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THE USE OF PORTABLE INSTRUMENTS FOR OPTICAL PROPERTY MEASUREMENT OF
SELECTIVE SURFACES
J. Whittle, J. Mason
To cite this version:
J. Whittle, J. Mason. THE USE OF PORTABLE INSTRUMENTS FOR OPTICAL PROPERTY
MEASUREMENT OF SELECTIVE SURFACES. Journal de Physique Colloques, 1981, 42 (C1),
pp.C1-365-C1-372. �10.1051/jphyscol:1981126�. �jpa-00220676�
THE USE O F PORTABLE INSTRUMENTS FOR O P T I C A L PROPERTY MEASUREMENT OF SELECTIVE SURFACES
J.E. W h i t t l e and J.J. Mason Inco Europe Ltd., B i r m i n g h a m , U . K .
Abstract.- The operation and accuracy of portable optical measure- ment equipment, the Willey Alpha Meter (Model 2150) for measuring
solar absorptance (as) and the Willey McDonald Emissometer (Model 2145) for hemispherical emittance at 100'~ (eh100) are presented.
The results obtained with the emissometer for a range of selective materials are compared with calorimetric results and shown to be significantly higher. A model explaining the difference in terms of the reflectivity of the emissometer detector is proposed. A high accuracy modification of the emissometer using a more sensi- tive detector, is described.
Results of as, obtained with the Willey Alpha Meter for black sur- faces like Maxorb, agree well with those calculated from spectro- meter results using A M 2 spectrum. However, because of the non-
linear response of the detector used in the alpha meter, mislea- ding results can be obtained when testing coloured surfaces. The magnitude of the errors involved is calculated and the limitations of the instrument discussed.
1. Introduction.- Portable instruments for the measurement of the opti- cal properties of selective and non-selective solar coatings have many advantages over conventional spectrophotometers and calorimetric instru- ments. Principal among these advantages are ones of low cost and ease of use. They are ideal for screenings large number of testsampleswhilst the low cost and simplicity of operation makes them suitable for quali- ty control purposes within a production environment.
Whilst there are now several portable instruments available com- mercially, this paper concerns our experience with two instruments, a Willey McDonald ( f ) Emissometer (Model 2145) for measuring hemispheri- cal emissivity at 100°C and a Willey Alpha Meter ( l ) (Model 2150).
Results obtained with the emissometer are cross checked with a calorimetric emissometer for several materials. The use of the alphame- ter in measuring both black and coloured selective surfaces is descri- bed.
2. Measurement of Thermal Emittance.- 2.1. W i l l e y - ~ ~ Q o _ ~ a _ ~ ~ - E m i s s p ~ ~ ~ ~ g Mode1 2145.- The model 2145 emissometer is designed to measure the to-
tal hemispherical emittance of flat samples, 35 mm in diameter, at a
( l ) Produced by International Technology Corporation, Satellite Beach,
Florida, U.S.A.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981126
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t e m p e r a t u r e o f 100°C. A s c h e m a t i c diagram o f t h e i n s t r u m e n t i s shown i n f i g u r e 1. I t c o n s i s t s o f an e l l i p s o i d a l r e f l e c t i n g c a v i t y w i t h a t h e r m i s t o r l o c a t e d a t one f o c u s . The specimen t o b e measured i s p l a c e d o v e r t h e measurement p o r t which c o r r e s p o n d s spatially w i t h t h e o t h e r f o c u s . The measurement sequence c o n s i s t s o f f i r s t l y e n s u r i l l g t h a t t h e whole o f t h e emissometer i s a t ambient t e m p e r a t u r e , t h e n p l a c i n g it o v e r t h e h e a t e d sample.
MEASUREMENT PORT
F i g . 1.- Schematic diagram o f W i l l e y Emissometer
The r a t e o f i n c r e a s e i n t e m p e r a t u r e o f t h e t h e r m i s t o r i s a measure o f t h e e r n i t t a n c e o f t h e sample. The o u t p u t o f t h e t h e r m i s t o r i s measured a s a f u n c t i o n o f t i m e and t h e o u t p u t a f t e r a f i x e d t i m e , which must be l e s s t h a n t h a t needed f o r t h e r m a l e q u i l i b r i u m , i s measured (Vspec).
