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Here is the math used by W3C for computing acceptable contrast: https://www.w3.org/TR/WCAG20/#relativeluminancedef

I suspect luminosity to mean percentage of brightness for a given frequency from a total light source assuming white light. For a more precise definition I would look to physics.

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That’s for sRGB, which as for most colour spaces is defined relative to CIE XYZ, which is itself defined in terms of the perception of a “standard human observer”. In XYZ, the Y coordinate is the luminance (brightness) of the colour and that first equation is computing Y from the RGB value

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