Joaquín Campos, Alicia Pons and Jorge Martín Instituto de Física Aplicada. CSIC. Spain

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

2. Outline • Introduction • Instruments used • Measurements • Results • Summary

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.

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 mreading with the mirror t reading with the standard M mirror’s reflectance Rt standard’s reflectance

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 mreading with the mirror t reading with the standard M mirror’s reflectance Rt standard’s reflectance

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.

7. Measurements Samples • Glossy white ceramic tiles • Matt white ceramic tiles • Spectralon type white standards • BCR matt white standards • BCR glossy white standards

8. Spectrophotometers Measurements Cary 17 • Internal diameter: 60 mm • Paint covering: barium sulphate • Incidence angle: 0º when excluding and 7,5º when including 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

9. Specular beam exclusion error

10. Specular beam weighting error

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

12. Measurements Corrected reflectance: sinstrument’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.

13. Specular included geometry • Differences < uncertainties • Procedure correct specular beam weighting error

14. Specular excluded geometry • Differences > uncertainties (shorter wavelengths) • Angles of incidence and sphere diameters • Different, function of , angular distribution of reflectance

15. 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.