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Monday, November 2, 2015

Color Measurement: What is Illuminant?

When making color measurements it is important to understand the terms Illuminant, Observer and UV-Level (or source). There will be three separate blogs addressing each of these points.

Illuminant
An illuminant is a light defined by a spectral power distribution, which may or may not be physically realizable.  We may use a certain light to shine on the actual sample when we make a measurement. The illuminant is applied as a calculation to tell us how the sample would have looked if it were under that standard lighting condition (or illuminant).

For example, most spectrophotometers use a pulsed xenon lamp to shine on the sample when measurements are made. The instrument then uses this reflectance and applies a calculation based on the illuminant selected to give us data that represents how the sample would look under that particular lighting condition. 

Some common illuminants:

Standard illuminant A - incandescent lamp light; has a red-orange tint; color temperature 2855.6 K

Illuminant C - northern sky daylight; spectral energy distribution is very similar to typical daylight but with less ultraviolet energy (like on a cloudy day); often used to simulate indoor lighting; color temperature 6774 K

Standard illuminant D65 - daylight illuminant; color temperature 6504 K; there are also D50 , D55 and D75 illuminants

Standard illuminants F1 to F12  - fluorescent lamps of different types differ widely in spectral power distribution. The CIE has published distributions for 12 fluorescent illuminants; F2 represents a cool white fluorescent lamp; color temperature 4200 K

Since a change in illuminant is just a calculation in a spectrophotometer, after the sample is measured, color values for different illuminants can be determined with just a click of a button. It is important to understand, though, that color values obtained under one illuminant will be different than those obtained under a different illuminant, This is because the lighting does have an effect on how people see color. 

In the Paper Industry illuminant C and standard illuminant D65 are most commonly used. They will give similar, but different results.  In general color applications standard illuminant D65 is most common.