MINISTRY bF AVIATION

AERONAU7KAl RESEARCH COUNCIL

CURRENT PAPERS

The Measurement of Absorptivity and Reflectivity

bY

E. T. de la Perrelle, Ph.D., A.lnst.P. and H. Herbert

LONDON: HER MAJESTY’S STATIONERY OFFICE

1962

THREE SHILLINGS NET

U.D.G. ITO. 535.32/34: 535. t.54

C,P, iio. 601

october , I 356

The jleasurement of Absorptivity and Reflectivit7y

by

k;. T. de la Perrelle, Ph.D. A. 1nst.P.

and

H. Ilerbert

To deduce the absorptivity from a measurement of reflectivi%y it

is necessary to measure the refleckivity in such a manner that both

diffuse and specular reflection are correctly included. A me!,hod of

doing this is described which makes use of a modified integrating sphere.

Results have been obtained for a varieQy of makerials from waveleng%hs -

0.33p to 2.3p.

Previously issued as R.A.E. Tech idote No. Rad 661 - A,R.C. 20,879.

I Introduction

2 'I'he Reflec?Aon, Absorptiow~ and Emission of Radia-kign

3 Use of Inh3gra45ng Sphc~.-e

4 Experimental Details

5 Results and Conclusions

Appendix

Table of Resultti

Det-.achable Abstrac-k Cards

Irhegrating Sphere for kkasmsemer;t of Ref lecf;ivit;y

Int.cgraJ-.in~ Sphere for Measurement of Reflectivity

1nl;egraM.n g Sphere Yor 1ieasuremenS: of Beflcctivitljr

Percentage Reflectivity EaH04 X53

Percentage Reflectivity Smoked Ng0; k&-Cl X74

Percentage Rerlectivity PbHO4 Yll PercenS;age Reflectivity 2X ~76

Percentage Reflectivity Al 0 * J x64

Percentage Rcfleckivity Silver

Percentage Reflectivity Brytal; Al Powder Yj5

Percentage ReflecMvity FbC+ x81 ZY-O x63 Percentage Refleckivity Ti.OZ Y-i3 x84

Percentage Reflectivity, &dative to ~.&$I of, TN 2 finatase Y13

and WI2 Rut;ile x8&

Percenkge Refleckivity, IkZat,ive lm IgO of, 3502 Anatase Y31

&p-e -“.-

3

3

4 - 6 6 -

8

13

Percentage Refle&ivity, Relative to &O of, Lead Carbonate (gel) Vj

Percentage EefLe&ivity, Relat;ive to P@ of, Aluminium in 827 Lead &rbonat,e Y33 16

Percentage Heflectivi-ky, Fbla$ive to &@I of, H and p2 17

Percenkage Xeflecl-iviky, Kela+ive to 1&O of, P3 and P4 18

-2-

1 InSroduction

Requirements exist for Ce measurement of emissivity or absorptivity of various materials. Emissivity may be measured directly by comparing the radiation emitted with that from a black body at the same temperature. Absorptivity may be measured directly by irradiating the specimen and observing the rate of rise of temperature. Eimever , this method is difficult when the result is required at; specific wavelen@?:a, because the radiation has firs+. to pass through a monochromator which seriously limits the radiant flux available and secondly one has to compare the specimen with a black body of identical thermal capacity and conductivity. In view of these difficulties it is more convenient to develop a general purpose appara+.us which measures reflectivity and to deduce the absorptivity or emissivity from the measurement of reflectivity.

2 The Refle&i.on,,,Absorption and FGlission of Radiation

When radiation of wavelength h falls on a surface a frac%ion AL is absorbed, a fraction R A is reflected and a fraction T A

is transmitted.

Evidently,

The body of a may occur Rh and Th angles. Th = 0.

RA i- Ah + TX = 1 (1)

reflection may occur at the surface or by scattering in the translucent inhomogenious medium, similarly the transmission with or without nca%terini;; but this is immaterial provided are the total fractions reflected or transmitted at all possible All the specimens examined to date have been opaque hence

In addition to the energy reflected and transmitted there will be thermal radiation from the surface, which will be a function of surfrace temperature and Ihe emissive power or emissivity* EL.

Kirchoff"~ law, which may be proved by considering the equilibrium of a body in an enclosure stakes that

EA = Ah (2)

If we measure A,, by measuring RX + TX and using (A), Rh and TX rust be measured for all possible angles i.e. for the whole hemisphere in the case of plane specimens.

