NRL eport 601

Measurement of Spectral Radianceof the HIzo Sky,

a~ -- 9

I-4,

NAV;AL RESEARCH LABORATORY

E Q Washinelonk, D

lhix do=wrtnt baa bftwiapir wea for palliczeleujeiM~ sale; its diaWtriiti is iacizmted.

I?

NRL Report 6615

Measurement of Spectral Radianceof the Horizon Sky

G. L K'ZsrmCK AND J. A. CURCIO

Radiorne.7 BranchOptical Phjsks Dvision

ij

1! December 4, 1967

I

0NAVAL RESEARCH LABORATORY

Washington, D.C.

I This document has been approved for public release and sale; its distribution is unlimited.

!f

7 CONTENTS

Abstract iProblem Status iJ Authorization i

I INTRODUCTION1

INSTRUMENTATION1

CALIBRATION

FIELD MEASUREMfENTS 2

RESULTS 2

CONCLUSION 13

ACKNOWLEDGMENT 13

REFERENCES 13

-r

I

ABSTRACT

Spectral radiance data were obtained for a rectangular por-tion of the horizon sky wider various weather conditions and atdifferent solar positions. Representative curves were selectedfrom 79 spectral radiance measurements which covered mete-orological ranges (at 665 mg) from 10 to 106 km and solar alti-tudes from -6 to 74 degrees. The north horizon at noon, with a30 km or greater meteorological range, typically produced a peakspectral radiance of 5 gw/cm2-srad-mng. The maximum spec-tralradiance value obtained inthe studywas 38 ILw/cm 2-srad-mgfor the south horizon in winter. The reciprocal dispersion variedfrom 1.8 mg/mm at 400 mg to approximately 8 mg/mm at 1000mg.A mixture of fog and haze produced a spectral radiance curvewith two maxima.

PROBLEM STATUS

This is an interim report; work on the problem is continuing.

AUTHORIZATION

NRL Problem AG2-17Project RR 004-02-42-5152

Manuscript submitted July 14, 1967.

1t

MEASUREMENTS OF SPECTRAL RADIANCEOF THE HORIZON SKY

INTRODUCTION

The spectral radiance of small areas of sky in the infrared region has been studiedby Bell, et al. (1), arriong others. However, very little data have been published on thespectral radiance of the horizon sky in the visible region. Such information is desirablein connection with over-the-horizon scattering studies now being made.

The spectral region of interest extended from the ultraviolet transmission limit ofglass (- 400 mgi) to 1000 mgi. Rectangular portions of the horizon sky were observed attwo or three well-spaced bearings, mainly over water. A method of calibration wasdevised to minimize experimental error, and curves were obtained which show the spec-tral radiance as a function of wavelength under different conditions, some typical andothers atypical. A short study was made of the effects that polarization of the horizonsky would have on the measurements because the monochromator exhibits polarization

* preference. Correction of the radiance data was unnecessary.

Reports on this work have been presented elsewhere (2,3) but with only a few spec-tral radiance curves included. it was felt that all the reduced data should be compiledfor reference purposes. From a total of 79 measurement runs, 45 were selected forreduction and are presented in this report.

INSTRUMENTATION

A Leiss double monochromator with glass prisms was used to disperse 'Ie radiation.The reciprocal dispersion varied from 1.8 mp/mm at 400 min to approximately 8 mg/mmat 1000 mg when a' three slits were fixed at 0.6 mm for all measurements. A field stopin front of the entrance slit limited the field of view to 75 mrad (4.3 degrees) wide by 25mrad (1.4 degrees) high. A beam-splitter reflected a portion of the incoming radiationto the monitor, an RCA 6342 (S-11 response) photomultiplier with a red filter.

The wavelength scan was divided into two parts: an RCA 7265 (0-20 response) pho-tomultiplier was used from 380 to 760 mg, and a Dumont 6911 (S-1 response) photomul-tiplier was used from 610 to 1000 ma. The dc output voltage of the detector was ampli-fied and recorded on one channel of a two-channel Sanborn recorder; the other channelwas used for the monitor output.

