NATL INST OFS|ANO&^J^

A111G7 allots

NBSIR 84-2916

Evaluation of Infrared Reflectance

as a Technique for Measuring

Absorber Materials Degradation

David Waksman

Willard E. Roberts

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Gaithersburg, MD 20899

September 1984

r

Prepared for: Active Heating and Cooling Division

Office of Solar Heat Technologies

. QQ -- J.S. Department of Energy

Afashington, DC 20585

.U56

34-2916

1984

NBSIR 84-2916

EVALUATION OF INFRARED REFLECTANCE

AS A TECHNIQUE FOR MEASURING

ABSORBER MATERIALS DEGRADATION

David Waksman

Willard E. Roberts

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Gaithersburg, MD 20899

September 1984

Prepared for; Active Heating and Cooling Division

Office of Solar Heat Technologies

U.S. Department of Energy

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CDNTENTS

Page

LIST OF TABLES iv

LIST OF FIGURES v

ABSTRACT vii

1 . INTRCDXnON 1

2 . EXPERIMENTAL 2

2.1 Property Measureanents 2

2.1.1 Solar Spectral Reflectance

2.1.2 Total Normal Einittance

2.1.3 Infrared Spectral Reflectance...

2.1.4 Surface Characterization Studies

2.2 Test Specimens 5

2.3 Measurement Reproducibility Studies 5

3 . RESULTS AH) DISCUSSION 6

3.1 Reproducibility Studies 6

3.1.1 Measurement Variability 6

3.1.2 Test Specimen Variability 7

3.2 Optical Measurement of Materials Property Changes 8

4. CONCLUSICWS AH) RECOMMEIDATIONS 9

5. ACKNOWLEJXSEMENTS 10

6. REFERENCES 11

iii

Ji.

(jO

U)

NJ
LIST OF TABLES

Page

Table 1. Series 1 Absorber Test S^jecimens 12

Table 2, Series 2 Absorber Test %>ecinens 13
LIST OF FIGURES

Page

Figure 1, The measurement spectrum of the Gier Dunkle Infrared

Ref lectometer. Model DB-100, (no filters) together with a normalized lOO^C blackbody spectrum as presented by Pettit,

reference [7]

Figure 2. Reproducibility spectrum for 45 degree specular IR reflectance,

rigid material AA, Condition 2 15

Figure 3, Reproducibility spectra for 45 degree specular infrared

reflectance, foil material AM, Condition 2 16

Figure 4. Variability in 45 degree specular infrared reflectance spectra for

several unaged sanples of rigid iraterial AA 17

Figure 5. Variability in ccatibined diffuse and specular infrared reflectance

spectra for several unaged sanples of rigid material AA, 18

Figure 6. Variability in 45 degree specular infrared reflectance for several

unaged sanples of rigid naterial AJ 19

Figure 7. Variability in conbined diffuse and specular infrared reflectance

spectra for several unaged sanples of rigid material AJ. 20

Figure 8. Variabililty in 45 degree specular infrared reflectance for

several unaged sanples of rigid naterial AP 21

Figure 9. Variability in conbined diffuse and specular infrared reflectance

spectra for several unaged sanples of rigid material AP 22

Figure 10. Variability in 45 degree specular infrared reflectance for several

unaged sanples of foil naterial AH 23

Figure 11. Variability in combined diffuse and specular infrared reflectance

spectra for several unaged sanples of foil naterial AH 24

Figure 12. Variability in 45 degree specular infrared reflectance for several

unaged sanples of foil naterial AW 25

Figure 13. Variability in corbined diffuse and specular infrared reflectance

spectra for several unaged sanples of foil naterial AW : 26

Figure 14. Comparison of changes in emittance with combined diffuse and

specular infrared reflectance spectra for aged and unaged sanples

of naterial AA 27

Figure 15, Comparison of changes in emittance with 20 degree infrared

reflectance spectra for aged and unaged sanples of natericLL AA . , .

.

28

Figure 16. Comparison of changes in emittance with 45 degree infrared

reflectance spectra for aged and unaged sanples of nateriaLl AA . . . . 2 ^

v
Figure 17.

Figure 18.

Figure 19.

Figure 20.