It is not necessary for the incident radiation to come from all angles, although 1:he value obtained for A A may be different for different directions of incident radiation. When stating the value of Ah it; is necessary to specify the direction of the incident radiation, although it would not vary ai)preciably Nith the angle of inoidcnoo ox~pt for rather unusual materials.

It follows from a generalisation of Kirchoffls law,that when Eh is deduced from Ah' the value applies only for measurement of the radiation in geometrically similar conditions to Yno irradiation used in the

* See definition. Appendix,

determination of % and TX from which -4, was deduced, i.e. if r;e irradiate a specimen normally and measure 5 and T h for all possible directions, we obtain a value of Ah which is the absorptivity for normal radiation, and this is equal to the emissivity I$ measured normal to the surface. If both the irradiation and measurement had been o,nnidirectional, we would have obtained the absorptivity for omnidirectional radiation and the emissivity integrated over all possible directions.

It should be noted that if we irradiate within some restricted range of angles and measure the reflected radiation within a limited solid angle, nothing can be deduced concerning the absorption except for certain surfaces whose characteristics are well !cnown. It is therefore necessary to use some optical system which can handle the radiation over a whole hemisphere, for which purpose an integratin> sphere suggests itself,

3 Use of Integrating Sphere

The usual method of using an integrating sphere to measure reflectivity is to send a beam of monochromatic radiation into the sphere so as to irradiate a specimen set in an aperture in the wall, the reflected radiation strikes the inside of the sphere and suffers multiple reflections, a portion reaching a photocell set in an aperture in the sphere wall (Fig I). The measurement is made by deflecting the beam so as to strike a reference specimen such as magnesium oxide and taking the ratio of the photocell output in the two cases (Pig 2). If an absolute measurement is desired, the apparatus is modified by placing a shield between specimen an3 photo- cell, so that radiation reflected from the specimen cannot reach the photo- cell directly but only by reflection from the sphere lining. The radiation : is tnen reflected from the specimen to tne sphere lining a certain fraction entering the photocell after single and multiple reflections .i'rall the lining. The reference beam is sri-anged to strike the sphere linila; directiy in such a position that the reflected radiation is not obstructed by the shield, a certain fraction enters the photocell as in the previous case. Thus in one case the radiation reaches the sphere lining directly and in the other case it reaches the sphere lining alter reflection from the specimen, and the ratio of the meter readings gives the reflectivity of specimen, This method is open to the following objections -

.

that it is not illuminated directly by the lamps. (Pig 3). This arrangement has the advantages that the lamps irradiate the sphere reasonably uniformly even without multiple reflections, and that tile sphere lining irradiates the specimen uni.forrLLy from all directions even if this lining is only a poor diffuser, since even if 3;he lining ~-as a specular reflector the radiation from the lamp would be approximteb focused onto the specimen after a single reflection frol,l the sphere lining.

The radiance (steradiancy, brightness) of the specimens is compared with that of the sphere lining, the reference line of sight passing t,o the sphere lining through the space previously occupied by the specimn. The total optical path 1engk.h is the same when observing the specimen as when observing the spnere lining. Since each psrt; of We sphere lining is always illuminated directly by radiation which has traversed one radius and secondly by multiple reflections and the initial irradiation is the same at; all points, each point receives the same contribution by multiple reflections as any other point. Then in one case this radiation has to cross a diameter to emerge from sphere and in the other case, il; has ?a firaverse one radius to reach the specimen, regardless of where it has come from, and one radius from specimen to hole in the sphere wall.

In this apparatus the specimen is irradiated uniformly from al.1 directions and the radiation reflected in one particular direction is measured. This is equivalent to a system in which the specimen is irradiated from that particular direction and the total energy reflected in all possible directions is integrated. This principle by which the special distributions of incident and reflected flux may be completely interchanged without altering the measured value of the reflectivity is known as the Helmholtz reciprocal relationship. It is equivalent to a statement that if we interchange source and detector and thus reverse the direction of all rays, the same fraction of the r;:diation from the source will enter the detector. The interchange of source and detector implies that the interchange is complete, i.e. t;he d&e&or has tie assume ?-he geometrical form of the source and vice verse.