CALIBRATION

The relative spectral response of the monochromator-detector system was measuredwith a working-standard lamp which had been calibrated against an NBS standard of spec-tral radiance. The signal, due to the light transmitted by the monochromator, divided bythe corresponding relative spectral radiance value for the lamp gave the relative spec-tral response of the system at a given wavelength.

h1

P6%

KNESTRICK AND CURCIO

A normalizing absolute radiance measurement was made at one wavelength at thetime of each run by viewing with an optical pyrometer the same area of sky observedwith the monochromator. The "temperature" thus obtained was converted to radiance at665 mg, the effective wavelength of the pyrometer (4).

Three sources of error were possible in the calibration: the standard lamp, theelectronics, and the pyrometer. The error in the relative spectral distribution of thelamp is estimated to be i 8%,, niade up of a ± 6% error in lamp calibration and a • 2%error due to lamp current setting. The error due to drift or fluctuations in the elec-tronics during the 90 sec required for a wavelength scan is believed to be negligible. Arecorder drift of 0.5% was de' ectable. The radiance values derived from pyrometer read-ings, most of which were in the neighborhood of 1000 0C, -had an uncertainty of ±4%. Atsunrise and sunset, when brightness temperatures were of the order of 800°C, the uncer-tainty was approximately ± 20%. Monochromator polarization was measured using anunpolarized tungsten light source and a Glan-Thompson prism. The light emerging fromthe monochromator was found to have partial horizontal polarization at all wavelengths,the degree of polarization 3 depending on wavelength. Calculation of the degree ofpolarization using the expression = (E,, - Erin)/(Emax + E i. ) yielded values of0.34 ± 0.07 from 460 to 1000 mu.

Since the absolute radiance of the horizon was measured at 665 mg with the pyrom-eter, the radiance values at all other wavelengths included the polarization effect at thenormalizing wavelength. Therefore, it was necessar'y to determine only the cnange as afunction of wavelength in the polarization effect of the monochromator relative to thepolarization of the sky radiation. The maximum variation observed was -5% of tWeradiance at a bearing of 100 degrees, a solar azimuth of 200 degrees, and a wavelengthof 462 mp, which was the lowest wavelength of polarization measurement The averagevariation was within ± 1% and, therefore, no correction for polarization effects wasdeemed necessary.

FIELD MEASUREMENTS

Measurements were made at three locations, all of which were approximately thesame height (100 ft) above sea level. The first measurements, made at NRL, were of thehorizon sky over land. Subsequent measurements were made of the horizon sky overwater on the western shore of the bay at the Chesapeake Bay Division (CBD) of NRL andat Tilghman Island, Maryland, on the eastern shore.

RESULTS

All spectral radiance observations were made on 16 different days. The range ofsolar altitudes covered was -6 to 74 degrees, and the azimuth of the sun varied from 58to 273 degrees. The meteorological range at 665 mti varied from 10 to 106 km and therelative humidity from 41 to 90 percent. The radiance distributions with wavelength areshown in Figs. 1-5, in order of diurnal solar altitude variation. In general, the spectralradiance aistribution varied with solar altitude as expected: the peaks of the curves oc-curred at wavelengths between 500 and 600 mg for higher sol-r altitudes and shifted to-ward red wavelengtns at low altitudes and/or near the sun's azimuth. One major excep-tion to this behavior (Fig. lb) occurred when fog and haze particles produced a maximumat about 450 ing equal to or greater than the red maximum. Figure Ic is a curve for thesame general direction on a clear day and shows a more typical sper ral radiance curvefor the sun just below the hcrizon, giving a peak value at 1000 mp of 0.65 gw/cm2 -srad-mg. The points which do not lie on the curve are a result of error in the relative re-sponse calibration. This error was greatest when the slope of the response curves wassteepest; this situation occurred in the overlap region of the two detectors.