Figure 21.

Figure 22.

Figure 23.

Figure 24.

Figure 25.

Page

Con5>arison of changes in emittance with 75 degree infrared

reflectance spectra for aged and unaged sairples of material AA 30

Comparison of changes in emittance with combined diffuse and

specular infrared reflectance spectra for aged and unaged samples

of material AJ 31

Comparison of changes in emittance with 20 degree infrared

reflectance spectra for aged and unaged samples of material AJ 32

Comparison of changes in emittance with 45 degree infrared

reflectance spectra for aged and unaged samples of material AJ.... 33

Comparison of changes in emittcince with 75 degree infrared

reflectance spectra for aged and unaged samples of material AJ . . . . 34

Comparison of changes in emitteince with combined diffuse and

specular infrared reflectance spectra for aged and unaged samples

of material AP 35

Comparison of changes in emittance with 20 degree infrared

reflectance spectra for aged and unaged samples of material AP 36

Comparison of changes in emittance with 45 degree infrared

reflectance spectra for aged and unaged samples of material AP, ... 37

Comparison of changes in emittance with 75 degree infrared

reflectance spectra for aged and unaged samples of material AP.... 38

vi
EVALUATION OF INFRARED REFLECTANCE AS A TECHNIQUE FOR MEASURING

ABSORBER MATERIALS DECJ5ADATION

David Waksman and Willard E, Roberts

ABSTRACT

Current ASTM standards concerned with the durability and reliability of

cibsorptive raterials rely on integrated solar absorptance and ^littance as the

primary methods for assessing changes in absorber optical performance

resulting from environmaital exposure. This study was undertaken to determine

if infrared reflectance measurements are a more sensitive technique for

detecting absorber materials degradation. Spectral measurements were mde to

identify factors that could affect the reproducibility of infrared reflectance

measurements and to conpare their ability to detect changes with currently

used methods for absorber materials, Recomnendations are made concerning the

use and limitations of infrared reflectance measurements for this purpose.

vii
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1 . INTRCDUCnON

At the present time, integrated solar absorptance per ASTM E 424 (1)* and

emittance per ASTM E 408 (2) are the primary techniques used by the solar

industry to measure changes in the optical performance of absorptive

materials, Uiese methods have been incorporated into ASTM procedures concerned

with the evaluation of absorptive materials used in solar collectors (3,4).

Studies conducted at NBS (5) have shown that integrated solar absorptance

measurements are a relatively insensitive indicator of absorber materials

degradation. Changes that occur in a limited portion of the solar spectrum,

i,e„ in the near infrared v^ere there is a small amount of energy, are oftoi

concealed by the integration process conducted as part of ASTM E 424, ASTM

E 408 measurements are a more sensitive indicator of changes in the

performance of many selective absorber materials; however, this technique

provides no information about the spectral distribution of changes in the

optical performance of these materials.

Preliminary measurenents made by the authors indicated that infrared spectral

reflectance measurements offer promise as a technique for the early detection

of absorber materials degradation. This study was conducted to determine

viiether or not these measurements are more sensitive indicators of changes in

the optical properties of absorber materials than current practice.

Measurement techniques evaluated included diffuse and specular infrared

reflectance, infrared emittance, and integrated solar absorptance.

* Nunt>ers in parentheses indicate references given at the end of this report.

1
2 . EXPE3UMENEAL

2.1 PROPERTY MEASUREMENTS

Uie property measurements used in this study are described in the following

sections. An alignment fixture was used in each instrument to ensure that

approximately the same sanple area was measured each time a measurement was

made on the same sairple.

2.1.1 Solar Spectral Reflectance

Spectral reflectance and solar absorptance were determined in accordance with

ASTM E 424, method A (1) using a spectrophotometer equipped with a 76 mm

integrating sphere coated with barium sulfate paint. Spectral reflectance

measuremaits were mde over the wavelength range from 300 to 2150 nm using a

lead sulfide detector above 700 nm and a 0iotoiTultiplier detector below this

wavelength. The incidoit monochrcxnatic radiation was 7 degrees off the normal

to the plane of the specimen and the reflected radiation was measured using an

integrating sphere. A pressed polytetraf luoroethylene powder (Halon)

reference standard vas used for the reflectance measurements. Solar energy

absorptance was calculated using the distribution of solar energy r^xjrted by

Parry Moon for sea level and air mass 2 (6). The weighted ordinates

calculation method described in ASTM E 424 was used to integrate the energy

distribution at 50 nm intervals. The spectral reflectance data were digitized

by the spectrophotometer and fed directly into a cc«iputer which performed the

integration calculations after correcting for the baseline values obtained

with the Halon reference standard.

