rescent lamps used in this intercomparison. Thiswas indicated, among other ways, by the fact thatthe standard deviation of an individual lamp mea-surement of spectral irradiance, determined by mea-suring sixteen fluorescent lamps at NRC, was onlyfour-tenths that of the standard deviation of mea-surements at all the participating laboratories. Ifone looks instead at the range of chromaticity coor-dinates at NRC compared to those for all laborato-ries, one finds NRC values on sixteen lamps for an18-day period in 1970 covering the area enclosed bythe solid curve of Fig. 2. The dashed curve of Fig. 2encloses the spread of measurements at NRC overthe period 1968-1972.

(2) There was an insignificant difference betweenthe results at 60 Hz and 50 Hz. This was shown byJerome's measurements at the two frequencies andby an insignificant difference between the means ob-tained by laboratories at the two frequencies.

The absolute value of spectral irradiance variesmuch more than the relative value. There was alarge variation in the reported relative irradiances ofthe mercury lines. This is not surprising, since it isrecognized that these line irradiances in a fluorescentlamp are strongly affected by mercury vapor pres-sure, which is affected by temperature and drafts.Normal spatial variations in phosphor coating den-sities may cause variations in the spectral irradian-ces of the mercury lines. However, this does notseem to have caused significant variations in thiscomparison because lamps measured in any labora-tory at one time were measured with little variation.One should look for this effect at the wavelengthswhere the phosphors absorb radiation.

Some laboratories reported data over restrictedspectral regions, e.g., 400 nm to 700 nm; therefore, inorder that all laboratories might be normalized forthe same region, the normalization was restricted tothat spectral region where all laboratories reporteddata.

Another cause of variability is the magnesium ger-manate peaks in the phosphor continuum in the630-670 nm region of the spectrum. The emission ofthis phosphor consists of broadened spectral lines.For this region where both the spectral irradianceand the rate of change of spectral irradiance change

rapidly with wavelength, the reported emission de-pends on the spectral bandwidth and the accuracy ofthe monochromator used in the measurement.Since the bandwidth was not standardized, consid-erable variation in the red region could be expectedand this region of the spectrum should be excludedfrom the region of normalization.

For the above reasons, the values from each labo-ratory were normalized to obtain equal area underthe continuum between 400 nm and 620 nm, i.e.,

620

ZS(X)AX = 100,000 (Ax = 10).400

The normalized measurements on the three fluo-rescent lamps were averaged at each wavelength toyield an average measurement for each laboratory.These measurements (one for each laboratory) wereaveraged at each wavelength to obtain the world av-erage for each supply frequency, 60 Hz and 50 Hz,and the total world average for the combined 60-Hzand 50-Hz measurements. The results are given incolumns WA(60), WA(50), and TWA, respectively,in Table II. To the right of each of these columnsappears the standard deviation of the result for anindividual laboratory calculated using the equation = [(DEV)i) 2/(NL - 1)]1/2, where NL is the num-ber of laboratories used in finding the mean andDEV is the percent difference between the laborato-ry measurement and the mean measurement. NL isindicated in Table II for each wavelength and eachgrouping of laboratories.

Comparing WA(60) and WA(50) and observingWMS(60) and WMS(50) at each wavelength, we seethat there is no significant difference between the60-Hz and 50-Hz averages. Therefore the two sets ofdata were averaged and the result appears underTWA. The percent differences at each wavelengthbetween each laboratory and TWA gave the devia-tions from which TWMS was calculated. FromTable II it can be seen that the standard deviation ofan individual value, TWMS, is dependent on wave-length. From 430 nm to 600 nm the variation isquite small (1-2%), but it increases to 10% at 670nm. In other regions TWMS is very high, up to 86%from 300 nm to 330 nm and up to 56% from 700 nmto 800 nm. The standard deviation for line mea-surements is also large, especially for the three linesin the UV (11-47%). Sometimes the high standarddeviation was caused by a very deviant measurementat one laboratory. Usually several laboratories havedeviant values in the same wavelength regions.

The average of the seven national standardizinglaboratories that participated is given in columnNM.

The standard deviation, NMS, of an individuallaboratory average was also calculated and the re-sults are shown in the next-to-last column of TableIL

The large standard deviations in the region 620-700 nm indicates wavelength errors and the effect of


Table II. Average World Measurements and Standard Deviations for 50 Hz and 60 Hz, Individually and Combined

Std.Dev. Std.Dev. Std.Dev.Ind. Ind. Ind.

