JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Solar Spectral Irradiance and Vertical Atmospheric Attenuation in theVisible and UltravioletL. DUNKELMAN AND R. SCOLNIK

U. S. Naval Research Laboratory, Washington 25, D. C.

(Received August 21, 1958)

The solar spectral irradiance outside the earth's atmosphere was determined by Langley's method ofextrapolation to zero air mass, from measurements taken on Mount Lemmon at an elevation of 8025 ftnear Tucson, Arizona, during October, 1951. The spectrum was produced and the energy scanned by aLeiss quartz double-monochromator, detected by a 1P21 photomultiplier, amplified, and presented on astrip chart recorder. About twenty-five spectra were recorded from sunrise to noon, with band widthsranging from 10 A at 3030 A to 170 A at 7000 A. The equipment was calibrated frequently by recordingthe spectrum of a standard tungsten lamp. Compared with earlier work performed in this field, our resultsagree best with those of Pettit. There is good agreement with the direct measurements from a rocket ob-tained by Purcell and Tousey in 1954 and with the Sacramento Peak ultraviolet observations by Stair andJohnston in 1955. The change of solar intensity with air mass showed that the attenuation of the atmosphereabove Mount Lemmon was approximately 15% higher than that for a Rayleigh atmosphere in the region3400 A to 4650 A, where there is no absorption due to ozone. A discussion is included which emphasizes theimportance of clear and constant atmospheres which are necessary to obtain accurate values of solar spectralirradiance outside the earth's atmosphere by the Langley method. The solar illuminance computed fromspectral data was 12 700 lumens/ft 2 .

I. INTRODUCTION

SINCE 1946 when rockets became available forscientific research, the Naval Research Laboratory

has made a number of measurements of the relativespectral intensity distribution of sunlight outside theearth's atmosphere, with the radiation measured fromthe entire solar disk. The spectral range has been con-fined to regions below 4000 A. At the present writing,data have been published to 2200 A." 2

During 1950-1951 intensive efforts were made to fixthe absolute level of the rocket-derived solar spectral-irradiance curve by adjusting it to match the mostreliable absolute data available from ground measure-ments. A search of the literature, however, showed agreat variation in the solar data obtained by differentexperimenters. This was particularly true near 3300 Awhere the match with the rocket curve had to be made.

In Fig. 1, there are shown the better known measure-ments of solar radiation from the entire disk madeprior to 1949. The Smithsonian data3' 4 are basicallyabsolute, but are usually shown in relative units only.Pettit5 normalized his relative spectral solar-irradiancecurve against the Smithsonian curve at 4500 A, therebyplacing his own on an absolute basis. Both the Smith-sonian and Pettit's published curves* have been read-

' Johnson, Purcell, Tousey, and Wilson, Rocket Explorationof the Upper Atmosphere (Pergamon Press, London, 1954), pp.279-288.

2 Wilson, Tousey, Purcell, Johnson, and Moore, Astrophys. J.19, 590 (1954).3 Abbot, Fowle, and Aldrich, Smithsonian Misc. Collections 74,

7 (1923).4 C. G. Abbot, Gerlands Beitr. Geophys. 16, 4, 343 (1927).6 E. Pettit, Astrophys. J. 91, 159 (1940).* Pettit plotted the Smithsonian irradiance curve for zero

atmosphere in the region 4500 A to 7000 A, and determined theratio of the area under the curve within a band of 100 A centeredat 4500 A to the area under the whole curve. Using the Smith-sonian value of 0.71 cal cm-2 min-' for the interval 4500 A-7000 A, he calculated the irradiance at 4500 A to be 22.4 Dew

justed downward to make them conform to the newabsolute energy values for the solar spectrum given bySmithsonian.6 Stair7 in 1947 made absolute measure-ments in the region 3000 A to 3300 A. The data ofHess,8 Reiner,9 and Gbtz and Schbnmann'0 were pub-lished only as curves of relative intensity distribution.We have normalized these curves against Pettit's at4725 A to make comparison simpler.

The large differences in intensity distribution below4000 A are not likely to have been produced by solarvariations. More probably they must be attributed tothe many difficulties besetting the measurements them-selves. Instrumental stray light is one of the moreserious problems. In a single dispersing system, theremay easily be sufficient stray light to produce an errorin the ultraviolet end of the spectrum where the energyis low. Other errors may arise from the inclusion of skyradiation from the region surrounding the sun when alittle haze is present. Still another problem is the accu-rate calibration of the equipment. This is usually per-formed with a standardized light source of establishedrelative intensity distribution. Unless standard lampsused by different experimenters have been comparedwith one another, there will always be some uncertaintyattached to the comparison of experimental data basedupon them.

Probably the most treacherous step, however, is the

cm' A-'. Repeating Pettit's calculations with the newer data inthe 9th edition of the Smithsonian Physical Tables,6 we foundthat the spectral irradiance at 4500 A should be 20.3 jMw cm2 A-'.

6 Smithsonian Physical Tables, Ninth Revised Edition (1954),Table No. 812.

7 R. Stair, J. Research Nat. Bur. Standards 40, 9 (1948).8 P. Hess, "Untersuchungen fiber die spektrale energiever-

teilung im sonnenspektrum von 350 mu bis 500 mA," Inaugural-Dissertation, Universitiit Frankfurt a.M. (1938).

9 H. Reiner, Gerlands Betr. Geophys. 55, 2, 234 (1939).10 F. W. P. Gdtz and E. Schonmann, Helv. Phys. Acta 21, 151

(1948).

356

VOLUME 49, NUMBER 4 APRIL. 1959

SOLAR SPECTRAL IRRADIANCE

4000 4500 5000

WAVELENGTH

FIG. 1. Solar spectral irradiance curves outside earth's atmosphere obtained by various observers prior to 1949. The Smithsonian andPettit curves are on an absolute scale. The curves of Reiner, Hess, and Gtz and Schonmann are normalized .to Pettit's curve at approxi-mately 4700 A. Stair's curve is on an independent absolute scale.

extrapolation of the data measured on the ground to alevel above the atmosphere. In the Langley method,measurements are made during all or part of a solarexcursion between the horizon and the highest positionattained by the sun during the day. The measuredintensity, I, is related to the intensity outside theearth's atmosphere, Io, by

I= Ioe-o-, (1)

where o is the coefficient of attenuation, and m, usuallyreferred to as the air mass, is the ratio of the pathlength through the atmosphere from the sun to theearth to the path length at vertical incidence. If themeasured values of Log I are plotted against air mass,a straight line should result, and an extrapolation tozero atmosphere can be made. As will be emphasizedlater, only under ideal atmospheric conditions do thedata measured over an entire day lie on a single straightline. Under nonideal conditions, if the measurements arecollected over a small range of air mass values, the datamay appear to fall on a straight line, but data over awider range of air mass would have shown an actualdeparture. In such cases extrapolation to zero air massleads to an erroneous solar intensity.