2
2.1.2 Total Normal Einittance

Total normal emittance was measured in accordance with ASTM E 408, method A

(2). The infrared ref lectometer used incorporates a rotating cavity

mounted below a 20 mm diameter sanple measuronent port. The inside of the

cavity is divided into two semicylindrical chambers, both coated black.

One* chamber is heated to approximately 45°C; the other is unheated but

reaches ^^proxinately 33®C. As the cavity rotates, a sairple placed over

the measurement port is alternately irradiated with black body radiation

corresponding to the teirperature of each chamber. The detector system is

positioned so that it views the sample in a near normal direction. The

sanple reflectance is averaged over a spectral distribution which is given

by the difference between black body spectral irradiance curves at the two

chamber temperatures. As shown by Pettit (7), this distribution, when

normalized, very closely approximates the 100°C black body spectrum, see

figure 1, and has maximum relative intensity at ^proximately 7.7/f m vhen

the instrument is used without a polyethylene filter in the optical path

between the sample and the detector. The instrument, as normally

manufactured, includes this filter v^ich was not used in this study. The

purpose of the polyethylene filter is to shift the approximately 100°C

black body spectrum to one of approximately 25®C, Gold (€= 0.045) and

black (f = 0.911) reference standards wefe used for calibration purposes.

The instrument is capable of reproducing emittance (f) values to within

0U)02 using these reference standards.

2.1.3 Infrared Spectral Reflectance

f

Diffuse and specular infrared reflectance of absorber materials were measured

using a fourier transform infrared (PT-IR) spectrometer. The instrument

3
incorporates a large removable sample chamber onto which various types of

attachments are fitted. Hie chairber rides on a track, and alignment pins and

stops are provided for precision alignment. Combined diffuse and specular

measuremaits were made using a Harrick Diffuse Reflectance Attachment*. Hie

design uses two 90 degree off-axis ellipsoids vAiich sibtend 20 percent of the

47T solid angle. Hie sanples were tilted forward at an angle of 30.75 degrees

to include both the specular and diffuse reflectance components of the

radiation. ^>ecular measurements were made using a Barnes* variable angle

specular reflectance attachment. Specular measurenents were made at angles of

20, 45 and 75 degrees off the normal to the plane of the test specimen to

determine if the reflectance measurements are angular d^iendent. A silver

front surface mirror was used to cbtain a background reference spectrum every

time a setting on the instrument was changed, or a mirror, or an attachment

was moved.

2.1.4 Surface Characterization Studies

Optical and scanning electron microscope studies were conducted on several

samples to determine if there was a correlation between surface

characteristics and infrared reflectance measurements. Optical observations

were made using a low power light microsccpe. Scanning electron microscope

studies were conducted at magnifications of 200 and 600 times.

* Product names are included, where necessary, for informational purposes

only. Hieir use does not constitute an endorsement by the National Bureau of

Standards.

4
2.2 TEST SPECIMElvIS

Two series of test specimens were used in this study. The first series

consisted of absorber iraterial sanples subjected to various types of indoor

laboratory environmental exposure tests in a previous study conducted at NBS

(5). These sarrples were selected for use in this study because of the broad

range of appearance and optical property changes induced in samples of the

same composition by various types of environmental exposures. They thus

provided a basis for evaluating the ability of the optical measurement

techniques studied to detect changes caused by absorber materials degradation.

The series 1 specimens, the aging conditions used and their solar absorptance

and emittance are listed in table 1. All of the absorber coatings were

applied on rigid, relatively flat, metal si±>strates; this is a consideration

that will become important in later discussions.