It should be noted that the reflectivity of the sphere lining does not influence the result except in so far as it influences tIie uniformity of the irradiation onto itself by determining the portion of this irradiation which is due to multiple reflections. ‘ire observe either the irradiation of the specimen by observing the radiation which is reflected from a portion of the sphere lining towards the SPfXimeil, or the radiation reflected from the specimen; the ratio of theso two giving the reflectivity of the specimen. A source of error arises if the portion of the sphere lining which we observe is not reflectin& the same quantity of radiation as the remainder of the sphere.

In t.ne case of diffuse reflecting specimens if the part of the sphere lining which is observed when making the reference measurement is brighter than the suitably weighted average of the remainder we obtain a value lower than true for the reflectivity. In the case of specular reflecting specimens we obtain a value lower than true for the reflectivity if the reference portion is bri&ter than the corresponding portion of *he okher half of the sphere whose imaLe is seen reflected by the specimen, There is, ho!vever, no reason why this portion of the sphere lining should differ appreciably either in irradiation or reflectivity from the reference portion.

4 Exper-imental Details

The sphere was made of aluminium, one meter in diameter. It; was coated internally with the following, in the order listed:-

Titanium Dioxide ;/bite Undercoat, DTD. 314A, Titanium Dioxide and Gelatin in Yater, Zagnesium Carbonate and Gelatin in ';t'ai;er.

The reflectivity of this sphere lining is unfortunately not perfectly constant with wavelength, and the fall in reflectivity at wavelengths greater than 2.2 microns sets the long wave limit to the range over which accurate measurements can be made. At the short wavelength end of the range, the reflectivity of' the lining, energy emitted from the lamps, transmittance of spectrometer and sensitivity of' the detector, all fall off. Since the observed output is proportional to the product of all these factors, it falls off exceedingly rapidly. The limit is approximately 0.4 microns with a PbS cell and 0.32 microns with an ultra violet sensitivity photo multiplier, even using the most powerful source practical. This consisted of a ring of 8 x 240 watt, 26 volt aircraft landing lamps. These &amps have thick filaments and can be run at high temperature, (3,000 K) hi 1 w cl is essential when making measurements in the violet and near ultra violet regions of the spectrum, but when work& at longer wavelengths the voltage on the lamps was reduced to lengthen lamp life and reduce the heating af %he sphere.

The radiation emerging from the sphere was focused onto :;he entrance slit of a Leiss Double Gonoohromator fitted wit& quartz prisms, which is described elsewhere (1) A chopper was placed in front of the ertr‘ance slit to interrupt the &diation at 000 c/s as the detector was used with an amplifier tuned to this frequency. The detector was situated in a light-tight box behind the exit slit of the monochromator.

The measurements were carried out by setting the spectrometer to pass a particular wavelength and moving the specimen to compare the radiance of the specimen with ihat of the sphere lining. A subsidiary experiment showed that the radiance of the sphere lining was not alizred by moving the specimen.

An experiment was oarried out to de+ermine the uniformity of irradiation of the sphere lining, by mounting a lead sulphide cell in the aperture in the wall and observing the variations of the irradiation of the cell as the ring of lamps was rota+.ed. The brightest part was a few per cent brighter than the dimmesk part, but the part of the sphere lining in the reference line of sight had a radiance which was very nearly eq;ml to the average for the whole sphere, (within I$) This measurement applied to the whole of the lead sulphide band, and it is possible t.hc* at wavelengths where the sphere lining is not a good reflector more serious variations may occur.

5 Results and Conclusions

The observed values of reflectivity of various materials are tabulated and graphs plotted of reflectivity versus wavelength. rligs 4 to A2 are absolute reflectivity. A specimen of smoked mamesium oxide was measured as a check on the apparatus when used to measure absolute reflectivity. It will be seen that the result for magnesium oxide is in good agreement with published data. '&en, later, the measurements were extended into the ultra violet a spurious absorption of some I$ was found in the observed refl.ectiviQ of PigO at 0.33 microns, all other substances examined

-6-

showed absorption at this wavelength, and to eliminate that fraction of the absorption'which was believed to be spurious the curves of Figs 13 -t;o 19 have been plott-ed relative to magnesium oxide as lOC& The results for polished silver are in reasonable agreement with published data, but the result,s for Brytnl appear to differ from results obtained by Pi.2.5. The reason for +his error is not known, but it is perhaps due to the dielectric interference layer on the surface of this material. Some small irregularities in the results between 0.5 and 0.6 microns may be due to the fact that the detector was ch::nged at this point, and slightly different results were obtained with PbS cell and phoko multiplier respectively, The curves have in this case been drawn between the observed points, iihen irregularities occur at longer wavelengths the curves have been drawn through the points because if, is to be expected that relatively sharp absorption bands may occur in this region.