NRL REPORT 6615

0.24 -__ I 2TIME 0645 0650

022 - SUN ALT -4' -3*

SUN AZ 114* 115*0,20- /1 BEARING I00 IO

0.18 -TEMP: -4C

0CLOUD COVER- 0

- 014-

Pd 0.12

LZ 0106C.

WAVLEGT (2)oo -

0.0S -02-

340 400 500 600 700 800 900 1000

WAVELENGTH I Am

Fig. la -Spectral radiance of clear eastern horizon

over water before sunrise

[( j~li SUN ALT. - "°9 001I SUN AZ. 58-" 0 61"011--BEARINGr 90" 90" 90

i MR: 8 kmTEMP 20*C

OI5- REL HUMIDITY 90%I\ CLCUD COVER 100% (NO HORIZON)

0120- (3)

u 010 131

00

(2).006-

0.04-

002 --

340 400 500 600 700 800 900 1000WACEL ENGTH (ms)

Fig. lb - Spectral radiance of fog and haze over wdternear sunrise showing additional maximum in blue region

KNESTRICK AND CURCIO

_ 08 TIME' 0655 - 0701

E 07- SUH ALT -2!5 TO-1.5SUN AZ." 116" TO i7

* BEARING: ICO"06 MR 60 km06 ---

TEMP.:-4"CREL HUMIDITY:

E 05 CLOUD COVLR"O

!ISI :L 04

WI~03S02-0

_ '.. 0 405060700 800 900 1000~~WAVELENGTH (n/g)

Fig. ic - Spect~ai radiance of clear easternhorizon over water before sunrise

S80

< TIME 0705

70- SUN ALT 2°6SUN AZ 89 °

" BEARING 120"60 MR24 km= 0-TEMP 14 °

REL HUMIDITY 58%'CLOJO COVER: 10% (AT HORIZON ONLY)

0

0--

340 400 500 600 700 800 900 1000

WAVELENGTH (mpd

Fig. Za - Spectral radiance of southeasternhorizon over land after sunrise

FNRL REPORT 6615

20_ iI 2

18- TIME 0650 0720SUN ALT. -03 5!5SUN AZ. 17 191I

I BEARING 0' 240 "0 07 20T 6 MR: 20 km 0 2 40

TEMP 14°C 02REL.HUMIDITY 58%S14 -- CLOUD COVER: 10% (AT HORIZON ONLY) H0 H20

~~E 1.2- . OOOQiF qG E T '4;(2

I. -KH 041

if I G

0.8 C (1

0.6--

340 400 500 600 700 800 C-00 1000WAVELENGTH (mi)

Fig. Zb - Spectral radiance of northern horizon at sunriseand southwestern horizon after sunrise over land

2.4~TIME 0605 :0615 0635

2SUN ALT 2.5 45 7?5SUN AZ 63 64 67'

2.0- BEARING 900 90" 1iE MR 8kin

1.8 TEMP 2I*C

REL HUMIDITY. 90%1.6 - CLOUD COVER. 10 TO C j rr'JCK HAZE)

EI4-

3:(3

08-

-j

06 -

o. 04 (2)

0.2(I

0-- _340 400 500 600 700 800 900 1000

WAVELENGTH (mI)

Fig. 2c - Spectral radiance of eastern horizon aftersunrise showing amoLnt of change in 30 minutes

6 !-NESTRICK .A.ND CUJRCIO

12-01 2j,.9'81 , U Z)

- 10-3 183'Ins-go- REL KnEDITY

-a V CLO.OVcOER J/I

z 5.0

4D

- 402.0 (3) ,

340 40050 070 090rMW-11ELENGTH (np0)

Fig. 2d - Spectra! radiance of east, south, and north horizonsover water at low solar altitudes through clear air

iX- ALT065107101su AZ.lS"Jz

- [PFARING!_ 90. t 1O-0 *,MR: 10 km

.-;. TE ;,P 23CE REL HUMIDITY- 86%

CLO D COVER: (No HopmZON) (- (2)