X WA(60) WMS(60) NL(60) WA(50) WMS(50) NL(50) TWA TWMS NL NM NMS NL% 0 0

_~~~~~~~~~ 0_

2 0.823 2.114 6.924 26.84 66.24 108.34 135.64 153.66 175.86 211.5

7 243.87 282.47 328.77 389.77 457.07 515.77 553.47 571.27 573.07 566.0

0.4 7 549.80.5 7 539.10.8 7 532.41.0 7 510.21.1 7 472.21.4 7 436.51.9 7 403.22.0 7 379.32.4 7 361.72.4 7 344.4

82.357.610.5

5.72.91.41.21.02.07.7

5 0.705 2.395 7.176 26.46 65.67 108.38 135.38 153.79 176.4

10 210.1

5.7 11 242.33.1 11 282.94.9 11 326.21.7 11 388.71.2 11 455.61.0 11 514.71.2 11 553.61.2 11 571.91.0 11 573.60.9 11 566.4

0.71.01.11.41.4

1.31.51.71.7

11 550.211 539.811 533.311 511.611 473.411 436.811 403.811 379.81 362.8

11 343.8

85.186.023.4

5.43.42.52.12.23.26.5

4.93.64. 3

1.61.2

1.21. 2

0 .9

7 0.448 1.329 6.58

10 26.610 66.111 108.912 135.612 154.615 175.916 208.1

18 239.418 278.718 323.918 389.218 453.018 515.418 553.918 572.618 574.418 569.3

0.6 18 552.50.8 18 543.41.0 18 536.71.3 18 513.81.3 18 475.21.9 18 436.61.5 18 403.11.6 18 379.52.0 18 361.12.0 18 343.5

600 326.4610 317.8620 342.4630 399.0640 341.8650 473.7660 650.7670 296.4680 174.0690 139.4

7 327.37 319.17 343.87 399.67 346.67 469.77 638.47 298.97 173.87 138.6

7 104.06 84.76 69.66 55.86 48.16 34.36 27.85 20.85 15.34 11.24 11.0

4 28.34 4.94 150.46 290.16 770.66 411.46 115.1

11 327.011 318.611 343.311 399.411 344.711 471.311 643.211 297.911 173.811 138.9

11 105.410 85.310 69.6

8 56.28 47.76 36.76 28.85 23.05 17.05 13.44 12.6

5 25.96 5.07 148.1

10 293.010 774.110 416.310 113.9

18 325.918 316.818 342.018 402.618 340.618 469.118 655.718 299.418 173.518 139.6

18 105.816 87.516 70.614 58.714 49.712 39.012 32.010 27.710 22.3

9 18.88 16.7

9 33.410 5.911 152.916 298.716 773.816 420.916 121.0


300310320330340350360370380390

400410420430440450460470480490

0.432.857.47

25.864 .8

108.2134. 6153 . 8177.4207.7

239.8283.9322.3387.1453.5513.1553 .8572.9574 .6567,0

87.0117.934.2

4.54.14.03.63.94.53.9

3.34.53.02.32.11.41.31.10.90 .9

500 550.9510 540.9520 534.6530 513.9540 475.2550 437.2560 404.8570 380.7580 364.5590 342.7

106.240.59.42.42.51.81. 11.21.62.4

1 .51 .31.20.81. 00.60.60.40.40.3

0.20.40.50.60.70.90.60.60.50.4

3334446677

7777777777

7777777777

2.62.52.23.82.45.19.76.94.94.9

6.27.27.79.0

10.213. 021.731.940.747.653.3

60.939.119.4

4.42.11.46.3

700710720730740750760770780790800

313334366405436546578

2.02.43.32.84.55.9

10.411.8

4.16.1

5.68.1

11 .115.524.217.423.028.039. 767 .056.1

40.3

7 .25.86.4

17.8

107.786.369.656.747.139.129.925.318.616.214.1

22 . 85.3

144.1297.8780.0424.4112.0

2.22.42.93.13.85.59. 9

10.04.35.5

5.97.69.7

12.619.016.021.630.539.556.452.4

46.736.111.4

6.24.75.2

14 .5

0.60. 91.81.41.33.55.45.91.72.7

3.33.25.49. 1

13.98.4

11.610.413.415.519.4

6.518.7

5.24.15.11.91.4

differences in bandwidth. A plot of the differencesbetween the laboratory results and the mean valueat 660 nm vs the half-bandwidth used by the labora-tory is shown in Fig. 1. A correlation between band-width and the deviation of relative spectral irra-diance is indicated by the closeness to the line of theplotted points. The scatter about the mean slope ofthe line could easily be caused by different wave-length errors in the different laboratories. The sym-bols GP, GR, PG, and QP indicate glass prism, grat-ing, prism-grating, or quartz prism, respectively, asdispersing components.

