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Data on total and spectral solar irradiance

Ann T. Mecherikunnel, James A. Gatlin, and Joseph C. Richmond

This paper presents a brief survey of the data available on solar constant and extraterrestrial solar spectralirradiance. The spectral distribution of solar radiation at ground surface, computed from extraterrestrialsolar spectral irradiance for several air mass values and for four levels of atmospheric pollution, is also pre-sented. The total irradiance at ground level is obtained by integration of the area under the spectral irra-diance curves. It is significant that, as air mass increases or as turbidity increases, the amount of energy inthe infrared relative to the total increases and that the energy in the UV and visible decreases.

1. Introduction

Quantitative data on solar irradiance at ground lo-cations are needed in the study of atmospheric optics,pollution, solar energy for energy conversion, climato-logy, and remote sensing. Total and spectral solar ir-radiance at a particular site at ground level depends onseveral parameters such as altitude, latitude, longitude,earth-sun distance, cloudiness, time of day, and atmo-spheric attenuation due to ozone, water vapor, carbondioxide, and dust particles. Solar irradiance data atground level necessary for several applications can beobtained from the extraterrestrial solar spectrum bycomputing the fractional loss due to various atmo-spheric constituents. Accurate data on extraterrestrialsolar irradiance and atmospheric attenuation factorsare needed for this.

This paper will present a brief account of the currentstatus of solar constant and air mass zero solar spectralirradiance values. Computed values of solar spectralirradiance at ground level for air mass values, 1, 1.5, 2,3, 4, 7, and 10 for four sets of af, Angstr6m turbidityparameters, 20 mm of precipitable water vapor, and3.4-mm ozone from NASA/ASTM Standard solarspectral irradiance are also presented in graphicalform.

11. Solar Constant and ExtraterrestrialSolar Spectral Irradiance

Solar radiation is usually described in terms of solarspectral irradiance and the solar constant. The solarconstant is the amount of total radiant energy (usually

Joseph Richmond is with U.S. National Bureau of Standards,Washington, D.C. 20234; the other authors are with NASA GoddardSpace Flight Center, Greenbelt, Maryland 20771.

Received 19 August 1982.

expressed in W * m-2) received from the sun per unittime, per unit area exposed normal to the sun's rays, atthe mean earth-sun distance in the absence of theearth's atmosphere. Air mass zero solar spectral irra-diance is the distribution of this power (surface) densityas a function of wavelength.

Earlier estimates of the solar constant and the solarspectral irradiance were obtained from ground-basedmeasurements. The most extensive investigations ofsolar constant and the spectral distribution of solarradiant flux were those made by the Smithsonian In-stitution from high altitude ground stations between1902 and 1950. The Moon' and Johnson 2 values of thesolar constant and the spectral energy distributionwidely used before 1970 were derived from the Smith-sonian data, with additive corrections for the UV andIR regions of the solar spectrum attenuated by the at-mosphere. Measurements made from the ground arelimited in accuracy due to the strong and highly variableabsorption and scattering characteristics of the atmo-sphere. In recent years several attempts have beenmade to minimize the errors due to the atmosphere bymaking measurements from high altitude platformssuch as aircraft, balloons, satellites, and rockets. Ex-perimental results obtained independently by severalgroups of investigators from high altitude flights duringthe 1960s resulted in the solar constant and air masszero solar spectral irradiance data 3 4 widely used in thedesign criteria for NASA space vehicles 5 and as an en-gineering standard of the American Society of TestingMaterials (ASTM).6 The NASA/ASTM6 standardsolar constant value 1353 i 21 W m-2 is an average ofthe results derived from nine series of measurementsusing different types of total irradiance detector, dataacquisition, and analysis technique. Two major sourcesof uncertainty are associated with the standard values.(1) Most of the solar irradiance determination requiredcorrections for the residual atmosphere and water vaporabove the aircraft or balloon and the transmittance of