The p r o c e s s i s r e p e a t e d f o r low e m i t t a n c e ( A l , VAl) and h i g h e m i t t a n c e ( N e x t e l p a i n t r V N ) s t a n d a r d s and t h e e n i t t a n c e o f ' t h e unknown (E spec 1 c a l c u l a t e d from t h e e q u a t i o n .
P e t t i t /1/ h a s measured t h e i n f r a r e d s p e c t r a o f N e x t e l and A 1 u s i n g a G i e r Dunkle I n f r a r e d R e f l e c t o m e t e r (Model DB
-
100) and c a l c u l a t e d t h e 100°C e m i t t a n c e v a l u e s u s i n g a 100°C blackbody s p e c t r u m t o b e 0.912 ( N e x t e l ) and 0.030 ( A l ).
S i n c e e q u a t i o n ( l ) , s p e c i f i e d by t h e emiss.ometer m a n u f a c t u r e r s , i s based on v a l u e s f o r N e x t e l p a i n t o f 0.965 and A l f o i l 0.060, t h i s equa- t i o n must be m o d i f i e d t o t a k e i n t o a c c o u n t t h e c o r r e c t s t a n d a r d v a l u e s .
The c o r r e c t e d e x p r e s s i o n i s g i v e n below : E c o r r e c t = 0.882 'spec
-
'AI + 0 3 0[
N'- 1
S i n c e e q u a t i o n (1) i s t h a t s p e c i f i e d by t h e i n s t r u m e n t manufactu- r e r , i t h a s been used t h r o u g h o u t t h e t e x t t o c a l c u l a t e t h e t e r m E ( W i l - l e y )
.
checked calorimetrically by Sikkens /2/, using a calorimeter havinq an absolute accuracy for small values of 0.003. The comparison is illustra- ted. in figure 2.
E calorimetric
Fig. 2.- Correlation between Willey meter and calorimeter
The Willey emittance gives consistently higher values than those deter- mined calorimetrically. Also illustrated in figure 2 is the relationship obtained by Pettit between a similar portable emissometer (Willey Ambi- ent emissometer Model 2158) and calorimetric results for a range of black chrome samples having actual 100°C total hemispheriaal emissivi- ties up to 0.4. The linear relationship obtained by Pettit
EH(lOOOC) = 0.810 (E Willey)
relates to E determinedby equation (2) and when transposed to equation (1)isillustrated in figure 2. The relationship between the two sets of results is very close.
2.2.~P~oretical-Consider~&,i~~g.- We have attempted to explain the diffe- rence between the absolute measurement of total hemispherical emissivi- ty by calorimetric methods and methods involving ellipsoidal cavities,
( 2 ) Trade Mark 1WD Technology Ltd.
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l i k e t h a t o f t h e Willey meter, i n terms o f m u l t i p l e r e f l e c t i o n s w i t h i n t h e c a v i t y .
I f we assume t h a t t h e a c t u a l e m i s s i v i t y o f a s u r f a c e i s E*, t h e r e f l e c t a n c e of t h e c a v i t y w a l l i s R and t h e r e f l e c t a n c e of t h e detec-
C
t o r i s F$,, t h e measured e m i t t a n c e Em i s given by a power s e r i e s l i k e
assuming 2 r e f l e c t i o n s w i t h i n t h e c a v i t y .
This c o r r e c t i o n cannot be a p p l i e d d i r e c t l y t o t h e Willey v a l u e s a s given by e q u a t i o n s (1) o r ( 2 ) because it can be seen t h a t a z e r o s i - gnal g i v e s a n e g a t i v e e m i s s i v i t y . We presume t h a t t h i s i s done t o o f f - s e t t h e i n t e r n a l r e f l e c t i o n enhancement. I f we r a t i o t h e Willey o u t p u t s i g n a l s t o t h e a p p a r e n t v a l u e f o r b l a c k p a i n t , a curve can be c o n s t r u c - t e d of a p p a r e n t a g a i n s t t r u e v a l u e s . F i g u r e 3 i l l u s t r a t e s t h e c o r r e l a - t i o n t h a t can be o b t a i n e d .