Chromaticity

The values of TWMS in Table II do not show thefact that when a laboratory had an error at one

wavelength, the values at adjacent wavelengths oftendiffered from the mean in the same direction. Thiskind of nonrandom deviation causes a chromaticitydifference when one calculates the color of thesource. Thus in the region from 400 nm to 700 nmone can find systematic departures from the meanspectral irradiance by looking in Table III at thechromaticity coordinates for the Hg lines and for thecontinuum that are given for each laboratory and forthe world average of both the first and second mea-surements. The chromaticity of the lines variesmuch more than the chromaticity of the continuum.The chromaticity coordinates x, y of the lines pluscontinuum (total lamp) are given in columns 6 and7, respectively, of Table III. Laboratory 18 repeatedthe measurements for a third time with modified

Table Ill. International Spectroradiometry Chromaticities Computed from Lab Averages; CRI, Ra, by 1971 CIE Method

Round 1

Lab No.

TWA1234567.89

1011131517182022

MercuryLines

x Y

0.22080.22390.22500.22530.22120.2229

0. 21940.22260. 22040. 21900.22220. 22200. 22400.22120.21930. 21930.2052

0.22190. 22970.23770. 23650 .21190.2279

0. 21090.22800.22090.21920.22350. 22320.23300.22050. 22430. 21910.1844

PhosphorContinuum Lamp

Tc Rax y x

0.29770. 29780.30130.29900. 29930. 29770.29970. 29730. 29770.29520. 29890. 29840. 29760.29590.29860.30210.29190. 2942

0.32760 .32740.32890.32930.32680.32970. 33110.32610.33150.32340.32800.32860.32850. 32530.32890. 33180.32260. 3219

Round 2

0.28660. 28750 .29090.2885G. 28770.2869

0.28550.28680. 28430.28710.28740.28670.28580. 28730.29030.28110.2822

0.31320. 31380.31650.31530.30970.3149

0.30860.31650.30810.31200.31340 .31330.31230. 31310.31650. 30740.3033

846583608042824684638377

K

86718338

84428375843685328392809090759137

91.491.791.991. 591. 990.9

91. 689.8

91.491.691.491.591.491.591.092.2

Lamp T Ra

x _____ a_

0.28740. 28770 . 28630.29200.28770. 28660.28440.28720.28300.28760.28620.28920.29050.28400.29270.2875

0. 31340.31450.31130.31770. 30960.31350.31180.31390.30970 .31490.31220. 31330.31660. 31100.32160.3135

837883258520794084618432865483778835832685008264

806987087809

K 91 . 491.391.991. 191. 891.390.091.391. 191. 591.4? / .t91.691.291.1


MercuryLines

x y

PhosphorContinuumx YLab No.

TWA12345789

11131518192118-3

0.22310.22530. 21990.22490. 2 2130.22260.21760. 22340.22050. 22440.22140.22460.21950. 21990. 24160.2131

0.22560.23280. 21060. 23400.21220.22390. 20430. 22990. 22110. 23400. 22040.23230.22480.22630.26390. 214 9

0.29830.29840.29770.30350.29940.29750. 29630.29800.29380. 29800. 29740.29960. 30230.29480.30080.2989

0.32840. 32850.32860. 33200.32680. 32870.33090. 32800.32500.32830.32800.32640.33180. 32520.33090.3291

