AD/A-004 080

RI71---LC Ti\NCI-. AND I-M1TTANC C. OF Sl Ll.CT ILDMATILRIALS AND COATINGS

Mt o r t i n D cl ,Tti bc d Li I I

-- Ac •' 5|', • c" or'|oratiOll

Nib ii L)o i cdidi

LN

F'repared to]:

'-n.1re I ,, nd Missile s$.steflls Ork;'•nizatio i

13 Jan uary 1975

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National Technical Information ServiceU. S. DEPARTMENT OF COMMERCE

hI'S V.ý ' h! -Ilea '

J 'SI C . ....... .

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YTsWTTins, Director A L Ljngve, AIociate WroupVehicl :- Design Subdivision DireciorVehicle Engineering Divisio, Di-,--lopment and S irvivability

Technology Di,,i sion

-'ublication of this report does not constitute Air Force approval of the

report's findings or conclusions. It is publi-lied only for the exchange and

stimulation of ideas.

D. W. Melanson, Ist Lt. , USAI}'Project Officer, Space DefenseSystem Directorate

SEccuIiTN CL SSIt ICAI ION OF THIS PAGE. (147-n Dole Enf-rd) jLJt ( 41 ~ J

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4. TITLE laild ubtiffe) TY EO EP R 6 tRI C V RE-

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Appvc. .ed for publi c r~ltea;e; di st Sibut ion uinljlimitetd.

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'8~ SUPPLEMENTARY NOTE;

PRICES '.30JECr TO CHANGE19. K.EY WORU)S (Confin,,e on fevtEravel.d. If necessary and Idontify by bl'ck nunibor)

in frared si gnaturies, direct jorial reflectance solar cellssolar absorplance directional emittance second sur face miirrorsspectral reflectanice aluminum blaclk paminspectral emittance t it anium anousiý,.d aluminliiniemimspherical eniittance wliiI~e p:i nlzi

20 AEIST ACT (Continue. aci reverse side it nac*86ary and idenifVIy byblc ob)

A 1,t ýraturc review was made to provide the refclht ivi and ther1 -nal. radiativeproperties (crniittaiicc) of a number of selected ri- at. rials and surface oatingvsapplicable to a few specific sat ellites. With the data provided, the infraredradiation signiature for cach surfax, may be evaluated fromi both a spectralanid directionial basis, Pro-perties defined include solar absorp~ance, normialand hemispherical ernittance, spectral reflectance and emjittapTCC, anddirectional (angular) reflectance and ernittzznce. I ex*'inient data which aid

? ~in pr-'icting degradation of the solar zbsorptan~c in the space einvironmen21t

DID FORM 77__UNCQLASS! lEDI SECUiRITYf CLA$O0iICAT C' Of THIS rAGE (14 !)enif Fnf*rPd)

I1I NIG, L ASS I1'V] l-,SECURITY CLASSIFICATION OF THIS PAGEt'Whun Pate Lrntl~red)

19 K LY W0140S LV.,i

20 AI3sTRACT (ConrItnud)

are also pr( sented. T1he ZSelC I CIk~ mate r Ial % iw hIdeLI IC alIIumI IinumII all 60 UO'lI

ti tanium alley (Al -4V, berylliumil, varioui; I itaniuml diexidc alld zuiwi oxide

Surl1faCe mirror.s), using buothi rigid and flexiblI. smb'itratcs, black puigmciitod

coa'tjingS, and( clear a,id blacký anodivzed alIumjflim1 coating-s

__ __ ______ Q

1 RE FACE

This i-cport dociiients Vesceai~li initiate'1 1)y Thei Aerospace

Corporat-ion's" Spa--ce lIefenISe Dil'eCCtoflittO of'I twhncc oohgv Divi sion' anid

carriHed out byý the eil Engirlecr-ir~g lDiVir-inn.l it shlould hc clearlv

emjdiasii.ect thaIt rhis report its riot inltelded as" a eerlrfeec frrdi-

ative arnd optical properties of' r.1bittfials. Ratheýr, it re~presents at corupilai-

Lion oit (rlle sele-ted nuiter jals and propertie~s org~anii.ed solely for)i use as a

pVolIimiiira ry gxiuikli in a spec ific Al s'ud] wiie il re n at of thec po .'O Ses is tc0

evaluatek tsigriatu re of- selec.,ted sat clit cs

corl 11';NUTS

I." .A G)..............7

op-1)I'ICAL ANI) i11-ER\MA1, RAl)Ie\-l\'l., RQ~A~~ .. ....

( ul ']1CAIL AND) -1 [IL;RMAL RADIATIVEK1i,1~L1LK

1)A C1A ...... ..... ...... ..... ...... ...........................

~an~ Surfa' k 2SW

D. O li"11 Slar37

Mivruyi)..................................... .......... 41

A IL D X S1,URVACE RADIATION f(Q1(I I wr v RON1 53........................,.........................................

NL)N IE'N C L AT :JR .. ... .. ... ............... .............. .. . . . 5

W. Xhitt, P'tinlt s SulitMab C fo vS ~acuc raft T1ý0 rinal C oni ro .......... . 76

3. Pcpvcc e ntat ive 0 Wacký 1)~ nt ( Cd for Spacec rot 3

Tvi.rnlal Cont.I v................................

4. o3.~ rj.bsorp"1 <iL'o alic jlicjii ýpic rical Enii talr ot-

Alim~iland SivrO1 ja olar Iteflectors............4

piacedinL- page bi~nk--

4

FI"GUI R." S

1. Sul.il Sp . Irull) at Zero A u \1 L",• . .. .. .. .. .. .. .. .. .. .. 1.2

2. Idcal , RL)) i skittion of L'oor iBasic SOf ',Icc :_ and1t itducl i Lt Io I NM tI.eri1l1 . .s . ..... . . . . . . . . . ... l..

3. N•,io nal Sncktrcal l{clholtu - of V\ 1ll5us AltllitillvilliAlloy 60Cl Speciniens ............. ............................. 1S

4n N(rnn l r S11cct ral ilefloctilnct. for0" Alwnitillli Alloy 6061 ..... . 19

5. Nuolrmai.l Sped cial Riefl' ctance fur Two Iitiiiutii All)\ ... . 19

D . Di ktt i in1ll l LnIt AI L'tC Variat-i n for 1 Se)1 " 1)r1-l M-I talls ........ . 20

7, Ratlio of l11 ,i.;phic vial to No inial Emxiossi iIt . ........... . 22

8. E'fffcl of Wave"\ l lcn tli on Dil'Ck2iilooil Ki-l1tussi'iVit,' of. .il n ................... . ............................... .. 23

9. Normal Spedc r,Vl Retl1ectince v" 1, Berylliunm .................... .... 24

10. CIi'.'-nge in Norm- al Sulir AAI rpl"MCC if 7 illk Xi('OxiPiginicitt~d Coat in 1 >.. . .......... .............................. 27

S, C llange in Solar A15so rpt lncec of Whit c T1c rmnat rol Plaint ...... ..... 8

12. Spcl tral Rvfl'l•,tai-c-, for DC 9)-007 White lPainil . ............ 28

13. >, :-,al Sp.ctral It efec t anc e fLA'r '- 1 3G Whit c Paint. ........... . 29

14. Norincl, I pect ral .. eflctanc'e for While lit'•rilat 0ol Paintl ...... 29

15. Spekctr,l 1 f*iccl ajIce of liV-100 WVhit, Paint ............... ,0

16. Dire ct i unal lElu to lcec Dat ta for Sev c.l rEi t i ric-aIN'uncondlu ct• kis . .N on n nri ot •......................0.• ............. 0,°........3

17, Notinal Spc_:t rl Eminiltance of GE DAD Aluiinuin Pa'int ....... 32

1. Nornial Speovct -al P cIeecr ac c fo)r 'ITW Alumionum Pa<int ........... 32

19. Dirt. ct jmoal Spect ral ELilt t anct of 3 M ]31ack V cel-vet - 4011' 1a1) .t ......................................................................... 34

20C. Solar Ainsorptanc kit 1Uak-k Anwodiztd Almilmniun .* 3-4

21. ý Sp kt. r aI v1(tft t 'Anl. k o] oI Ikt I I No ,rI I IalIE Ih Iiit t .tI I ' I to

lltk I, Antoli iAd :\lumimit . .. .......... .... ...................... 31

22ý. Iota1 Normnal lnititjt t Atyotliwkd Aluiaitinititl 38

2 A. Vt.Y1 1l'irisphirntal I-,tiuttaitur and Aflsorp'aiik Nursus'1 vnipt'ratiirt of Aiiodizcd Aluii,,mii ... ... ... . 3,4

Z-1. Sola1r Alorttc -gr.tdat toii of Anui th,. 6 i.tiuf

C outtIinlgs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39)

25. SpoCkt I,1 ral l ilt' tAi'' (f Aniodiz/in( Alunitinuino....... . 39)

2(, INorinatd Sjt.. t tal RclckI~. tutiu- foýr a at5utlmil Sol 11i1k k

27. lDiret- intml Spc'-t it1 Littittittv tof Acro PuttSttl So rfaer

28. lDirck-ioa j 1 -)I mtttl 1-Innttanttt of A.1-0 1. St o Surlfat-cM\1ir ror * 42

