8.')6

J. Opt. Soc. Am.IVo!. 71. No. 7/Ju ly 1981

V. R. Weidner and J . J . Hsi

Reflection properties of pressed polytetrafluoroethylene powder Victor R. Weidner and Jack J. Hsia Rudiomet ric Physics Division, National Burea u af Stan dards, Washing ton , D.C. Received Decemb er 18,1980 ; revised man uscript received Feb ruary 12, 1981 T he reflection propert ies of pressed polytetrafluoroethylene powder have been under investigatio n by the Radiomet ric Ph ysics Division at the Nati onal Bure au of Standards for the past five years. T his mate rial has a great pote nt ial use, both as a sta nda rd of diffu se reflectan ce and as a coating for integrating spheres for applications in reflecta nce spect rophotomet ry and ot her signal-averaging devices. It possesses certa in physical and opt ical proper ties that make it idea l for use in these application s. T echniques are given for prepar ing reflection sta ndards and coating integrating sp heres with the pressed powder. T he effects of powder den sity and thickness on its reflectance are reported, and observat ions of possible problems with fluorescence th at are du e to the presence of conta minants in the powder are discussed . The absolute reflectan ce (6°/hemispherical reflectance factor relative to a per fect diffu ser) is reported for the spectral range of 200- 2500 nm. T he d irectional/h emisph erical reflectan ce facto r relat ive to 6°/ hemispherical reflectance is given for several wavelength s in the ultr aviolet and visible spectrum and for angle s of incidence bet ween 5 and 75°. T he bidirectio nal reflectance facto r is repo rte d for 300, 600, and 1500 nm at angles of incide nce of - 10, - 30, -50, and - 70° and at viewing angles at 10° intervals from - 80 to + 80° .

INTROD U CTION T he reflect ion p rop er t ies of pressed polyte trafluor oeth ylen e (PT F E ) powder! in t he ultraviolet, visible, and near-inf rared spectral regions, com bined with certain desirable physical characte ristics, mak e thi s material exceptionall y useful when the re is a need for a good white diffuser or a standar d of diffuse reflectance. T he diffuse reflectance of pressed PTFE powder is remark ably high over the spect ral ran ge of 200-2500 nm and is probabl y highe r t ha n th at of an y ot her kn own material , its reflectance being 99% or higher over t he spect ral ra nge of 350- 1800 nm. It is par t icu larl y useful as a coati ng for inte grati n g spheres commonly used in diffu se reflectance spect rophotometry . Hi stori call y, the most commo nly used mate rials for t his purpose have been magne sium oxide and bar ium sulfate powders or bari um sulfate paint.? Much has been written on the optical pr oper ti es of t hes e mat erials, and the adva ntages and disad van ta ges of t heir use in spec t ro photomet ric ap plicat ions have been experienced by nearl y everyone involved wit h reflect an ce spectro photo met ry. Certai n ly, the re is no longer any advantage in coati ng integrating spheres by the old method of burning magnesium an d collecting the magnesiu m oxide smoke. T he standards that still specify th at reflecta nce measure ments be reported on a ph ot ometric scale re lative to smo ked magn esium oxide sho uld be rewritt en to specify that reflectan ce measur em ents be rep orted on an a bsolut e reflectance scale (relative to a perfect d iffuser-'). A review of p ubli cati on s on pr op erties and reflecti on values of material reflection standards is given in Ref. 2. T he purp ose of thi s pa per is to make available the techn ical findings of several years of resear ch on the optical propert ies of pr essed PTFE powde r. Ot he r authors" have rep orted on t he pot ential usefu lness of this mat eri al as a reflecti on stan da rd . The sp ectrophotomet ry grou p of the Rad iom etric Physics Division at the National Bureau of Standards (NBS)