1354 APPLIED OPTICS / Vol. 22, No. 9 / 1 May 1983

the optical window materials. (2) The radiation de-tectors used in these measurements were calibrated inthree different radiometric scales: the InternationalPyrheliometer Scale 1956 (IPS 56), the Absolute Elec-trical Units Scale, and the Thermodynamic KelvinTemperature Scale.7 The difference between the threescales is not known accurately. Only three of the ninetotal irradiance measurements were performed withelectrically calibrated absolute radiometers. The otherdeterminations were made with pyrheliometers or ac-tinometers radiometrically calibrated by comparisonwith a reference instrument using IPS 56 scale. Thestability of the reference instruments for the IPS 56 wasquestioned at the International Pyrheliometric Com-parisons, and a series of solar irradiance determinationswith several absolute cavity radiometers resulted in anew pyrheliometric scale which is -2.2% higher than theIPS 56.8,9 Since the different experimental results usedin the derivation of the NASA/ASTM standard valuecould not use the same primary standard for the cali-bration of the irradiance detectors, a reduction to acommon reference is needed to enable direct compari-son of the values obtained by different experimenters.Frohlich reviewed the various solar constant determi-nations and reduced the values to a common referencescale (the Solar Constant Reference Scale, SCRS), andthe most probable value of the solar constant is pro-posed to be 1373 W * m 2 .8 A similar reduction of theindividual measurements used in the derivation of theNASA/ASTM standard solar constant to a commonreference scale based on electrical power equivalencewas reported by Forgan.10 According to him the solarconstant value is 1375 W . m- 2 compared with 1353W.m- 2 .

The uncertainty of solar constant measurements hasimproved significantly of the order of 0.5% in recentyears, because of the development of self-calibratingpyrheliometers and the availability of space measure-ments. Several long-term solar constant monitoringprograms have been started in recent years to measurethe solar constant and its variability with self-cali-brating pyrheliometers from space to study the effectof solar irradiance variations on weather and climate ofthe earth. The results from Rocket Experiments of1976,11 1978,9,12 1980,9,13 Nimbus-7 Earth RadiationBudget Experiment (ERB),14 and the Active CavityRadiometer Irradiance Monitor (ACRIM)15 on theSolar Maximum Mission (SMM) satellite are summa-rized in Table I.

Table 1. Average Value of Solar Constant Measured from Rocket Flights,Nimbus-7 ERB and the SMM ACRIM Experiments

Solar Estimatedconstant error

Platform/sensor Year W . m- 2 (%)

Rocket experiment" June 1976 1367 +0.5Rocket experiment/ACR 9 Nov. 1978 1367.6 +0.5Rocket experiment/ACR 9 May 1980 1367.8 +0.5Nimbus-7/ERB,H-F1 6 Nov. 1978 to 1372.7 40.4

July 1981SMM/ACRIM 9 Feb. to 1367.7 +0.2

July 1980

In many applications of solar irradiance values, bothtotal and spectral, a question of major concern is thevariability of these values. The solar constant is de-fined for the average sun-earth distance. As the earthmoves in its elliptical orbit around the sun, the totalsolar energy received varies by t3.5%. There are alsosmall and undetermined variations due to cyclic andsporadic changes in the sun itself. Temporary de-creases of 0.1-0.2% in solar total irradiance lasting fora few days and associated with the passage of sunspotgroups have been reported by Willson et al. 9 1 5 andHickey et al. 14 16 But the analysis of the solar constantdeterminations from 1965 to 1980 by Frohlich andBrusa revealed no indication of any significant changein the solar constant value.17

