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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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Page 1: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

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

Page 2: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

AIC Colour 05 2

Outline

Introduction

Instruments used

Measurements

Results

Summary

Page 3: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

AIC Colour 05 3

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.

Page 4: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

AIC Colour 05 4

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

Page 5: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

AIC Colour 05 5

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

Page 6: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

AIC Colour 05 6

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.

Page 7: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

AIC Colour 05 7

Measurements

Samples

– Glossy white ceramic tiles– Matt white ceramic tiles– Spectralon type white standards– BCR matt white standards– BCR glossy white standards

Page 8: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

AIC Colour 05 8

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

Page 9: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

AIC Colour 05 9

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

Page 10: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

AIC Colour 05 10

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

Page 11: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

AIC Colour 05 11

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

Page 12: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

AIC Colour 05 12

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

Page 13: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

AIC Colour 05 13

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

Page 14: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

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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

Page 15: AIC Colour 05 1 Reflectance changes in white reflectance standards measured in different instruments with 0/d geometry Joaquín Campos, Alicia Pons and

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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.