29. Spcct ral .A~bsot-p~tanc.k Ot Sil ýk r C(Auitdi I'-lulon ......

30. Nturmial Exnliicttt of FEPl loflon v's Titilnoss- 4-14

31. Normal potRI -cflcctan'~ of Aluntiniv.cd Mlylair.......... ..4

32). Ro\,flc taneck of 0, N-l~i~c~~i~ ylar...............45

33. Apparent lkgryu on. Aliuxnini,.cui !1IIP .'f4

34. Spot ra-,I Rcflcctanlcv Cha1-nges, inl Kaptonl 11 lilinl, Yl~llomx l

EX 1 )tstre1(t ) Uit [-,\ 10111 ](adi -tioi...................................................46

35. Nt-i r lI Spcct Ira 1<11 -ci anctt off Cojt ralalt Solar Cell... .. .. ....... 47

36. ijirect inial Spect ral Enuttanck- for~ AelrOjot Solar1 (tCll at

37. Di rod lonal Spoct ral EmittIancw for Aerojet Solar Cecll atzoo' 1K.. .... ........ .......... ........ ........ .......... ........ ....... 48

iI actcvt I sibit s) oi a parnik ilar sac( r ai It i i nk,ý ssary 10 01htýin I IItAtniI

basic Pro I crt ics 0 1 vxtcrtta]1 bllpactira'1ht Sur la(I's . Ali bat tliltc iiiaturials

cxlkibit rcfl"kIn tat ll: Id cliliitttucc tltt crtis that1 kal ,I akc 111k %-'tll.It

dCctk'.~talh~ Iby iV!1Zillt sIn So rs opt-rating ill a pa--i'l iulr spwttIral relgik'n hi)

thei \i!ýii)Jc through ticani-inli rar-.'d bspcktt rumI1, flt- delcci abut! ' (if a saicilit c

i~i a I-uncltimon di th di rccliott alld jllttuls-itV 01 tilt' SOlarliry tart 1 alCdo0,

and(/or artif ilil illuininat ion tttt rgv' rii~tcd hy flit- 'xicrinai skt irýik k Of Illk

study, Hiht i-atialk (I clitrgy plrimlarily dIt ermint-s tilt~c lt-abtil ity.

lDttektabl~jlt is llask(I kil a LOrolllol rit (lt st rip? ion of I llo sat Illilt ;it)( a

gitral dIcsk ript it'l (A tilt' Iit, rtfopllysical propt, rl its 0f tilt vth~lol butiatis

togc-tilut will) flilt- asik. orbital paraitni 1-s and hin rtial llh''t gi-itirat ion ralts'.

'or alt a( uti altc i-cprt-ttit at ionl of intrau-1trd sýi-tao~ 11. 11t Isnit Ilk, SOk-f1 td

di rect i'mla ciiaract riizal Ion of- lit'c bsatfak Ycl Cth Iat LI Cor t~lli I'liatilc shoulld

1)1-cldtd Tllh dOi?! tilitiat -,11~ tilics propi-i its tori a selkttted p roup of

rtiatcrials and cotnswas; l'k, primary put-post. of this Study.

3'or furtictr dliscuxss lol of flit, lh-cory of radi at j\' proptrl icti, Ilikt rnader~

is diri-tCOVl tO it~l 5kOUI*CS ilSItI :11 lilt' liblik~lorpiy

Prec~eding page blank

1j; order th-ii presentation of d~ita Iw ns cha~r as possihl~t, O1w

Properties urilizked, their colrrespondling dvifinit ions mnd syzil ho1 ai e swoo

IEiiittaiic('(' 1ý;It o of III( ratdianit ctIUssi lil ncr 1Phint '17'c" ulthec si' JwciU'Ii to thatl etolited hy ;I~a1radintot- ;' the Ti-i templt, rat o~rC

1Reflecl-arce (1)) Rlatio of reflected radiant fluxc i ink-ident flux,Ii function of rad iation \vavkti e ngt1- i all so rfa c t'

4 C nI pV rat tror

Absori ane (~) lt in of thw ahsorhied radiant fluix t) tilc incident! flux, a fun t ionl of radiat ion Wa zvolk .1gil 111idsurfaceý t('.ii)('rattre.

Thle follo0WingP te rln01nolgy isý kisc! to cpml.1if) these cS0 propertit's in

termns of the xvrvelen-gtb.

ci ra , Foc t~on f -wakve cn u12thI sx 11 I.-lv ill a niarr owband of vv'('levnpthis (also refert-ilr I"tIioilciO 71 oilla tic1

Total, T Ordinz rily refe rs to eioitt ao-e onlv. ieteit -

peratare of the spocinlienl should hI)( !pecifiedsince tht. total tniitt~mce va)ries, withiteflpe rato Xe.

Integ rd ed Rein tive to sonme spe cifitcd wýavel1 ng!h~ di st rim-hilion or spec ifi edi band.

Sola r, s Having the wave1Lengthi (listrilomion vi uiiicprirnal ing thc stil. WhenI uISed inl eon1 urc iU on'_I wit ija the spectral absorpt,11C, Of the SPeckimenv hasbeenl i nt cgratedt ovc ti 1lt: wvavel c igilh d 1st r iut iono1filhe sunl.

Ill IdditOll, a 111111ber Of g(2oinutt nc subscriptsi or- qualifiers associaicd

With theL inc ident , reflected or enuittced flUKes art'- alSo 1'i~iiZed tIS ful lOWkS:

Normnal, N Conlditions for viewving thrliriig an angle2 0 thati! esstntially nornmal to thev spetimen.

Preceding page blank

~ (rire~~ion1~ corlditfios to], '11~in a! )Illke::gl 0.

I Ii~i:~h~V itlIt (Ct~lirl0t i0, f or- i~iClCii( 01- ViewAing 0%:-TFhe~ii5pli11c~il reg~ion (i.ec. , 2 TT Ltcraoi~ins;).

br aalyi~;of ,~frredsi~itucs.the anlgi.lot (dircc-iviial) nld i;pvc-

tlral t-1:tivti.ý;Ce IS 01 priT11flly toterC:i. Ihiailed t '.intt.-titve data oti .1li of tlhe

11.tae-t 1.111!t(, itleItt'lSt areý limlited. IIlwc\cr the citfa1cI,( Of I llii~tti' Io-

facc ca: 1)k re'jitcd !c ih) r)i'ct ~~p.iC 'A'¾JC1 . ' YIV 11u101l C,)!L lv 01h4tA il

or* lixore( readily ;sabbe Vvi-lxcu! gontg 111to Ilw doliilis of thec prcof !I frn

Ki-rchiotfs lx it fi11ov.,!4 thait ltt~ an opmclue. 1tnilt'til in t'(pllilibriutll vviti its

cuivironnien!

L (\, T, (1fl I (\, TF, U)

In other wvords if the n rop ri-ic rs are e-ahia t ed tt t H t' s,- ow tvrnpe rat or .i

\\'a\elcem, ll, and angles ot inci~dence' or v-iev-ing, thec emittaa1cc is equIAl 10 teI

absorptariccý and the ciniittsrice is uq-,ml 'o onle minusl. tl'- refe tii-( , rhus,s

uinde r prcpe iiy col txrolled c ondi tic nr n i'rei of the anogul ar (di rt-ct jot-al)

spec~t val Illetaw is quivl t ti. tneak)I ring. il,, angulr - di rectiona.i spec -

trnl e'iiillanrie. The' reCSU.tillp.urc froin this assuim~p~ion is adcqu rte foi-

l) sC) S 011g i I-,CV1 ri ng Ippl i Ct n1t' a C)1 1A Ii,1 ,onJS ie (V I l s at ii L; ",ct o ry f or t h is s --id

The x-alucs givtn for total enxittance (eith-- n;o-rtt.il or heruisphieviic1)

'o r theI \,.rI I. i cI sII)If t-r ials a nd oa t in gs i n ! li: s evporI ta r-c y'cn 2r alIIy d u : r cd

for- s oni inominal tein pe r attire: eithier at 1`0011 t eope raiu re , at 701'F, or some

other so.jcific tetfl~p(riutlitq, 1upcinding oln the s( arce ind -oniditions uinder -wlich

the da iin vie rz ubt aind. E a sed on lie spectral emlit!an ce dat a- given for the -e2

runt era Is, the total em it i nce rrn bx cotoput ed for any nt-her tempova ture de -

sired by (-alculat ing the blackbo Jy clia racfc -istics -s g ;ven by Planck's Loiw.

The actual powver eri-ii~zed or a'-iborbed is obtained by ta-king the product of Ht.-~

bin ehbody radiant inftenr.'ity and thle emiti ance at each -;va~-clengthI. Thel( total

crildtaflcc is t': deterwtined bNy inicgrat itg this t.rnfllet (Iv(v All %\\Av('lcnlth1s

anid div'idinig by' tht'- 1hi'.ackhodv uncerý,y over Owe s, -I.#-~vlegh

exrperirii.'ntal tt.-~¾l1inique lot tiu-asotritg the thcrilal r.mdiativt'

lpruperti('b of iiiaterialsi have h)f~l '-l escrilied im, 'he' hiteramo~krc '11d !Io 'Attermip!

,vill be 1macde to iis~.uss tll(e tIlthiods.. or illst ruliem't'i in (10t 'Ail hr(. It will hie

tiocd. ontv thiat the mth~o&;ý fall riwo two genev r~i Ikatug.irie; rad'Ioiltt I c Ind

Ccalorinilet ic. III callolictric~ 1cjO t ,i liiq( i~ Ot- r.A,r:mtw !II%- ab~sorbied or wtc

is cvalkuatvd in term., of hleat lost, or gainled by thf. !?pl. I ra'ljmnwt ric

teClin~ptes. 0,cl radii;kn' flu.x is nieasurcd direccly or roimip tred %vith a enýli -

bral ed relt rt-ricesaiiul.