In an attempt to resolve the differences among the

early investigators, and thereby to fix the absolutelevel of our own rocket results, we undertook, in 1951,to determine the relative solar spectral intensity ex-trapolated to zero atmosphere, taking advantage ofthe most recent advances in detecting and recordingequipment, and selecting a site where measurementscould be made under most ideal atmospheric conditions.

II. THE SITE

A basic requirement for applying the Langley methodaccurately is that the atmosphere be homogeneous overthe portions of the sky covered by the sun-to-instrumentpath during the course of an entire morning, or after-noon, or both. For this condition to exist, the sky mustbe stable with passing time, and uniform over a widegeographical region. These requirements are most likelyto be met when the sky is very clear, that is, free fromclouds, haze, and dust. A high altitude is important,not only for the gain in energy reaching the instrument,but also because the less of the earth's atmosphere thatremains overhead, the shorter and the more accuratewill be the extrapolation to zero atmosphere. The samereasoning suggests a season and latitude when the sunwill reach a high altitude at noon.

Mount Lemmon, in southern Arizona, was selected

25

20 F-

15f-

I

U

0-Ja:

H

Uj

0

cr

cLn 5

10 I-

.. . SMITHSONIANREINER

-::STAIR-- PETTITGOTZ SCHONMANN

- ;- HESS

013000 3500 5500

(A)

6000 6500 7000

357April 1959

/

I . I

L. DUNKELMAN AND R. SCOLNIK

as the most promising site within the United Statesfrom the points of view of atmospheric clarity andstability. Reaching an altitude of 9180 ft, it is thehighest peak in the Santa Catalina Mountain Range inthe Coronado National Forest. The observing stationwas located on a huge flat-topped rock, altitude 8015 ft,jutting out from the top of the headwall of UpperSabino Canyon. To the east, the view of the sun wasunobstructed from about 15 min after sunrise, while tothe west, the view was open until 30 min of sunset.To the south, the rock overlooked timbered SabinoCanyon immediately below, while in the distance laythe desert east of Tucson. About 100 yards to the northand separated by a pine grove lay the modem, thenrarely traveled, Hitchcock Memorial Highway.

The abrupt 5600-ft rise of the terrain from the desertfloor to the observatory rock, the wooded nature of thesurroundings, and a prevailing north breeze ensuredthat the instrumentation was well above the level oftransient dust and smoke. The winds, the temperatures,and the altitude were moderate enough to cause neitherpersonal discomfort nor instrumentation difficulties.The humidity was always extremely low. The mountainwas easily accessible and electric power was available.There were comfortable living quarters within twomiles at the privately owned Summerhaven tract.

Contrary to expectation the sky above MountLemmon was found to be unsuitable for accurate solarmeasurements during most of the period September 20to October 17, 1951 spent on the mountain. During thefirst ten days, only calibration checks and test measure-ments were made, for clouds overhead and smoke fromforest fires in California made the sky totally unsuitable.

SUN

60 CY.

SIDEROSTAT QUARTZ

DOUBLE MONOCHROMATOR

FIG. 2. Block diagram of the apparatus. Sunlight was intro-duced into the Leiss double monochromator from the magnesiumcarbonate block C. The block was illuminated either by sunlight,through the siderostat, or by the tungsten-in-quartz standardlamp L by interposition of mirror ML. The lamp current andvoltage wcro monitored continuously by means of voltmeter Vand ammeter A and adjusted whenever necessary by variac VA.The 1P21 multiplier phototube detector was operated at a con-stant 600 v obtained from the regulated high-voltage powersupply HV. The phototube current was amplified by the chopperamplifier and presented on the strip chart potentiometer recorder.The dark current was subtracted by means of the bucking box B.

At last, aided by the only day of rain encountered onthe entire trip, the atmosphere cleared; and on October 4we were able to make a full day's measurements withthe clear stable sky required. Following this day,although the sky often seemed at first glance to bequite clear, careful examination showed faint wispy skydiscontinuities which could not be described as trueclouds, but which appeared and disappeared during thecourse of the day. Subsequent examination of the dataconfirmed our suspicions that the skies on these dayswere not sufficiently stable and uniform to permitaccurate extrapolation to zero atmosphere.

In this paper we are reporting the results of theOctober 4 observations as the best data obtained. Thereare presented also data and curves showing how theaccuracy of the measurements deteriorated on dayswhen the sky appeared to be clear, but the atmosphericattenuation was not actually constant. These data maygive some insight into the lack of agreement among theresults of the many prior investigators.

III. APPARATUS

The equipment used for the solar measurements,shown in block form in Fig. 2, comprised basically adouble monochromator that could be illuminated eitherby the sun through a siderostat, or by a standardizedcomparison lamp. Energy emerging from the exit slitwas measured with a 1P21 multiplier phototube whoseoutput was amplified and directed into a strip chartpotentiometer recorder. The monochromator made byCarl Leiss was selected because of its compact andportable nature. Stray light was minimized throughdouble-dispersion. The prisms were of quartz, and theradiation was collimated and focused by means ofaluminized mirrors. Exit and entrance slits were main-tained at 0.25 mm and center slit at 0.5 mm, resultingin a band pass that ranged from 10 A at 3030 A to170 A at 7000 A. The monochromator was convertedto automatic scanning and recording by coupling amotor drive to the wavelength-change drum. Theinstrument was installed in a light-tight box thermo-statted to maintain a temperature of 850F, which wasthe maximum reached during the heat of the day.

Sunlight was introduced into the instrument bymeans of a siderostat. This consisted of a mirror, M.,which was rotated with a synchronous motor driveabout an axis coinciding with the axis of rotation ofthe earth. With the siderostat, the angle of incidence onthe mirror remained constant all day, thus obviatingcorrections for change of reflectance with angle ofincidence.

