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Contrast Variables

Explanations > Perception > Visual Perception > Contrast Variables

Amplitude | Frequency | Variation | Combination | Discussion | So what?

There are many ways of understanding contrast. A simple way to

Frequency (hue, color)

Much of what we sense is based on frequency, or speed of vibration. Light has frequency that shows as hue. Sound also has frequency that is heard as pitch. Temperature, too, has frequency, with higher frequency associated with heat. When we detect difference between two frequencies, we perceive this as contrast.

Visually, some hues contrast with strongly with others. Hues can be arranged in a circle (the 'color wheel') where hues opposite one another are at maximum color contrast. In this, opposites are blue and yellow, green and magenta, and red and cyan. These are also called 'complementary' hues.

Here is a color wheel. Note the high complementary contrast between hues on opposite sides of the wheel.

Amplitude (luminance, tone)

Given any frequency detected by our senses, we also can determine the amplitude or height of this vibration. Physically, a higher amplitude is felt as a greater degree of movement in the 'shake'. Visually, higher amplitude is seen as brighter light or hues, as in the picture below. Aurally, it is louder sounds.

As well as frequency contrast, contrast can also be detected between low amplitude and high amplitude, for example between dim light and bright light.

Visual amplitude what photographers call luminance or tone, which is within the luminance or lightness-darkness spectrum (as in the gradient example below). The basic tone spectrum is the shades of gray between black and white. With this, a light tone contrasts more with a much darker tone, than with a slightly darker tone. The ultimate tonal contrast is between the ends of this spectrum, black and white.

There is also contrast of tone within hues. Hence, for example, you can have a light red, light blue and so on, as well as a dark red, dark blue, etc.

Variation

Frequency and amplitude can vary across both space and time, creating sharp and soft changes.

Contrast can be sharp, such as in a step change from black to white or silence to loud noise. It can also be gradual, such as in a slow gradient from white through every shade of gray. The sharper the contrast, the easier it is for us to detect. Yet we can also detect even subtle contrastive changes.

This rate of change of contrast is like the gradient of a hill. The steeper the hill, the greater the gradient. The steepest change is a step, where it goes from one level to the other, in one go, like a physical step, as below. In the example below, there is an initially shallow gradient, which then changes to a steeper gradient. You can even see the contrastive 'line' at the point of change between these. Note also how your brain tries to make sense of this and may even see the right gradient as looking down on something that is curving away from you.

Combination

As all children and artists know, you can mix paint and light together to get different hues. When the eye detects several frequency-amplitude (hue-luminance, color-brightness) light beams at the same time, it combines them to form a single perceived overall color.

The reason paints and lights mix differently is because paint is reflective. White light (typically from a lamp or the sun) bounces off it and all frequencies except the paint colour are absorbed, so only the remaining frequencies enter the eye. For example yellow paint absorbs all other frequencies except yellow. Mixed lights work in the opposite way, hence blue and red lights mix to form a perceived magenta.

Not all 'white' light is pure white (which is a mixtures of equal amplitudes of all visible frequencies), which is why things look to be a different color indoors (under lamps) and outdoors (under the sun). While the frequencies absorbed are the same, the light does not provide all frequencies, resulting in the reflected light being different in each case. Illustrating this, an object that appears blue in white has a blue light shone at it, it will appear black as it absorbs the only available frequency, reflecting none.

Discussion

Visually, contrast is detected by the differing signals from adjacent sensors in our eyes, the rods and cones that detect luminosity and hue of the incoming signal. We will detect a tiny white light in a dark room (though the overload on our optic nerves of all white makes a black dot white harder to see).

Contrasts are sometimes due to our physiology. For example red and green can appear to be 'opposites', as are blue and yellow. This is because of the way the 'wires' that carry the optical signals back to the brain from the eye have one wire that is red-green and another that is blue-yellow (and a further that is luminance, or black-white. This is why we cannot see a reddish green or a bluey yellow. Note how this form of contrastive opposition is not quite the same as that in a color wheel.

Contrast is also detected over time as we compare succeeding mental images. In nature, this is a critical ability for predators who look for movement before static image contrast. The natural camouflage that their prey have evolved weakens the contrast between them and their backgrounds. Yet this advantage is lost when they move, which is why many have learned to freeze all motion.

So what?

There are many different systems of classifying color and other senses, yet by understanding frequency and amplitude, and how these change within space and time, you can make significant use of contrast.

If you want to gain attention, use sudden changes in contrast, with significant steps in amplitude and/or frequency.

See also

Attention