The series 2 test specimens consisted of the five types of selective absorber

materials ^plied on metal foils listed in table 2. The foil samples had a considerable amount of wrinkling. Samples of these materials were

mechanically attached to rigid aluminum backing plates. Optical property

measurements were made on unaged replicate samples of these materials to

assess the influence of this foil wrinkling on measurement reproducibility.

2.3 MEASUREMENT REPRODUCIBILITY STUDIES

Studies were conducted to determine the influence of sample and reflectance

attachment placement on the reproducibility of diffuse and specular infrared

reflectance measuremaits. All parameters within the FT-IR spectrometer were

5
neintained the same, i^. gain, iris diameter, nuiiber of scans, etc, The only

changes made were to the specimen, specimen fixture, reflectance attachments,

and chamber. For each condition evaluated, ten runs were made and cxxipared.

The conditions were as follows;

1. Specimen, specimen fixture, reflectance attachment, and chamber all fixed

in position, with ten runs made without touching any part of the system.

2. Specimen taken out and replaced between runs; specimen fixture,

reflectance attachment and chamber fixed.

3. Specimen and specimen fixture taken out and replaced; reflectance

attachment and chamber fixed,

4. Reflectance attachment and specimen removed and then replaced; chamber

fixed.

5. Chamber removed and replaced; reflectance attachment, specimen fixture

and specimen fixed in place inside chamber.

RESUnrs AM) DISCUSSION

3.1 REPRg)IX:iBILITy studies

3.1.1 Measurement Variability

Infrared reflectance measurement reproducibility studies were conducted

primarily using the specular reflectance attachment set at an angle of 45

degrees. The multiple curves obtained overlapped one euiother for the

conditions in vhich all of the sample measurement parameters were kept fixed

and in vhich just the sample chamber was moved in and out (conditions one and

6
five, respectively). Condition two, in which the speciiren was taken out and

replaced between runs, with all of the other parameters remaining fixed,

produced overlapping curves for an AA specimen, which had a rigid metal

substrate, and an approximately three percent spread in the curves for the

thin foil sanples of material AM vdiich had a wrinkled surface (figures 2 and

3, respectively). Moving the test specimen and alignmait fixture, with the

specular reflectance attachment and chamber fixed (condition three) also

produced a family of curves that overlapped one another for an AA (rigid)

specimen. With the diffuse attachment, there was a one to two percent spread

for foil material AM and slightly less for material AA using the condition

three parameters. Condition four, vAiere the specimen, fixture and specular

attachm^t were moved and the chamber remained fixed, also produced a one to

two percent spread for material AM and slightly less for material AA.

3,1.2 Test Specimen Variability

Figures 4 through 9 illustrate the type of spread in spectral infrared

reflectance curves that can be expected for samples of several different rigid

absorber materials taken from a single larger sheet of each material. All of

the samples were unaged. Factors that could influence the relatively large

^read in the spectral curves include the surface roughness and uniformity of

the absorber material and the size of the sample area viewed by the

spectrometer. Hie combined diffuse and specular infrared spectral reflectance

curves appear to have considerably less spread than the 45 degree specular

curves. Figures 10 through 13 show that the spreads in infrared curves

cbtained for unaged foil specimens are somevbat greater than those obtained

for absorptive materials on rigid substrates. Ihis is most likely due to the

combination of wrinkling of the foil and the small sample area measured by the

7
PT-IR spectrometer. The conbined specular arid diffuse curves measured for the

unaged foil sairples also had considerably less spread than the 45 degree

specular measurements. The spreads in specular curves measured with an

incident angle of 75 degrees were approximately the same as those obtained at

45 degrees.

3.2 OPTICAL MEASURE^EOT OF MATERIALS PRQPERTy CHA^HEq

Figures 14 through 25 coirpare emittance with the specular and combined

specular and diffuse infrared reflectance curves measured on sanples of three

different absorber materials. These samples were subjected to the

environmental exposure conditions listed in table 1 prior to the infrared

reflectance measuremaits. All of the absorptive materials discussed in this

section were applied on rigid substrates. Measurements made on absorptive

materials ^5plied on thin metal foils ^speared to give inconsistoit results on

aging and are not reported for this reason. This may be due to wrinkling of

the foils and the small sanple surface area viewed by the spectrophotometer

which does not average out the effects of the wrinkling.