Ill. Air Mass Zero Solar Spectral IrradianceSolar constant determination from space with active

cavity radiometers has significantly reduced the un-certainty in solar constant values of the order of t0.5%.But no definitive measurements of solar spectral irra-diance have been made from space. Measurements ofextraterrestrial solar spectral irradiance in selectedspectral bands in the UV, visible, and near-IR regionsof the solar spectrum have been made by filtered solarchannels of both Nimbus-6181 9 and 720 ERB experi-ments. Preliminary analysis of the data from bothERB instruments shows that the filters have undergonesignificant degradation in space.21 22 According toPierce and Allen23 the best presently available data ofsolar spectral irradiance in the interval from 0.3 to 3.0um are those given by Thekaekara et al.,3,5 Arvesen et

al. ,24 and Labs and Neckel.25 The spectral irradiancedata reported by Thekaekara et al. and Arvesen et al.are based on NASA Convair 990 aircraft (11.6-12.5-kmaltitude) measurements between 1967 and 1969. Labsand Neckel data for the spectral region from 0.33 to 1.25,m were derived from absolute intensity measurementsat the center of the solar disk made in the early 1960sat Jungfraujoch Scientific Station (3.6-km altitude) inSwitzerland. Beyond 1.25 m, Labs and Neckel usedthe values reported by Pierce and Allen.23 26 Neckeland Labs recently published a revised version of thesolar spectral irradiance data given in Ref. 25 using moreaccurate values for center to limb variation of the solardisk in the conversion of center disk intensities intomean disk intensities.27

Figure 1 shows a comparison of solar spectral irra-diance values for the 0.3-1.2-,um solar spectral regionreported for NASA/ASTM3,5,6 Standard (Thekaekaravalues) and Neckel and Labs revised solar spectrum.27

There are significant differences between the twospectral curves, especially for the 0.53-0.8-,m spectralregion where differences amount to more than 10%.However, the revised data of Neckel and Labs for the0.3-0.5-gm spectral region bring it closer to theNASA/ASTM values. The 0.2 7-2.6-gm solar spectralinterval contains over 96% of the sun's energy, and themajor input into the energy budget of the earth comesfrom this region.

1 May 1983 / Vol. 22, No. 9 / APPLIED OPTICS 1355

I

I-

E

w

0.C)

U,

2000 Z - NASAIASTM (THEKAEKARA)2000 si- -NECKEL AND LABS

II'.1500 11

1000 X

a I __ _ x

0.2 0.5 1.0 1.2WAVELENGTH MICROMETER

Fig. 1. Comparison of the NASA/ASTM and Neckel and Labs datafor air mass zero solar spectral irradiance.

A conclusion that can be drawn from this survey isthat the uncertainty in solar constant is about +0.5%.The uncertainty in the best available data on solarspectral irradiance is as much as 10-15% at certainwavelengths.

IV. Solar Total and Spectral Irradiance at GroundLevel

The air mass can be computed from Equation (2):

m = secz = (sinO sin5 + cosO cos5 cosh)- 1 , (2)

where 0 is the latitude of the place, is the solar decli-nation for the day, and h is the hour angle of the sun.

The Rayleigh optical depth cl and the ozone opticaldepth C3 used in this study are based on the data de-veloped by Elterman.3 0 The total amount of ozone ina vertical path is assumed to be 0.34 cm (at STP). Theattenuation coefficient c2 due to turbidity is given byEq. (3):

C2 = ,a (3)

where X is the wavelength in micrometers, a and : areAngstr6m turbidity parameters related to the size dis-tribution and the number of particles of aerosols perunit volume of air.31

In the infrared there is the selective absorption bypolyatomic gaseous constituents of the atmosphere,mainly water vapor and carbon dioxide, and the con-tinuum attenuation due to scattering and absorptionby particulate matter and water droplets. The selectiveabsorption is characterized by many thousands of linesof the vibration-rotation spectrum of the molecules.The total effect over finite bandwidth is not simpleenough to be expressed by Eq. (1). In the infrared, Eq.(1) has to be modified as

A. Atmospheric Attenuation of Solar Radiation

Direct solar radiation reaches the surface of the earthconsiderably weakened and with its energy distributiongreatly changed due to the attenuation along its paththrough the atmosphere. The attenuation is small inpure air but increases with the amount of contaminationor turbidity due to variable components such as dust,aerosol particles, and water vapor. The attenuationincreases both with extinction coefficient and the op-tical absolute air mass. Therefore, the solar radiationis found to vary with time of day, season, geographicallatitude, and altitude, even when the extinction coef-ficient remains constant. It is necessary to know theamount of attenuation of direct solar radiation in theatmosphere to compute solar flux available at theearth's surface. The discussion given below is takenfrom Thekaekara.282 9 Throughout most of the solarspectrum the attenuation of a monochromatic beam oflight is governed by the logarithmic decrement lawknown as Bouguer's law:

E = E exp- (cl + 2 + C)m, (1)

where E° and E are spectral irradiance at a givenwavelength outside the atmosphere and after trans-mittance through air mass m, respectively. The coef-ficient c1 is due to Rayleigh scattering, c2 is due to tur-bidity, and C3 is due to ozone optical depth. The airmass m is defined as the ratio of the slant path lengthof the solar rays through the atmosphere to the pathlength if the sun were in the zenith. The ratio is usuallyequal to the scant of the solar zenith angle z, except forlarge zenith angle (z ' 620) where atmospheric refrac-tion and earth curvature need to be taken into account.

E = E' exp[-(c, + c2 + c3 )m]Txi, (4)

where Ti is a transmittance factor to account for themolecular absorption bands. No single expression isapplicable to all the absorption bands. T can have oneof the three forms:

Tx = exp[-c 4 (Wm)1/2 ; T = exp(-c5Wm); T3 = 1 - 6m12

,

(5)

where m is the air mass, W is the amount of precipitablewater vapor along the path in millimeters, and C4, C5,and c6 are empirical constants.32

The expression Txj is for a strong random model andholds true in the main body of a water vapor absorptionband. The expression TX2 is for a weak random modeland holds true for the wings of the bands and for smalloptical depth. The third expression, T 3 holds truewhere the effect of water vapor is negligible but whereother molecular species such as CO2 and 02 in the at-mosphere influence the transmission values.32-34 Forthe present computation the spectral transmissionvalues Tx1,TX2,TX3 for the 1.0-4.0-Am spectral regionare based on the empirical constants reported by Gatesand Harrop.32 For the 0.7-1.0-gm spectral region, thespectral transmission values are based on the spectralparameters and the spectral water vapor transmissiondata reported by Koepke and Quenzel.35

B. Computation from Extraterrestrial Solar Spectrum

The solar irradiance received on a surface has twocomponents: (1) that received directly from the sunand (2) that diffused by the sky. Direct solar spectralirradiance at ground level can be computed from theextraterrestrial solar spectrum and the atmospheric


E

-

c

t1600t\XXA Mass Zero Solar Spectru. 1353 Wi'

2

Black Body Cura 5762K INormolized) 1353 Win2

t \ vArMassTmo21SoarSpatr-r0.66.0.B5H2

O2Om 03 0.34cm.

g~l A< A Mass Two Solar Sp-tr-n Withot Moleclar Absorplio

0

63

UV VISIBLE 115

2 -

j 360H0.O 2.00.8 1.4

WAVELENGTH (MICROMETER)

2.0 2.6

Fig. 2. Spectral distribution curves related to solar irradiance.

ES

5

It

0)

2250 Solar Spectral Irdiace for Diferent Air Mas V. U.S.Standard Atnosphere, Ptrecipiteba HaO Vapo 26 mm. Ozone

2000 3.4 mm 11. 3= 0.02)

Air Ma Idadianca

-0 1353 W.1_ 91.0

/ -,\ 1.5 9.0.I |/ a t - 3726.9

1000

250 \ ,Zz..

