Color Relative Luminance Calculator

What is a Color Relative Luminance?

Relative luminance follows the photometric definition of luminance, but with the values normalized to 1 or 100 for a reference white.[1] Like the photometric definition, it is related to the luminous flux density in a particular direction, which is radiant flux density weighted by the luminosity function y(λ) of the CIE Standard Observer. The use of relative values is useful in systems where absolute reproduction is impractical. For example, in prepress for print media, the absolute luminance of light reflecting off the print depends on the illumination and therefore absolute reproduction cannot be assured.

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What is WCAG?

Web Content Accessibility Guidelines (WCAG) is developed by W3C WAI (The World Wide Web Consortium Web Accessibility Initiative) with a goal of providing a single shared standard for web content accessibility. The WCAG documents explain how to make web content more accessible to people with visual, auditory, physical, speech, cognitive, language, learning, and neurological disabilities.

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How does WCAG Define the Relative Luminance?

Relative luminance is the relative brightness of any point in a colorspace, normalized to 0 for darkest black and 1 for lightest white. For the sRGB colorspace, the relative luminance of a color is defined as L = 0.2126 * R + 0.7152 * G + 0.0722 * B where R, G and B are defined as:

• if RsRGB <= 0.03928 then R = RsRGB/12.92 else R = ((RsRGB+0.055)/1.055) ^ 2.4
• if GsRGB <= 0.03928 then G = GsRGB/12.92 else G = ((GsRGB+0.055)/1.055) ^ 2.4
• if BsRGB <= 0.03928 then B = BsRGB/12.92 else B = ((BsRGB+0.055)/1.055) ^ 2.4

and RsRGB, GsRGB, and BsRGB are defined as:

• RsRGB = R8bit/255
• GsRGB = G8bit/255
• BsRGB = B8bit/255

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