In general, the results obtained with the rigid sanples were consistent with

those obtained by infrared emittance measurements, with the lower emittance

specimens having higher infrared reflectance values. The large spreads

obtained for some of the reflectance curves speared to be closely related to

factors such as coating flatness and smoothness, the ^jpearance of corrosion

or other degradation products on the sanple surface and the exteit of cracking

and blistering of the coating. For absorber material AJ, there was a large

spread in the reflectance curves measured for sanples that had the same

8
measured emittance. ^is appeared to be directly related to blistering of the

absorber coating and, for the high emittance sanples, to the formation of

cracks. The reflectance curves of coatings having a similar surface

appearance tended to cluster together.

Integrating the infrared spectral reflectance values with the black body

energy distribution shown in figure 1 gave pittance values that were in poor

agreement with those measured in accordance with ASTM E408 using an infrared

reflectometer

.

4, CONCLUSIONS AND RECOMMENDATIONS

Infrared reflectance spectra are a more sensitive indicator of changes in

absorber material surface tc^)ography and conposition than emittance measured

in accordance with ASTM E 408, as listed in tables 1 and 2, However, the

phenomena causing changes in the reflectance spectra are not obvious from

examining the spectra.

Direct observation of changes in surface tcpography using optical and scanning

electron microscopy, and compositional analysis where warranted, in

cc«±)ination with the current practice of performing absorptance and emittance

measurements per ASTM E 424, method A, and ASTM E 408 appears to be a more

effective use of resources, with regard to the information provided, than

infrared reflectance measurements.

To minimize the influence of sample variability, optical measurements should

be performed on the same test specimen before and after aging using an

alignment fixture to ensure that the same sample area is viewed by the

9
spectrometer. Wrinkling can cause a considerable amount of spread in the

infrared reflectance spectra measured on replicate samples of absorptive

coatings applied on foil substrates.

Alignment variabilities can be minimized using the FT-IR spectrometer if the

reflectance attachment is kept in a fixed position in the sanple chairber and

used without changing its alignment until all measurements are conpleted. If

this is not done, extreme care must be taken in attachment alignment, to

ensure that the same reflected energy is going into the spectrometer when

successive measurements are mde.

The spectral infrared reflectance measured for materials AA, AJ and AP was

considerably higher at an incident angle of 75 degrees than at 20 or 45

degrees. This is consistent with data given by Meinel and Meinel (8) which

show a large decrease in emittance (increase in reflectance) at angles near

grazing incidence for dielectrics.

5. ACIOCWLEDGEMENTS

The financial support of the Office of Solar Heat Technologies of the U.S.

Departmait of Energy is gratefully acknowledged. Hie authors also thank Dr.

Kent Reed of the National Bureau of Standards for his many helpful

discussions. Foil backed absorber sairples were obtained frcxn Stan Moore of

Los Alamos National Laboratory.

10
6 . PfFERENCES

1. "Test for Solar Energy Transmittance and Reflectance (Terrestrial) of

Sheet Materials," ASTM E 424-71, American Society for Testing and

Materials, 1916 Race Street, Philadelphia, PA 19103,

2. "Test for Total Normal Emittance of Surfaces Using Inspection - Meter

Techniques," ASIM E 408-71, ibid.

3. "Practice for Evaluating Solar Absorptive Materials for Thermal

Applications," ASTM E 744-80, ibid.

4. "Practice for Evaluating Absorptive Solar Receiver Materials When Exposed

to Conditions Simulating Stagnation in Solar Collectors with Cover

Plates," ASTM E 781-81.

5. Waksman, D., Thomas, W.C. and Streed, EJl,, "NBS Solar Collector

Durability/Reliability Test Program: Final R^jort," NBS Technical Note

1196, National Bureau of Standards, Washington, D.C. 20234. (In

publication.)

6. Smithsonian Physical Tables, Table 1, Vol. 815, 1954, p. 273 or Journal

of the Franklin Institute, Vol. 230, 1940, p. 583.

7. Pettit, R.B., "Optical Measurement Techniques ^plied to Solar Selective

Coatings," Report No, SAND-77-0421, Sandia National Laboratories,

Albuquerque, N.M. 87115, August 1977.