bega n studies of the refle cti on pr ope rtie s of pr essed P T powde r in 1975. At t hat time, this group was engaged in co st ruc t ing a new reference spectro photo met er for diffuse r flect an ce" along with a nu mber of accessory devices for me suring diffuse reflec tance. T he se accessory devi ces incl ud integrating spheres for mea suring 6°/hemispheri cal reflee tanc e facto r, 45 % ° refle ctance facto r, and di recti on hemispherical reflectance factor and a specular reflectome ter. At the sam e time , a more accurate absolute diffuse reflectanc scale was being established by the auxiliary sphere or Van d Akker method." A technique was de velope d for coating i tegrating spheres with the PTFE powde r, and exte nsi measur ements of the optical proper ti es of this ma teri al we ca rried out over a five-yea r peri od . T he results of the studies are repor ted here along wit h some descr ipt ive detai of the tec hniques used in preparing reflect ion standa rds fro PTFE powder and its application to integrating spheres.

PREPARATION T he PTFE powder is somewhat lumpy as it comes in a shi pi ng drum. It ca n be pr epared for optical-coating purpos by reducing it to a uniform low-density powder. T his can acco mp lished wit h a blender or othe r cho pping devi ce, pref era bly one with stainless-ste el blades and a glass container T he powder should be kep t in glass containe rs and handl wit h too ls mad e of materials such as stainless steel t hat less likel y to contamina te the material. T he powder will a her e to itself on pre ssing, bu t it does not adhere well to me glass, or plastic. One techni que for makin g it adhere to me (or ot he r ma terials) is first to coat t he metal with a t hin fil of high-vacuum silicone grease. Once this is done, the powd can be p ressed into place in thickness es varyin g from 1 to 1 mm without mu ch diffi culty. The silico ne grease has a 10 volatility and does not affect the reflectio n pro perties of t

Vol. 71. No. 7/J uly 1981/J . Opt. Soc. Am,

V. R. Weidner and J . J . Hsia

Fig.1. The NBS integrati ng-sph er e-coat ing ap parat us showing th e hemisphere man ipulat or with t he electronica lly cont ro lled tamping 'bl!ad used to pr ess t he PT FE powder int o t he hemisph ere with a uniform radiu s.

PTFE powder becau se it contacts only a thin layer at t he powder-met al interf ace. PTFE powder has been successfully applied to integratin g spheres ranging in diameter from a few cent imete rs to 45 cm and to large, flat sur faces used as reflecti on standards. T he coatings are not affected by conditions of high humidity because the pre ssed powder repels water. Exposur e to highintensity ultr aviolet radiation," such as that encountered near uenon arc, will slightly degrad e t he reflection propert ies of ,:the coatings in th e ultraviolet spectral region. The reflectan ce will be degrad ed as a resul t of cont aminat ion from smoke, dust, or other contamina nts that may be pr esent. The time required for coating an integ rati ng sphere of llO-cm diameter with a 6-mm-t hick coating is typically about '. h. Thi s includes pr epa ring th e PTFE powder with a blender, t rimming up t he finished coat ing around t he ports, IIId assembly of the usual two hemispheres. At NBS, spheres up to 45-cm diamet er are coated with the aid of a special b!tnj~phere manipulator (Fig. 1) t ha t enables the te chnician to rotat e and pivot the hemisph ere about th e cente r point of the sphere and pack the PTFE powder into t he curva ture of the hemisph ere with a light pressur e, using hand tools such 18 a stainless stee l spoon or a round-bott om glass flask. In 8OI1le special application s, such as in th e preparation of an Illliliary sphere for determining the absolute reflectan ce of the sphere coating, the rad ius of curvat ure of the coati ng lUrface is caref ully controlled by tamping t he PT FE powder lIDder an electronic tamping head before it is rolled to a final ilniah. A retain er rin g is attached to th e hemisph ere flan ge 4uring the coating procedur e to retain th e powder at the open of the hemisph ere. When t he coati ng is completed, this 'ning ring is removed, and the two completed hemispheres attached to each ot her by mean s of th e flan ges. T he . E coating expands slightl y at the hemisph ere edge when 11 ~reta ining ring is removed . This is an advantage because atesults in t he form ati on of a t ight fit when th e t wo herni~res are combined to form th e sphe re. ;8 Aa with any sphere-coat ing material, PTFE powder has its of Idvantal!'es and disad vantaz es. Some pra ctice is requ ired in