After reflection from M., the sunlight was incidenton a magnesium carbonate block placed in front of

the monochromator entrance slit. The diffuse reflectorserved two functions: it eliminated the slight polariza-tion arising at mirror M,; and it caused the mono-chromator optical system to be filled at all times. If the

358 Vol. 49

SOLAR SPECTRAL IRRADIANCE

sunlight were not depolarized, an error would be intro-duced because the transmittance of the monochromatoris slightly polarization dependent. Thus as mirror M.rotated, the plane of polarization would have beenchanging continuously relative to the monochromator.The importance of keeping the optical system filledlay in the fact that the transmittance of the prisms wasnot constant across the aperture. Therefore, had thesystem been used in an unfilled condition, it would havebeen necessary to make sure that both the sunlight andthe light from the calibrating lamp always passedthrough the same part of the optical system.

A system of baffles restricted the acceptance angleat the carbonate block to a cone of one degree diametercentered on the sun. This minimized the radiationaccepted from the sky surrounding the sun. When thesky is hazy and an aureole is present, considerablelight comes from the sky close to the sun. Inclusion ofsky light may account for some of the discrepancies inthe literature. In the Mount Lemmon work the skycontribution is negligible, because of the small accept-ance angle used, and because no observations weremade when the sky near the 'sun was appreciably hazy.

Since the measurements of solar radiation consistedof direct comparisons with radiation from a tungsten-in-quartz lamp," it was necessary that both light beamsbe introduced under identical conditions. This wasaccomplished by reflecting the lamp radiation to thecarbonate block by interposing the mirror ML. Thebeams from the sun and from the lamp were of aboutthe same angular subtense, and were incident on theblock at the same angle. Mirrors M, and ML were frontsurfaced, aluminized together, and handled identicallythereafter.

The tungsten reference lamp was calibrated bytwo completely independent procedures. The NationalBureau of Standards determined the current at whichto operate the lamp for a true temperature of 2805'Kwhich corresponds to a color temperature of 2854 0K.They also furnished a set of emissivity values basedupon measurements of Hamaker'2 in the visible and theinfrared, of Worthing" in the visible, and of Hoffmannand Willenberg:4 in the ultraviolet. In 1954, Devos'5

confirmed the emissivity values to 1% between 7000 Aand 4000 A. Below 4000 A, Devos' values are lower,

the difference being approximately 2% at 3400 A andreaching nearly 5% at 3100 A. This is well within ourexperimental error. From the emissivity values and the2805'K blackbody distribution, the intensity distribu-tion of the lamp radiation was calculated. A secondcalibration was performed at the Naval Research

11 R. Stair and W. 0. Smith, J. Research Natl. Bur. Standards30, 449 (1943).

12 H. C. Hamaker, Thesis Utrecht (1934).13 A. G. Worthing, Phys. Rev. 10, 377 (1917); Phys. Rev. 25,

588 (1925); Z. Physik 22, 9 (1924).14 F. Hoffmann and H. Willenberg, Phys. Z. 35, 1, 713 (1934).15 J. C. Devos, Physica 20, 690 (1954).

Laboratory with the assistance of Packer and Lock 6

using equipment they had assembled to measure thespectral radiance of the carbon arc. They used a quartzdouble monochromator of known spectral transmittance,and a phototube whose absolute spectral response hadbeen determined by comparison with a thermocouple.The two sets of calibration results were in agreementwithin 5%.

IV. PROCEDURE

A day's work began at a predawn hour when tar-paulins were removed and electric blankets were takenaway, or not, according to the early morning tem-perature. The electronic instruments had been pre-warmed to stable operation by use of a clock relay.The siderostat was capped, and the photomultiplierdark current balanced out by means of a microcurrentbucking circuit. The reference lamp was brought to theproper operating condition and a spectral traverse wasmade from 3000 A to 7000 A. By the time this wascompleted, the sun had cleared the trees to the east,and the day's first solar spectral trace could be started.The lamp mirror was withdrawn from the baffle allowingsunlight to enter the monochromator, the multiplierdark current was checked for zero balance, and thesolar spectrum was recorded. At the end of the firstscan, the wavelength drum was quickly returned byhand to the starting point, at 3000 A, and a new tracewas begun immediately. All scans were made con-tinuously in the direction of increasing wavelength.

Until about 10 A.M. spectral scans were made inrapid succession, with a lamp calibration interposedapproximately every hour. A scan required 5 min, thereturn to starting point a matter of seconds. From 10to 11 A.M., as the rate of change of the sun's pathlength became smaller, the scans were made at quarterhour intervals, and from 11 A.M. to noon at half-hour intervals, ending with a final lamp run. When thesky remained clear, measurements were continued intothe afternoon, first at half-hour intervals, then everyquarter hour, and finally with traces obtained as rapidlyas possible again, ending with a final tungsten calibra-tion just after sunset.

When scanning the solar spectrum, a neutral filterof density 1.2 was always interposed at 3800 A, in orderto keep the multiplier phototube anode current below2 pa. This was necessary to prevent phototube fatiguewhen the more intense portion of the solar spectrumwas traversed. Even though the current limitation didnot require it, the filter was also introduced at 3800 Afor the reference lamp scans in order that optical condi-tions be maintained identical to those of the solarradiation measurements.

From 11:45 A.M. to 12:45 P.M. the air mass changedfrom 1.26 to 1.25 and back to 1.26, a negligible varia-tion. Therefore, the noon hour was used for deter-mining instrument stability and signal reproducibility

16 D. M. Packer and C. Lock, J. Opt. Soc. Am. 42, 879 (1952).

359April 1959

L. DUNk-ELMAN AND R. SCOLNII Vol 4

X I '/ '. ~-'Ai' f; J W 0 : \

F .8- , , ..6-

I J4

-°I2 -AK I- 2 - INSERTEDFIG 3.1600 00 20 A DENSITY FILTER ISRE

7000 6000 4500 4000 3800 3600 3400 3200 3000-

WAVELENGTH -ANGSTROMS

FIG. 3. A solar trace made at noon on the strip chart recorder. An amplifier recorder zero shown by 0 was recorded before each runAnother zero shown by the 0 with solidus was recorded just before a run was begun and when the amplifier was set at the highestsensitivity and the sunlight blocked momentarily to insure that the dark current was entirely subtracted. Timing marks along the topwere used for wavelength registration. Typical of solar features are the calcium H and K absorptions. The sharp spikes (10-, X, Y)are sensitivity range changes, not solar features.

and for checking the constancy of the sky. For ex-ample, the solar intensity at a single wavelength wasrecorded continuously for about 20 min to detect anyvariation in output or any sign of instability. On severaldays control voltages and temperatures were deliber-ately varied during the period of constant air mass todetermine the magnitude of the effects of instrumentalinstability on the solar measurements. The precisionestimate for the solar spectrum is based in part on this.