The principal source of er',*or is the lack of perfect uniformity of radiance of the sphere lining at iuavclengths at which it has low reflectivity. An experiment to measure the variation in irradiation of the: lining ks baen rcftirred to above and the vCariation does not zppeC'tr to bc serious, neither does it effect the observed reflectivity of eif;kr magnesium oxide or polished si.1~~. AZi,,ost any surface can bc: regar&d as intermediate bctwetin these two cases of diffuse a& speoul.~ul reflectors, so we conckdc that the apparatus givLs reasonably accurate results for &any surface.

-7-

Zxflcctivity

This is thi: fraction of the incident radiztion which is rtf'loc%od from the specimen.

Absorptivity

This is the fraction of thd incident rtldiction which is absorbtid by the specimen.

Tmnsmissivity

This is the fraction of the incident radiation whioh is fz;l~cj- mitt;ed through tht: specimen.

Since those are the propertix of 3 particular object, mthcr than 2. particular medium, it I;GJ have been more logical to use the terms reflectance, absorptraco and transmittance, reserving f,hi: lxrm trans- missivity for unit thickness (c.P. use of the terms rcsistivity and resist2acc etc.). This has not been doni because the terms Lbsorptanco and transmitiance arc not gencral1y accepted, while r&%xl;ivity, absorptivity and cmissivity are widely used.

Emissivc Power

A surf&e at tompcraturi: T emits an o_p,lount of onurgy per unit SC:? equal to Eh d h ergs par second, between the ~avulcn@~s h C& h + d h. E7 is called the Emissivc; Powcr for thz wavelength L qLntity of energy emitted in ergs per

Eh io thus the sticond par unit rangi: of 1vc~~~1~21gbh~

This cxcludcs phenomena such as lulainesccnce and limits consideration to tcmporaturi: radi3tion.

Emissivity

The Emissivity of a surfzco is the ratio of the radiation emit&d by that surface to that, emitted by -t,hc- szmc area of' a Black Body nt t;ho same temperature.

Emissivity at a wavolcngth h

This we define 2s Exfor the specimen E

h for a Black Body

Table of Results

Percentage Ref lectf vi ty nt varf 0~s wavelengths

h IPI - 0.3'

0.4

0.4

0.4

0.4

0.5

0.5:

0.5i

0.5

0.6

0.6,

0.7

0.7:

0.8

0.81

0.9

O-9!