F i g . 3 . - C o r r e l a t i o n between a p p a r e n t and t r u e e m i s s i v i t y
2.3. Modified Emissometer.- A more s e n s i t i v e i n s t r u m e n t , working on t h e same p r i n c i p l e s a s t h e Willey meter, has been c o n s t r u c t e d and i s shown s c h e m a t i c a l l y i n f i g u r e 4 . A r o t a t i n g chopper i s mounted w i t h i n t h e c a v i t y a l l o w i n g t h e t h e r m i s t o r t o be r e p l a c e d w i t h a more s e n s i t i v e p y r o - e l e c t r i c d e t e c t o r . The d e t e c t o r i s mounted behind t h e e l l i p s o i d a l c a v i t y and r a d i a t i o n i s d i r e c t e d t o it v i a a l a r g e a p e r t u r e h y p e r b o l i c c a s s e g r a i n m i r r o r l o c a t e d a t t h e r e a r f o c a l p o i n t .
This i n s t r u m e n t g i v e s r e p r o d u c i b l e r e s u l t s t o w i t h i n +- 0 . 0 0 1 e- m i s s i v i t y u n i t s . The c o r r e c t i o n s n e c e s s a r y t o c o n v e r t t o a c t u a l t o t a l
P ~ Y
/U
PYRO!&ECTRIC DETECTOR
Fig. 4.- Modified emissometer
Examples a r e given i n t a b l e I.
Table I.- R e s u l t s from modified emissometer
3 . Willey Alpha Meter f o r Measurement of S o l a r Abs0rptance.- The Willey Alpha Meter (Model 2 1 5 0 ) c o n s i s t s o f a n e l l i p t i c a l c a v i t y p a i n t e d white
i n s i d e which i s a s i l i c o n p h o t o e l e c t r i c d e t e c t o r . The l a t t e r i s p o s i - t i o n e d away from t h e incoming l i g h t and o u t of l i n e from d i r e c t r e £ le c - t i o n from t h e specimen. The specimen i s l o c a t e d over a measurement p o r t and t h e d e t e c t o r o u t p u t measured when t h e sample i s i l l u m i n a t e d from a s u i t a b l e source. The meter manufacturers recommend use w i t h a Kodak Carousel P r o j e c t o r b u t because of a v a i l a b i l i t y we have used a Xenon Arc Source f i l t e r e d t o remove t h e s p e c t r a l l i n e s . Such a source h a s an i n - t e n s i t y spectrum r e a s o n a b l e c l o s e t o t h e AM1 s o l a r speckrum.
A zero r e a d i n g (Vo) w i t h t h e measurement p o r t open and a 1 0 0 % r e a d i n g o b t a i n e d w i t h a MgC03 s t a n d a r d (Vs) o v e r t h e measurement p o r t completes t h e measurements. I f t h e meter o u t p u t f o r t h e sample t o be measured i s Vi t h e n t h e f r a c t i o n a l r e f l e c t a n c e of t h e sample F i s g i -
'
.
C a l o r m e t r l c Values Beens0.017 (at 29.3'~)
0.079(at 97.4'~) i Material
A I foil
Cu for1
electroplo~shed
N I foll
Mdxorb
Uncorrect d Value Eh 100gC
0.058
0.036
0.090
0.153
Correc8ed Value Eh 100 C assuming r o f l c c t l v ~ t y product 0.526
0.028
0.0175
0.045
0.079
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ven by
vs
- v.
and r e £ l e c t a n c e c a l c u l a t e d from
a s given i n t h e i n s t r u m e n t i n s t r u c t i o n s .