The noon' hour was an ideal time for checking themonochromator for scattered and stray light. To dothis the monochromator was set at 2850 A where nosolar energy could possibly penetrate the earth's atmos-phere. When the siderostat mirror was alternatelycapped and uncapped, the photocurrent change couldbe only that caused by radiation scattered within themonochromator. This was found to be less than 10, pa,an entirely negligible amount.

V. THE DATA

It cannot be over-emphasized that in order to obtaindata valid for extrapolation to zero atmosphere, idealsky conditions must prevail for the duration of a com-plete set of solar traces: i.e., from sunrise to noon, orfrom noon to sunset. In four weeks at Mount Lemmon,only three days were sufficiently clear and stable sothat both morning and afternoon scans appeared worthanalyzing. Three other days were satisfactory for themorning scan only. On the remaining days, sky con-ditions proved entirely unsatisfactory.

On October 4, 1951, the day of the nearest approachto the ideal stable sky, 26 traces of the solar spectrum

and three of the tungsten standard lamp were recordedfrom noon to sunset. Figure 3 is a reproduction of asolar trace made at noon. The range change discon-tinuities in the solar curve are not to be confused withreal spectral features. The actual full scale currentvalue in microamperes is given at the point of eachdecade range change, while intermediate range changesare shown by X and Y representing factors of 2 and 5,respectively.

The reduction of the traces was carried out at nearly100 wavelengths between 3030 A and 6900 A. Thewavelengths were those of distinct spectral features:maxima, minima, and points of inflection. All featureswere marked on each of the 26 traces, and the photo-currents corresponding to each of the selected wave-lengths were tabulated. Next, for one wavelength at atime through the 26 traces, the log of the photo-current was plotted against the air mass. The curvesfor 4850 A, 3585 A, and 3118 A, shown in Fig. 4, aretypical and were selected as representative of regions ofwidely different character. For solar zenith angles from0 to 750, the air mass values are the secants of theangles. Beyond 750, Bemporad's modifications17 cor-recting for the curvature of the atmosphere and refrac-tion were used.

Because of the clear, homogeneous, and constantatmospheric conditions on this day, the straight linerelation held extremely well, even to air masses greaterthan ten, as can be seen from Fig. 4. This was true forall the wavelengths selected. Under these conditions,the extrapolation to zero atmosphere could be carried

17 See reference 6, Table No. 811.

360 'Vol. 49

SOLAR SPECTRAL IRRADIANCE

TABLE I. Solar spectral irradiance data. The wavelength X is inangstroms and the zero air mass spectral irradiance Hx is in 4wcm--2 A-'. The sun is at the mean solar distance.

3032305230563078308630883116311831413152315831683182319232043212

HX X

7.3 32217.7 32257.1 32426.9 32557.3 32796.5 33018.4 33087.9 33228.3 33308.3 33356.8 33689.7 33807.5 34108.1 34149.4 34307.2 3442

8.1 34567.5 34708.9 3508

12.7 352211.1 355013.2 358511.5 360011.3 361011.3 363711.2 366810.4 368311.3 369812.3 371611.7 374011.9 375210.3 3780

Hx X HA

11.2 3807 13.511.3 3830 9.212.7 3848 12.111.3 3870 10.413.1 3908 13.49.2 3933 8.6

13.0 3950 12.211.0 3968 10.112.0 4019 19.414.4 4062 18.513.9 4094 20.013.0 4140 19.613.5 4190 18.210.9 4220 20.211.5 4272 19.014.5 4300 16.1

?X Hx

4380 20.14390 19.44439 22.04546 22.04640 21.44720 21.94808 21.54850 20.34948 20.75002 19.55088 20.05174 18.75230 19.45260 19.15312 19.5

5380545055255604570057585800584259806021612262806500

Hx

19.720.019.419.018.818.718.718.418.218.017.116.615.9

out with little error, and yielded io(X), the photo-multiplier current that would have been obtained abovethe earth's atmosphere at each wavelength. The slopeof the line gives the attenuation coefficient defined byEq. (1) for the particular wavelength.

The current values io(X) were converted to relativevalues of solar spectral irradiance outside the atmos-phere, H, (X), by means of the traces made with thestandard lamp. The several traces were found to benearly identical, therefore, the average trace could beused. The relation is

H, (X) = io ()HA (\)/iA (X), (2)

where HA (X) is the spectral irradiance on the mag-nesium carbonate block at wavelength X produced bythe standard lamp, and iA (X) is the correspondingphotomultiplier current as read from the average trace.

The relative values of H, (X) are considered to beaccurate to z3%, except at the extremes of the wave-length range covered, where the error may be as highas 6%. The chief reason for the increased error nearthe short wavelength limit is the feebleness of the photo-multiplier current resulting from the low level of irradi-ation from the standard tungsten lamp. Near the longwavelength limit the increase in error is caused by thelow sensitivity of the photomultiplier.

The original objective of the Mount Lemmon experi-ment was to obtain the solar intensity distribution ona relative scale only, and this is all that is intended byEq. (2). However, an absolute calibration was obtainedby measuring with a MacBeth Illuminometer theilluminance produced at the magnesium carbonateblock by the tungsten standard lamp. This was com-pared with the illuminance computed from the relativespectral irradiance curve for the lamp, the conversionfactor, 680 lu/w, and the CIE. Standard luminositycurve. The error in this process was judged to be t 10%.Later, Johnson,"5 from a study of the Smithsonian workon the solar constant, the intensity distribution curveobtained from rockets, and the present Mount Lemmonresults, concluded that our original scale should be

18 F. S. Johnson, J. Meteorol. 11, 431 (1954).

I

, .

I _c1

TIME OF DAY (MST)

AIR MASS

FIG. 4. Langley plots of logarithm of photomultiplier currentagainst air mass for both morning and afternoon of October 4,1951. The selected wavelengths represent a region of strong ozoneabsorption (3118 A), a region of no ozone absorption but moderatescattering (3585 A), and a region of relatively little scatteringand weak ozone absorption (4850 A). Each pair of A.M. and P.M.curves yielded nearly identical values for zero air mass. In theplot for 3118 A, the squares below the crosses and circles representthe values obtained after correcting for the curvature of theozonosphere.

raised by 9%. The data presented here are on theabsolute scale arrived at by Johnson.