1.0

1.1

1.2

1.3

1.4

1.6

1.8

2

2.1

2.2

-

Silver snm1u3c &ZO

X84 Yl3 Tll$

69

79.2

84.2

91.5 83 57.’ 91.3

96 82.7 85.. 87.5

96.6 86.8 89.’ 91.2

90 98.5 91.4 92 93.5

90 98 94.5 92.1 93.7

91.75 98.5 93 92 93.3

92.7 98.8 93.7 92’ 94

94 99.0 93.7 93-s 94.1

95.5 98.8 94.5 94 94.4

96.2 98.25 93.5 94rd 94.5

96.7 98.5 EL75 94.: 95

97 96.0 93.4 94.1 94.4

98 98.8 94.2 94.1 94.6

98.5 98.7 94 95 95

97

99

99

99

99.5

too

98

98

100

99 94.5

99.2 94.2

99 93.2

98.3 88

97*2 ss

99 89.5

97.5 84.5

95.5 73*5

97.8 71

99 65

94.: 94.3

94.d 34.4

93-l 93.6

89.’ 90.9

86 86

85.1 85.2

82.' 81.9

68.: 67

63 62.5

-

I

I L

Yll PbWO4

X64 ~$3

X76 X63 !&X3 x0

X81

‘bo4

82

88.4

go. 1

84.8

s7

90.7

78 74.G 76.1 39.3

846 83 82.5 89.6

86.3 87.9 88.3 91.4

92.7 92.7 88.9 91.6 89.6 94

93.6 956 95 2 95.2 92.8 Y-l.1

94*9 .%9

94.8 95.8

94.3 953

94.8 948

94.3 98.5

94.6 95.5

94.4 95.3

95.0 96.0

go.8 93.7

go.4 93.4

93.0 94;6

92.3 9x5

91.8 96.2

92.7 97

92 96.9

92 97.5

91.4 96.5

93.7 96.6

90.9

97.4

91.1

91.6

91.2

91.3

91.0

91.3

90.6

90.0

95

95.3

96

97.1

97.2

97.3

srl.1

98.2

97.8

37.9

93.8 95.3

94.9 96.0

93.7 95.2

09.4 93.1

l3zi.g 89.8

67.6 90.7

78.5 86.4

70.8 79.0

70.3 79.7

92.5 97.5 91.1 98.9

91.7 97.4 89.9 99.0

90.6 96.e 89.2 99.1

82 92 gl.2 95.2

77.1 87.5 82*4 95-l

76.1 89.4 82.5 96.9

69.5 82.4 79.8 93.4

63.9 74.c VS.3 8525

58.5 76.1 60.5 85.7

54.4 77.5 58 79

3ryttll --

87.5

85.5

83

87

86.5

86.2

05 5

88.5

85

04

83

80

83.7

85.5

89.2

91 ) 90.6)

93

94

94.5

95

96.2

97.8

96

94

Y15

-I

67.6

61

60

GO, 4

59.5

f>O. 2

$3.6

28.2

57

552

53.8

,i*3

2

556

58.1

53.6) 59.5)

61.2

Cl.5

62

61.4

61.5

62.75

Ea.75

61

92.5

-9 ”

Cf;, w? 11102 TN2 Tl02

mtlle/gel betase/gel (Rutile In 8.~7) unatase in 827) lpad "xzym Igel) 222 22 pl- &O Ma;0 &CO 3