857

ord er to mast er t he coat ing tec hniques. T he powder should be packed in lightly at first, to a dept h of ap pr oximat ely 2-3 times the desired final th ickness before it is pressed to a finish. T he application of added powder to an already hard -pressed coating may result in a peeling and separation of t he material into layers. T he best results are obta ined with a sand-blasted or ground -glass pressi ng tool, such as a round -bottom flask of small diameter. The coating should be done in a relat ively clea n, du st -free environment because t he PTFE coat ing usually becomes electrostatically cha rged during the pressing and will hold sma ll specks of dirt or lint. T hese can be picked up or removed with a small, clean art ist 's bru sh . PTFE powder is very fine and easily mixes in the air about the working area. Alth ough the powder is believed to be nontoxic, it is a good practice to use a du st mask t o avoid un necessary br eath ing of the material. PTFE may form toxic gases at thermal decomp osition temperat ures above 400° C. T hese toxic pr odu cts may be produ ced inadverten tly by contact of t he fine powder with a flame or ot he r high-temperature source. For further infor mation on t he safe handli ng of fluorocarb on resins, refer t o Ref. 8. It should be noted th at t here are U.S. patents dealing with the use of fluorocarb ons as coatings for integ rating sphe res. Th ese U.S. patents ar e given in Ref. 9.

DENSITY Th e reflectance of PTFE powder is influenced by th e density to which the powder is pressed. Th is relat ionshi p is illustrated in Fig. 2, in which t he results of a number of measu rements of reflectan ce versus powder density are plotted. T he reflectan ce scale in thi s illustr at ion is a relativ e one with the da ta normalized at a density of 1.0 g/c m-', because t his is the density at which the powder reflectance is t he highest. T here seem to be no noticeabl e wavelength -relat ed effects for wavelength s less than 2000 nm. T he vertical bars in Fig. 2 show t he spread in reflect ance values at th e 10 selected 1. 0 0 5, r - -- - -- - - - - - - ,

PTFE POWDER 1.0 00

, \

'+\ \ \

0 .985

,

• 300 to 180 0 nm (overage of 10 wave-

\

\

lengths)

.2000nm

\

\

(averoge of 2 ru ns)

O.980,L.---J._-'-_.L...-----L-=--...........__=_' 1.0 1.5 2 .0 0 .5 DENSITY (o/cm 3 )

Fig. 2. Pressed PTFE powd er reflectan ce as a fun cti on of powde r den sity (lO- m m t hickne ss) .

858

V. R. Weidner and J. J . Hs

J. Opt. Soc. Am.N o!. 71. No. 7/J uly 1981

Table 1. Reflectance of Pres sed PTFE Powd er (0.8 g/cm 3 ) Relative to T hat of a lO-mm-Th ic k Layer at Each Wavelength T hickness (rnm )

400nm

1 2 4 6 8 10

0.991 0.995 0.998 1.000 1.000 1.000

Relative Reflectance 550 nm 0.989 0.994 0.998 1.000 1.000 1.000

700 nm 0.986 0.993 0.997 0.999 1.000 1.000

wavelengths. The maximum reflec tance is obt ained by compressing the powder to a density of from 0.8 to 1.2 g/cm''. T he loose powder has a density of approximately 0.5 g/cm''. A sample of the pres sed powder having the maximum reflecta nce can easily be prepared by compressing 2-2.5 volumes of loose powder int o 1 volume. This resul ts in a pr essed sample that is rather soft. A sample compressed to a den sity of 2 g/cm" will have a much harder surface and slightly lower reflectance. Higher den sities require higher pressures, and a mechani cal pre ss must be used . As is shown in Fig. 2, the reflectan ce does not vary much with change in powder density for the den sity range of 0.8-1. 2 g/cm '', Repeated preparation of pre ssed samples having reflectances that are the same to within a few te nths of a per cen t can easily be ach ieved , and a fresh sample can be prepared in minutes. At wavelen gths greater t han 2000 nm the reflectance dr ops more rapidly with increasing powder den sity.

