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For each 8-bit RGB octet triple contained in each 24-bit color value in the input image, the NTSC luminance is computed using the following formula: [1] [2] [3] = + + The image is now subdivided into 4-pixel by 4-pixel blocks, and, the arithmetic mean of the luminance of each pixel in the block is used to select a representative luminance value.
RGB values of example colors with the same relative luminance as the lightest primary color (green) using BT. 709 primaries for ' (gamma correction) = 2.2 Luma is the weighted sum of gamma-compressed R′G′B′ components of a color video—the prime symbols ′ denote gamma compression .
The first three numbers are the white point to use, then the average surround lighting, in this case 200 cd/m², then the relative luminance of the surround on the same scale as the white point, in this case 18%, then the surround conditions, where 1 = average, 2 = dim and 3 = dark, and then XYZ coordinates of the color to check.
On a typical standard 2.2-gamma CRT display, an input intensity RGB value of (0.5, 0.5, 0.5) only outputs about 22% of full brightness (1.0, 1.0, 1.0), instead of 50%. [19] To obtain the correct response, a gamma correction is used in encoding the image data, and possibly further corrections as part of the color calibration process of the device.
N bb is a fudge factor that is normally 1; it's only of concern when comparing brightness judgements based on slightly different reference whites. Here Y is the relative luminance compared to white on a scale of 0 to 1 and L A is the average luminance of the adapting visual field as a whole, measured in cd/m 2.
Relative luminance follows the photometric definition of luminance including spectral weighting for human vision, but while luminance is a measure of light in units such as /, relative luminance values are normalized as 0.0 to 1.0 (or 1 to 100), with 1.0 (or 100) being a theoretical perfect reflector of 100% reference white. [1]
The candela per square metre (symbol: cd/m 2) is the unit of luminance in the International System of Units (SI). The unit is based on the candela , the SI unit of luminous intensity , and the square metre , the SI unit of area .
A comparison between a typical normalized M cone's spectral sensitivity and the CIE 1931 luminosity function for a standard observer in photopic vision. In the CIE 1931 model, Y is the luminance, Z is quasi-equal to blue (of CIE RGB), and X is a mix of the three CIE RGB curves chosen to be nonnegative (see § Definition of the CIE XYZ color space).