Light is the visible part of the electromagnetic spectrum. Light has three characteristics: Hue, Saturation and Lightness.
Figure 1. Color Wheel of Hue
Figure 2. Saturation
Figure 3. Lightness gradient
Figure 4. Lightness example
Hue is what we usually call color. For example, the hue of a light blue
sweater is blue. What determines hue is where it falls in the spectrum. Using
the diagram above, light on the right, which has a shorter wave length, will
have blue/violet hues and the light on the left, which has a longer wave length,
will have orange/red hues. The color wheel, as seen in Figure 1, is the
typical method of portraying hue.
Saturation is how pure hue is. The saturation 'range' goes from vivid to dull
to grayscale (the absence of hue). A hue that is 100% saturated would be
vivid. As the hue is desaturated, it will become duller and if further
desaturated, it will eventually become grayscale. In
tristimulus color models, a color is desaturated by adding its opposite
color. Adding red and cyan together will desaturate both colors. The
same is true with blue-yellow and green-magenta. Using the sweater
example, if the sweater is pure blue, then it is a saturated blue. If the
blue has some yellow in it, then it is of lower saturation because yellow is the
color opposite of blue and will cause the blue to be less saturated.
Figure 2 shows a red hue and various states of saturation. From left to
right, it is 100% saturated, then 80%, then 60%, then 40%, then 20% and finally
Lightness is relative brightness. In the purest sense, it is a
black-white gradient with all shades of gray in between, as seen in Figure 3.
When combined with hue, it is what gives us our dark greens, our medium yellows
and light reds. Figure 4 shows a red hue and various states of lightness.
In the sweater example, since the sweater is light blue, then the lightness of
the sweater is light. If the sweater were dark blue, then the lightness
would be dark. Lightness is determined by how much, or the
quantity of, light there is.
Other names for lightness are brightness, luminosity and tonality. The actual measure, or quantification, of
lightness is called luminance or tone.
To summarize, hue is determined by the wavelength. Saturation is
determined by how pure the hue is in the light we are seeing. Lightness is
how much light we are seeing. It is the Lightness characteristic that we
measure with exposure meters.
When working with lightness and exposure, we are working with