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Making Text Legible: Designing for People with Partial Sight.

Big Type is Good Business

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Effective Color Contrast

Designing for People with Partial Sight and Color Deficiencies

by Aries Arditi, Ph.D


This brochure contains basic guidelines for making effective color choices that work for nearly everyone. To understand them best, you need to understand the three perceptual attributes of color: hue, lightness and saturation, in the particular way that vision scientists use them. Full explanations of these terms are provided in the pages that follow.

 


How does impaired vision affect color perception?

Partial sight, aging and congenital color deficits all produce changes in perception that reduce the visual effectiveness of certain color combinations. Two colors that contrast sharply to someone with normal vision may be far less distinguishable to someone with a visual disorder.
 


A color solid oriented so that colors go from dark to light from the top to the bottom.  

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


Picture illustrating that black on light pink is an effective color contrast where as equally light red on equally light green is not.

Don't assume that the lightness you perceive will be the same as the lightness perceived by people with color deficits. You can generally assume that they will see less contrast between colors than you will. If you lighten your light colors and darken your dark colors, you will increase the visual accessibility of your design.

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Picture of a hue circle, oriented so that  colors that may appear darker to someone with acquired color deficits are shown on the bottom half of the circle.  Colors that are relatively unaffected by color deficiencies are shown on the top half of the circle.

Choose dark colors with hues from the bottom half of the 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.


Picture  showing that dark purple, (one of the colors in the bottom half of the hue circle), contrasts effectively against a light green background,(one of the colors in the top half of the hue circle).  Light green does not, however, contrast effectively against pink.

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

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Picture showing a hue circle.  It is not effective to contrast colors that are next to each other on the hue circle, like red and orange.  It is, however, effective to contrast colors that are opposite each other on the hue circle, like blue and yellow. 

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


Picture  showing that colors of similar hue like yellow and orange do not contrast effectively, whereas colors of different hues like yellow and blue do contrast effectively.

Color deficiencies associated with partial sight and congenital deficiencies make it difficult to discriminate between colors of similar hue.

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Hue, lightness and saturation - the three perceptual attributes of color - can be envisioned as a solid.

 


Picture  illustrates the three perceptual attributes of color as a solid as the three dimensions in space.

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

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Picture  of the hue circle, illustrating that hues follow a natural sequence based on the similarity of their appearance.

Hue is the perceptual attribute associated with elementary color names.

Hue enables us to identify basic colors, 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.
 


Picture  showing red, blue, and green, horizontal bands which vary from dark to light.

Lightness corresponds to how much light appears to be reflected from a surface in relation to nearby surfaces.

Lightness, like hue, is a perceptual attribute that cannot be computed from physical measurements alone. It is the most important attribute in making contrast more effective.

With color deficits, the ability to discriminate colors on the basis of lightness is reduced.

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Picture  showing three bands of red, blue and green which are less saturated at one end appearing gray and more saturated at the other end.

Saturation is the degree of color intensity associated with a color's perceptual difference from a white, black or gray of equal lightness.

Slate blue is an example of a desaturated color because it is similar to gray. A deep blue, even if it has the same lightness as slate blue, has greater saturation.

Congenital and acquired color deficits typically make it difficult to discriminate between colors on the basis of saturation.
 


Picture  simulating the appearance of a dark brown square against a lavender background, to a person with color deficient partial sight.  To such a person the lavender appears much darker and does not contrast effectively with the brown.

To a person with color-deficient partial sight, the left-hand panel might appear like the right-hand panel appears to a person with normal color vision.

With color deficits, 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.

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Lighthouse International has a companion brochure entitled "Making Text Legible: Designing for People with Partial Sight."

Follow this link to find out about Lighthouse International's Large Type Campaign


Dr. Arditi is an expert in vision science.  This brochure is based on his earlier work with Kenneth Knoblauch.

©1995-1997 The Lighthouse Inc.

©2002 Lighthouse International. All rights reserved.

Arlene R. Gordon Research Institute
 
 

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