(aloriniiciri- me(thods g.'nvrailly tend11 teh'- rel '1%. v buipl 1 11

Icimer P:cq-eisn, vvhibc radaxlh'tric ni'-.d.s re'1 uive riiwre ll'ri tqolill)-

nlerit andl arc. canalAv of hiighcr precis ion 'wit suliiect to sVwefliat ic e~rors

that can be difficul' to evaluatc. Calorinict rjc t1wfwis art, ii~td to measure

*abslo-w'ance an-! '.1ititarcu (Usually the lot.1l hexinisphe -ricall vilxitt-Ankcv' authalk

lh'-dioxxictri&- zviviods measure the emiil ted or in,-id* ncr raidioril Iluw:

di r(.t ty -4s 1 futiction of angle fronui the( normal. and ;or \avu'ength 1 Im's

abso rpt an rcc , -,pe't ra I a; id di recýt iofl I 7r,? I fectanc I ri d vli ii ittac dOCC )rivn r I i,

* can 1,vetruind In the as- oi t ~t un* n sol., r Oisorp! .. rce (' -Y . 'h

sp,_cl r:1 r f-ýclec ance is t-g gratecd -in a n ene xgy & \¼igp!no ri diS us i,; ng the olr

SWrct Y'ýl di s ribu! I ai qur'h as showni ill F' git1týu' I -' and1 021 f I m e!s St alld .

re ~pecti vel v.

NVvhi~i the' vinfittmi-vt is, the property of inturest , tiiie~suruwviots arc

usuahi I I) keri at nea r -noriiial in,. idcnct, and liericc the %. allm of t1w Tio rui ml

I Snsr 11 I'l, Pe So la r Cons r a nt 3 af Mi v fco rq j Iug\ Vol .I , I 1 )c (e c 1 -

* ~ StIr, P. ''Thermal Testing of Spacecraft .'' PRport No. 1O01{ -017 2(24-11 -01) 4,-I -i Aerios pace Corp. , 1:1 Segundo, Calif. ,Scpteihe r 11)71

03

G.2

-J

z 01 --

Ic -I . I ... I I i

0.1 02 (14 0.6 1 2 4 6 10

WAVELENGIH. IIcronI,

100

(IL>/

-4 (-)"98

- - < 94

ua -j z

9uJ

> 2 3 4 C , 7

WAFLENGTH

-D I

0 0.4 0.8 1.2 1.6 2.0

WAVELENGTH, fmcrons

•. • ., I" I I~i Ik Tl • ,I', I • l~ tl 'll h:1- 1- x{'1..,'

Figure 1. Solar Spectrunm at Zero Air Mass

-12-

emittance (cN) is obtained either on a opectral or total basis. Although the

directional emittance can also be obtained, it is nut ordinarily mcasured in

the longer (IR) wavelengths.

In fliese cases the normal emittance ( N) is uised to est oimac' the henmi-

spherical emittance (Ei 1) based on ratios of ( N to f1tha!t have bee,' estab-lished for similar maferials either on an experimental or analytical basis.

G Given nic and 1NF together with various optical constants of the miaterial,directional emiffance can then be compi;ted (•although qiiite complex) from

electromagnetic theory (Ref. 3). These derivations are quite 1,lengthy andthus are not included in this report. Ilowever, a sunimary of the significant

equations are provided in the appendix. The interested reader is directedfurther to specific texts on radiative heat transfer cited in the bihliography.

In some applications, a rigorous analytical treatment of the radiativeenergy interchange between surfaces requires k,,nowledge of the reflectancedistribution function which provides the flux reflected in a particular direc-

tion as measured by angles 0 and 0 rel.ative to the flux incident from some

fixed direction. (See sketch)

OBSERVER NORMAL SOURCE

Bidirectional Angles for an Area Element

3 Herring, R. G. and T. F. Smith, "Surface Radiation Properties from Flee-tromagnetic Theory,'' Int. J. Heat and Mass Transfer, Vol. 11, pp 1567-71,1968,

-13-

*o, f-

This reflectvnce is termed the bidirectional reflectance. In the case

of signatures in the visible arid near infrared, the solar energy reflected from

an object in ,pace is of prinmary interest and bidirectional reflectance data is

obviously important.

In contrast, in the case of the thermal or far-infrared signatures of

interest for this study, self-emission of external surfaces of objtcts pro-

vides the predominate source of energy and bidirectional reflcctance data

is of less significance. Also, as the wavelength increases, the mangnitude of

the surface roughness dimensions become smaller relative to the wavelength

dimensions and the bidirectional reflectance effect is diminishied.

Another factor is th( complexity and quantity of data req'uired for appli-

cation of bidirectional reflectance. If the bidirectional reflectance of a sur -

face is mapped at reasonably close-spaced angular increments, a rather large

set of numbers is required. For example, in a case where polar increments

are set at 10' (0 and 0') and the azimuth increments ,& are set at 20', there

are 1458 (i.e. , 9 X 9 X 18) individual refleetance.s required for just nne wave-

length. As the wavelength regions or bands of interest are increased, the

number of data points for each surface of interest becomes substantial.

_Ilherefore, due to these factors and the relatively large mlnber of

materials involved, the inclusion o&' bidirectional reflectance data was con-

sidered to be beyond the scope of this preliminary study.

-14 -

111. OPTICAL AND T11•IRMAL RADIATIVE PROPIIT.]E.,S DATA

Ideal'y, thermal control surfaces can be divided into four b)asjk classes:

solar absorbers, solar reflectors, flat absorbers and flat reflectors, as

illustrated in Figure 2 from Ref. 1 . The solar" absorbers are priwarily

polished metals which exhibit a high a and a relatively low a in the longer

11R wavelengths. Flat absorbers such as black paints generally exhibit a

high ta over both the solar and IR wavelengths. This characteristic is also

typical of solar cells.

1Flat reflectors which exhibit a relatively flat but moderate a' over the

wavelength range through the TB 3re typical of aluminum paints. Solar re-

flectors with a low a and a high ( in the 1l. region are typical of specifically

devcloped white paint&, and second surface mirrors which are also referred

to as opti.l solar reflectors OR 's). The 0SR s . onsist of vapor deposited

silver, aluminum or gold overlaid with clear transparcnt layers of fused

silica, Teflon, Mylar, etc. The transparent layer allows transmission of

solar energy to the metal film where a large percentage of it is reflected.

This transparent layer is, however, relatively opaque in the 1R wavelengths,

and therefoi e, exhibits a high emittance. Thus, the OSR composite reflects

most of the solar energy while at the same time providing an efficient radi-

ator to maximize heat rejection capability.

In a space environment, the white pail ts exhibit the least stability0.e. , they exhibit an increase in a ) while thc second surface mirrors tend

5

to be the most stable. (Quantitative data are presented in Paragraphs ITI-l

and III-D.) The significance of this is that with white paint the equilibrium

temperature of a spacecraft surface increases with time on orbit. A vari-

ation of the conventional second surface mirror involves anodized aluminum,

4 fBroadvay, N. 3., "Radiation Effects Ilandbook, Section 2, Thermal ControlCoatings," NASA-CR-1786. Prepared by Battelle Memorial institute,Columbus, Ohio, June 1971.

S~-15-

W C:

k- E

E I

_j s C

<: 2j >:uC: UJ >

C C

* 0

coC

U)U

C) <

_j_ _ _ _>__j

CC

c-

C: vy dJO U C.f ID d 3D '-d Of3V- V i)

whe rein a transparent aluminum oxide filni is chemically ormited on a bright

polished aluntinunt substrate. The stability of this surface tends to be somne-

what better than most of the white paints (quantitative data are presented in

Paragraph Il-C).

For the purpose of presentation, data are grouped into the following

material ci;tcgories: metallic surfaces, coatings of white, black, or alumi-

nnum pigments, anodized aluminum , second surfak'. mirrors and solar culls.

A. MNETALIIlC SURFACES

The nornm-l (i.e. , 90 degrees from the surface) spectral reflectance

of various aluminunm alloy 6061 specimens is sho\vn in Figtire 3, bascd on

Ref. 5. The strong influence of the surface characteristics of the specimens

on reflectance is clearly visible. A second source of data (Ref. 6) is pre-

sented in Figure 4 for 6061 aluminum, pla-te with a surface condition defined

as unpolished as received from the supplier. For this s-,pecimen, ar ý 0. 37

and c_, = 0.042 at room temperature. These wvill be the assumned char-

actrristics for 6061-A1 tt ,in hrt sirfaces

Normal spectral reflectance for two comnrtonlv used titanium alloys

(6AI-4V and 5A1-2. 5 Sn) are presented in Figure 5. Data from Re. 5 for

6AI-4V did not specify wavelengths beyond four microns, thus the curves

shown for this alloy for wavelengths beyond four microns has been estimated

to be the same as that shown for the 5Al-2. 5Sn alloy since the difference in

reflectance values between specimens of titanium at the longer wavelengths

tend to be relatively small. The unpolished 6A1-4V specimen is selected asthe baseline titauium alloy with properties of a 0- 0. 57, ( 18- 0 from

5 Touloul<ian, Y. S. , et al,"Thermophysical Properties of .Matter, ThermalRadiafive Properties,'"Thermophysical Properties Research Center, PurdueUnivevs ity, 197?.

6Thermophysical Properties Measurement, Unpublished Data, TRW, RedondoBeach, California.

-17-

< U)

-. 41

U.

< a)

C) C,

amE~ cuX 0

E .