340 400 500 600 700 800 900 1000WAVELENGTH (mp)

Fig. Ze Spectral r.adian-ce toward north and cast horizons. over water at low solar altitudes through haze

NRL REPORT 6615 7

50~ I T2 34 TIE i

OF / R: 6,0kn-o - " , W - 3C

C LOW =,R 0

~30* 3 5 -"''

25-

-33

'C

340 400 500 600 700 800 900 Km0~~~WAVE LEMTH; ' (mpl

Fig. 3a Spectral radiance of horizon in threedirections over water through clear air

TIME 108S20 0e835-- SUN ALdT 126.5 29!5

I" MR: 10 km1)TEMP: 24C

z 12 t 12

</

o - I ! ! I J340 400 500 600 700 800 900 OO

WAVELENGTH ((2)

Fig. 3 - Spectrl radiance tofar horo ind teasdrecions over watr" through clarze~

6 -

U) (40.

340 400 500 600 700 800 900 1000wAVELLNGTH (mI)

Fig. 3b - Spectral radianc,- toward north and easthorizons over water through haze

8 KNESTRlCK AND CURCIO

1*1- 90. 0*T,,,,. __TVIP 25 IC

CLOW~ COVER EN~O ICP2Cd)

121

V-I-'Z

2A-

J

Fig. 3c -Spectral radiance to north and east over

,-ater ;t high solar ak.itudes zhrough haze

16 I ____ ___2 1 3

4KO ;TV 1230: 22C 1240!SUU ALT 53' 52-5 53 *T ;i'.'Y €Z-j168* '164° 172'

120 N fliAR;Gi2024O" 1 0*

- '1 MR 34 krr

RFL hUMiDITY 49%E tLOUO COV'FR 10%

z60

fk 1x40

a- 40

340 400 500 600 700 SOO 90C 1000WAVELENGTh (m(3)

Fig. 3d - Spectral radiance of horizon in threedirections over land at high solar altitudes

NRL REPORT 6b!5

40

1 2 1-TW Z . 2551 ,13 o 113201,J .~T 2 . 12 °

25*5

£ I4LeG 9 IiDr-]oo. d35~REL HUDITY 51%

COVUZ060lk

CtcOLcO~ 0.

C r

(3,

WAVEfLEWTH (m i.)

Fig. 3e - Spectral radiance of horizon in three directions over, water shom-ing high for ,;.rd scattering in direction of sun

T I o rE 1 3 0 0 ':'.I"

ALT I . -Lt i ,1SUfl 7-.! 4 7 "

gubn AZ8EARMNG 190* L. _-. 50 MR:90 kmTEMP;24 *REL HUMID'Y 3E %

7 CLOUD COy -R 1001%

40 -

- 30 -

20 (2)

Ld 0O-

340 400 500 600 700 800 900 000

WAVELENGTH (m ))

Fig. 3f - Spectral radiance of north and east horizons overwater through clear air on overcast day

10 KNESTR CK AND CURCIO

-lu. |;eoori5s g532 1

's~a~z I236234~ ~

OR : 49 I90V- T-WU 41C

• , co!- OLOu COVE£R 0

70-

360--1S.0

• '30- "2

2S 70 1" 693:73L6 °- 240-r0,,12

M0

11 t 30 wAVELEtNGTH (In,. \/=]

TP ig. 4a - Spectral radiance of horizon in three* drections ovrwater in late afternoon

lo-e

2.01

Z SUNAZ 171"69"273B6lEARNGI240" o" 120" JT MR : 30ki

4- REL.HUMIDITY 48% 1/I

ILI

-. 10%(O ,RZO OLY

u) 082 --

Id I: 06

o.(3

L __' 02'i

340 400 500 600 700 800 900 1000WAVELENGTH (mp)