.320 -

.315 e

y

.310

.305 F-

.280 .285 .290 .295X

Fig. 2. Chromaticity diagram showing average chromaticities offluorescent lamps measured at various laboratories indicated bynumbers. 11-1 and 11-2 indicate first and second measurementsat laboratory 11. M-1 and M-2 are values for the mean laborato-ry of first measurements and second measurements, respectively;v indicates that the laboratory used 45' illumination on the dif-fuser; x indicates a national laboratory value; the solid curve R1is the range of thirty measurements on sixteen lamps at NRC inthe period 1-17 June 1970; the approximate range of measure-ments at NRC from 1968 to 1972 is indicated by the dashed curveR2; 18-3 indicates an additional set of measurements at laborato-ry 18 with improved mechanical motion of pickup sphere; 19-3 in-dicates values with improved black paint on baffles before pickupsphere; the dashed lines marked 8000 K and 8500 K are the iso-temperature lines for 8000 K and 8500 K, respectively; and theline marked Planckian locus is the Planckian locus for complete

radiators.

input geometry and found the values indicated by18-3 at the bottom of Table III.

The chromaticities of the fluorescent lamps asmeasured by each laboratory are also shown in Fig.2. The national laboratories' values are indicated bycrosses. An approximate idea of the spread of mea-surements made at NRC on sixteen lamps during 18days is shown by the solid curve in Fig. 2, which in-cludes all the thirty measurements. These measure-ments include the scatter of measurements plus thespread of the chromaticities of the lamps. The ap-proximate spread of chromaticity of all the lampsmeasured at NRC in the period 1968-1972 is shownby the dashed curve.

All but one of the measurements at the nationallaboratories are inside the long-term spread of mea-surements at NRC. The other one is just outsidethe spread of NRC in the first set of measurements.

For comparison the chromaticity coordinates ob-

tained by several national standardizing laboratoriesin measurements of the chromaticity coordinates of acolor-matching fluorescent lamp in a comparison or-ganized by NPL' in 1958-1960 are shown in Fig. 3.It will be noted that the spread in chromaticitycoordinates in the present comparison is considera-bly reduced. Part of this improvement is no doubtdue to all the laboratories using spectral irradiancestandards calibrated in one laboratory. The nation-al standardizing laboratories are grouped much moreclosely now than in 1960 and all of the laboratoriesare making measurements inside the spread foundfor the national laboratories in 1960. It should benoted, however, that line measurements at laborato-ries 6 and 22 in the latest comparison were very de-viant. The cause has not been resolved but is be-lieved to result from difficulties in using Rossler'smethod 2 of line measurement.

The solid line in Fig. 2 shows the Planckian locus,i.e., the chromaticity coordinates of complete radia-tors. The set of dashed lines shows the loci of corre-lated color temperature for 8000 K and 8500 K. It isobvious that the correlated color temperature ob-tained for the fluorescent lamps varies considerablyin this comparison but even more in the NPL com-parison.

The other difference in the NPL comparison wasthat the spectral radiance of small sections of thefluorescent lamp was measured by four laboratories.One laboratory measured the lamps in an integratingsphere and one laboratory measured the relative dis-tribution from a short section of the lamps. Some ofthe larger error in the previous measurements mayhave been caused by the supports for the fluorescentlamp that touched the bulb and that later in thatcomparison were found to change the lamp outputby causing condensation of Hg inside the wall at thepoint of contact.

.351-

y

.34

.33 1-

.32L.30 .31 .32 .33

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Fig. 3. Chromaticity coordinates of color-matching fluorescentlamps measured by national laboratories in the NPL comparison,

1958-1960.


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A figure prepared by J. Moore and presented tothe subcommittee at its meeting in Barcelonashowed how deviations in measurements at variouswavelengths affect the chromaticity of the lamp.From his figure the chromaticity deviations of thelaboratories or of the measurements at NRC aremainly caused by deviations in measuring the 436-nm or 546-nm lines or by a slope across the spectrumsuch as might result from using the wrong current onthe incandescent laimp (thus producing a color tem-perature error in the standard lamp) or by drift ofeither lamp during the scan of the wavelength re-gion.

There has been a tendency to blame the inabilityto measure fluorescent lamps consistently in differ-ent laboratories on variations in the fluorescentlamps. Neither this comparison nor the earlier NPLcomparison indicates systematic, uncontrollable, orirreversible changes in the fluorescent lamps as beinga major cause for the interlaboratory differences inmeasurements. At any one time in any one labora-tory, lamps repeat well and all are measured similar-ly.

Temperature

Drafts on the fluorescent lamp may cause errorswhen the normal to the 25-cm baffle is in or at 450 tothe vertical plane. The baffle is close enough to thelamp that the air passing the baffle may cause cool-ing. In general no fans were operated near the fluo-rescent lamps.

