Colour

Explanation

Luminance is a measure of how light or dark a colour is. A colour of any hue will approach a pure black as its luminance approaches zero or pure white as its luminance approaches 100%. This limits the saturation of any colour that has a low or high luminance.

Luminance refers to human perception of the colour quality, not to the power output of a light source. I can only make approximate adjustments to luminance settings when my program adjusts the output levels of the pixels on this computer screen.

Some observations

• When you adjust the luminance of the background colour, the luminance of the patch in the middle seems to change in the opposite way. This effect is called simultaneous contrast. How we perceive a colour may be dramatically affected by its surroundings. The colour of the patch does not actually change at all! If you don’t believe me, test it—cover the background with a piece of paper with a hole in it and watch the patch again as you move the slider. (That’s why the mid grey background on these webpages allows you to see the colour illustrations consistently and with the least interference.)
• The terms lightness, brightness and value are often to mean luminance, usually when it is measured with a different scale. Many computer programs, including Photoshop, use a brightness scale that goes from 0% at black to 100% when one of the RGB values has reached its maximum. This scale is particularly convenient for computer calculations but the scale used in these notes corresponds more closely to our perception of colour. (There is no perceptual reason for black and white to be treated asymmetrically.)
• Unfortunately the term brightness is ambiguous. Paint manufacturers, for example, use brightness to refer to a combination of luminance and saturation—a well saturated, light colour is called a bright colour.
• The perception of luminance and hue result from activity in distinct parts of the brain. Luminance alone can provide considerable visual information, which is why we can understand black and white photographs and why we can get around at night when we can’t distinguish hues.