LI)

0

ui c E

ci cCN E

~, - LU

LUC N >

Inc<C:

1~. C*t(0(

T f

UC) J-. C:) C

S2 CD Q) (

0.4 CC (0 d

.0 E E E E~ .2C) Q., Cd m

LI) - F- U) U) CI)~

00

3:JNV1371138 1VUAiZdS IVLJHON C

100 1 . -

80 ROOM I3I MPLR.AI URL

cgs 031S60 -(H (0 4

2

fCN 0 033

SURFACE CON D',ION

- 40 UNPOLISHEDu-,J

20

0.3 0.4 U6 1 2 4 6 10 20WAVEL[ NGT[I, microns

.'igure 4. Normal Spectral Reflectance for Alumninmum Alloy 6061

if'in ;., .... - I "" * I I ", I , [ -

p ~ROOM 1EMPERATUFIET i -S A I 2 .5 S n (C 1 1 0 M (R e f . T .O-- ,- --0

80

.60

u 40 PULIbHLD .298 Ki. 6 91-j

u-

020 UNPOLISHED 1298 K , 9 9

0.3 1. 4 0.6 1 2 4 6 10 20

WAVELENGTH, microns

rigure 5. Normal Spectral Reflectance for Two Titanium Alloys

-19-

IIm

Ref. 5: and is assumned to have the same rct'cctance properties beyond tour

rxiicrons as that shown for the SAl-2. 5Sn speciien, from Ref. 7.

No angular (directional) reflectancu or en mittance data were avaiil;,ble

for either of the specific aluminum or titanium alloys. llovever, represen-

tative data for several metals are presented in Figure 6 (Ref. 8). In gen-

eral, all nmetals follow the anwe form as shown in Figure 6: i e . , f remains

quite uniforn, at angles less than 40" and then increases sharply at larger

angular inclinations. Although the experimen!al data stIown were not carried

out to 900, the directional emnittance would apjmroa e zerit. In geWnral, thu 1)1L"-

centage increase in ( with increasing angle is more pronounced when the value

of ( is low. This is opposite to that for nonconductors (e.g. . glass, paints,

plastics, etc. ), wherein the emnittancec norinally is qiiite uniforli up to about

50' and then decreases with large angles of 0 away from the normnal.

ANGLE Or LMISSION 0

60 40 20 0 20 40 60

S80 M NAEE80

0.14 0.12 010 0.08 006 004 002 0 002 0.04 006 008 010 012 0.4

_- ( 0 DIRECTIONAL EMITTANCE 9-

Figure 6. Directional Emittance Variation for Several Metals (Ref. 8)

7 Cubareff, G. G. ,'Thermnr1 Radiation Properties Survey,"2ndEd., Honey-xwell Research Center, Minneapolis -- lloneyw(l Regulator Co. , Minneapolis,Minn., 1960.

8Schnmidc, E. and Echert, E. , "tiber die Richtungsverteilung der Warmestrahlung," Forsch. Gebeite Ingenicorwesen, Vol. 6, 1935.

'.20-

lDased on Figure 6, the directional emittance for aluminuin at angles

less than 40' corresponds reasonably well with thell and 1 N values defined

for the 6061 specimen in Figure 4. Thus, as an approximation, it can be

assumed that the directional emittance variation given for Al on I'igxre 6 can

be applied to the normal spectral emittance values given on Figure 4. On

this basis, the ratio of I /" and the correspo:,ding value of '0 can be esti-•. 0 N

mated as shown in Table 1.

For metal surfaces, the ratio of (I/ (N is generally greater than 1.0.

Although this ratio can be calculated from electromagnetic theory, an cnipir-

ical relation is given in Figure 7 based on measured data as a function of the

normal emittance. Although no directional ernittance data were found for the

specific titanium alloy of interest (i.e. , 6A1-4V), directional spect ral data

Table 1. Calculated Directional Emittance for 6061 Alutr.inuni

Angle from '0 U-)!N ffr F0, A/

Normal (0) (from Figure 6) (from tFigure 6) (EN = 0. 033)

0 0.04 (fN 1.0 0.033

20 0.04 1.0 0.033

40 0.041 1.02 0.034

50 0.044 1. 10 0.036

60 0.050 1.25 0.041

70 0.065 1.62 0,054

80 0.105 2.62 0.086

90 0.00 0.00 0.00

Based on 1-1/*N 1.28 for metallic surfaces and ttH 0.042.

-21-

1 4

Al0

t

"Cr0 N, POLISHIF [12 - __ANGANLSL

0" N,. DULL Al PAI' PCI

CONDUCTORS

1 0 - ELECi RICNONCONDUC TOHS

C11 OXIDE "CLA_

09 1 1 1 GIASS

0 C2 04 06 08 10

EN

]iiurc 7 . P atin of lenm ii-phe rical to N o rm , . F',in issiv ityS(R ef. 7)

for pure titaniumI arc illust rated in Figure 8 (Ref. 9). At very short

wavelengths it can be seen that the directional spectral eruissivity actually

decreases with increasing angle 0 which is sonmewhat contr.ry to the normal

behavior of metals as predicted by electromagnetic theory. llowever, for

ihe region of interest, i.e., the longer IR wavelengths, the emissivity does

increase v witi increasing U. 'The directiona! em irtancc for Ti-6AI-4V can be

estimated utilizing the coml,ined data presented in Figures 5, 7, and 8.

9 Edwards, D. X. , and Ivan Catton, "Radiation Characteristics of Roughand Oxidized Metals," Advances in Thermophysical Propertic- at ExtremeTemp erature and Pressures, ASME, 1965, pp. 189-199.

-22-

ANGLE Of EMISSION, 0

0 "15 ,SURFACt HOUGIINLSSWAVELENGTH 0.40 iim:.min ts

rlmicrt1s0, 340

0.43

45-

1.0 /1.62 60

[ ~40

812

20

6 0.2 04 o0.6 089

DIREC1iONAL SPECTRAL EMISSIVITY. X. 0"

Figure I. ILffect of Wavclength on Directional !-mis-s ivityof Pure Titanium (!ef. 9)

Beryllium is used for a wide variety of purposes incInding heat shields,optical mirrors and in nuclear reactor technology as a neutron reflector sur-face. Spectral reflectance data from Bef. 5 a'e presemed in FigUre 9 1,ran extruded sample of beryllium. The total normal emittance ((N) for th;ssample is conmputed as 0. 14 at 500F, ".The corresponding value for cyl is

estimated aa 0. 17 utilizing Figure 7 as a basis. The a' for beryllium isassumed to bc 0.70, from Ref. 10.

' 0 Gaurner, R. Y. and L. A. McKeder, "The- nial ,adiative Control Surfacesfor Spacccraft," LMSC-704014, Lockheed Missiles and Space Co.,March 1961.

-L3 -

100 1 I I I " ! 1

c 80

LU

7- 0I--

-J

0.3 0.4 -102741 0 2L

U,,

-24

z 40•

0.3 0.4 0.6 1 2 4 10 20

WAVEL[- NG1 H* itrn

Figure 9. Normal Spectral Reflectance of Be~rylliumn (IRef. 5)

• -24--

B. COATINGS

1. SO(LAE RII.,CI'LLCTORS - WtI I'I'YE PAIN lS

A number of wvhite paints have bcn devvloped which aNr v utilizid as

solar reflectors. A listing of sonme of the iioie coriun ones are presented

in Table 2. All of these coatings, however, anr dog radcd to different dog'recs

b>F the space environment. The p'ii:.ary dam iaging fdict,)rs inclide ITV radia -

tion, clect ron and proton bombardmnent and nuclear radiali•n. The change in

soltr absorptance (aY ) of three common zinc oxido pigncnti. d (oatings. i.e.

Z-93, S-13 and S-13G, as a result of exposure to -armous environments are

sunirnarized in Figure 10. The acce,,crated degr;,dation effec, of tho added

particle boinliardment associated with high altitude/cdoep spiet' oe ironnmen!s

is indicated. The clhange in a for a titanium di•ixide base paint (whiteS

Thernmatrol, {,efs. I I and 12) is shown in Figure ! I. Tl' hi gher degrrida -

tion ra t e for the synchronous crbital altitude associated wi'ti the AATS-I is

also illustrated here.

-- Of the white paints illustrated, 7-93 is cons idered to b, the ifl ao-t stable,

The major problems with this coating are tle dtfficulty of application, the

more complex curing process and ease of soiling durirng proeflight conditjons.

Therefore, the use of the S-13, SA-I3G or the titaniuii1 dioxide base paint,

Dow Corning ()C-92-007) is prefe -'able. Spectral reflectance data are pre.

sentf.d for DC-92-007 and S--13C on Figure 12 (Ref. (;) and Figures 13 and 1.4,

from Ref. 13. 'rhese data as noted are measuied at angles at or near norn.al.

Reflectance data for PV-lfl) ns ut ill-rd (in te.0 NinhllZ and .l' "' ,- :,-,.:..

are slhown in Y'igure 15 from 1Ref. 14. hlowever, a significant chnirge in the

hlultquist, A. E. , et al, "Advanced Therwal Control Materials lDevelop -

ment," LMSC-A967871, Lockheed Missiles and srace Co. , May 1970.t 2 Personal Communication, Aerojet hYlectro-Systems Co., Azusa, Calif.,

±3 July 1973.1 3 fBair, M. , et al,"Opticcl Properties of Satellite Materials, 'III- f- Copy).

Document No. 194100-6-F. Infrred and Opt-es lDi ision, En'.Research Institute of Michigan, APnn Arbor, Mich. , July 1973.

