"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
- The color opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner.
- The three types of cones (L for long, M for medium and S for short) have some overlap in the wavelengths of light to which they respond (L, M, S have spectral sensitivity around Red, Green, Blue respectively), so it is more efficient for the visual system to record differences between the responses of cones, rather than each type of cone's individual response.
- The opponent color theory suggests that there are three opponent channels: red versus green, blue versus yellow, and black versus white (the last type is achromatic and detects light-dark variation, or luminance).
- Responses to one color of an opponent channel are antagonistic to those to the other color. That is, opposite opponent colors are never perceived together - there is no "greenish red" or "yellowish blue".
- Though the trichromatic and opponent processes theories were initially thought to be at odds, it later came to be understood that the mechanisms responsible for the opponent process receive signals from the three types of cones and process them at a more complex level.
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?