Mgo

0.328 87.4 0.338 52.9 60.1 57.7 59.9 88.6

0.348 27.4 43.4 34.0 36.3 87.7

0.362 15.9 26.8 17.9 25.9 89.4

0.373 12.2 39.3 -13.0 38.0 90.5

0.385 12.7 54.0 14.5 52.8 41.6

0.393 17.6 72.1 21 .o 66.8 92.9

0.402 36.7 75.3 92.7

0.413 65.0 91.5 60.1 83.9 92.0

0.470 95.9 95.7 93.8 91.5 96.5

0.470 90.5 89.5 72.3

0.540 92.4 92.2 93.4 90.0 vG.0

0.595 93.3 94.5 94.8 91.4 97.2

0.645 94.4 74.7 96.4 93.7 98.4

0.700 92.0 93.4. 95.8 94.3 97.0

0.761 93.4 95.0 97.1 74.t3 79.4

0.890 93.4 74.7 96.8 92.4 38.2

1 .ooo 94.2 94.2 96.4 71.2 79.4

1.076 94.3 94.3 95.2 87.9 100

1.315 73.5 92.5 93.3 87.0 100

1.55 85.4 84.0 85.4 79.7 76.4

1.69 86.0 83.0 75.9 67.2 97.2

1.89 82.3 78.8 76.4 70.5 94.2

2.068 67.6 65.5 67.3 61.2 80.8

2.293 59.7 55.2 39.4 ; 32.5 80.7 ,

1

- IO -

I 0.328

0.338

0.348

4362

0.373

0.385

0.393

0.402

0.413

0.470

0.470

0.540

0.595

0.645

Q.700

0.761

0.090

1.000

l.OgS

1.315

I.55

1.69

I.89

2.068

2.293

53.7

50.8

47.9

48.6

49.9

50.1

52.2

52Ez.o

54.5

551

51.1

fio.6

!3L6

(47.41

47.3

45.2

50.6

53.5

56.3

56.2

55-8

55.8

8.2

54.1

Lead Carbon

/ i$i

76.0

58.7

47.9

43.4

47.7

51.0

3.4

64

70.6

83.2

79.2

84.3

88.6

91.7

90*3

92.0

90.5

90.5

es.7

90.4

81.9

74.6

74.8

6.5.1

40.8

nate in 827 PI P2 9 W Y42 Y47 PI p2 1 P4

xs pi y42

Ho i-id p

MgO

I

I 25.3 70.4 64.3 ~ 32.5

I 19.7 46*9 48.0 ~ 31.3

1 1592 29.3 33.5 29.9

14.3 19.5 26.5 23.7

10.8 IS.6 37.5 25.1

8.43 15.4 50.6 29.2

8.83 21.1 66.5 3EO

Ii-J.1 37.8 75.6 81.7

15.3 69.8 83.9 49.8

31.2 93.7 89.3 66.7

28.2 87.7 84.9 62.7

42.1 90.4 88.7 65.8

40.4 92.0 91.4 7004

51.0 94.2 Yl.9 72.4

49.3 93.1 92e76 7206

45.6 94-2 94.0

34.8 90.8 91.4

29*9 30.3 e9.2

29.4 e.9 8893

33.8 07.5 85.3

31.6 82.6 nil

12.2 67.7 64.5

18.7 71.8 67.0

16.1 64.7 57.8

6.2 38.2 31.7

32.2

31.4

31.4

32-O

35.4

37.9

41.3

42.8

448

49.5

5Q.7

56.4

600.7

61.7

62.0

63.5

62.5

63.2

62.7

62.7

57.4

51.1

52.6

40.3

20.6

40.0

38.2

36.9

39.5

43.5

47.0

49.9

52.4

55*6

61.6

62.8

67.2

71.3

71.4

71.1

72.0

69.2

67.9

84.0

68.5

65.0

59.6

62.9

60.2

34.3

7%7

6907

7108

72.7

73.5

60.1

43.7

46.1

39.6

17.5

IJ.2. 2078 C.P. 301. K3 - Printed in England

FIG.2. INTEGRATING SPI-IERE FOR h&6l)R&,ENT

OF REFLECTIVITY .

OF LAMPS.

FlG.3.

INTEGRATING SPHERE FOR MEASUREMENT OF REFLECTIVITY.

IO0

90

F

80

76

IO0

80

60

100

80

70

3 0*4 o-5 0.6 O-7 O-8 O-9 I I*5 2 2.5

‘IG. 4.

FIGS.

FIG. 6.

h (MICRONS)

100

FIG.7

FIG. 8,

I 1

SILVER

I

701 O-3 O-4 O-5 O-6 0,7 0.8 0.9 I I.5 2 2.5 3

60 60

+Oo3 +Oo3 . . 04 o-5 O-6 0.7 08 09 I 04 o-5 O-6 0.7 08 09 I l-5 l-5 2 2 z-5 3 z-5 3

FIG.9.

FlG.10.

100

90

80

70

60

90

80

70

60

o-3 0.4 O-5 0.6 07 0.8 O-9 1

Y

FIG.1 I.

FlG.12.

h (MICRONS)

100

90

00

70

I I I I ! I . .

I \q

\

0 I I I

o-3 0.4 05 O-6 O-7 O-8 C%9 I I.5 2 2-5 3

loo

90

80

70

60

50

4c

3c

2t

I(

I

1

I I I FUTILE IN 027 --4+4-H

I I I/

FIG. 13.

F IG.14.

loo

90

80

70

6c

SC

40

3c

zc

IO

C I I I I I I I I I o-3 04 05 O-6 0=2 O-8 0*9 I 15 2 2-5

WAVE LENGTH (MICRONS)

FIG. I

60 -

10 w V-j 1 0. -- nl txe -# Am -a -a. ,C -- -

WAVELENCiTH (MICRONS)

100, I I I I I I I I

100

I

90- . -4

A^

FIG

70

60

50

4-O

30

20

IO

0 0.3 o-4 O-5 0.6 O-7 O-8 019 I 15 2 2-5 3

FIG.19.

,

- - - - . I _ . . _ I - . . .

1 A.R.C. c;.P. No. 601. October, 1956. 535.32/34 : 535.84

I de la Perrelle, E. T. and Herbert, H.

j THE l.$LAXJmT OF AE!SORPTIVITY AND REFLECTIVITY I

To deduce the absorptivity from a measurement of reflectivity it is necessary to measure the reflectivity in such a manner that both diffuse and specular reflection are correctly included. A method of doing this is described which makes use of a modified integrating sphere. Results have been obtained for a variety of materials from wavelengths - 0.3% to 2.q.l.

-

s.R.C. C.P. No. 601. October, 1956. 535.32134: I 535.84

de la Perrelle, E. T. and Herbert H.

THE MEASUREMENT OF ABSORPTIVITY AND REFLECTIVITY

To deduce the absorptivity from a meesurement of reflectivity it is necessary to measure the reflectivity in stxh a manner that both

1 diffuse and specular reflection are correctly incluaed. A method of ’ ’

doing this is described which makes use of a modified integrating sphere. Results have been obtained for a variety of materials from wavelengths - 0.33/J. to 2.w.

C.P. No. 601

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C.P. No. 601