which an area appears to be similar to one of the perceived colors: red, yellow, green, and blue, or to a combination of two of them". Radiance (Le,Ω) The radiant power of light passing through a particular surface per unit solid angle per unit projected area, measured in SI units in watt per steradian per square metre (). Luminance (Y or Lv,Ω) The radiance weighted by the effect of each wavelength on a typical human observer, measured in SI units in candela per square meter (). Often the term luminance is used for the relative luminance, Y/Yn, where Yn is the luminance of the reference white point. Luma (Y′) The weighted sum of gamma-corrected , , and values, and used in , for JPEG compression and video transmission. Brightness (or value) The "attribute of a visual sensation according to which an area appears to emit more or less light". Lightness The "brightness relative to the brightness of a similarly illuminated white". Colorfulness The "attribute of a visual sensation according to which the perceived color of an area appears to be more or less chromatic". Chroma The "colorfulness relative to the brightness of a similarly illuminated white". Saturation The "colorfulness of a stimulus relative to its own brightness". Brightness and colorfulness are absolute measures, which usually describe the spectral distribution of light entering the eye, while lightness and chroma are measured relative to some white point, and are thus often used for descriptions of surface colors, remaining roughly constant even as brightness and colorfulness change with different illumination. Saturation can be defined as either the ratio of colorfulness to brightness, or that of chroma to lightness. General approach HSL, HSV, and related models can be derived via geometric strategies, or can be thought of as specific instances of a "generalized LHS model". The HSL and HSV model-builders took an RGB cube—with constituent amounts of red, green, and blue light in a color denoted —and tilted it on its corner, so that black rested at the origin with white directly above it along the vertical axis, then measured the hue of the colors in the cube by their angle around that axis, starting with red at 0°. Then they came up with a characterization of brightness/value/lightness, and defined saturation to range from 0 along the axis to 1 at the most colorful point for each pair of other parameters. Hue and chroma