8. Meinel. A3, and Meinel. MJ>., Applied Solar Energy; An Introduction.

Reading, Massachusetts: Addison-Wes ley Publishing Company; 1976. p.

2174.

11
Table

1.

Series

1

Absorber

Test

Specimens

OJU

c

fO »— O O O VO r— LOcva CM CSJ CSJ CSJ CM CM ^ ^ ^ CM CM ^

VO ^ to CM ^ 00 00 O »— ro r- •— o O

S.<0 o. ^ u o o

<) vn <

r*. VO r*^ ^

00 CO GO 00 a^

00 00 00 VO VO Ch ^ O' o>

VO uo vo vn LO lo LO

O' O' O' O' O' O' O'

0) u

3

(/) O

o.X

cc o o o o • o o o z o o ^ CM <— CM ^ o o o o o o • o o o o o o z o o o o o o ^ CM ^ CM t— CM

JZ JZ ^ JZ ^ JZ <000000 * o o o o o o

z o o o o o o ^ CM ^ CM »— CM

c r— f— 1— o o o o o o o o o o o o o o ^ o o ^ P O 0 0 ^ 0 OOP 4-> -MOO •M o o o o O o o o C O' o» coo O' o» c o o o o

o o o o

c O OQ «0 CJ oO oO pQ oS CJ oO oO o9 «e o O o o o o o o *3000 o o o u o 0 •3 U u e 0

0) L. u to m 01 ^ LD tn 01 W tn tn

0) v/> CO »0 f**s. V/) <Q «0 (/> fO &. O f— F- o z z o z z 3 Q. C C O. C C Q£ CL C C QC QZ

</i X o o X o O X o o

o OJ c c 01 C C ^ ^ 01 c c ^ ^ a C OJ 01 c 01 01 tn tn C 01 01 LD tn X O X X O X X o o O X X o o LU

o

l/» JZ<

o<u

rM

«/> 3

v/^

<D *—

O) ro

•4-» 0>

oo ^

o

u£o

u

<L» cx

Q.Oo

O)Eo

uo

CO

Q.*0 E O

rO O OO

vn CM CO CM CO ^ vO vO O' 0>

un CM ro CM CO CM ro tn cm ro cm ro cm m ^vOvor^r^O'O' ^vovor*«.r*«.0' O' Q.a.O.O.a.CX.0. < C C <C <C < < <<£<<«£««««

12
Table

2.

Series 2

Absorber

Test

Specimens

O) KO

O CM CM CO CO

u o o o 1— r— o o o o c

03 00 CM ro p*^ p^ o o o o o o o o

F= UJ

<uu

c:03

A-) Q lo LO CM CM CO r*^ S. (T> CTi CJi o^ cTi cn CT> CTi o

CO -O

OO o o d

d

ddd o c ^ LD CO CM CM CM

CTi CTi CTi CJ> <T> CT» CJ^ 03O

UO o o o d

dd

d

d

d o . —.

“O ~o

CD CD CD +J x:

03 to to

Si. 03 03 +J

to <+- O) ^-

-Q S E CD 3 3 3 f—

00 t- CD

CC S- 00 0) C> • 1— •p— <D r— Cl u E E Q. “O a

•r~ Q.-r- 3 3 Q. o O c r— O •- c

u.

O

— < <c C-) ^ i-

-*-> CD c C “O

•r- c X

03 o CL CL s.

d

CJ o

03 03C •1— > > -r- to

•1— oo s.

4-> 4-> 03 i- 03 U CD S- c_> >

03 03 1 c

I— >- o

03 03

X z u

cn to s- i/i o 1 c 03 a. o r—

r— 03 “3 03 03

+-> > C > 3 u

T3

O -*-> -Q 03 03 c=

C_3 •r— •r- *r- 4-> 03

01

C tO M- jC

C •*“

C -r- to

U

CD CD to 03 -O o 03 E uO 03 to O CL >- E O S. E c= jD O

S- to to

O

to 03 to 03

u

O o 03 C 03 C 03 CJ C O C O a U U ^ CJ CD 3 u U u -r- a "O u •r— •r- 1— 03 o

-C T- ^ *r- s. s. CO 1— to to C3 CL CO

(D X>O

C_3

CD r— CO 3 :e s: o Q. < < < «=t <c

13
t

IU9D J9<j • AiiSN3iNi oaznvwaoN

14

Figure

1.