chromatic illum ina tion of t he PTFE powder and the n wi polychr omatic illumination. In Table 2 the norm alized di ference D between t he refle ctances obtai ned by t hese t mode s of illumination is given as D = (R m - Rp )!R p, whe Rm is pr oportional to the refle ct an ce of the PTFE powd sample relative to that of an aluminum mir ror with mon chromat ic irradiation and R p is the same relative reflectan with polychromati c irradia tion. There appeared to be a slig amount of fluore scen ce with excitation at wavelen gth s I than 290 nm and emission in the wavelength ra nge of 310-3 nm . Subsequent measurements with a spectrofluorometer confirmed these conclusions. T he levels of fluorescence observed may be negligible f most rad iometric and spect rophoto metric applicatio However , in applications in which a small am ount of fluor cence can cause a significant err or, it would be wise not to any kind of pressed -powder coat ing with out first checking f the presence of fluorescence t hat is du e to contaminan espec ially when ultraviolet radiation below 300 nm is i volved.! '

REFLECTION PROPERTIES The 6° /hemispherical reflectance factor of a diffuse refl tance st andard is probably t he most imp ort an t calibratio The term 6°/h emispherical reflectan ce refers to a measur men t geomet ry in which the sample is illuminated by a col mated source incident upon the sample at 6° from th e norm

Table 2. Wavelength (nrn)

THICKNESS T he diffuse reflectan ce of pressed PTFE powder is influenced by the thickness of the powder layer . Its translucency is such that at least a 6-mm t hickness at a density of approximately 1 g/ cm'' is required to obtain maximum refle ctance. T able 1 shows the influence of thickness on the reflectance of PTFE powder at 400, 550, and 700 nm. These mea surements were made wit h an aluminu m backing having a reflectance of approximat ely 60%. Later mea surements in the infrared out to 2500 nm indicate that 6- to 8-mm t hicknesses and a density of 1 g/cm'' are adequate in order to achieve maximum diffuse reflectanc e values for the pressed powder. Coatings this thick are not required in applications where the coating is used in a signal-averaging sphere. However, when the PTFE powder is used as a reflection standa rd , a 10-mm thickness will be cert ain t o give t he maximum reflectance at all wavelengths.

FLUORESCENCE T here ha s been no clear evidence to show that the PTFE powder itself fluoresces. However, coatings made from PTFE powder show a very weak fluorescence. This fluorescence may be associated with a slight contami nation from the plastic bag used to contain t he PTFE powder in its shipping drum. As a t est of the cont ribution of fluorescence to th e reflectance of P T FE powder, a specimen was first measur ed on a doublebeam refle ctance spect rophot ometer with t he use of mono-

Da ta fr om F luorescence Check Wavelength D

(n rn)

D

250 260 270 280 290

0.019 0.016 0.022 0.Ql8 0.005

500 510 520 530 540

0.000 -0.002 0.000 0.000 0.002

300 310 320 330 340

- 0.004 - 0.Ql8 - 0.011 - 0.021 -0.015

550 560 570 580 590

0.000 0.000 0.000 0.005 0.001

350 360 370 380 390

- 0.009 -0.007 - 0.006 -0.00 3 -0.002

600 610 620 630 640

0.006 0.004 0.001 . 0.001 0.000

400 410 420 430 440

- 0.006 - 0.002 - 0.002 0.002 0.002

650 660 670 680 690

-0.002 0.000 - 0.002 - 0.002 -0.006

450 460 470 480 490

0.000 0.002 -0.002 0.002 0.002

700 710 720 730 740

- 0.003 - 0.004 - 0.003 0.002 -0.008

750

- 0.007

:v.R. Weid ner and J . J. Hsia

Vol. 71, No . 7/J uly 1981/J . Op t . Soc. Am .

