aic colour 05 1 reflectance changes in white reflectance standards measured in different instruments...
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AIC Colour 05 1
Reflectance changes in white reflectance standards measured in
different instruments with 0/d geometry
Joaquín Campos, Alicia Pons and Jorge Martín
Instituto de Física Aplicada. CSIC. Spain
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Outline
Introduction
Instruments used
Measurements
Results
Summary
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Introduction
• Spectrophotometric errors in measurement of reflectance 0/d widely studied.
• Calibration standard (white standard) is assumed to be constant.
• 0/d reflectance measurement are done with an integrating sphere. Sphere error corrections
• Procedure developed at NPL: correction factors for the correct inclusion and exclusion of the specular component of reflectance.
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Specular beam exclusion error (gloss trap error)
A gloss trap which does not completely eliminate light specularly reflected can lead to an error that depends on the specific instrument.
Measuring the reflectance of a mirror in the specular excluded geometry
)()(
)(
)(
4)(2
t
t
m RM
K
m reading with the mirror
t reading with the standard
M mirror’s reflectance
Rt standard’s reflectance
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Specular beam weighting error
In many integrating spheres, light specularly reflected may not be evaluated with the same efficiency as light diffusely reflected. An error will be present in total reflectance measurements.
Measuring the reflectance of a mirror in the specular included geometry
1
)(
)(
)(
)(4)(3
t
mt
M
RK
m reading with the mirror
t reading with the standard
M mirror’s reflectance
Rt standard’s reflectance
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To test the goodness of this procedure by measuring the reflectance of several types of white reflectance standards in different high quality spectrophotometers and compare the values obtained for every standard with every instrument, once the corrections have been applied.
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Measurements
Samples
– Glossy white ceramic tiles– Matt white ceramic tiles– Spectralon type white standards– BCR matt white standards– BCR glossy white standards
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Measurements Spectrophotometers
PE Lambda 900
– Internal diameter:
150 mm
– Paint covering:
Spectralon
– Incidence angle:
7º for exclusion
and inclusion
PE Lambda 9
– Internal diameter:
60 mm
– Paint covering:
Spectralon
– Incidence angle:
7º for exclusion
and inclusion
Cary 17
– Internal diameter:
60 mm
– Paint covering: barium sulphate
– Incidence angle: 0º when excluding and 7,5º when including
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350 400 450 500 550 600 650 700 750 8000,00
0,02
0,04
0,06
0,08
0,10
0,12
0,14
0,16
0,18
0,20 Lambda 900 Lambda 9 Cary 17
Co
rrec
tio
n F
acto
r K
2
Wavelength (nm)
Specular beam exclusion error
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350 400 450 500 550 600 650 700 750 800
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8 Lambda 900 Lambda 9 Cary 17
Cor
rect
ion
Fac
tor
K 3
Wavelength (nm)
Specular beam weighting error
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Measurements
• Reflectance of every sample was measured (including and excluding the specular component) in the three instruments taking one of the matt standards as a reference, the same for all instruments
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Measurements
Corrected reflectance:
s instrument’s reading when sample is measured
t instrument’s reading when reference is measured
Rt reference standard’s reflectance (in practice not used)
Ki correction coefficient for exclusion (i=2) or inclusion (i=3) of specular component
and coefficients related to sample and standard, 1 for glossy specimens and zero for matt ones.
)(
)()()(
)(
)()(
t
sit
t
s KRR
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400 500 600 700 800
-1,00
-0,75
-0,50
-0,25
0,00
0,25
0,50
0,75
1,00
Lambda 900 Lambda 9 Cary 17
Dif
fere
nces
fro
m th
e m
ean
(%)
Wavelength (nm) (glossy to matt)
400 500 600 700 800
-1,00
-0,75
-0,50
-0,25
0,00
0,25
0,50
0,75
1,00
Lambda 900 Lambda 9 Cary 17
Dif
fere
nce
s fr
om
th
e m
ean (
%)
Wavelength (nm) (matt to matt)
400 500 600 700 800
-1,00
-0,75
-0,50
-0,25
0,00
0,25
0,50
0,75
1,00
Lambda 900 Lambda 9 Cary 17
Dif
fere
nces
fro
m t
he m
ean
(%
)
Wavelength (nm) (glossy to glossy)
• Differences < uncertainties
• Procedure correct specular beam weighting error
Specular included geometry
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400 500 600 700 800
-1,00
-0,75
-0,50
-0,25
0,00
0,25
0,50
0,75
1,00 Lambda 900 Lambda 9 Cary 17
Dif
fere
nces
fro
m th
e m
ean
(%)
Wavelength (nm) (glossy to matt)
400 500 600 700 800
-1,00
-0,75
-0,50
-0,25
0,00
0,25
0,50
0,75
1,00 Lambda 900 Lambda 9 Cary 17
Dif
fere
nces
fro
m th
e m
ean
(%)
Wavelength (nm) (matt to matt)
400 500 600 700 800
-1,00
-0,75
-0,50
-0,25
0,00
0,25
0,50
0,75
1,00 Lambda 900 Lambda 9 Cary 17
Dif
fere
nces
fro
m th
e m
ean
(%)
Wavelength (nm) (glossy to glossy)
• Differences > uncertainties (shorter wavelengths)
• Angles of incidence and sphere diameters
• Different, function of , angular distribution of reflectance
Specular excluded geometry
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Summary
• In specular included geometry, the adopted procedure allows to correct for the possible specular beam weighting error.
• In specular excluded geometry, significant differences can be observed not only for different angles of incidence, but also for different spehere diameters.
• In the example glossy to matt, the differences seem to indicate that the adopted procedure does not correct adequately for the possible specular beam exclusion error; but this assumption cannot be used to explain the differences in the matt to matt example.
• A more exhaustive research is necessary. BRDF.