Lighthouse International - Effective Color Contrast

http://www.lighthouse.org/accessibility/effective-color-contrast/

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Effective Color Contrast
Designing for People with Partial Sight and Color
Deficiencies
by Aries Arditi, PhD
This Web page contains three basic guidelines for making effective
color choices that work for nearly everyone. Following theguidelines are explanations of the three perceptual attributes ofcolor -- hue, lightness and saturation -- as they are used by vision
scientists.
How does impaired vision affect color perception?
Partial sight, aging and congenital color deficits all producechanges in perception that reduce the visual effectiveness ofcertain color combinations. Two colors that contrast sharply tosomeone with normal vision may be far less distinguishable tosomeone with a visual disorder. It is important to appreciate that itis the contrast of colors one against another that makes them moreor less discernible rather than the individual colors themselves.
Here are three simple rules for making effective color choices:

1. Exaggerate lightness differences between foreground andbackground colors, and avoid using colors of similar lightnessadjacent to one another, even if they differ in saturation or hue.

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Lighthouse International - Effective Color Contrast

http://www.lighthouse.org/accessibility/effective-color-contrast/

simulation of acataract.
View more vision
simulations here.

Dont assume that the lightness you perceive will be the same as
the lightness perceived by people with color deficits. You cangenerally assume that they will see less contrast between colors
than you will. If you lighten the light colors and darken the darkcolors in your design, you will increase its visual accessibility.

2. Choose dark colors with hues from the bottom half of this hue
circle against light colors from the top half of the circle. Avoid
contrasting light colors from the bottom half against dark colors
from the top half. The orientation of this hue circle was chosen toillustrate this point.

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Lighthouse International - Effective Color Contrast

http://www.lighthouse.org/accessibility/effective-color-contrast/

For most people with partial sight and/or congenital color
deficiencies, the lightness values of colors in the bottom half of thehue circle tend to be reduced.

3. Avoid contrasting hues from adjacent parts of the hue circle,
especially if the colors do not contrast sharply in lightness.

Color deficiencies associated with partial sight and congenital
deficiencies make it difficult to discriminate between colors ofsimilar hue.
Hue, lightness and saturation -- the three perceptual
attributes of color -- can be envisioned as a solid.

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Lighthouse International - Effective Color Contrast

http://www.lighthouse.org/accessibility/effective-color-contrast/

Hue varies around the solid; lightness varies from top to bottomand saturation is the distance from the center.

Hue is the perceptual attribute associated with elementarycolor names.
Hue enables us to identify basic color catagories such as blue,green, yellow, red and purple. People with normal color vision
report that hues follow a natural sequence based on their similarity
to one another. With most color deficits, the ability to discriminate
between colors on the basis of hue is diminished.

Lightness corresponds to how much light appears to bereflected from a colored surface in relation to nearby

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http://www.lighthouse.org/accessibility/effective-color-contrast/

surfaces.
Lightness, like hue, is a perceptual attribute that cannot becomputed from physical measurements alone. It is the mostimportant attribute in making contrast more effective.
With color deficits, the ability to discriminate colors on the basis oflightness is reduced.

To a person with color-deficient partial sight, the left-handpanel might appear like the right-hand panel appears to aperson with normal color vision.
With color deficits, the ability to discriminate colors on the basis of
all three attributes -- hue, lightness and saturation -- is reduced.
Designers can help to compensate for these deficits by making
colors differ more dramatically in all three attributes.
Aries Arditi, PhD, is Senior Fellow in Vision Science, Lighthouse
International; this brochure is based on his earlier work with
Kenneth Knoblauch.

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