0.0~~~~44 ~ Z'1.0 1.5 2.0


Fig. 3. Solar spectral irradiance for different air mass values as-suming U.S. Standard Atmosphere, 20 mm of precipitable watervapor, 3.4 mm of ozone, very clear atmosphere (a = 1.3; f = 0.02).

optical parameters using Eq. (4). An example of theresults of such computation is shown in Fig. 2. It showsfour curves related to solar spectral irradiance. Thetopmost curve is the NASA/ASTM Standard solarspectral irradiance for air mass zero, that is, irradianceoutside the earth's atmosphere at the average sun-earthdistance on unit area exposed normal to the sun's rays.It gives the E° of Eq. (4). The area under the curve isthe solar constant 1353 W m-2 . Below is the nor-malized blackbody curve. The curve with many sharpdips is the solar spectrum for air mass 2, that is, forspectral irradiance on unit area on the ground at sealevel exposed normal to the sun's rays, assuming rela-tively clean air, no clouds, and the sun at 600 from thevertical. This curve is computed from the air mass zerocurve for atmospheric parameters, 20 mm of precipi-table water vapor, 3.4 mm of ozone, and turbiditycoefficients a = 0.66, / = 0.085. The total direct solarenergy transmitted by the atmosphere in this case isobtained by integrating the area under the curve andit is found to be 726.9 W m- 2 or 53.7% of that receivedabove the atmosphere. The relatively smooth curve is

E

E

ZS

_ ~~~~~~~~~~Solar Spectral Iradiance Io Diferent Air Mas Vales. USStandard Atmosphere., PrecipiableWat Vapor 20 mn, Ozone

000 hi3_4_13 ._4)

Ai, Mas Irradiance

500 33 We-a

Soo1 96.22 787.7

00

1.0 1.5 2.0


Fig. 4. Solar spectral irradiance for different air mass values as-suming U.S. Standard Atmosphere, 20 mm of precipitable water

vapor, 3.4 mm of ozone, clear atmosphere (a = 1.3; / = 0.04).

'El

2 1

W E 1.5


Fig. 5. Solar spectral irradiance for different air mass values as-suming U.S. Standard Atmosphere, 20 mm of precipitable watervapor, 3.4 mm of ozone, turbid atmosphere (a = 0.66; fl = 0.085).

-E

E

4E

41II5

Solar Spectral Irradianca for Diftamt Air Mass Values. U.S.Standard Atmosphere. Precipitabla W.ar Vapor mm. Goe

2000 0 -i 3.4mmt(o.6B. 1=.17)

| | | | 5 Alr M. Irradiance

'\ _-O 135) Wma

-50 -....1 829

1.5 697.8

50 00 -3 _ _ _ _ __8

0.01.0 1.5 2.0


Fig. 6. Solar spectral irradiance for different air mass values as-suming U.S. Standard Atmosphere, 20 mm of precipitable watervapor, 3.4 mm of ozone, very turbid atmosphere (a = 0.66; / =

0.17).


Solar Spectral Iradiacce for Difares Air Mass Vale. U.S.Standard Atmosphere Precipitable Water Vapor 20mm

2000 _ l Air . 34m ( 066. =3 085} i Mass.. Irradiance

0 1353

1 934

1.58149

2 7269

0.0

50 0 - --__ _ __ _ __ _ _

I

I

0.2 0.3 0.5, 2.5 2. 7 3. 0

0.3 0.5 2.5 2.7 3.0

0.2 0.3 0.5 .C 2.0 2.3 2.7

0.2 0.3 0.6 2.5 2.7 3.0

the computed air mass 2 solar spectrum in the absenceof molecular absorption.

Spectral irradiance values for air mass 1, 1.5, 2, 3, 4,7, and 10 computed from the NASA/ASTM standardsolar spectral irradiance outside the atmosphere (E°)(assuming U.S. Standard Atmosphere), 20 mm of pre-cipitable water vapor, and 3.4 mm of ozone using Eq. (4)are given in Figs. 3-6 for the 0.3-4.045-gm wavelengthrange. As the solar zenith angle or air mass increasesthe transmitted energy decreases, and the wavelengthcorresponding to the maximum energy is found to shiftto the red end of the spectrum.