One laboratory's data was outside the ellipse inthe first measurements when 32-360 C was used forits measurements. In the second measurements,when 250C was used, it obtained data well inside theellipse. The chromaticity change was -0.005 inboth x and y in going from first to second measure-ments. Measurements of chromaticity coordinateswith a photoelectric colorimeter at NRC showed thata change of +0.002 in y occurred for a temperaturechange from 21.60 C to 300 C. It is possible for achanging temperature to cause an over-all change inthe spectral irradiance produced by the fluorescentlamps. This change would show up as a differentslope of the spectral irradiance. To avoid this, somelaboratories measured the illuminance from the lampsection and adjusted the spectral values to compen-sate for the drift.

Geometry

When one looks at the measurement conditionsused by the most divergent laboratories of the pres-ent comparison, one finds that five of them used450/00 or 450/450 illumination and viewing angles.Only two laboratories using 450 illumination hadchromaticities inside the dashed curve of Fig. 2.One of these two obtained a chromaticity outsidethis region for its second measurements. Althoughthe goniophotometric properties of the diffuse reflec-tors used may be spectrally selective, measurements

at NRC suggest that the differences resulting fromthis cause will be less than 0.002 in x and y. Inorder to cause larger errors, nonuniformity of trans-mittance in the entrance cone of the monochromatoror nonuniformity of sensitivity across the photomul-tiplier must interact with nonuniform irradiance onthe diffuser to cause the larger errors. The divergentlaboratories using 450 illumination were on bothsides of the less divergent laboratories in thechromaticity diagram and in the direction of color-temperature shift. One might expect such varia-tions if they are caused by nonuniform spectral sen-sitivity of the photocathode or by prism absorption.The error caused by nonuniform irradiance on thediffuser can be reduced by using a lens to image thediffuser on the entrance slit. However, care shouldbe taken that the image be kept exactly in the sameposition relative to the entrance slit. Measurementsat PTB indicate that even with the use of a lensthere is a small systematic difference between using0°/45° viewing with the two sources both on one sideof the optical axis of the entrance aperture of themonochromator or both on the other.

Some of the laboratories using spheres as receiversobtained measurements that agreed with the meanof the laboratories using 0/45° measurements.However, two laboratories using sphere receivers atthe entrance to the monochromator obtained datadiffering considerably from the mean. These twosets of data were in opposite directions on thechromaticity diagram. Both these laboratories latermodified their arrangements and obtained considera-bly better agreement with the world mean. Onelaboratory's error seems to have been caused by amechanical difficulty that caused the integratingsphere to treat the test and standard irradiances dif-ferently. The other laboratory's error seems to havebeen caused by the fact that the paint on the cone orthe cylinder used to limit stray light from the roomwas spectrally selective and did not affect both testand standard irradiances equally. In this case theuse of a better quality matt-black paint brought theresults for the laboratory closer to the world mean.

In general the causes of the discrepancies cannotbe definitely determined but must remain hypotheti-cal until a set of experiments is made to show, ineach divergent laboratory, that changing geometry oranother condition does in fact produce differences inmeasurements similar to those found in the inter-comparison.

The one laboratory using 0°/0° conditions with aquartz diffuser obtained chromaticity coordinates forthe fluorescent lamps that indicated a correlatedcolor temperature higher than average. This mightbe expected from the spectrally selective scatteringproperties of ground quartz, which would affect dif--ferently the spectral irradiances from the test andstandard lamp because of the different angular sizesof the sources as seen by the diffuser.


Measurement of Line PowerThree principal methods of measuring line power

were used by the laboratories. The one used bymost laboratories was the integrated net area meth-od,3 which involves using a fairly broad half-band-width and making measurements at a number ofwavelength settings on both sides of the line's peakreading. Some measurements were made at wave-lengths far enough from the line energy that no lineenergy was still included. The spectral irradiance ateach measured wavelength was calculated and theninterpolated values of the continuum were subtract-ed. These values were obtained either by linear in-terpolation between the two outside continuumvalues or by fitting several continuum values to apower function and then using the power function tofind continuum values at each of the measuredwavelengths. Some laboratories repeated the linemeasurements at all wavelengths for every test run.Others determined the half-width on one occasionand thereafter used only the peak measurement tofind the line energy.

