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
<0 -
~w2 . 0
->
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).