It has been discussed earlier that the ca,/ Angstromturbidity parameters are related to the size distributionand the number concentration of aerosol particles in theair. When the wavelength is given in micrometers, thevalue of the Angstr6m turbidity coefficient correspondsto the attenuation coefficient c2 at gim. Theoreticallythe exponent xx should vary from 0 to 4, but experi-mental results show ranging from 0.5 to 2.5 andAngstr6m suggests an average value of 1.3.31,36,37 Ancv value of 0.5 indicates a greater than average propor-tion of large particles. 3 8

Solar spectral irradiance values given for av = 1.3,/= 0.02 in Fig. 3 correspond to a very clear atmosphere.Figure 4 shows spectral irradiance values for av = 1.3, = 0.04. A higher value of corresponds to a more tur-bid atmosphere. Considerable higher levels of pollutiontypical of large cities and industrial centers are repre-

_500 Air Mass 1.5 Solar Spectral Irradiance For Foot Levels ofAtmopheric Torbidity

1250 2 2 HO 20 mm. 0 3.4mm

3 Torbidit C-H~ieot

0.3 0.5 1.0 .524 1. 1.32 0022.F1.3 0.4r3. 0.66t0r0

1000 4. 0.66 0.t7

.5 0

0.3 0.5 1.0 1.5 2.0 2.5 2.7 3.0


Fig. 7. Air mass 1.5 solar spectrum for four levels of atmosphericturbidity.

sented by a = 0.66, = 0.085, and av = 0.66, = 0.17 areshown in Figs. 5 and 6, respectively. 2 8 As the turbidityincreases, the transmitted energy decreases, and thisdecrease is more pronounced in the visible region of thespectrum. Figure 7 illustrates the spectral effects of thediffering amounts of aerosol represented by cv,/Angstr6m turbidity factors. Detailed spectral irra-diance data for air mass values 1, 1.5, 2, 3, 4, 7, and 10for four sets of cv,/ Angstrom values are available inMecherikunnel and Richmond.39

Total direct solar irradiance at ground level is ob-tained by integration of the area under the solar spectralirradiance curves. Table II gives direct solar total ir-radiance as a function of air mass and cv,: Angstromturbidity parameters. It is significant that, as air massincreases or as turbidity increases, the transmitted en-ergy decreases, and this decrease is more significant inthe ultraviolet and visible portions of the spectrum thanin the infrared. Solar spectral measurements in theterrestrial environment reported by Bird et al. 4 0 andMatson et al. 4 1 support this. The spectral irradiancevalues presented in the preceding sections refer only tothe direct beam solar spectral irradiance received atground level and do not include any scattered or diffusesolar radiation. As in the case of direct irradiance, thediffuse radiation received at ground surface depends onthe optical properties of the atmosphere, the sun's ele-vation in the sky and cloudiness. Computed values ofthe energy distribution in the diffuse radiation spectrumfor various model atmospheres have been reported byMatson et al.41 and Dave et al.42 The ratio of the dif-fuse to direct solar radiation is very high in the UV anddrops off rapidly in the visible and IR. Representativevalues of this ratio based on Ref. 41 for a model atmo-sphere (air mass 1.5, ozone 3.4 mm, precipitable watervapor 14.2 mm, turbidity 0.27, and ground albedo 0.2)are 162% at 310 nm, 76% at 400 nm, 48% at 550 nm, 25%at 650 nm, and 16% at 900 nm.

V. Results and Discussion

The spectral distribution of direct sunlight at sea levelon the earth's surface is computed as a function of airmass and turbidity parameters from NASA/ASTMstandard solar constant and solar spectral irradiancevalues. The computational procedure is essentially thesame as that used by Thomas and Thekaekara,29 withmodifications for the spectral water vapor data for the0.7-1.0-gm spectral region. The modification involvesthe incorporation of the spectral water vapor data re-

Table II. Direct Solar Irradiance E in W m-2 Received on the Ground at Sea Level for the O.3-4.045-,im Spectral Region as a Function of Air Mass anda,/3 Angstrom Turbidity Parameters; U.S. Standard Atmosphere, 20-mm Precipitable Water Vapor, and 3.4-mm Ozone

