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"L" for lightness and "a" and "b" for the color-opponent dimensions, based on nonlinearly compressed (e.g. CIE XYZ color space) coordinates.

Lab color is designed to approximate human vision. Its L component closely matches human perception of lightness, although it does not take the Helmholtz-Kohlrausch effect into account.
RGB or CMYK spaces model the output of physical devices instead.
Thus, L*a*b*-model is device independence.

The lightness, L*, represents the darkest black at L* = 0, and the brightest white at L* = 100. The color channels, a* and b*, will represent true neutral gray values at a* = 0 and b* = 0.
The red/green opponent colors are represented along the a* axis, with green at negative a* values and red at positive a* values.
The yellow/blue opponent colors are represented along the b* axis, with blue at negative b* values and yellow at positive b* values.

CIE L*a*b* (CIELAB) is a color space specified by the International Commission on Illumination (French Commission internationale de l'éclairage, hence its CIE initialism).

Conversion from/to RGB and CMYK

The L*a*b* color space is used when graphics for print have to be converted from RGB to CMYK, as the L*a*b* gamut includes both the RGB and CMYK gamut.
Lab space is much larger than the gamut of computer displays, printers, or even human vision.

There are no simple formulas for conversion between RGB or CMYK values and L*a*b*, because the RGB and CMYK color models are device-dependent. The RGB or CMYK values first must be transformed to a specific absolute color space, such as sRGB or Adobe RGB. This adjustment will be device-dependent, but the resulting data from the transform will be device-independent.
sRGB is a standard RGB color space created cooperatively by HP and Microsoft in 1996 for use on monitors, printers and the Internet.


gray = 0.299 ×  red + 0.587 ×  green + 0.114 ×  blue

Opponent process



If the trichromatic theory was the only way to explain color processing, then red/green colorblind people would also be UNABLE to see yellow (as red/green cones work simultaneously to create yellow). However, by saying that ganglion cells contribute to our color experience, even someone with poorly active red/green cones could potentially see yellow.

Afterimages also provide evidence for the opponent process theory. When a person stares at an afterimage, it is believed that their ganglion cells get excited. If a person is seeing BLUE, the ganglion cell excites BLUE and inhibits the opposing color YELLOW. Once the staring ends, the ganglion cell will achieve balance by inhibiting BLUE and exciting YELLOW. In other words, after staring at one color, we often see the opposite color appear. THIS IS AN AMAZING EXPERIENCE and there are many examples in this unit.

First stare at the colored blocks. Count 30 seconds and try not to blink and then shift your eyes to lower, white block. Do you see OPPOSITE COLORS in the white block?