VI. SOLAR IRRADIANCE

The final values of the solar spectral irradiance forzero atmosphere, with the sun at the mean solar dis-tance, are given in Table I for the wavelengths of allthe identifiable features of the records obtained atMount Lemmon. These data to 4000 A are plotted asthe solid curve in Fig. 5 which reveals all the Fraunhoferabsorption detail that could be obtained from themeasurements in this wavelength region. This may becompared with the two other recent curves shown. Onewas obtained by Purcell and Tousey'5 from the analysisof a solar spectrum photographed from a rocket at104 km altitude on February 22, 1954. In this work theabsolute energy scale was set by adjustment to theMount Lemmon curve at 3600 A. The other curve isthe result of the work of Stair and Johnston2 0 carriedout at Sacramento Peak in 1955. These data are to bepreferred over the similar earlier work of Stair2 in 1951and of Stair, Johnston, and Bagg22 in 1953. In 1955 theentire monochromator was automatically directed atthe sun, thus avoiding the necessity of making a cor-rection for polarization introduced by the siderostatused in the earlier work. The instrument used in 1955was a Leiss double monochromator similar to that usedat Mount Lemmon.

It may be seen that all three curves of Fig. 5 are in

19 Purcell and Tousey (private communications).10 R. Stair and R. G. Johnston, J. Research Natl. Bur. Standards

57, 205 (1956).21 R. Stair, J. Research Natl. Bur. Standards 49, 227 (1952).22 Stair, Johnston, and Bagg, J. Research Natl. Bur. Standards

53, 113 (1954).

April 1959 361

L. DUNKELMAN AND R. SCOLNIK Vol. 49

3400WAVELENGTH (A)

FIG. 5. A comparison of the solar spectral irradiance curves obtained by Stair and Johnston at Sacramento Peak, by Purcell andTousey from a rocket at 104-km altitude, and by the authors at Mount Lemmon. The Mount Lemmon and the Stair and Johnstoncurves are plotted each according to its own absolute scale. The rocket curve, which begins at 3600 A was determined in relative units,and is plotted to give the best fit.

good agreement over this wavelength range. Thevariation below 3100 A is not unexpected because ofthe reduced energy available at the ground. Here therocket curve, which actually extended to still shorterwavelengths, is considered to be the most accurate.There is some difference in detail, but no more than isusually encountered when comparing data taken withinstruments whose resolving powers are not identical.

TABLE II. Integrated solar spectral irradiance data. The wave-length X is in angstroms and the zero air mass spectral irradianceHx is in uw cm'-2 A-1. The integrated values were obtained byaveraging the detailed solar distribution data over a 100-A intervaltaking points separated by 10 A.

A

30803090310031103120313031403150316031703180319032003210322032303240325032603270328032903300

HA A

7.4 33107.5 33207.6 33307.6 33407.8 33508.0 33608.0 33708.2 33808.3 33908.3 34008.2 34108.3 34208.5 34308.9 34409.2 34509.4 34609.8 34170

10.3 348010.5 349010.8 350011.1 351011.4 352011.6 3530

HA X

11.5 354011.3 355011.2 356011.2 357011.2 358011.1 359011.2 360011.2 361011.2 362011.1 363011.1 364011.3 365011.5 366011.6 367011.6 368011.7 369011.6 370011.6 371011.6 372011.8 373012.0 374012.0 375011.8 3760

Hx

11.6 377011.7 378011.6 379011.5 38001.5 3810

11.5 382011.6 383011.7 384011.9 385012.2 386012.6 387013.0 388013.1 389013.3 390013.4 391013.3 392013.3 393013.2 394013.2 395013.2 396013.2 397013.2 398013.1 3990

Hx X

12.8 400012.5 401012.3 402012.4 403012.4 404012.2 405012.0 406011.7 407011.6 408011.6 409011.4 410011.2 411011.2 412011.4 413011.3 414011.2 415011.4 416011.6 417011.9 418012.4 419013.0 420013.7 421014.7 4220

HA X HA

15.4 4230 19.216.0 4240 19.016.7 4250 18.917.5 4260 18.718.2 4270 18.418.8 4280 18.219.1 4290 17.919.3 4300 17.819.3 4310 17.719.3 4320 17.819.4 4330 17.819.4 4340 17.919.4 4350 18.219.4 4360 18.619.3 4370 19.019.2 4380 19.419.1 4390 19.919.2 4400 20.219.2 4410 20.619.2 4420 20.919.2 4430 21.119.2 4440 21.319.2 4450 21.5

Since the resolution in Fig. 5 is too fine for easy com-parison with earlier work, and the data of Table I aretoo detailed for many purposes, a low resolution curvewas prepared by averaging the curve of Fig. 5 over a100-A interval, taking points separated in wavelengthby 10 A. These averages are presented in Table II, andare shown as the solid curve of Fig. 6. The data ofTable I was used to extend the curve to the wave-lengths longer than 4450 A. Comparing this curve withthe earlier work shown in Fig. 1, the agreement is bestwith the data of Pettit which are replotted in Fig. 6.Shown also are Stair's latest (1955) results. Comparingthe Mount Lemmon results with Pettit and with Stair,below 5000 A the agreement is best with Stair, and tolonger wavelengths with Pettit. The results of Stairbetween 5000 and 7000 A are high, and are not inagreement with any previous work, including his ownearlier measurements. Furthermore, they lead to avalue of the extra-terrestrial illuminance that is higherthan the recent measurement of Karandikar,2 ' andmost previous solar illuminance measurements.

VII. ATMOSPHERIC ATTENUATION AND OZONE

The vertical atmospheric attenuation coefficient forthe total atmosphere above Mount Lemmon is givenby the slope of the log current vs air mass curve (cf.Fig. 4) for the particular wavelength considered. The

B3 R. V. Karandikar, J. Opt. Soc. Am. 45, 483 (1955).