1 4 Personal conimnunication, A. E".agles, Gem ral Electric Valley Forge _ipaceCenter, Philadelphia, P,. , 2 Oct 1974 (IERTS-l Surface Coating OpticalProperties Test Data).

!- -'U

Table 2 White Paints Suitable for Sp.acocraft Thermal Control

I'•.j t I

A IIIS(I., ..

P II

IIlV

II~d i-~I nIIIT 1- I I.h

PV IW I' i,' Va r I E,. NI

1II T In A

I o I I

y " , .1 I. ), , i t.a.i h./ D x Iv/ A 't , , d n• r om , x•,•. t; o, . ul.

C r-i , ,IN .F +.•1 , I VMryu V:•, Tempe atur Contro V.lf e ig ,T s

,.,n. Perorm nc ," JPI Pa'acr!- Calif Pape prsetd'tX1A 3

16 it mh s , , q J . t, •'13 . . ptI al, "Spac M ateri al H and ook , N A SAI S1 3025 ,

, . ItI. t I,,, - Cnt.5 ApTT l, C .

SiK•\ T .| i I Itr i:T•fl 1)i,. jielt S''Ter'. n- V.r 2.";t,- , - 1 , -Ic

St'pr~xVy bind•,r P .,'r,,' Co.

At 5

SCarroil, W. .'. ,"Marirnei V T+emperature Control Reference Design, Testand Performance,' J PL, Pasader, ', Calif. Paper presented at PIAA 3+rdThermo>nhysies Conferenec:, L. A., Calif. , June 24-2.6, 1968.

16 Pittanhouse, 3. 13. . et al, "Space Materials Handbook," NASA StP-3025,

Supplement 1, t966.

-266-

01) 0a .

0- 0

z 0 CL '0

0 C)

C) <C)

- a.Q..V 1.

CC

ir _N

C) '2

C) C) 0~ C 0

IN C) C)

-3-)NVidki0SýIV BlVl)S IvLNHON NI 39AJV117i

C3 CC

LfN C)

CC

C jr 0C0 -_Ccc a I020

CD x

ZVa-0uL

_27-

Joe-.~-

060 * =.1-

/ •"AIS 1 FLIGHI DATA Ref. 12'

0 V,Uz/< 0.40

Q,.a / •LABORATORY 'JV EXPOSURE:

0< 0.30

(n

0.20

0.10 I I

0 100O 2000 3000 4000

EXPOSURE lIME ESH

Pigtre 1. Change ill i Amorpt1nCe of White Thernatrol Paint

Q 15

-80 EMPERATURE -. 70 F

0.20, CN 0.86,

x 60 - CH -0.B2 lestimawd hasede n EH.,CN - 0.951

z

40

U-LU

20

0.4 0.6 1 2 4 10 20

WAVELENG1 H, microns

Figure 12. Spectral Reflectance for DC 9Z-007 White Paint

-28-

100

S80 09 - 15

L ROOM TEMPLRAlUR[

60

Ldi

wi--

"c40

0 5 30 45

_ * WAVELENGTH, mro.is

Figure 13. Normal SFectral Reflectance for S-13G WhitePaint (Ref. 13)I100

. 1580

ROOM IEMPERATORE

S60 10

z

---------------- •lWAVELENGTHt, microns

Fi~gure 14. Normal Spectral Reflectance for WhiteTherrnatrol Paint (Ref. 13)

- 9

ii•- a:

l = I I i i i i i I! 20

100

80280-- N 086, Eli 0 82 etmindic

,H"( N 09")

S60 -

a-w

U

- 40U-

c-

20-

C /-,. , I . I I [I.

02 0.4 06 1 2 4 6 ii 10 20

WAVELENGTH. micro.),

Figure 15. Spectral Reflectance of PV-100 White Paint (Ref. 1.')

directional reflectance would not be expected for these <-oatings at angles

less than about 50' since most nonconductors exhibit characteristics as

illustrated in Figurc 16.

Inspection of results obtained by Schmidt and Eckert (Wef. 8) in Fig-

ure 16 shows that the directional emittance remvains essentially uniform1 for

angles between 0' and 50°; then drops off quite rapidly to zero. This re-

lationship is, in general, in accord with that pred cted by electromagnetic

theory. The relationship between c t and cN is relatively complex, depend-

ing on the wavelength, refractive index and dielectric constani. In general,

however, the ratio Et/( N for nonconductors falls bctwecen 0. 93 and 1. 0 as

indicated in Figure 7. For analysis purposes, the dirck-tional cmittance of

any of these white paints can be assumed to followv th• relationships typical

of those given in Figure 16 for nonconducto-s.

-30 -

I 4

a WET ICE e. CLAYtV WOOD 1. COPPER OXIDE /--ANGLE OF EMISSION, 0c. GLASS g ALUMINUM OXIDEd. PAPLR 0

20 20

80 8

1.0 0.8 0.6 04 0.2 0 02 0.4 06 0.8 1.0DIRECTIONAL EMITTANCE, t0

Figure 16. Directional Emittance Data for Several E.lectricalNonconductors (Ref. 8)

2. PIAT REFLECTORS - ALUMINUM PAINTS

Materials which exhibit telatively flat wavelength characteristics are

typically referred to as flat reflectors. These properties are characterized

by aluminum paints.

* Normal emittance characteristics of GE-D4D aluminum paint which is

extensively used on the Nimbus and ERTS vehicles are presented in Figure 17.

These curves were plotted from raw laboratory data (Ref. 14) which yields

properties of a = 0.28 N = 0.27 and c 0.28 (estimated) at room

temperature.

The normal spectral reflectance of a second aluminum paint is shown

in Figure 18 based on Ref. 6 before and after UV exposure. The initial prop-

erties of this aluminum paint (Rinshed-Mason) were computed as as = 0.26,S~S

CH = 0.ZZ. After 2000 hours of laboratory UV exposure, a degraded to 0.32,

while t at room temperature is essentially unchanged.

-31 -

1 I - - I I 'r

0.8 - c 028

N 029. (H 0221 estinitiLd2"

04

Cc0.0

0

WA-_LLNGTH, n'crors

Figure 17. Normal Spectral Enmittance of GE D4D Aluminum Paint (Ref. 1-1)

100. -

S80 N A

S60'•" / ALItR 200 ESH• iUV VXo'r)su'e!

-7

S40-j v SUPPLIER. RINSHLD MASON

U-W C1 0.26, -H 0.22

20

AFTtR 2000 ESH UV EXPOSURE, a 0.32

ROOM TEMPERATURE

04 0 1 2 4 6 10 20 30

WAVELENGTH. microns

Fiure 18. Normal Spectral Refleciance for TRWAluminum Paint (Ref. 6)

-32-

3. FLAT ABSORBERS

a. Bla ck Paints

Representative coatings used as flat absorbers and their thermal

properties arc summarized in Table 3. In general, these classes of coatings

have been shown to be generally stable in the space environment. Directional

emnittance for 3M Iltack Velvet paint is illustrated in Figure 10. The cxpqri-

mental data shows that the directional e, "iittance is essential]y uniform for

angles up to 450 from the normal. At greater angles, the einittanhe decreascs

quite rapidly. No data are shown for angles grteater than 80', howNever, the

emittance would be expected to decrease sharply to zero, typical of the curves

presented on Figure 16.

Table 3. Representative B'lack Paints Used for SpacecraftThermal Control

.Sola.r I. emisoheri cal nRefDesignation Manufaee r er AbsorptAnce Emittance (El

Cheniglaze, I{ughson 0.95 0, 88 17Z -306 Chemical

Black Velvet 3M 0.95 0,9Z 18(401)

Black Keinacryl Slerwin- 0.93 0.88 4Lacquer ;'-lisrns(M49RC-1Q)

Black Siliconc W. P. Fuller 0.89 0.88 4Paint (517-B-?)

Cat-a-lac flat Finch Paint & 0.85 0,90 18black Chemical Co.

At room temperature

t17Personal communication, R. Wallace, Lockheed Missiles and Space Co.,Sunnyvale, Calif. 10 Oct 1974.

f 8 Personal communication, G. Borson, Aerospac- Corp., Materials ScienceLaboratory, 5 Sept 1974.

-33-

8 0 0 70t9-7

20 0 80

U.

-1-

40 P i- 40e. 3

20 ROOM 1tMf'LHAIURL

00 U30 45

VVAVfLl. NO 1ii. <rll

Figtire 19. l)irectionial Spectral inj of 3MBak\elxct401 Paint (Ref. 1 3)

b. Black Anodized Aluminum

Anodized aluminum blackened by the use. of conwmercially available

dyes has btor pr011 duced and tested for Use as solar abso'lbers (R(ef. 19).

Tests conducted therein have indi cafed that these surfaces experience only

negligible changes in thermal radiation characteristics when exposed to sim-

ulated space environnment. Because the blacli paints typical of those Identified

in Table 3 ai"e generally limited to temiperatures on the -trder of 400O02, a blaclk

anodized aluminum surface is ideal where a high lenmperature surface with a

high emittance and an aluminum substrote are required such as in the case of

t19Wado, W. R. ard I). 3. Progra, "Effects of Simulated Space Environmenton lhermnal Radiation Pro erties of Selected Black Coatings,' NASATN-D-41 16, September 1967.

-34-

a nuclear reactor radiator. In addition, the organic-based black paints tend

to deteriorate rmore rapidly whden exposed to high intensity radiation in close

proximity to a nuclear reactor.