The

measurement

spectrum

of

the

Gier

Ounkle

Infrared

Ref

lec

tome

ter.

Model

DB-100.

(no

filters)

together

with

a

normalized

100 C

blackbody

spectrum

as

presented

by

Pettit

reference

17).
o

o

15

Figure

2.

Reproducibility

spectrum

for

45

degree

specular

IR

reflectance,

rigid

material

AA,

Condition

2.
o

o

16

Figure

3.

Reproducibility

spectra

for

45

degree

specular

infrared

reflectance,

foil

material

AM,

Condition

2.
17

Figure

4.

Variability

in

45

degree

specular

infrared

reflectance

spectra

for

several

unaged

samples

of

rigid

material

AA.
o

o

18

Figure

5.

Variability

in

combined

diffuse

and

specular

infrared

reflectance

spectra

for

several

unaged

samples

of

rigid

material

AA.
19

Figure

6.

Variability

in

45

degree

specular

infrared

reflectance

for

several

unaged

samples

of

rigid

material

A

J

.
o

Q

20

Figure

7.

Variability

in

combined

diffuse

and

specular

infrared

reflectance

spectra

for

several

unaged

samples

of

rigid

material

A

J

.
QO

3;u —

C3 C3 u

u. U

C 1) —

r: u e

-3 u ac

i) —

.

o. u

X

<

01

OJu

scu

-o

kT'

SQI 06 SZ. 09 Sh 06

30Nbli03nd3d%

SI

sc

r3C

to» u >

t3 1) > to

00

U

3

00

21
o

Q

22

Figure

9.

Variability

in

combined

diffuse

and

specular

infrared

reflectance

spectra

for

several

unaged

samples

of

rigid

material

AP.
23

WAVENUMBeiflS
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aaao0) oooo-0) o z n UJ ao

-jt* (Oao

<j)cnoo

24

Figure

11.

Variability

in

combined

diffuse

and

specular

infrared

reflectance

spectra

for

several

unaged

samples

of

foil

lAaterial

AH.
25

WAVENUMBERS
26

Figure

13.

Variability

in

combined

diffuse

and

specular

Infrared

reflectance

spectra

for

several

unaged

samples

of

foil

material

AW.
LD

27

Figure

14.

Comparison

of

changes

in

emittance

with

combined

diffuse

and

specular

infrared

reflectance

spectra

for

aged

and

unaged

samples

of

material

AA.
Unexposed

Control , f-«

OO

O NO

U) I ^ ^ rH lO • • (0 CB 0 CO W U u u js j: JZ X o o o o o o o o o o O O <H ^si •H <M u u U o o O O u-t %M VM o u CJ O O o OP ID O O 9n ON ^ ^ u *o *o

1-1 0) 0 0 00 00 0 0 C C 0 0 0 0 > > X X o 6 ^ <N •»» ID

r

09

28

Figure

15.

Comparison

of

changes

In

emlttance

for

20

degree

Infrared

reflectance

spectra

for

aged

and

unaged

samples

of

material

AA.
Unexposed

Control

o O O vO

1^

, m OB

go a u lx U xz JS JZ £ o o o o o o o o o o o o CN

rH <N| M M

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29

Figure

16

Comparison

of

changes

in

emlttance

for

A5

degree

Infrared

reflectance

spectra

for

aged

and

unaged

samples

of

material

AA.
o

o

30

Figure

17.

Comparison

of

changes

In

emlttance

for

75

degree

Infrared

reflectance

spectra

for

aged

and

unaged

samples

of

material

AA.
oo

CM <N <N «N <M <N r*4 m| ^ i-M fH ^ <N CM O o o o o O f-4 CM O O • u oc0 4-» u «

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u

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M S > in o CT' 9S ^ CM

31

Figure

18.

Comparison

of

changes

in

emittance

with

combined

diffuse

and

specular

infrared

reflectance

spectra

for

aged

and

unaged

samples

of

material AJ.
Uaexposed

Control

u|

T 1 r 0^ OE 02

33Nyi33nj3y %

32

Figure

19.