Fig. 4b - Spectral radiance of horizon in threedirections over land near sunset

NRL REPORT 6615 11

22

2 T0 E i6271 :642

I o - AZ 240- 242*S38 5Ri'GI9O 119O

MR .49 kjE TEiW 4°C16REL PIMIVITY 51 %

!--CLO=JCOVER: 0

3 UI~

* Ei

' d

Q 0_6

0.2

9340 400 500 600 700 800 900 K)00WAVELEWTH Imp)

Fig. 4c - Spectral radiance of south horizonover water near sunset

TIME 1647 1653'

o.4F LGl gg",o-0-MR 49K

'/ TEMP 4

L REL -'11IT 51%_L 02.4-

:1~~M CLOD OVR

E

W

C., (I)

.(2)

0

--

340 400 500 600 700 800 900 1000WAVELEN3TH imp)

Fig. 5a - Spectral radiance of sotuth horizon

over water after sunset

ia

12 KNESTRICK ANiD CURCIO

012

Oil TE 1 1735- " 011 -- SMt ALI -3*5 -4*4 /

SUN AZ 2451 246*

0D SEARING M-0 190'o R-49.km!, TEW.. 4 "CREL HUMDITY 51%

0.09 - CLOW 0OVER. 0

' II002 M0.07

006

002-

340 400 500 SI0 700 800WAVELENGTH (mp)

Fig. 5b - Spectral radiar;ce of south horizonover water after sunset

TIME: 1711

SUN ALT: - 5*5

SUN AZ. 247

0020 - BEARING 190

MR- 49km0.018- TEMP. 3>C

V REL AJ )ITY 53%0016 - CLOUD COVER: 0

'E 00,4-

0012 .

I I iZU 0.0,10 -

00021

3000 -

.th 0004 - . -

340 400 500 600 700 800 900 1000

WAVELENGTH (mp)

Fig. 5c - Spectral radiance of south horizon

over water after sunset

NRL REPORT 6615 13

V The earves in Figs. lb and ic represent eastward observations at sunrlise. Figure2b presents data for northward and westward observations at or near sunrise. Most ofthe atmospheric and solar amsorption structure is identified in Fig. 2b. Several of theFraunhofer lines are masked by atmospheric absorption bands; e.g., the sodium-D linesat 589 mg are masked by the 590-mp water vapor band. The Fraunhofer C line at 656mg, however, Is resolved from the water vapor band at 650 mg. The bands attributed towater vapor are at 505, 545, 590, 650, 730, 823, and 945 mg, and their strength dependsupon the humidity and the length of air path, which in this case depends only on the sun'saltitude. The strength of the oxygen bands at 573, 630, 690, and 765 mg is dependentonly upon the air path traversed by the light.

A group of data (Fig. 3) which represented relatively high solar altitudes showed aninverse relationship between radiance and meteorological range which was more pro-nounced toward the east than toward the north. Little correlation existed Detween hori-zon radiance and solar altitude within this group o! data. The radiance values shown inFig. 3e are typical for a clear sky and atmosphere near noon with peak values of 6 gw/cm 2 -srad-mp for the north horizon to 13 pw/cm2 -srad-mg toward the southeast. In-fluence of the sun's position is shown in the upper curves of Figs. 3a through 3c, wherethe low altitude of the sun, combined with the proximity of its azimuth to the observationbearing, produced peak radiances which were approximately three times the radiancevalues in other directions.

The data in Fig. 3f were obtained when the sky was completely overcast and showdecidedly stronger water vapor absorption due to the high humidity and to the increasedair path caused by the diffusion of light in the clouds. The oxygen absorption also hasbeen enhanced by the increase in air path. MacAdam (5) noted the same phenomenon toa lesser degree when observing light from the zenith of an overcast sky. The greateramount of blud light in curve 1, compared to that in curve 2, was noted in two other in-stances (Figs. 2d and4a)and is believed to be a multiple-scattering effect.

