For the normalized sRGB calculations here, relative luminance is the linear-light weighted sumwhere are decoded linear-light components, not encoded code values. This is the sRGB calculation specified in W3C's WCAG 2.2 definition of relative luminance. It places reference black at 0 and reference white at 1; it does not by itself specify an absolute display light level.[ref]
https://www.w3.org/TR/WCAG22/#dfn-relative-luminance. W3C, WCAG 2.2, definition of relative luminance: sRGB decoding and weights
For a neutral sRGB shade, , and the weights sum to 1, so . For a coloured pixel, relative luminance can be encoded as a neutral sRGB shade with all three encoded components . This preserves the specified while discarding colour differences; it is one precisely defined grayscale conversion, not a unique way to turn all colour images into grayscale.
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Relative luminance is a measure used to describe the perceived brightness of a color or image in relation to a reference white point or a defined scale. It reflects how bright a surface appears to the human eye, considering factors like the surrounding environment and specific color characteristics. In the context of digital media, relative luminance is often defined in terms of the RGB (Red, Green, Blue) color model.