For this type of application, anodized aluminum blackened with an in-

organic dye, CoS (cobalt sulfide) has b,-en selected as a logical representa-

tive surface. The spectral reflectance fnt a black (CoS dyed) anodized ale-

Ininuim surface is illustrated in Figureb 20 and 21 hoth prior and aftcr cxpe -

sure to a sinulated space enl-ironniliclt. '1lc ellcvi rolintielt is spiecifiedcol ti le

subject figurcs. The saunple' used in tliess test-s was 0. 125-inch thick conm -

Inc rcially pure altiminuni 1100 sheet anodized under the following conditinns:

Electrolyte 15"0 sulfuric acid

Current Density 0. 1775 amperes/cm 2

Temperature 305 r K

Time 120 minutes maximum

To produce the black coating, the pores formed on the aluminum surface by

the anodizing process are impregnated by the CoS dye. The initial surface

properties prior to the radiation exposure were a =0.957 and EN = 0.93.

The properties changed only slightly after exposure. The thermal emittance

was obtained utilizing a heated cavity reflectomgter (wVith the sample at

approximately 300"IK) which yields the normal emittance (fN). The value

of is estimated as 0.87 utilizing Figure 7.

C. CLEAR ANODIZED ALUMINUM

Bright polished aluminum surfaces which have been anodized provide

a basic solar reflector material which is relatively stable in the space envi-

ronment. Alurninurn is an excellent reflecting material for radiation in all

Darts of tle spectrum, while the oxide film produced in the anodizing process

is transparent in the visible region and relatively opaque in the IR region.

The normal ernittance of this coating is a direct function of the aluminum

-35-

p

C.(3 DYLD

ROOM 1LMPLILAIUOL

40C SOL/"R ABSOt'1ANCL a

- UNLXPOSLO 0.957

Q 3160 ESH AND 0 %33 3 101b u cm 2 LXPOSUtL

I--(_)Lu,J

20Ln lot-

0 0 4 08 1 2 1 6 20 24

S ~~WAVE'LENGI H, rmi.'m~l

"Figui'C 20. Solar Aeb8rptanC,ý of Black Anodized Aluminum (Ref. 19)

1 11

CoS DYEDROOM TEMPERAIURE

40 NORMAL

TOTAL IMIITANCE (N

0)

UNEXPOSED 0930

ul

' 30 3760 ESH At. 0924

101be Ct12 EXPOSURE

L)10-

#' 240

0 4 8 12 15 20 24

SWAVEL ENGT IH, mrns

Figure 21. Spectral Reflectance and Total Normal Emittanceof Black Anodized Aluminum (Ref. 19)

oxidc layer thickness as illustratcd in Figlirca 2'. Tlhe effect of the oxide.

layc r thikness and t en, p errature oll the l ie splicrical cilitta nec and solar

absorptancc is illustrated in ligure 23. The coating. is rt'lativc]ly stable but

does undergo an inc'rease in solar ab so r)t anlc N whiiCh is CauIs ed p ili Iri y by

1JV exposure. Laboratory and flight data suggest thiat there is a significant

variability ill the degriadation characteyristics frol)l batch to bit)ly t' vell n whien

an identical process is utilized.

Typical ch1anges ill Y8, i -sd 0n Orbiting Ast roitoliical Ohistrvattoi y

(OAO) and Orbit ing Solar CObscr'at Ory (050) flighlt data, are shown ill 1.it'Fur 2.1,

hbased on Ref. 20. T1le OAO panels consistcd of 1199 alun)iuii subst rate withli

hiet alumininu oxide layerie CStiluat ed at 0. 26 iiils v. ith an initial o 03. 1 6,5

0. 76. The thickness of the OSO-li oxidc layer is undcfincd.

I lic no rnial spectral reflectance of anodized aluillinun' as a function of

th" A1Z0 3 layer thickness is shown in Figure Z5, from data in Ref. 21.

D. OPTICAL SO1 UR REF1,ECTOIIS (SECOND SURFACE IIRROIS)

\Tac.uu, deposited films of silver or aluminu,, co-.'eredwit, wh% trans-

parent layer of sufficient thickness to achieve the desired high eniittance pro-

vide an ideal theronal control surface commonly referred to as OSl('s. These

surfaces have shown excellent stability in both near earth and synchronous

altitude orbital flights.

A typical OSR consists of a film of vapor deposited silver (-.1000 ang-

stroms) on a 6 mil fused silica facesheet with an overcoating of vapor de-

posited inconel metal which protects Hit qilver from corrosion or damage

2 0 Fine, tI. , An Insight into the Features of the OAO Thermal l)esipn, ASME' 3-ENAs-46, 20 Aug 1973.

1 Weaver, J. II. , "Bright Anodized Coatings for Temp-rature Control ofSpace Vehicles," Plating, 51 (19), 1165-1172, December 1965.

-37-

1i 10 - I I I I I I I - I

AlI lOIL ANODIZLD IN 15'1 SULI UIRIC ACID

1LMt'LItAIUtL 311 K

0.8-

Lu

h 0.6

0

a-

02

02

C - I , I I , 1i I I_0 2 4 6 8 10 12

COAl INC lIttCKNESS. rni'rons

Figure 22. Total Normal Emitace of Anodized Aluminum (Ref. 5)

10 PF I-- I Y " I I I I I I " I 1 -10'.-13L OTAL HEMISPHEtICAL 1 HICKNESS 01 OXIDF I AYE.H

[-ITA•C E - H 0.0001 in.

" 13---- 0,00027 m08 0-...-- 00005 in.

-0- --- Q 0.001 i.

nz 06

-- -. A - 1

€.- "~IHICKNESS OF OXIDE LYt

C o00001 10 00005 in.

(1, SOLAR0001 In ABSORP1ANC[. ccl

t 4

I _ . J . t L ___,_____,__1

0.2 _ _ _ _ _ _ _ _ _ _

-100 0 100 200 300 400 500 600

TEMPERATURE. 'FYigure 23. Total Hemispherical Emittance and Abscrptance Versus

Temperature of Anodized Alumninum (Ref. 16)

-38

0.32

Al 1199 ANODIZED IN I LUQBOH'C ACID

F LIGHT DATA

OSO HI SIPACCHAI 1

~- 0.24

C)

(f)

0 1000 2000I 3o00( '40a)

FXPOSURL H MI. I SH

FigureŽ 24. Solar Ahsorptanc'e ])cgradmlion of AnodizediAlhininuin Coatinf-, (Ref. ZU)

lOCDL I AY 1iIHIK I S

80 000WL026

0 000112 "~ \~ ~" L AI1I IKES

40-U'

01- --- j

0 2 4 6 8 10 12 14 16

WAVELENGTH, flHcrofl,Figure 25. Spectral Reflectance of Anodized Alurrxinum (R~ef. 21)

-39 -

while being handled. These mirrors, approximately 1"X I ', are then applied

to a substrate generally with a silicone adhesive such as, ]\TV 615. For ap-

plication on curved surfaces, where lhe rigid mirro.rs are not :deal, lhe

ý'ietallic films are deposited on flexihle transp;t.rcnt films such as fluorin.ited

ethylene-propylene (FEI') Teflon, IKapton or Mylar.

Typical properties of rigid O, Z's with 8 nil facesheets are summar-

ized in Table 4.

Table -1. Solar Abborptance and ILviiii.phlri-cal Emlinla.ce of Aluminumand Silver Optical Sol'a , I, -ctle t,or (oBi 'f. 4)

i51 liTemperature a t-(R) s l t /

Slver 160 0.050 0.744 0.067

360 0.050 0. 800 0.063

460 0.050 0. 8,13 0.061

Aluminum n60 0.2 0 0.800 0, 125

360 0. 10 0.810 0. 124.

460 0.10 0.800 0. 124

Norn.a] spectra] reflectance of a typical OSR with silver on 6 mui fused

silica is shown ii Figure ,16. Directional spectral ernittarce for an Aerojet

silver O.SR measui ed at angles fromn 0' to S0' is presented in Figures 27

and 28. Flcxllle OSR's utilizing Teflon or Mylar are produced with varying

thickness which strongly affects the emittance and to some extent tlhe solar

ab-orptance. The spectral absorparnce of silver coated Teflon is illustr,,ted

in Figure 29 for varying thickness of Teflon, based on data fronm Ref. 22.

L:nder, W., "Series Emittance Thermal Control Coatings,' Proceedings

or the Joint Air Force-NASA Thermal Control Working Group, 16, 17August 1967, AFML-TR-68-198, Aug 1968.

-40 -

100 '-• • I••• • "

80-

C- 60• SIt.VFR ON FUSE. SILICA

(-)I IEMI'LRAIURL - 70F

C 40 Of :0.07, ftt 0.80'4

-iU- F

20-

004 06 1 2 4 6 10 20 30

WAVELENGT;,, IcIoi[

Figure 26. Normal Spectral Reflectance for a SecondSurface Mirror (Ref. 6)

100 *I II

0 0 1030

z

° CCN)

F--

L1EMPIRATURE = 373 K

0 is 30 45

WAVELENGTH, icrons

Figure 27. Directional Spectral Emittance of Aerojet

Second Surface Mirror (Ref. 13)

-41 -

Q 0 !)O

S50

F-

"4.