CONCLUSION

The spectral radiance of the horizon sky was found to be a function of thE altitude ofthe sun, the azimuth of the sun relative to the direction of observation, humidity, cloudcover, and meteorological range. Radiance values at noon when the sky %-.as clear andthe meteorological range was 30 km or greater were of the order of 10 uw/cm 2-srad-mg. No difference could be found between land and sea horizons, but a cifference wouldbe quite possible if the land horizon were viewed in an industrial area becaazse of the dif-ferent particle-size distribution.

ACKNOWLEDGMENT

The authors wish to acknowiedge the help of T.H. Cosden, A.G. Rockman, and C.V.Acton.

REFERENCES

1. Bell, E.E., Eisner, L., Young, J., and Oetjen, R.A., J. Opt. Soc. Am. 50:1313 (1960)

2. Knestrick, G.L. and Curcio, J.A., J. Opt. Soc. Am. 56:1455 (1966) (Abstract)

3. Knestrick, G.L. and Curcio, J.A., Appl. Opt. 6 (to be published Dec. 1967)

I°

14 KNESTRICK AND CURCIO

4. Curcio, J.A. and Knestrick, G.L., J. Opt. Soc. Am. 47:113 (1957) (Abstract)

5. MacAdam, D.L., J. Opt. Soc. Am. 48:832 (1;58)

-rcun Cla-smfacation

DOCUMENT CONTPOL DATA- R &D

'I Os"'If, ATING AC TIVT (.V..r4 autlo) T o"ae. REPORI SLCVP'T1T CLASSIFICA 10,

Naval Research Laboratory UnclassifiedWashington, D.C. 2039(.o

3 RICOOR TITLE

MEASUREMENTS OF SPECTRAL RADIANCE OF THE HORIZON SKY

4 OCSCRIP-IvC NOTES fnT'pe o1epceft and onclus#. dares)

This is an interim report: work on the problem is continuing.5 AU tHOAtS) (Ff:sj "n.. ,,'l -,j ja. lasj n*Mr)

G.L. Knestrick and J.A. Curcio

C REP IORT OAT"S Ta. TOTAL NlO O1F PAGC Cs NO 01 REV€S

A December 4, 1967 18 5

04 CONTRACT OR GC R

ANT NO 9&. ORIGINATO'S REPORT . ERCISF

NRL Prob!em 73A02-17b. P oJEC NO NRL Report 6615

RR 004-02-42-5152C- 9b O'TER REPORT "O!S) (Any othef' u ~bers thel may be .. lged

th's repoeI)

d

10 ;JIST'IDUTION STATEmEnT

This document has been approved for public release and sale, its distribution isunlimited.

11 SUPPLCtEuCiT ARV NO ES 1I. SPm1,1SOR,,.O PIL TAR AC*lIlT

Department of the Navy (Office of NavalResearch), Washington, D.C. 20360

13 ABSTRACT

Spectral radiance data were obtained for a rectangular portion of the horizonsky under various weather conditions and at different solar positions. Representa-tive curves were selected from 79 spectral radiance measurements which coveredmeteorological ranges (at 665 mg.:) from 10 to 106 km and soiar altitudes from -6 to74 degrees. The north horizon at noon, with a 30 km or greater,_,neteorologicalrange, typically produced a peak spectral radiance of 5 gw/cfn2 -srad-mA:, Themaximum spectral radiance value obtained in the study was 38 Mw/cm 2-srad-mfor the south horizp in winter. The reciprocal dispersion varied from 1.8 hiji/mmat 406' n io approximately 8 m/mmii at 1000,mp. A mixture of fog and haze pro-.duced a spectral radiance curve with two maxima.

DD NOV ,1473 (A 15S/N 0101.807-680, Securltv Cla's*l(i( aton

A

Security Classification

14 1 INK A LINK C LINK CKEY WORD-Z.. .

ROLE" WT rOI " T R~OLE WT

Spectral radance L

Horizon skyMonoch-romator

Sun azimuthMeteorological rangeSolar altitude

DD FR,.. 1 4 7 3 (BACK) 16 GPO 934.220

(PAGE" 2) Security Classification