Effect of Illuminant/Observer on Calculation of Color Values

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FAQ: "Last week we had discussed measuring specular reflectance with my company's Ultra Scan VIS device. You had recommended to do Total Reflectance Measurement in RSIN mode and subtract Diffuse Reflectance Measurement: RSEX mode. Our need is to measure % Specular reflectance for a range of wavelengths (400-700 nm).

I have done this using A/2 illuminant/observer angle.

My question is will the Total Reflectance and Diffuse Reflectance be different at A/10 illuminant/observer angle than A/2? The Y Brightness value did change when I selected A/10 but reflectance values at range of wavelengths did not.. Do you know why this is?"

All colorimetric values - X, Y, Z and all the follow-on color metrics like L, a, b* are calculated from the same spectral signature of the material.

The choice of illuminant adjusts the calculated colorimetric values to reflect what the color would look like under different sources of white light - D65 noon daylight outside versus F02 cool white fluorescent in the office versus illuminant A representing tungsten lighting in the home.

The mathematical representation of the human observer is one of two definitions - 1931 2 degree and 1964 10 degree. They are similar but not the same.

More on this at AN 1031 Identification of Measurement Parameters at http://wp.me/P24zt3-iY

FAQ 1: "Will the Total Reflectance and Diffuse Reflectance be different at A/10 illuminant/observer angle than A/2?"

Reflectance being spectral is not affected by the choice of illuminant or observer. It remains the same.

FAQ 2: "The Y Brightness value did change when I selected A/10 but reflectance values at range of wavelengths did not.. Do you know why this is?"

The Y Brightness values for A/2 and A/10 calculated from the same reflectance signature (specular or diffuse) from the material will be very close but different reflecting the difference in standard observer.

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