M i [M i I , I kU R[L 313 K,

0 1 30 45

WAVLL f N m(1I(.M,

-•100, 1 1 1 ', 1 1"

,/• 0 600

/ 0-70

LU

UEM)PERAIUR1 313 K

015 30 45

WAVELENGI H, micion;

Figuire 28. Directional Spectral Ernittance of Aerojet SecondSurface. Miror (Ref. 13)

-42-

SOLAR EMITTANCE YTHICKNESS A3BSORPTANCEmils 0 (N

05 0.055 0422 0 131.0 0059 0 515 0.112.0 0.059 0 675 0095.0 0090 0802 01i

100 • '

_• z ~~~~[ill-1's'

Q

2z

CIDtr•"/5 il . 2

, 0 0 . 1 2

0.2 0.6 1.0 14 18 4 8 12 16 20 24

WAVELENGTH, microns

Figure 29. Spectral Absorptance of Silver Coated Teflon (Ref. 22)

The corresponding values for a and .N' and the ratio /cNP as evaluated

by the original source, are also shown. An approximate ,-.urve fit to deter-

mine the f N as a function of thickness is shown on Figure 30. Normal spec-

tral reflectance of aluminized Mylar is presented in Figure 31. The reflec-

tance of a number of metallized Mylar films are compared in Figure 32. rhe

FEP Teflon films appear to be relatively stable, based on laboratory UV ex-

posures. A 5 rnim thick, silvered Teflon specimen showed no detectable

degradation of a after 4600 hours of solar exposure in a 500 nmi orbit,

based on Ref. 4. Aluminized FEP Teflon (1 rnil thick) flown on the deep

-43 -

_=.

08

U

04

0.2

0 I I I I It I I II0 2 4 ~ '8 10

1lC;KNESS (31 lI P 111 ON I,

,jpigure, 30. No-irwal Fniittame of TE cfbii Thi r11 Cl~ss (Ref. 22

100 1 1 1 1 1 1111 1 1 1 111 1

8') cx.i~l01 0?2t). ( 14 0A[. 'ER 2000 £SII OF UV EXPOSURE,

CKs039

- 60 A ' ~I I. 2000

2oJ

OL I 1 11 1 1 11 111 L0 i 0.6 2 4 6 10 20 30

WAVELENGTH, microns

Figuire 31.. N\ormnal Spectral Reflectance of.Aluminilzed Mylar (Ref. 6-)

-44-

=!c

SILVER

ALUMINUM

C1 C, L D

S60 -COPPEP

ME1 AL~4 -

0COPPER 0.47

LGULL) 0 3020 ALUMiNUM 0 18

SILVER 0006

02 0.6 1.0 14 18

WAVELENGTH, microns

Figure 32. Reflectance of 0, 'i-MIL-MetallizedMylar (Ref. 22)

space Mariner V, however, showed some degradation as shown in Figure 33

from. data of Ref. 1 5. This is apparently due to particle bombardment which

is expected to far outweigh the degradation effects of UV exposure alone.

Aluminized Kapton, which is used in place of aluminized Teflon where higher

temperature requirements exist, is somewhat more sentitive to UV radiation.

Chlanges in reflectance following exposure to UV radiation are shown in

Figure 34.

F. SOLAR CELLS

Normal spectral reflectance for a Centralab silicon solar cell with a

6-mil fused silica facesheet is presented in Figure 35. The solar absorp-

tance (a s) of thi' specimen is 0.83. The corresponding value of eli based on

a 70*F sample temperature is also 0.83.

-45-

I) 3 1 1

32MAF0N1 PI V iAl A

,2

ý3' c 3o 0 j

000

028

0 120 30 40I XI'OSULJ IUEM fS 1,Xi

I NIJI. YEP feflun (I1cf. I 'ý)

10

2p , 131) LS1

u C41 ________II-_t______-

03 0 7 1 1 1 5 19 2.3

WAVFLEN(;TH. imu

V'igire,34 Spectral Reflectance Changes in Kapton11 Film, Following Exposu re to Ultra -

violet Radiation (Ref. 4)

-46-

100 -r-- r

S8 TEMPEH AI URL 70 f

of 0 0 ,8•f 083

K 60u-i

0 40

200 ' l , t ,I * . I. . * , , i ., ,

04 0 , 1 2 4 6 10 20 30

WAVELENGTH. ,crons

Figure 35. Normal Spectral Reflectance of CentralabSolar Cell (Ref. 6)

Directional spectral emittance for Aerojet solar cells is presented in

Figures 36 and 37 measured at 373'K (100'C) and Z00'K t..73°C), respec-

tiL'ely. Cover-glass thicknesses are not specified for the Aeroj.t cells,

Nominal cover glass thickness ordinarily ranges between 6 and 20 mils.

Degradation of solar cell conversion efficiency may vary significantly with

cover glass thickness and the specific orbit altitude and inclination. tHow-

ever, the solar absorptivity (a and hemispherical emittancc (f ', are rela-Stively insensitive to cover- glass thickness beyond about 6 mils and these

properties would not be expected io vary significantly.

Cell conversion efficiencies may degrade an order of ZO percent over

a three year period. However, since the initial conversion efficiency is on

the order of 10 percent, this degradation would represent only a 2 percent

increase in absorbed thermal energy by the cells. The corresponding in-

crease in solar cell temperature would be about 3°F. Therefore, the accu-

racy of the data presented for solar cells should be more than sufficient for

utiliztion in satellite infrared signature studies.

-47 -

100( 14

--- 60

C, -.- 0 70)

0~ 8oII

0 S304

, 0 30 405

100~~. 1 10

070

00

0 1530 45

WAVELENGI H, mcos

Figure 37. Directional Spectral Ernittance for AerojetSolar Cell at ZOO'I( (Ref. 13)

-48-

R ': IPR I:NC I'S

1. F .5. Johnson, -Thq Solar Constant, ' J. of M t'teorology, Vel. 11(l)cceniber 1154).

2. R. Stark, lhcrrinal T'c1sting of ]p'clcraft. Pce -rt No. TOR-0172(24.1 .0f1)-4,The Aerspacv Courporation, 1I:1 Scgundo, Calit. , (Scptenibbre 197 1).

3. R.. C. ;'eirrinx and T- Y. Smith, "Surface I<adiation 1Propcrtics frotmElectromagnotic Tlbcory," Int. .J. 1eat and Mass I rianstri. Vol . 1 1,pp 1567-71 (I3)68).

4. N. J. Broadway. Radiation Effects ttandbook. Section 2, ThermalControl Coatings, NASA-CR -1786. lPrepa red by Battelle MemorialInstitule, Columbus. Ohio (Aunt' 19711

5. Y. S. l'Iouloukian, et_;tl. , TIhe rniophysical 1ropc'rtes of Matter, TherrnalRadiative Prt.t'itics. Thcrnmophysical Propcrti'a Rt'Search Center,Purdue University ('1972).

6. Theirniophysical Properties Measurement, Unpublished Data, TRW,Redondo Beach, Calif.

7 . G.. Gubareff. Thern.al Radiation lPropertic's Survey. 2d Ed.,ttoneywell Ressearch Center, Minneapolis-Iloneywell Regulator Co.,Minneapolis, Minn. (1960).

8. E. Schmidt and I.. Eckert, "Uber die Richtungsverteilung derWarmestrahlung. " lForsch. Gebeite ingvnieorwesen, Vol. 6 (1935).

9. D. K. Edwards and 1. Catton, "Radiation Characteristics of Rough andOxidized Metals." Advances in Therniophysical Properties at Extreme"I emperatures and Pressur's, ASME, pp. 189-199 (1965).

10. R.. . Gaumer and IL. A. McKeder. Thermal Radiative Control Surfacestor Spacecratt, 1,MSC-704014, ILockheed Nlissile.sand Space Co.(March 1961).

11. A.-E. Hultquist, et al. , Ad'.'anced Thermal Control Materials Develop-ment, LMSC-A967871, Lockhecd Missilesand Space Co. (May 1970).

12. Unpublished D, ta, Aerojet Electro-Systems Co., Azusa, Calif.(13 July 1973).

-49 -

1 3 MI. Pa iir, ct al . , 'Opt ica I I ropc itics of 'sat cfl it c Nlit c ialIs,' (D raftCopy). Doumk m %Ný. 1 '1 1 Oo- ")-Y lilt I;I 1-d '1n1d Cpi tis Division,1:ti\i1tk~iittictO1 I<scalrc1 IrSlitutic uf Mikl iganl, Anti Arbor, Mci( I ilý P. 17, ) .

14. A. Eag Ic - ,(G;t'i riI IKlck t rnt Vail1cy l'orgýc sp C c Ctuic r, Phiil ad,,phi it,P; .i , sukbjt- ct : L. P-]I S - 1 Su rii ~t. (c (_o;.t ing Op It it, il I Pr o)'c t i cs 'I , st Dat a,Ilkurn comittnhiicdt ion (2 OCt. 19 7 -1)

15.~ . I C 1( o 1, NI,..tin V 'I Ciiipcrn-_tcirt COnlt I'0l Icf01CitCc l.S igt41,'I it -Ind lPtirt*(tllIt-ic' - Paipcr ptt1scuitt'd at \IAA 31-d i1 tlriopysics(mnfcrcn'll k, J Pl., Los Angclcs. Calif. 24 -26 lunt, P19o8.

10. .. !j. P jtltcnlioU.C. C1 11l., Vpw Mlaltn1iaIS I lit uldbukW, NPASA S')- 3025,supip. I (1,)60).

IT 1,. Wallktc k, Ik~o ]Jtcd Nlisbilec-.aml Spdc"c Co. , SkinnityvaIc-, Calif.pir1sonil.I (_ llnu1111itliatioln. ( 10 Oct P174 ).