859

Ta ble 3. 6°/Hemispherical Reflectance Factor of a lO-mm-Thick Pressed PTFE Powder R el ative to a P erfect Diffuser A

A

A

(nm )

p

(nrn)

p

(ri m)

p

200 210 220 225 230

0.962° 0.964° 0.967 0.968 0.969

950 1000 1050 1100 1150

0.994 0.994 0.994 0.994 0.994

2140 2150 2160 2170 2180

0.964 0.965 0.967 0.970 0.973

240 250 260 270 275

0.971 0.973 0.976 0.978 0.979

1200 1250 1300 1350 1400

0.993 0.993 0.992 0.991 0.991

2190 2200 2210 2220 2230

0.975 0.977 0.977 0.978 0.978

280 290 300 310 . 320

0.980 0.982 0.984 0.985 0.987

1450 1500 1550 1600 1650

0.992 0.992 0.992 0.992 0.991

2240 2250 2260 2270 2280

0.977 0.977 0.976 0.976 0.975

325 330 340 350 360

0.988 0.988 0.989 0.990 0.990

1700 1750 1800 1850 1900

0.990 0.990 0.990 0.986 0.985

2290 2300 2310 2320 2330

0.974 0.972 0.971 0.970 0.968

370 375 380 390 400

0.991 0.991 0.991 0.992 0.993

1950 2000 2010 2020 2030

0.984 0.981 0.979 0.978 0.976

2340 2350 2360 2370 2380

0.966 0.965 0.964 0.963 0.963

450 500 550

0.993 0.994 0.994 0.994 0.994

2040 2050 2060 2070 2080

0.975 0.973 0.972 0.971 0.970

2390 2400 2450 2500

0.962 0.962 0.961 0.960

0.994 0.994 0.994 0.994 0.994

2090 2100 2110 2120 2130

0.969 0.968 0.967 0.966 0.964

600 650

IOO'..----r-

and in which the reflected flux is averaged by an integ rating here-detector system. An angle of 6° off t he normal is t'OInmonly used to allow for including the specular component In the measur ement. The 6°/ hemispherical reflectance factor of samples of P1'FE powder pre ssed t o a t hickness of 6 mm or more with a 'ty of 0.8-1.2 g/cm '' is very repr oducible. For a set of - pIe)! made at differen t t imes from different lots of P T FE ~er. a stan dard deviat ion of less t han 0.001 in measured ce was found for the wavelength range of 400- 750 nm. ~r measur ements will have to be made in order to esr-_~ ~ what t his dev iati on will be in the ultraviolet and in" veQ regions. The 6°/ hemispherical reflectance factor of PT FE powder is given in Table 3 for a PTFE coating IllInin thickness and having a density in the range of 0.8-1.2 ClIl3. The data listed in T able 3 are plotted in Fig. 3. T hese are reported on an absolute reflectance scale (relative to Peifect diffuser ) and were obtained by the aux iliary sphere

-

-...-

--T'"- --r--

--,

99

E ~ 98

z

t:!

(,)

w

~ 97

w a::

96

6°/HEM ISPHERICAL REFLECTANCE OF PT FE POWDER

.L...._""""__-'

95 L-...J._ _ _ _ 200 500 10 00 1500 ~

2000

2500

WAVELENGTH (NM)

Fig. 3. T he 6° /h emi sph erical reflect ance fact or (relat ive to a perfect diffuser ) of 10-mm -thick, I -g/cm't-den sit y pr essed PTF E powder .

860

V. R. Weidner and J. ,J. Hsia

,J. Opt. Soc. Am.IVol. 71. No. 7/July 1981

Table 4.