A c l o s e c o r r e l a t i o n , b e t t e r than
*
0.02 absorptance u n i t s , was o b t a i n e d between t h e Alpha meter and c a l c u l a t i o n s f o r AM2 from t h e re- f l e c t a n c e s p e c t r a f o r black s e l e c t i v e and n o n - s e l e c t i v e f i n i s h e s . For example, a sample o f llaxorb f o i l gave as=0.97 w i t h t h e Willey meter and 0.97 w i t h a Perkin Elmer spectrophotometer.Use of t h e i n s t r u m e n t w i t h non b l a c k s e l e c t i v e s u r f a c e s g i v e s r e - s u l t s which a r e f a r from a c c u r a t e . Since t h e i n s t r u m e n t u s e s a s i l i c o n d e t e c t o r , t h e response of which peaks a t about 0.9 p n and c u t s o f f a t about 1.1
vm,
t h e r e s u l t s a r e b i a s s e d towards t h e r e f l e c t i v i t y over t h e range 0.7 t o 1.1m.
To i l l u s t r a t e t h i s e f f e c t , samples with a l a r g e v a r i a t i o n i n re- f l e c t i v i t y over t h e wavelength range 0 . 3 t o 1.1 p m were s e l e c t e d . These samples were chemically c o l o u r e d t y p e 304 s t a i n l e s s s t e e l s which had been p r i o r p o l i s h e d t o a m i r r o r . f h i s h . The r e f l e c t a n c e s p e c t r a of t h e s e samples a r e i l l u s t r a t e d i n f i g u r e 5.
Fig. 5.- T o t a l r e f l e c t a n c e s p e c t r a of t y p e 304 s t a i n l e s s s t e e l s
The samples a r e b l u e coloured and have r e f l e c t a n c e minimum r a n g i n g from
o f t h e AM2 s o l a r spectrum. The v a l u e s o f s o l a r a b s o r p t a n c e f o r v a r i o u s samples h a v i n g s l i g h t l y d i f f e r i n g c o l o u r s a r e compared w i t h v a l u e s c a l - c u l a t e d f o r AM2 s p e c t r u m from t h e r e f l e c t a n c e c u r v e s ( s e e F i g . 6 ) .
8'
A
6 0 7 I 08 Q9 I 1.0 I'NAVELENGTH OF FIRST REFLECTANCE MINIMA p m
P i g . 6.- Comparison o f a s between W i l l e y m e t e s and s p e c t r o m e t e r f o r d i f - f e r e n t c o l o u r e d s t a i n l e s s s t e e l s
The specimen w i t h t h e optimum a p p a r e n t r e f l e c t i v i t y h a s a r e f l e c t a n c e minimum c o r r e s p o n d i n g t o t h e maximum s e n s i t i v i t y o f t h e s i l i c o n d e t e c -
t o r . I n o r d e r t o c o r r e c t t h e problem i t i s n e c e s s a r y t o modify t h e i n s - t r u m e n t by p r o v i d i n g a PbS d e t e c t o r i n a d d i t i o n t o t h e s i l i c o n d e t e c t o r a s s u g g e s t e d by P e t t i t .
4. Conclusion.- The p o r t a b l e W i l l e y i n s t r u m e n t s a r e s a t i s f a c t o r y f o r mea- s u r i n g b o t h t h e r m a l e m i t t a n c e and s o l a r a b s o r p t a n c e w i t h i n c e r t a i n r e s - t r i c t i o n s . The t h e r m a l e m i t t a n c e v a l u e s need c o r r e c t i o n t o c o r r e l a t e w i t h c a l o r i m e t r i c t e s t r e s u l t s . The Alpha m e t e r g i v e s s a t i s f a c t o r y va-
l u e s f o r n o n - s e l e c t i v e b l a c k and some s e l e c t i v e b l a c k samples a s l o n g a s t h e r e i s no s i g n i f i c a n t i n c r e a s e i n r e f l e c t a n c e i n t h e r a n g e 1.1 t o 2 . 2 Mm. No c o r r e c t i o n s c a n be made t o t h e s e and c o l o u r e d s e l e c t i v e o r n o n - s e l e c t i v e f i n i s h e s . I n o r d e r t o o b t a i n s e n s i b l e r e s u l t s t h e equip- ment needs m o d i f i c a t i o n .
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References
/l/ Pettit,R.B., J.Eng.Power. Trans ASME 097%) 1-8
/2/ Sikkens,M., Private communication /3/ Beens, W. W., Sikkens, K., Verster, J.L.,
J. Phys. E.