The second method used was that described byRssler,2 in which measurements are taken with sev-eral sets of different slit widths. The third methodwas developed by L. Morren4 during this comparisonand was described by him at the CIE meeting inBarcelona. This also involves measurements withdifferent slit widths on a pure continuous spectrum,on pure spectral lines, and on the line-plus-contin-uum of the fluorescent lamp. It is claimed to yieldreproducible results within 1%.

The laboratories using Rssler's method obtaineddata that were inconsistent with the other laborato-ries and that also had a larger scatter on repeat mea-surements. Morren's measurements agreed wellwith those obtained using the integrated net areamethod.

Wavelength ErrorsWhen the monochromator has a wavelength scale

that is being used to set the wavelength, there is apossibility of systematic wavelength errors because oferrors in the scale markings. Checking with Hg andCd lines or other common elements is not adequatebecause the scale may be good only at the wave-lengths of the lines that were used in defining thescale. There may be serious departures at interme-diate wavelengths.

Stray Light and NonlinearityThe ratio of test to standard varies by a factor of

100 in going from 460 nm to 770 nm. In going from460 nm to 700 nm it varies by a factor of 10. If thedistances are as specified, the above ratios are 0.1 to0.001 and 0.1 to 0.01, respectively. By increasingthe distance for the incandescent lamp some labora-tories have modified the above ratios so that theyvary from 1.0 to 0.01 and 1.0 to 0.1, respectively.The actual response recorded will depend on the

spectral sensitivity of the spectroradiometer timesthe spectral irradiance of the source and thus nonlin-earity may cause different errors even when the mea-sured ratio is nearly constant as it is in the rangefrom 380 nm to 460 nm. It is not possible to sayfrom the analysis whether this was a problem, butthe larger standard deviations of the measurementsabove 700 nm may be associated with nonlinearity orstray light problems.

In the comparison, the measurement of a ratio of0.003 is made with a standard deviation of 0.0003.This is as good as or better than the standard de-viation in measurement of transmittance in spectro-photometry. At 530 nm the ratio of 0.057 is mea-sured with a standard deviation of only 0.0006. Themeasurement of the same ratio at 330 nm has a stan-dard deviation of 5%, indicating the effect of in-creased slope in the curves and of the lower sensitivi-ty and lower irradiances on the ratio measurements.The standard deviation at 530 nm will be reducedbecause the process of normalization of the area thatwas used tends to improve agreement in the regionfrom 400 nm to 620 nm. That is to say that theerror in measuring the ratio of 0.1 is partly eliminat-ed by the normalization process.

In order to provide more information about theability to measure ratios it would have been useful tomeasure a small ratio in the region where the spec-tral responses of the spectroradiometers was high.

Stray LightWhen one reads of stray light of 1 part in 105 in

the specification for a monochromator, one may feelsafe in using the monochromator in a spectroradi-ometer without worrying about stray light. How-ever, suppose that one is measuring the red part ofthe spectrum from a fluorescent lamp and comparingit with an incandescent lamp where the ratio is1:1000. Even if the linearity and detectivity of theinstrument is adequate, can one rely on the results?The photomultiplier may have a response 1000 timeshigher in the blue than in the red at 750 nm. Doesthat mean that the stray light is 10 times the quanti-ty to be measured? Obviously we need to knowmore about the meaning of stray light being1:100,000. Is it independent of slit width? Is it foran equal energy source or independent of wave-length? If it is not already defined by a reputableorganization, we need to work out a meaningful defi-nition (one that can be used to predict what errorwill be caused by stray light). This comparison didnot solve the problem of stray light. A detailed at-tempt to find the causes of the large deviation in thered and uv region should be made with this in mind.

NBS ComparisonThe NBS conducted a comparison5 in 1968 that

concerned itself mainly with flux measurements byphysical photometers and by spectroradiometry butincluded also spectroradiometric measurements on a


25-cm-long section of lamp on three colors of fluo-rescent lamps. There were seven American labora-tories making measurements. If one excludes onevery divergent laboratory, the remainder showed de-viations that were similar in size to those found inthis comparison. The NBS comparison also showedthat the chromatic deviations produced a similarpattern in the chromaticity diagram. The measure-ments at NBS indicated that the chromaticity coor-dinates of the lamps did change slightly with time.The x chromaticity coordinate increased by almost0.0007 and the y decreased 0.0009 for one cool whitelamp. However, the lamps changed much less thanthe difference between laboratories and the lampsserved as useful sources to test each laboratory'sability to make spectroradiometric measurements.

