Color Brightness Calculator

Brightness models exist because equal RGB numbers do not produce equal sensations. The eye responds far more to the middle of the spectrum than to its ends, a fact captured by the CIE photopic luminosity function peaking near 555 nm, so green contributes most of the perceived light, red less, and blue very little. The BT.709 weights of 0.2126, 0.7152, and 0.0722 encode that imbalance, and pure green (0,255,0) looks several times brighter than pure blue (0,0,255). A second complication is gamma. Values stored in sRGB are encoded on roughly a 1/2.2 curve, so a stored 128 emits only about 22 percent of the light of 255, and the weights above are strictly valid only after that curve is reversed. Applying them straight to encoded values, the common luma shortcut, gives a usable ranking but not a true luminance. The alternatives on this page answer different questions. Relative luminance linearises first and is the quantity WCAG requires for contrast work, running from 0 for black to 1 for white. The HSP model squares the RGB values before weighting and takes a square root, which approximates the gamma curve cheaply and predicts light-versus-dark judgements well. HSL lightness and HSV value suit colour manipulation but are poor perceptual measures: HSL reports 50 percent for pure red and pure green alike, and HSV reports 100 percent for white, red, and yellow. Where perceptual uniformity matters, CIELAB L* is the reference scale, and it is worth noting that mid-grey at L* 50 corresponds to only about 18 percent relative luminance, not 50.

Quantifying brightness began with visual photometry, where observers matched an unknown source against a reference until the two appeared equally bright. Averaging many such matches across wavelengths produced the standard luminosity function that the Commission Internationale de l Eclairage adopted in 1924, and the CIE built its 1931 XYZ colour space around it so that the Y component would carry luminance directly. Albert Munsell had meanwhile approached lightness from the perceptual side, publishing his color notation system in 1905 with a value scale spaced by equal visual steps rather than equal physical intensity, an approach that CIELAB formalised mathematically in 1976 with its cube-root lightness function. The specific coefficients used in imaging came from television engineering. The NTSC colour system standardised in 1953 used phosphor primaries whose luminance contributions worked out to 0.299 red, 0.587 green, and 0.114 blue, and those numbers were carried into ITU-R BT.601 for standard-definition digital video. High-definition television changed the primaries, and BT.709, published in 1990, recalculated the weights as 0.2126, 0.7152, and 0.0722, the values the web inherited when sRGB was defined on the same primaries in 1996. Charles Poynton spent much of the 1990s arguing that the industry was conflating luma, computed from gamma-encoded signals, with true luminance, a distinction now embedded in accessibility standards. Darel Rex Finley later published the HSP model to give developers a practical brightness estimate suited to deciding whether text over a colour should be black or white.