362

20

c'J

0

0

0

0

-J

ir0w0L U)

SOLAR SPECTRAL IRRADIANCE

5000WAVELENGTH (A)

FIG. 6. A comparison of solar spectral irradiance outside the earth's atmosphere obtained by Pettit, by the authors at Mount Lemmonin 1951, and by Stair and Johnston in 1955. The curves below 4500 A have been drawn using (1) the integrated values for the MountLemmon data shown in Table II, (2) the integrated distribution as determined by Stair and Johnston, and (3) the Pettit data. Forwavelengths longer than 4500 A nonintegrated values are used in each case. The scales of Fig. 6 and Fig. 1 are identical and permitfurther comparison with the earlier workers.

coefficients listed in Table III, and plotted in Fig. 7,are for the data of the afternoon of October 4, 1951.The attenuation coefficient a-, is defined as the expo-nential coefficient for the total vertical atmosphereabove Mount Lemmon, which has an equivalent thick-ness of 5.96 km. For comparison, the attenuation curvefor a Rayleigh atmosphere of pure air of equivalentthickness 5.96 km is also shown. Use of the log-log plotwould make the Rayleigh curve a straight line, exceptfor the small curvature produced by the dispersionof air. The experimental curve departs markedly fromthe Rayleigh curve in the 4650-A to 7000-A region andalso below 3400 A, because of ozone absorption in theChappuis and Huggins bands, respectively.

From 3400 A to 4650 A, where there is no absorptiondue to ozone, the observed curve is nearly parallel to,but approximately 15% above the Rayleigh curve.Expressed in terms of the vertical atmospheric trans-mittance, e-, the atmosphere was approximately 5%more opaque than a Rayleigh atmosphere of pure air.This difference must be ascribed to atmospheric con-taminants. So far as we are aware, there have been nocases observed of a perfectly pure Rayleigh atmosphere,

at least in recent years. Tousey and Hulburt,2 4 frommeasurements of sky brightness from aircraft at 10 000

TABLE III. Spectral exponential attenuation coefficients of thetotal vertical atmosphere above Mount Lemmon on October 4,1951. The wavelength X is in angstroms and the attenuationcoefficient a is to the base e for an equivalent atmospheric thick-ness of 5.96 km.

X a X X a

3030 2.54 3301 0.658 3807 0.377 5088 0.1323032 2.39 3308 0.650 3830 0.361 5174 0.1313052 2.06 3322 0.644 3848 0.359 5230 0.1233056 2.04 3330 0.629 3933 0.310 5260 0.1183078 1.70 3335 0.618 3950 0.308 5312 0.1203086 1.632 3368 0.584 3968 0.299 5380 0.1233088 1.591 3380 0.592 4019 0.293 5450 0.1243116 1.335 3410 0.575 4062 0.279 5525 0.1163118 1.323 3414 0.562 4094 0.267 5604 0.1333141 1.121 3442 0.549 4220 0.241 5700 0.1123152 1.081 3456 0.521 4272 0.228 5758 0.1183158 1.043 3470 0.526 4300 0.213 5800 0.1123168 0.960 3508 0.489 4380 0.208 5842 0.0983182 0.936 3522 0.483 4439 0.203 5980 0.0963192 0.897 3550 0.471 4546 0.178 6021 0.0973204 0.849 3585 0.455 4640 0.167 6122 0.0813212 0.789 3600 0.455 4726 0.159 6280 0.0753221 0.792 3637 0.437 4808 0.152 6502 0.0653225 0.811 3683 0.409 4850 0.1483242 0.751 3716 0.399 4948 0.1373279 0.703 3740 0.392 5002 0.139

'A R. Tousey and E. 0. Hulburt, J. Opt. Soc. Am. 37, 78 (1947).

April 1959

25

201_.fl

4c'JN

0()

L:

00

PIi0z

0

0U)

101-

363

7000

/

. -- STAIR & JOHNSTON--- PETTIT

-~ MT LEMMON

I I I ram4000 6000

15 _

I

3 000I l

L. DUNKELMAN AND R. SCOLNIK

z0

z

x

-I

0

0'

0.I

0.0

0.0

3000 4000 5000

WAVELENGTH A

6000 700

FIG. 7. Spectral vertical attenuation of the total atmospheabove Mount Lemmon from an elevation of 8015 feet on October1951. For comparison, the attenuation curve for a theoreticalpure (Rayleigh) atmosphere of equivalent thickness (5.96 kris shown.

ft and Packer and Lock,2 5 from similar measurementsup to 38 500 ft, concluded that the atmospheric attenu-ation coefficient for visible light was approximately 35%higher than for pure air at 5500 A. The Mount Lemmondata for October 4, depart from Rayleigh by approxi-mately 30% in this region. The data of Stair at Climax(1951) and of Stair et al. from Sacramento Peak (1953)depart even more from Rayleigh.

The amount of ozone present was determined fromthe absorption in the Huggins band region between3400 A and 3000 A, and derived from Fig. 7 from thedifference between the observed curve and the extrapo-lation of the nearly linear segment to wavelengthsshorter than 3400 A. In Fig. 8 the values of absorptionare plotted against the Ny and Choong26 absorptioncoefficients, for the various wavelengths in the Hugginsregion. The points fall on a straight line, as they should,from whose slope the total ozone was determined as2.004z0.05 mmn (NTP).t In the Chappuis band region,absorption by ozone is relatively weak and the accuracyof the data is less because of reduced resolution of themonochromator and diminished sensitivity of the photo-tube. Therefore, the ozone content cannot be deter-mined with precision from these data. Nevertheless,the absorption in this region using coefficients deter-

25 D. M. Packer and C. Lock, J. Opt. Soc. Am. 41, 473 (1951).26 Ny Tsi-Ze and Choong Shin-Piau, Compt. rend. 195, 309

(1932); 196, 916 (1933).t Recently the International Ozone Commission [International

Ozone Commission Circular No. 0.4 (July 5, 1957)] adopted thecoefficients found in 1953 by Vigroux E. Vigroux, Annales dePhysique 8, 709 (1953)] and stated that his values have hadample confirmation by independent methods. We used thesecoefficients and our points again fall along a straight line with aslope yielding a value of 2.4340.03 mm of total ozone at NTP.

z2 L> LB

I

01.21 14

I-o .

I. O.

° O.

Z, O.:0

I 0 20) S,

Vol. 49

2 3 4 5 6 7 B 9 10 - II 12

EXPONENTIAL ABSORPTION COEFFICIENT - 1.0 CM OZONE

FIG. 8. A plot of the absorption by total ozone above Mountc Lemmon vs the Ny and Choong exponential absorption coefficients

which yields the amount of ozone above Mount Lemmon onOctober 4, 1951.

re

41 mined by Vassy and Vassy2 7 was found to be consistentn) with the values expected from 2.0-mm ozone.