The ability to make flux measurements by spec-troradiometric means was shown by the NBS com-parison to be poorer than the ability to do it byphysical photometry using fluorescent lamp stan-dards. In the NBS comparison some laboratoriesmade spectroradiometric measurements with thelamp in an integrating sphere or box and also mademeasurements on a 25-cm central section. NBS'sown relative values from the two methods, using dif-ferent parts of the lamp and different pickup geome-try, differed at most by 0.002 in x and y but one ofthe other laboratories using the two methods ob-tained differences up to 0.006 in x and y. Most lab-oratories differed by up to about 0.0025 in x and y.See pages 162-3 of Ref. 5 for more details of theNBS comparison.

ConclusionsThe large differences in the measurements of the

spectral irradiance of the red phosphor did not affectchromaticity, color temperature, or color-renderingindex very much. The standard deviation deter-mined in measuring the red phosphor can be used tosome extent to predict how the measurements ofsources with narrow-band emission, e.g., light-emit-ting diodes or high pressure sodium, will differ fromone laboratory to another. More rigid requirementson wavelength accuracy and bandwidth will be nec-essary for measurements of such sources than for thisred phosphor because they have only the energy inthe narrow band and its absolute values are impor-tant. Measurements at closer wavelength intervalswould be necessary to characterize such sources.

The effect of noncosine and spectrally selective re-ception by the diffuser may shift the chromaticitybut the effect is small, 0.002 in x or y. Care must betaken to ensure that nonuniform transmittance ofthe monochromator or nonuniform sensitivity of thecathode of the photomultiplier does not interact withnonuniform irradiance on the diffuser at the en-trance of the monochromator unless the nonuniform-ity is the same for the test and standard lamps.

Another method of controlling the temperature ofthe fluorescent lamps may be required to reduce the

difference between measurements at different labo-ratories, e.g., lamps with Amalgam might be lesssubject to changes with temperature.

Although an intercomparison can show how muchdifference there is in the measurements made in dif-ferent laboratories, it is not an efficient method ofdetermining the effect of changing a measurementcondition or of determining the stability of the lampsused. The method described by Youden6 of varyinga number of conditions in a controlled manner andin a systematic pattern is much more efficient in de-termining how much variation can be permitted inthe operating techniques. The use of lamps of thesame type in different laboratories does permit You-den's kind of experiment to be verified in other labo-ratories. Only after all the measurement conditionsthat may cause problems are established should thelaboratories be asked to measure the lamps whilecontrolling the conditions as specified. If all the sig-nificant variables have been identified and con-trolled, the measurements should be the same in alllaboratories.

In this comparison many laboratories could notmeasure in exactly the manner prescribed for all theconditions. Furthermore, no bandwidth was speci-fied for the measurements. The result of this situa-tion was that the results obtained differed and it hasnot always been possible to assign a cause to the dif-ference found. In addition, or as a consequence, it isnot certain that there are not unidentified sources ofsystematic differences.

Dependence of spectral irradiance on geometryand temperature suggests that no matter what capa-bility exists for measuring spectral irradiance, therewill be difficulty in predicting the relative spectralirradiance that exists in a practical case, unless onemeasures under exactly the conditions to be found inthe practical case. An integrating sphere should beverified as giving the ideal reception properties.

The anticipated causes of variation implied byspecifying certain conditions of measurement wereverified by the larger differences obtained by labora-tories that departed from the specified conditions.

The inability of the laboratories to provide thespecified conditions shows that the choice of fluo-rescent lamps as the test sources was not completelyjustified. The different sizes of test and standardsource caused a problem. For this reason, filtersmight be a better method of testing spectroradiome-tric capability. There is, however, no evidence thateither the fluorescent or the incandescent lampschanged over the intercomparison period by morethan 0.002 in x and y when used as specified.Because both NRC and Sylvania spectroradiomet-ers produced varying data with time, we do notknow exactly what the lamp repeatability is.