Direc t iona l/Hemispherical Reflectance F actor of P r essed PTF E Powde r fo r a IO-mm -Thick Coating (Re lative to Hem ispherical R eflec tanc e at 6° Incide nce)

Angle of Incidence

250 nm

300 nm

Reflectance :350 nm

450 nm

600 nm

750 nm

6° 10° 15° 20° 25° 30° 35° 40° 45° 50° 55° 60° 65° 70° 75°

1.0000 1.0003 1.0009 1.0017 1.0026 1.0038 1.0052 1.0066 1.0082 1.0099 1.0117 1.0134 1.0153 1.0170 1.0186

1.0000 1.0002 1.0005 1.0010 1.0015 1.0022 1.0030 1.0038 1.0047 1.0056 1.0065 1.0074 1.0083 1.0090 1.0097

1.0000 1.0001 1.0004 1.0007 1.0011 1.0015 1.0021 1.0026 1.0032 1.0038 1.0044 1.0049 1.0054 1.0058 1.0061

1.0000 1.0001 1.0002 1.0004 1.0007 1.0010 1.0013 1.0016 1.0020 1.0023 1.0026 1.0029 1.0031 1.0032 1.0032

1.0000 1.0001 1.0002 1.0004 1.0006 1.0008 1.0011 1.0014 1.0017 1.0020 1.0023 1.0025 1.0027 1.0028 1.0029

1.0000 1.0001 1.0002 1.0004 1.0006 1.0008 1.0011 1.0014 1.0017 1.0020 1.0023 1.0025 1.0027 1.0029 1.0029

1.025r---- - -- - - - - - , DIRECTIONAL/HEMISPHERICAL REFLECTANCE OF PTFE 1.020 POWDER 250 o (NM) W

W Uo z l- 1.0 15

~

U W w~

...Jti

~...J 1.01 0 a::: W a::: 1.005

350 450 -:::::--750

1.000L..1lliiiI~~---L--L.....L."""'..J

6 0 20° 40° 60° 75° ANGLE OF INCIDENCE Fig. 4. The directional/hemispherical reflectance factor of pressed PTFE powder as a function of angle of incidence and wavelength.

method." T hese absolute reflectan ce data ar e believed to be accurate to wit hin ± 0.002 on t he basis of the analysis of t he kn own sources of error as discussed in Ref. 7. T he direct ional/hemispherical reflectance factor of pressed PTFE powder given in T abl e 4 shows how the hemi spherical reflecta nce of the material varies as a fun ct ion of the angle of incidence. These measurem ents wer e made by mean s of an integrating sphere accessory to the NBS refer en ce spectroph otometer for d iffuse refle ctance. This accesso ry consisted of a 45-cm -d iame te r sphere designed so tha t the sam ple can be mounted in the cente r of the sphere where it can be rotated to cont rol t he a ngle of in cidence . The measurem ents were made at several wavelen gths in the ultraviolet and visible spect ra for both vert ica lly and hor izon tall y polari zed sample

beams . The results list ed in T abl e 4 and illus tr ated in F ig. 4 are an average of th e tw o polarizati on s. The valu es for eit her polarization did not vary by more than ± 0.5% from the ave rage. T he most noti ceabl e changes in reflectan ce as a function of angle of incidence occur in the ultraviolet spect ral region . T he data are relati ve to the 6°Ihemispherical reo flectance fact or valu es. The bidi recti onal refle ct an ce factor of pr essed P TFE powder listed in T able 5 shows how the reflectance varies over a wide ran ge of viewing angles for fou r d iffer en t a ngles of incidence. These measurem ents wer e ma de by mean s of the NBS specular refle ctorneter .f This ins trument is ordina rily used for sp ecular reflectance measurem en ts as a fu nct ion of angle of incidence. It can be used to measure t he bidirectional refle ct an ce of a diffuse sa mple in t he plane of t he inciden t sam ple beam . Measur ements were mad e at 300, 600, an d , 1500 nm for angles of incid en ce of - 10, - 30, - 50, and - 70°. The ang les of viewing were selected at 10° inte rva ls from - 80 to + 80°. The bidir ectional reflectance data reported in T able 5 are an ave rage of t he verti cal and hor izon tal polar izat ions. The values for either polari zat ion varied from this average by as little as ± 0.2% to as mu ch as ± 20% dep end ing on the combination of beam in cidence and viewing angle. T hese da ta are relative to t he reflect an ce values obta ined at 0° (norma l) viewing. T he results list ed in T a ble 5 are repr esen tati ve of the bidirectional reflect an ce of a " rough" sur face prepa red by pr essing the PTFE powder with a coarsely ground glass plate. Sa m ples pr essed with a polish ed glass plate exhi bit slightly higher valu es of reflectance, par ticularly at viewing angles greate r th an 75°. Figur e 5 illustr ates t he bidi recti onal reflectan ce pr oper t ies of p ressed PTFE powd er at 600 nm. Simi lar results are shown for bari um su lfate powder in Fig. 6. Further studies of the reflect ion propert ies of pressed PTFE powder are bein g mad e at NB S. Among these pr opert ies are the 45° 10° or 0°/45° reflectan ce factors for the visible spect ral region. The resu lts of these stud ies will be pu blished at a futur e date.