I 3i. GC. Boi-sor, Atcros-pzt t Co rporation, Nlatcri al s Sc ivnkc 1. auoratory,tit run~o1m)1m cýln nt itoll, (5Sept 1974

ii. X PX . W' .\adu '111 1. .1 'r ugii, E*ffect s of S~imula~tt(d Spt.1C v hnvirjr -hootm (31) Ici:i,;t1 P tolitionl 1Popcirtics uf Sclcctcd Black CuatirnL~s

NASA 1 -l)-4 in .Sptttj r16)

20 . 11 . I inc. 'An Ins ight into ta c V ceatur cs of tht, c GA Thecrnial De)s ignt,ASVN1: 73-lKNAs---16. 1,20 Aug 1',7ý).

21. .1. 11. \Vv~iv.k r, -PIti-ght Aiodjz~cd Co)atings it- ' lt-iipcrature Control ofSpa. c\clicls. Plating~ 51 19), 1pp. 1165- 1172 (1)ccctnbcr 1965).

2 .' . W. Ij.itd Cr, SL'iý lt s1Kmitt 3 flU c Ihic rit t I C fil t C.01,11 Cotting-S. r oceCCClingSof Uthc J1oinit A il- 1--w-cc -NASA Tb yr ina C otit ml Wurking Group, AFliv I.-IT. -68-1)8 (Augus.-t 1168).

23 . WX'. Fische r, Ae ros pace ýorporatioii. personal (. iittuttlication

( 17 Nov 19 7 4).

24,. J . F. T raub, It crat ivo Mt-cthods for the Solution of ELI Luat iOtis, P renticec-H all Inc . E ngle~wood Cliffs, N.J3. , 196-1. pp. 22 1-224.

-S0 -

B11BLIOGR APlHIY

Caldwell, G. R. and 1'. A. Nel:son, '1Thermal Contrail Expt-rinlient till t-hCIunar Orbiter Spacecraft , Prop. Astronautics and Acrontamtics 21(1969), pp. S19-52.

Childors, ]i. MI. and 3. N. Cerceo, -Electron Beam Tecilliqi'Ms folrMeasuring Th,,rmophvs i cal ropertie. of Ni na er a WADI)D-TIR-0-19)0(AD 272691).

Dunkle, R. V., "''hernial R adiation Characteristics of Surfaces," inTheory and Fundamental Res.a rch ini lHeat Trailsfer. (.1 . A. Clark, cd.),Pergainon tPress, New vork (1 (o 3).

Ecke rt, E. R . G. and R. M. Dra1ke, .1r. , IHeat and Mass Transfer, McGraw,-H ill, New Ytrk t1959).

.lakob, M., Hleal Transfer, Vol. 1, Wilt-y, Now York ( 194')).

Langley, R. C. , et al. , inorganic Films for Solar Eme rgv Abn' ]pt ion,ASD-TDR--62-92, (1963) ,AD424099),

Love, T. .l. Radiative Ii, at Transf1er, E. Merrill Punbl ishing Go.Columbus, Ohin (19 68).

Siegel, R. and 1. R. Itlowell , Thermal Radiation lni at Transfer,McGraww-till 1973, New York (1972).

"Wcibelt, J. A., En'i neerintj" Radiation 1eal T1ransfer, Hloll Rinellart andWinston, New York (1966).

Wolfc, W. L. , IHandbook of Military infrared T'Fc-hnol,)gy, Office of NavalResearch, University o)f Nicjirn1, IT... Gov't trintino Office ( 965

-51 -

APPENDIX

SURlFAC,; RAI)IATION PROPIERT IES FRtOM

'ILECTROMAGNIETIC TllCOtlY

-ito obtain accurate infrared signatures ol satellites ill orbit, detailed

spectral a1nd dLrectional prot' Wrties are required. 1These surface. properties

are betst obtained experItlirLontally aind to the ext cnt , a'-ilable have oeen pre-

sented in thi,s; report for a selected n1umber of uxiterials and coatings. Iow-

ever, in some, cases, the experinmental daita are Intomlplete te r itnsufficient

in Scope or dtail. In these cases, one may, resorl to physical iltodels to

predict sor'acep•, p rt iris either empirically or tit-rough fundanmental re-

lations, to suippleneit or replace exxperimteuntal d13ta!

A method of providi-ug this capability to obtain directional surfacei -~~~~~~~~:' -- :-^ ••," ai sn•,Irse

properties from nt'xi ' ja, and normnal errittance dat-a, sinn.ros nel

relations derived from electromagnetic theory is presented herein abstracted

from Ref. 23.

M ETITOD

Pasic electromagnetic theory predicts that directional surface

prope rties are related to the optical constants defined by the complex index

of refraction, Pý:

ii = n(I - ik) (1)

where v and k are refraction index and absorption index, respectively.

and i = .- i

2 3 W. Fisch~er, Aerospace Corporation, personal communication, 17 Nov. 74.

Preceding page blank

D)irectional 0e'ijttalice of 1olt~l 11nagriitic ~illital)5aiccý jul ullitormtl)

pol.arize(1, incidenit rfldj~ti'.)n iti given iii Rcf. 3 by (2):

(0 - i. 1 (Oj(.

Wile (,r

(0) aU.i~d (0) aethk' dire ClitiaJ~l elctneipallad

pope 'n die ul a to the planec of 11W id C lCc' , ieaectiV'

and

() (lip - cos 0) 2 + T) [t 12)(0) 211 (n 4 ,- cus 0) n [1 ]2

(Y__ 2?(1 1 -Y

0 is tho p)olatr angle of incidence relative to gIhe so ifarm- no rmlal and a, fp, ad

are defined by:

2 + &sin? 2) 0 (4 s~ ~ ill 01

2 1~2 ,.2 z2 (1k II~k + R jI (6)

54-

r2

-Y 1 (7)1 k i a-F(I + -k ) (1 4 1k )

Normnal cmittance ib obtained for the case where 0 0 arn is .ven by

- = - 4n6

IN (n + 1)2 + n2 k z

Hemispherical e-nxittance is calculated by integrating directioral emittarice

over half space.

I

J ((0) d (sin 20) (9'0

Often only hemispherical and normal ernittance data arc presented in

tables :)t experimental data. To obtain value,, for optical constants, n and k,

which correspond to values o, k&1 and EN' E-;qn (8, and Eqn (9) must be solved

siinultaneously. Solt,tion to these expressions is a formidable task and

numerical techniques are employed to obtain values for r. and k.

For perfect dielectrics, absorption index, k, is zero. Thus either

refraction index, n, or hemispher-ical ernittance, E1 1 ' or normal cinittance

(N) wil' uniquely define dielectric surface properties.

Expressions for normal emittance and hemispherical eniittance

sirmiplify to

N 4n (10)(n + 1)2

-55.

and

] 1 ( 3 n 4 1)(n - ) n - 1) 1 n -n

6 (n 4 i 2 1n2 ) n

3 12 (144 1in-L Zn - 1) 8nI ((4 4 (I(n 2 + 1)(n 4

- 1) (n2 4. i )(1 n )

Often only hemispherical emittance or normal ein'ittance values are

provided for dielectrics. If normal emittance is provided, a simple solution

for refraction index, n, results. Conversely, the soluation for refractive

index, n, becomies more involved when only hemispherical ernittance is pre-

sented. Us ing the Newton Raphson method (Ref. Z.A) a value for it may be

quickly obtained.

n =m r -

If experimental data are not available to provide spectral or tetal

cnnilttance property values, empirical models, semi-empiricaJ models, or

classical mechanical mcde2ls may be employed. Selection, (t an appropriate

model to evaluate surface properties often depends upon the spctra rge

ovei whici data arc: rcqoir.ed or availability of model parameter value s. Care

should be exercised to apply models over applicable spectral and temperature

ranges and to use the most accurate source of data available.

±ATraub, .1. F. . Iterative Methods for the Solution of tI;QUatio na, Prentice-

Ilall Inc., thingt _w od Cliffs, N. J., 1964, pp. 221-224.

-56-

ACRONYMS AND SYMBOLS

Al symbol for the element aluminuin

ATS Applications Technology Satellite

ERTS Earth Resources Technology Satellite

ESII equivalent sun hours

e/cxn integrated exposure of electrons per square centimeter ofsurface area

FEP fluorinated ethylene propylene

OAO Orbiting Astrononical Observatory

0SO Orbiting Solar Observatory

OSR optical solar reflector

Sn symbol for the element tin

T temlperat-ure, the scale as defined

Ti symbol for the element titanium

V symbol for the elcment vanadium

aabsorptanee, the ratio of the absorbed radiant flux to theincident flux

"emittance, ratio of the radiant emission of a surface to thatemitted by a blackbody radiator and the same temperatureaand wavelength

p reflectance, ratio of reflected radiant flux to the incidentflux

0, 0' the angle measvred from the normal to a surface to describethe directional location of the viewer and incident ray,respectively

the azimuth angle to describe the orientation of the viewerwith respect to the in ident ray as measured in the plane ofthe surface

-57-

A

Subscript s

N conditions for incidencte or viewing through an angle e that isessentially nornald to the surface

SII conditions for incidence or vielding over a hem'iisphericalregion, i. c. , 2rT steradians of the surface

s Having the wavelength distribution of the sun. When used inconjunction with ac the spectral absorptance ot the specimenhas been integrated over the wavelength distribution of thesun.

T signifies total, When used in conjunction witlh ey or E it repre-sents integration over all wavelengths from 0 to infinity

SX wavelength or a narrow band of wavelengths

0 the angle measured from the normal to the surface definingthe geometric conditions of viewing

-58-