VIII. EXTRAPOLATION TO ZERO ATMOSPHERE

Now that the instrumental error in solar measure-ments can be made very small by present day refine-ment in techniques, the principal error remaining indetermining the solar curve for zero atmosphere is thatintroduced by the Langley extrapolation procedure,especially if the atmosphere is not uniform and stable.The most accurate extrapolation will be obtained onthe day with the clearest atmosphere, this being thebest guarantee of homogeneity and stability. Therefore,we have chosen to report final data from the results ofthe best day only (October 4, 1951). A reduction inaccuracy will result from averaging the data of the bestday with those obtained on days that were also good,but showed definite signs of changing atmospherictransmission. Data from several less perfect days arealso presented to illustrate the manner in whicherroneous results may be obtained.

There are two principal criteria that can be appliedto determine the constancy of the atmospheric trans-mission on a given day. The measurements taken onOctober 4, 1951 will be used in the discussion of thesecriteria. The first is the straightness of the log energy vsair mass (Langley) plots, which are shown in Fig. 4 forthree selected wavelengths. In the ideal case, the datafor each wavelength would fall on a single straight linefrom sunrise through noon and back to sunset. Thedata for both the morning and afternoon runs ofOctober 4 fell exactly on straight lines. There was asmall difference in slope between the two sets of runs,showing that the atmospheric attenuation was slightlyless in the morning than in the afternoon. However,the extrapolated intersection at zero atmosphere was the

27 A. Vassy and E. Vassy, J. Chem. Phys. 16, 1163 (1948).

364

* OZONE ABSORPTION

H WUGGINS BANDS

I

X -,--- OZONE ABSORPTION-

RAYLEIGHN CHAPPUIS BANDS

.~~~~~~ e¢c

0

/0

SLOPE SHOWS O C

0 I

I I

1.

I

!O CM OZONE

I I I

SOLAR SPECTRAL I

101I

0.1

FIG. 9. Langleyplots revealing errorsintroduced when theatmosphere is nei-ther Rayleigh-likenor constant. In eachplot there is shownthe point for zero at-mosphere for the par-ticular wavelength asdetermined from theOctober 4, 1951 data.

AL

c

0

0:L:

LdI

I

0.01

(.1

0 1 2 3 4 5 6 7 8 9AIR MASS

(c)

0.01 =

0.001

0.0001 I I I I I I I I0 1 2 3 4 5

AIR MASS6 7 8 9

a-

0

1.0

0.1

0.011c

(b)

)Z L

-_Z _

I I I I I I I I1 2 3 4 5

AIR MASS

1.0F

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L,

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I-ZLUCEa:0:U

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0.01

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AIR MASS

same for both sets. This is good evidence that no errorwas introduced because of the slight difference inatmospheric attenuation.

Data reaching large values of air mass are of greatimportance in establishing valid straight lines. This isparticularly true because the sun's altitude is changingrapidly over this range, resulting in a large change of airmass in a short period of time, thereby minimizing thepossibility of any significant change in atmosphericattenuation during the measurements. The region nearnoon, on the other hand, when the air mass is practicallyconstant, is of least value in establishing the extrapola-tion. In actual measurements, the maximum value ofair mass that could be reached, even on a very clearday, depended upon the wavelength. In the visible anddown to 3400 A, it was possible to reach m= 12; whilewithin the Huggins Band region the range was reduced.For the wavelength range below 3400 A, where absorp-tion by ozone becomes important, it was necessary tomake a correction for the curvature of the ozone layer,following the method of Dobson.28 After this was done,these Langley plots were found to be straight lines.

28 Operator's Manualfor the Dobson Specirophotometer (R. and J.Beck, Ltd., London, England).

The second criterion for atmospheric constancy wasthe plot of atmospheric attenuation vs wavelength,Fig. 7. From 3400 A to 4600 A where ozone attenuationis negligible, the lower limit to the attenuation is setby the Rayleigh curve for pure air. The closer the actualattenuation curve lies to the Rayleigh curve, the lessthe possibility for variation in transmittance due to achanging atmosphere. As previously noted this curve isthe closest to Rayleigh that has been obtained to ourknowledge, in recent years. Furthermore, the pointsfor the different wavelengths lay on a straight line towithin experimental error. This would be expected,because attenuation produced by scattering by ordinaryatmospheric contaminants would not show irregularchanges over small wavelength intervals, and the knownrare absorbing constituents of the atmosphere, such asN2 0, NO, SO2, NO2 , do not absorb significantly at thesewavelengths.

Within the ozone absorption regions the plot of theabsorption produced by ozone vs the ozone absorptioncoefficient, Fig. 8, serves as a criterion for the reliabilityof the results. The straight line obtained checks thecorrectness of the Langley plots for the several wave-lengths.

(a)

_

I I I I I I I I6 7 8

6 7 8 9

._

365RRADIANCEApril 1959

- In

L. DUNKELMAN AND R. SCOLNIK

The types of error introduced when the atmosphere isneither Rayleigh-like nor constant are illustrated byLangley plots for several selected days. Figure 9(a)shows what happens when there is present a suggestionof haze, or perhaps very thin high cirrus. Most of thesky appeared cloudless; however, in sky areas far re-moved from the sun-to-instrument path, there were afew faint wispy clouds. We always succumbed to thetemptation to continue measurements on this type ofday, but invariably the data yielded unsatisfactoryLangley plots. In the example shown, a single beststraight line through the data could not be determined.The point for zero atmosphere determined from theOctober 4 data is shown, and it can be seen that thedata from m= 5 to m= 7.5 fall on the correct straightline. However, the points for m<5 do not lie on astraight line, and their use would lead to an erroneousvalue for m=0. For example, a solar irradiance value23% too high would result if the only data collected onthis day had been those between m= 1.25 and m= 2,whereas the data between m=2 and 4.5 would lead toa value 25% low.

Figure 9(b) illustrates data falling on one line for mvalues less than 3, and on another for the larger values.If one postulates that a single abrupt change in theatmosphere occurred at m=3, it should be possible toextrapolate each straight line segment (m= 1.3 to 3;and m= 3 to 6) to zero atmosphere and arrive at thesame value of solar energy. However, the lines do notmeet at m=0, and on the basis of this day's run only,it is impossible to decide on the best value for zero airmass. It can be seen, however, that the solar irradianceobtained from extrapolation of the segment betweenm= 3 and 6 is within 4% of the indicated correct valuebased upon the October 4 data, whereas the extrapola-tion of the measured points at m<3 yields results thatare 45% low. Inspection of our field notes shows thatthe sky, though cloudless most of the day, was slightlyhazy with some suggestion of inhomogeneity.