Total solar irradiance E,\, W -2

Angstrom turbidityfactors Air mass/solar zenith angle (z°)

a /3 1/0° 1.5/48.2° 2/60° 3/70.5° 4/75.50 7/81.80 10/84.3°

1.3 0.02 991 908 839 727 639 456 3421.3 0.04 958 865 788 663 569 378 2660.66 0.085 934 815 727 587 482 282 1740.66 0.17 829 698 592 439 325 147 72


ported by Koepke and Quenzel35 for the 0.7-1.0-gmspectral region, and the computational details areavailable in Mecherikunnel and Richmond.39

The spectral distribution of solar irradiance at groundlevel based on various model atmospheres has been re-ported by several investigators. The spectral datapublished by Moon in 19401 was extensively used inengineering applications. The spectral energy curveswere calculated from the then available estimates ofatmospheric attenuation and the extraterrestrial solarspectrum. The spectral distribution of solar radiationat the earth's surface reported by Gates in 196643 wasbased on the improved extraterrestrial solar spectrumderived by Johnson.2 The extinction coefficients forthe infrared absorption bands have been derived byGates and Harrop from observations of the solar spec-trum.32 The Gates procedure has been used by The-kaekara28 and Thomas and Thekaekara2 9 to producesolar power density spectra at ground surface from theNASA/ASTM Standard solar spectrum. The spectraldata reported by Gates and by Thekaekara et al.showed a strong attenuation between the 0.84- and0.95-gm spectral region; whereas the experimental in-vestigations in the efficiency of solar cells by Matson etal.41 and by Ireland et al.

4 4 showed only absorption fora narrowband at 0.94 gim. The discrepancy is tracedto water vapor absorption data for the spectral regionbelow 1.0 gm. In the present computations, the spec-tral water vapor transmission data reported by Koepkeand Quenzel35 have been used, and the spectral irra-diance data at ground level presented in the precedingsections are in agreement with the experimental andcomputed terrestrial spectral data reported in Refs. 41and 44.

The increase in solar zenith angle or turbidity resultsin the decrease of the total amount of solar irradiancereceived at the ground level, and the observed decreasein irradiance is more significant in the UV and visibleregions of the spectrum than in the infrared. Thewavelength corresponding to the maximum energy isfound to shift to the red end of the spectrum with anincrease in solar zenith angle or air mass.

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Sci. 6, 229 (1971).4. M. P. Thekaekara, Sol. Energy 14, 109 (1973).5. Anon, "Solar Electromagnetic Radiation," NASA Spec. Publ.

8005 (1971).6. Anon, "Standard Specification for Solar Constant and Air Mass

Zero Solar Spectral Irradiance ASTM Standard," E 490-73a, 1974,Annual Book of ASTM Standards, Part 41 (ASTM, Philadel-phia, Pa., 1974).

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(Colorado Associated U.P., Boulder, 1977).9. R. C. Willson, Sol. Phys. 74, 217 (1981).

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Radiation, Davis, Calif. (1978), p. 91.19. J. R. Hickey and F. J. Griffin, "Space Simulation," NASA Spec.

Publ. 379 (1975).20. The Nimbus 7 Users' Guide, Goddard Space Flight Center,

Greenbelt, Md. (Aug. 1978).21. R. E. Predmore, H. Jacobowitz, and J. R. Hickey, Proc. Soc.

Photo-Opt. Instrum. Eng. Technical Symposium, 338, in press(1982).

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the Sun-Solar Technology in the Seventies, Winnipeg, Canada(American Section of the International Solar Energy Society, Fla.1, 338, 1976).

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New York, 1958).34. W. M. Elasasser, Harvard Meteorological Studies No. 6 (1942).35. P. Koepke and H. Quenzel, Appl. Opt. 17, 2114 (1978).36. G. W. Sadler, Sol. Energy 21, 339 (1978).37. H. A. McCartney and M. H. Unsworth, Q. J. R. Meteorol. Soc. 104,

699 (1978).38. T. Bilton, E. C. Flowers, R. A. McCormick, and K. R. Kurfis,

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