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Monday, March 9, 2015

From the Testing Lab: Brightness

A question recently came from a customer: When we talk about color we have to specify an Illuminant. Why don't we have to specify an Illuminant when we talk about brightness?

Think about the sequence of a color measurement using a spectrophotometer:
1) An emitter with a specified UV Level shines on the sample.
2) The reflected light is then split into its components across the spectrum.
3) This reflectance spectrum is then multiplied by appropriate CIE tristimulus functions (red, green and blue) corresponding to the desired Illuminant/Observer condition.
4) The resulting x, y. z values can be converted into any color system, typically L*a*b*.

Think about the sequence of a brightness measurement using a spectrophotometer:
1) An emitter with a specified UV Level shines on the sample.
2) The reflected light is then split into its components across the spectrum.
3) This reflectance spectrum is then multiplied by the brightness function from 400-500 nm.
In the brightness measurement sequence, the brightness function takes the place of the CIE tristimulus functions in the color sequence.  Brightness is the reflectance of blue light under a certain UV Level condition. Color is how does this sample looks under a certain UV Level condition compared to the average Observer in 1931 (2°) or 1964 (10°) as if it were under a certain Illuminant condition (C, D65, A, FL2, etc.)

Let me know if this explanation makes sense. Contact me, Todd Popson.