Vol. 71, No. 7/J uly 1981/J . Op t. Soc. Am.

V. R. Weidner and J . J . Hsia

Table 5. Angle of Viewing - 80 0 - 70 0 - 60 0 - 50 0 _ 40 0 - 30 0 - 20 0 _ 100 00

100 20 0 300 40 0 500 60 0 70 0 80 0

861

Dir ectiona l/Direc t ion al R efl ectance Factor (Bidir ect iona l Reflectance ) of lO-mm -Thick Pressed PTFE Po wd e r 300 nm - 30 0 - 50 0

- 100

0.792 0.867 0:914 0.949 0.979

0.763 0.836 0.882 0.919 0.946 0.967 0.988

0.872 0.937 0.980

1.000 0.991 0.982 0.970 0.960 0.943 0.921 0.882 0.817

5.0r---

1.148

0.743 0.822 0.872 0.909 0.939 0.963 0.986

~~

- - - - - PT FE POWDER (60 0 NM) ANGLE OF INCIDENCE : -100 - - - - - -

WW

-300- - - - - -

w ~~ 4.0 u~

-

-10 0

1.074 1.022 0.996 0.982 0.981 0.988 1.000 1.021 1.052 1.098 1.163 1.265 1.426 1.69] 2.190

1.000 0.993 0.991 0.994 1.000 1.008 1.022 1.036 1.056 1.085 1.125 1.167 1.2] 4

1.005 1.000 1.000 1.001 1.001 0.999 0.997 0.996 0.992 0.974 0.937

- 70 0

0::0

1.000 0.991 0.983 0.971 , 0.957 0.938 0.914 0.872 0.802

-

-50°- ' ~>3.0 wo

Angle of Incidence 600 nm - 30 0 - 50 0 0.827 0.898 0.946

0.761 0.840 0.891 0.931 0.967

0.982 0.980 0.984 0.991 1.000 1.012 1.027 1.048 1.076 1.114 1.168 1.227 1.291

0.998 0.998 1.000 1.002 1.003 1.004 1.004 1.002 1.000 0.985 0.946

- 70 0 1.048 1.019 0.987 0.974 0.971 0.974 0.983 1.000 1.027 1.067 1.125 1.212 1.343 1.549 1.897 2.55]

5.0-

---,

1500 nm -10 0 gnhr - 30 0 -50 0 0.702 0.760 0.819 0.875 0.920 0.959 0.978 1.000 0.998 0.986 0.971 0.942 0.895 0.832 0.754 0.660

0.830 0.882 0.918 0.952 0.974 0.980 0.990 1.000 1.005 1.012 1.015 1.018 1.022 1.030 1.035 ]. 040

- 70 0

0.820 0.855 0.886

1.045 1.000 0.992 0.980 0.980 0.985 0.995 1.000 1.010 1.060 1.155 1.270 1.440 1.695 2.075 3.090

