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I have always heard the word value used to describe the lightness of a color. Does luminosity refer to the same thing?

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Luminosity and value occur in different polar presentations of RGB color system and they are calculated differently fom RGB numbers.

Value of an RGB combination is the biggest of RGB numbers and luminosity tries to present how much light a human observer sees.

Luminosity concept is defined for color television. It was an attempt to make color television signal properly visible as black and white in black and white televisions. It's a weighted sum of RGB numbers and the weighting coefficients are different in different systems. Generally the weight of blue is smallest and the weight of green is biggest.

Read this (long) writing of HSL and HSV. https://en.wikipedia.org/wiki/HSL_and_HSV It contains not only one, but several different complexity level explanations of the subjective ideas, history and also calculation formulas.

If you happen to be a Photoshop user you probably have met HSB presentation of RGB colors in the color selector. Adobe's B (=brightness) means the same as value in many other programs, it's the biggest of Red, Green and Blue said in percents. In Photoshop layer blending mode Luminosity copies the luminosity of upper layer to the pixels of the lower layer. Luminosities are calculated from RGB numbers with Adobe's own calculation rules which they keep well hidden, you cannot find them from Photoshop User's Guide. But definitely they are not the same as brightnesses (=Values).

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