Figure 9(c) describes an atmospheric change thatappears to have progressed in stages. On the otherhand, in Fig. 9(d) the change in atmospheric attenu-ation seemed to be more continuous. In these cases itis quite apparent that if measurements had been madefor low values of m, or for a limited range of m only,straight lines would have been obtained which wouldhave led by extrapolation to incorrect values of thesolar intensity for zero atmosphere.

Even if the Langley plots are perfectly straight linesover a wide range of values of m, there is still a possi-bility that the extrapolated value of solar intensity atm=0 may be in error. This possibility arises because a

may vary during the course of the spectral scans in justsuch a manner that a straight line Langley plot willstill be obtained. but this straight line is rotated so asto intersect n=0 at an incorrect value of Io. Consider

Eq. (1), which may be rewritten in the form

In (I/o) = - um. (3)

Since we are considering only observations for whichstraight lines are obtained, that is, for which:

In (I/o) = - am+b,

we may compare these expressions, obtaining

a= a- b/n. (4)

Thus a straight line will result not only when a isconstant (b= 0), but also when b is not zero. The latter,of course, will lead to the erroneous extrapolation valuesof solar intensities. However, since, the variation in amust follow this specific and highly improbable func-tion of n, we are probably safe in disregarding thispossibility, provided that measurements are not madeover too limited a range of air masses. And once againthis emphasizes the importance of the use of the clear-ness of the sky as the best criterion for the constancyand uniformity of the atmosphere.

The applicability of the Langley method has beencriticized by Deirmendjian and Sekera,29 largely on thebasis of the work of the Smithsonian and of Pettit.Their argument is that values of atmospheric trans-mission were reported that were somewhat greater thanthe corresponding values for a theoretical pure Rayleighatmosphere. In the case of the Smithsonian results, thiscommenced at about 4000 A and increased to shortwavelengths. Deirmendjian and Sekera explained thisby assuming that a large amount of light was producedby Mie scattering from the sky around the sun andwas accepted by the equipment. Our explanation forthis is that the greater-than-Rayleigh transmittanceswere caused by the presence of instrumental stray light.In the Smithsonian work, only a single dispersing mono-chromator was used. The detector was a bolometer.These are conditions that would produce such an effect.Stray light would also explain the low values for extra-terrestrial irradiance reported by the Smithsonian inthe ultraviolet.

In the case of the work of Pettit, values of trans-mittance greater than Rayleigh occurred in the 1934Mount Wilson fifteen-day mean data below about4000 A, and persisted to 3300 A, beyond which theattenuation caused by ozone makes it meaningless tocompare the results with Rayleigh. The departure fromRayleigh is a small one, but it cannot be explained ascaused by instrument stray light because a doubledispersing system was used. Inspection of Pettit'spublished Langley plots for June 8, 1939 suggests,however, that these departures are not significant,since the five points obtained at each wavelength rarelylie on the straight line required to give a precise value oftransmittance. It is also quite likely that among the

29 D. Deirmendjian and Z. Sekera, J. Opt. Soc. Am. 43, 1158(1953); 46, 565 (1956).

366 Vol. 49

SOLAR SPECTRAL IRRADIANCE

15 days there may have been included seemingly gooddays when the atmospheric transmittance was actuallyvarying, such as we encountered so frequently at MountLemmon. These could easily have led to false values ofaverage transmittance.

Deirmendjian and Sekera have concluded that thereis a trend toward higher values of the extra-terrestrialenergy as the elevation of the observation station in-creases, and have suggested that the NRL rocket andMount Lemmon data, which do not conform, are toolow. Their conclusions were based not only -upon theSmithsonian work (Mount Wilson and Mount Whitney)and the Pettit work (1934 Tucson) but also upon theearlier work of Stair and co-workers (SacramentoPeak, 1953, and Climax, 1951). Since the more recent(1955) ultraviolet results of Stair and Johnston, therocket and Mount Lemmon data are in agreement, itwould appear that there is little justification for theirconclusions.

IX. SOLAR ILLUMINANCE

The solar illuminance outside the atmosphere wasdetermined by multiplying the absolute spectral-irradi-ance curve from the Mount Lemmon measurements(Fig. 6) by the CIE Standard Luminosity Curve andintegrating the area under the resulting curve. Thisyielded 0.0201 light watts cm-2 . Converting to units ofilluminance by using 680 lu/w, the reciprocal of themechanical equivalent of light, we obtained a value ofthe solar illuminance outside the atmosphere of 12 7005t400 lumens ft-2 . This supersedes the preliminaryvalue 11 500 reported by Dunkelman and Scolnik,3 0 the

0L. Dunkelman and R. Scolnik, J. Opt. Soc. Am. 42, 876(1952).

change being caused by the adoption of Johnson'svalue of the solar constant, as described earlier. Theestimated error includes the probable error in solarconstant reported by Johnson. It is noted that this resultis in agreement with the recent work of Karandikar,who computed the solar illuminance from measurementsof the luminance of the solar disk. The value obtainedwas 12 200:1:300 lumens ft-2 .

The efficiency of the sun in producing visible lightmay be defined as the ratio of the solar illuminanceexpressed in light watts cm-2 to the total solar irradi-ance which is the solar constant expressed in wattscm-2, whence

E = 0.0201/0.1394= 0.144.

This value is dependent only upon the solar spectralintensity distribution and the CIE Standard Luminositycurve. It is independent of the absolute value of thesolar constant and of the value of the mechanicalequivalent of light.

ACKNOWLEDGMENTS

We wish to express our thanks to Dr. RichardTousey, in whose group this work was conducted, forhis many helpful suggestions and to Francis S. Johnsonwho suggested this investigation. We are grateful alsoto the U. S. Forest Service, the Steward Observatory,and the Trico Electric Cooperative of Tucson for aidin determining the location for the experiment and forproviding the necessary facilities. Finally our thanks areextended to Helen Gish Drake of Summerhaven Lodgeand to Forest Ranger John Brinkley for their co-operation.

"<-4

367April 1959