The main deficiency in our specification for opera-tion and measurement was in omitting the exactbandwidth that was to be used in the measurements.This should be agreed upon if measurements need to


be compared more exactly. For most purposes themost useful data would be the average spectral irra-diance over a 5-nm or 10-nm band obtained by mak-ing a large number of measurements with a narrowbandwidth at close intervals or by adjusting the dataobtained to approximate such data.

The accuracy of measurements at wavelengthsfrom 300 nm to 420 nm and from 700 nm to 800 nmis very limited. The measurements in both the uvand the red may be limited by wavelength or band-width errors, the nonlinearity, hysteresis of PMT oramplifier, or by stray light. The larger standard de-viation existing in the far red may be partly psycho-logical because the participants realize that the redfrom 700 nm to 800 nm will play a very insignificantrole in the practical use of the fluorescent lamp.Hence no extra care is exerted in measuring thispart of the spectrum. Evidence regarding the im-portance that this far-red radiation has in plantgrowth would encourage more care in this region. Ifthis region is being used to test the laboratory's abil-ity to measure small ratios, then additional attemptsneed to be made to improve these measurements butit would seem more useful for that purpose to pro-vide filters with a low transmittance somewhere inthe spectrum rather than to use fluorescent lamps.A low ratio in the far red makes the stray lightproblem severe. By using materials with a lowtransmittance in different parts of the spectrum onecould attempt to distinguish between nonlinearity,hysteresis, and stray light. The stray light in spec-troradiometry needs to be considered more carefullyif the measurements in the uv and far red are to beimproved. Apparent nonlinearity caused by hyster-esis can be eliminated, or at least reduced, by usingphotomultipliers designed for low hysteresis.

The color rendering index calculated from resultsin the different laboratories is sufficiently accurate.Before specifying better accuracy of lamp measure-ments one should consider the practical use of thedata to see if a closer tolerance is necessary.

The use of a series of measurements at close wave-length intervals on each line during each scan seemsto improve the chances of having a better reproduci-bility of measurement. If the line power is critical,it would be advisable to specify how to interpolate inthe continuum near the lines. Voltages on incandes-cent lamps should have been measured and reportedin all cases. The color temperature of the fluo-

rescent lamps is not measured accurately enough inall cases.

The comments in this paper express the conclu-sions of the authors as a result of the comparison.The writing of a CIE document expressing the con-sensus of members of CIE TC-1.2 on the subject willafford an opportunity to further test and discuss thepresent conclusions. One should see Nonaka et al.

7

for a detailed report on the equipment and proce-dures used and on the corrections applied in one lab-oratory where results were obtained that were closeto the mean of all the laboratories.

It should be noted that the spectral irradiancestandards were mounted in all laboratories using alamp holder similar to that shown in Fig. 1 of Ref. 8.This may have increased the reproducibility of thestandards over that which would be obtained with-out a specified holder.

It is a pleasure to acknowledge the measurements,reports, comments, and discussion of those at theparticipating laboratories mentioned at the begin-ning. The assistance to H. Einhorn by a researchgrant from the South African Council for Scientificand Industrial Research is thankfully acknowledged.We would also like to acknowledge the assistance ofW. Gaw, at NRCC, and G. Spears, at SylvaniaLighting Center, in making the measurements and ofJ. Eby, Sylvania Lighting Center, and R. Burton, atNRCC, in making the analysis.

References

1. J. R. Moore, Report on the International Intercomparison ofPhotometric and Colorimetric Measurements on FluorescentLamps (NPL Report, September 1963).

2. Fritz Rossler, Z. Phys. 110, 495 (1938).3. C. L. Sanders and W. Gaw, Appl. Opt. 6, 1639 (1967).4. L. Morren, "Une mthode de separation des raies et du fond

continu dans les spectres mixte: le proc6d largeur de fentevariable," Proceedings CIE, Barcelona (1971), paper 71.08.

5. D. A. McSparron et al., Spectroradiometry and ConventionalPhotometry, An Interlaboratory Comparison (NBS TechnicalNote 559, Nov. 1970, U.S. Government Printing Office).

6. W. J. Youden, in NBS Special Publication 300, Precision Mea-surement and Calibration (U.S. Department of Commerce,1969), p. 151.

7. M. Nonaka, K. Kinameri, and M. Ishibai, Metrologia 8, 133(1972).

8. Radlph Stair, William E. Schneider, and John K. Jackson,Appl. Opt. 2, 1151 (1963).


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