Comparison

of

changes

In

emlttance

for

20

degree

Infrared

reflectance

spectra

for

aged

and

unaged

samples

of

material

AJ.
33

Figure

20.

Comparison

of

changes

in

emlttance

for

A5

degree

infrared

reflectance

spectra

for

aged

and

unaged

samples

of

material

AJ

.
Unexposed

Control

VO

w|

CM tM fN «s <N <M

•-* r-l iH r-l tH CM (N

. . (0 Q (0 bo o o o

o o o o o ^ »H <N M

b W O o o ^

m

u

JC (0

o

u

9

c c

o o

c c

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o o o CM O O o o U ^ CM o

<4^ id b

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CJ H-t VM

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<0 Q 4 4J a 4 CJ «t3 -o

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r SOI

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06

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SL

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SI

a

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o

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z:D

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(VJ 5

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cn

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o

0

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o

<J)

34

Figure

21.

Comparison

of

changes

In

emlttance

for

75

degree

Infrared

reflectance

spectra

for

aged

and

unaged

samples

of

material

AJ.
aa

>0 I-* OO 00 ^

U)|

O *-i o O iH

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X

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4) 0) • • 00 00 X s

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35

Figure

22.

Comparison

of

changes

in

emittance

with

combined

diffuse

and

specular

infrared

reflectance

spectra

for

aged

,and

unaged
Unexposed

Control oo

36

Figure

23.

Comparison

of

changes

In

emlttance

for

20

degree

Infrared

reflectance

spectra

for

aged

and

unaged

samples

of

material

AP
Unexposed

Control

) »-( 00 OO (N

' l-t o o •H

. 4 4

o QD u W • • u Ui JZ X 4 4 W M o o X X o O o o o o o o o o

o o rsj o o

<N o o

u M tN u M 0 O

o 0 4-1 4h u u

0 o O u 44 4-1 o a 0 o

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u iJ U

m 4 4 4 u 4J 4 4

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4 4 • •

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CM m sO

o

Q

0 X 08 09 Oh 02 0

33NU1331J3y y.

37

Figure

24.

Comparison

of

changes

in

emittance

for

45

degree

Infrared

reflectance

spectra

for

aged

and

unaged

samples

of

material

AP.
fn

38

Figure

25.

Comparison

of

changes

In

emlttance

for

75

degree

Infrared

reflectance

spectra

for

aged

and

unaged

samples.

of

material

AP.
NBS«n4A tREv. 2«ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions) 4. TITLE AND SUBTITLE

1. PUBLICATION OR

REPORT NO.

NBSIR 84-2916

2. Performing Organ. Report NoJ 3. Publication Date

September 1984

EVALUATION OF INFRARED REFLECTANCE AS A TECHNIQUE FOR MEASURING ABSORBER

MATERIALS DEGRADATION

5. AUTHOR(S)

David Waksman; Willard E. Roberts

6. PERFORMING ORGANIZATION (If joint or other than MBS, see in struction s) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State. ZIP)

Active Heating and Cooling Division

Office of Solar Heat Technologies

U.S. Department of Energy

Washington, D.C. 20585

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Current ASTM standards concerned with the durability and reliability of absorptive

coatings rely on integrated solar absorptance and emittance as the primary methods

for assessing changes in absorber optical performance resulting from environmental

exposure. This study was undertaken to determine if infrared reflectance measurements are a more sensitive technique for detecting absorber materials degradation.

Spectral measurements were made to identify factors that could affect the reproducibility of infrared reflectance measurements and to compare their ability to

detect changes with currently used methods for absorber materials. Recommendations

are made concerning the use and limitations of infrared reflectance measurements

for this purpose.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons^

absorptive coatings; emittance; infrared reflectance; materials degradation;

optical performance; solar collectors

13. AVAILABILITY 14. NO. OF

PRINTED PAGLS

1 Xl Uni imited

1 1 For Official Distribution. Do Not Release to NTIS 46

1 1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402. 15. Price

[~j^ Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $8 .

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^ tr. OKAk.4. r ' • 4 » • •