0.936 0.945 0.966 0.982 1.000 1.010 1.018 1.040 1.066 L.100 1.155 1.235 1.340

- - - -- - - BARIUM SULFATE POWDER (60 0 NM) ANGLE OF INCIDENCE: - 10 0 - - -- - -

-

-,

- 300 - - - - - 500 _0 _ 0 - ' - ' - 7 00 •. • 0. • 0 •• '0 0

-0_ ' - ' -

-70°0• • . •• •• .• • 0

....J O

1-

" ROUGH"

~t:i

. ..

M

.>: .:>

---

0:: ....J

Q~ 1.0

· H' ..... - _ ....

1Il ~

-400 00 400 80 0 ANGLE OF VIEWING ..,...,......,.. Fig. 5. The bidire ctiona l reflectan ce factor at 600 nm of pressed PT FE powder having a rough sur face finish .

- 800 - 4 0 0 00 400 ANGLE OF VIEWING

-800

6.

1. The polytetrafluoroeth ylene powder used in these investigations is manufactured by Allied Che mical Company. The material is

S. 4' . 6 .

commonly refe rred to by the t rad e name Halon and is fur t her identifi ed as type G-80 tet rafluoroet hylen e resin . The commercial name of t his mater ial is given here for purp oses of identification only and in no way implies endorsement by the Nati onal Bureau of Stan dards. Bureau Cent ral de la Commission In tern ationale de l'Eclairage, Review of Pub lication s on Prop ert ies and Reflect ion Values of Material Ref lecti on S tandards (Pa ris, 1979), CIE P ub licat ion 46 (TC-2.3), pp. 15- 72. The ter m " perfect diffu ser " is defined in Bur eau Centra l de la Commission Intern ationale de I'Eclairage, In ternational L ighting Vocabulary (Paris, 1970), CIE Publication 17 (E-1.1), p. 95. F. Gru m an d M. Sa ltzman , P-75-77 N ew Wh ite St andard of Reflectance (P aris, 1976), CIE P ub licati on 36, 91. W. H. Venabl e, J. J . Hsia, and V. R. Weidn er, " Development of an NBS reference spectrophoto met er for diffuse reflectance and

800

Fig. 6. T he bid irectional reflectance factor at 600 nm of pres sed barium sulfate powder having a rough surface finish.

REFERENCES

2.

ROUGH "

7. 8.

9. 10. 11.

t ransmitta nce," Nat. Bur . St and . U.S. T ech. Note 594- 11, 47 pp. (1976). V. R. Weid ner and ,J. J . Hsia, " NBS specular reflectometerspectrophoto mete r," App l. Opt. 19, 1268- 1273 (1980). W. H . Venable, J . J . Hsia, and V. R. Weidner, "Esta blishing a scale of dir ect ional- hem isp herical reflectance factor I: t he Van den Akker met hod ," J . Res. Nat. Bur. Stand. 82, 29- 55, (1977). E. I. Du Pont de Nemours and Company, Inc., Fluorocarbo ns Division, T efl on Flu orocarbon Resins- Safety in Handling and Use (Wilmingto n, Del., 1970), pp . ] -1 0. T his pub licati on references technical pap ers dealing with t he t herma l decomposition of fluoroca rbons and associated safet y problems. T he following U.S. pate nt s are assigned to J . A. Seiner of P P G Industr ies, Inc., Pi ttsburgh , P a.: U.S. Patent No. 3,956,20] , May 11, 1976, and U.S. P at ent No. 3,764,364, October 9, 1973. From uncor rected emission spect ra supplied by K. Mielenz and R. Velapoldi using t he referen ce spectrofluorimete r in t he NBS Cente r for Anal ytical Che mist ry. R. D. Sa unde rs and W. R. Ott, "Spect ra l irrad iance measur emen ts: effect of UV produced fluorescence in integrati ng sphe res ," App l. Opt . 15, 827- 828 (1976).