Dear Dr. Color:

For the last few decades photographers have been infatuated with saturation. I suspect that if prints were compared with the original views that they would be more saturated than reality. It certainly does ‘punch up’ the picture though.

What is saturation, though? How about the term “vibrance” that Adobe is throwing around?

Believe it or not, there are actually ISO standard technical definitions of most color terms. Here’s a definition from the CIE’s “International Lighting Vocabulary” (1987). Saturation is “colorfulness of an area judged in proportion to its brightness. Note: For given viewing conditions and at luminance levels within the range of photopic vision, a color stimulus of a given chromaticity exhibits approximately constant saturation for all luminance levels, except when the brightness is very high.” Colorfulness is a perceptual attribute of how chromatic something is.

So you can decouple chroma (more or less the same as hue) from luminance and saturation. The farther you get away from neutral for a given hue, the greater the saturation. Essentially, if something seems very colorful (now matter how bright it appears) it’s more saturated. So you pretty much have that right. But it’s not exactly the same as color purity in a spectral sense, because many colors (like the purples) can be quite saturated but are always a combination of other colors.

It’s true that in RGB space, when you want to saturate colors you have to move R, G, and B away from neutral, which can have undesirable consequences for other colors. Different color systems are much better at these color transformations, in particular HSV (or HSL) and L*a*b*. I’m a big fan of L*a*b* for saturation manipulation because it’s completely decoupled from luminance/brightness, and it’s easy to perform via scalar multiplication for all colors at once; there’s no need for curves and no impact on overall color-balance as long as a* and b* are multiplied by the same amount.

As far as I can tell, “vibrance” is a completely nontechnical term. In Adobe products it “is a more selective version of the Saturation slider. Whereas the Saturation slider increases the saturation of all of the tones in your image, the Vibrance slider limits its saturation boost to primary colors, leaving skin tones and other secondary shades untouched.” Sounds like an adjustment in L*a*b* space that is greatest along the a* and b* axes.