SIMULTANEOUS AND SUCCESSIVE CONTRAST

The terms "simultaneous contrast" and "successive contrast" refer to visual effects in which the appearance of a patch of light (the "test field") is affected by other light patches ("inducing fields") that are nearby in space and time, respectively. The names are somewhat misleading since both simultaneous and successive contrast involve inducing fields that are close in both time and space. The figure below demonstrates both.

A Demonstration of Simultaneous and Successive Contrast

Simultaneous contrast. Simultaneous contrast is demonstrated in the lower left panel. The three small squares are the same physical lights as in the top panel. The labeled squares are physically identical, and in the top panel they have roughly the same appearance. In the lower left panel, however, the pair that are physically identical have different color appearances due to the simultaneous contrast effects of the green and magenta backgrounds.

Successive contrast. Successive contrast is the effect of previously-viewed color fields ("inducing fields") on the appearance of the currently-viewed test field. To demonstrate successive contrast we again use the above figure, but now we will ignore the color of the small patches and pay attention to only the colors of the background fields. Stare at the top left blue square on the green background for 5-10 seconds, then look at the small outline square below. You should clearly see the aftereffect of the green and magenta background fields as faint magenta and green fields, respectively. Now repeat the 5-10 seconds to build up the afterimage again and then look at the upper left square on the pastel green and magenta backgrounds (upper right panel). The afterimage should be strong enough to make these backgrounds look gray or even reverse their appearances, with a pale magenta appearance on the left and pale green on the right. Several characteristics of the phenomenon are apparent in this demo:
1) The effect fades after a few seconds.
2) The afterimage can be multicolored and preserves the spatial shapes of the inducing field.
3) The afterimage can alter the appearance of not only white fields, but of colored fields as well.

Implications for Design of Graphics

The parameters of the stimulus patterns above were chosen to produce large successive and simultaneous contrast effects. The figure demonstrates that the effects can even be strong enough under some conditions to risk misidentification of the labeling color of a symbol. In graphic applications the effects can be reduced by setting one or more parameters differently:

Color of the test field. It is sometimes possible to find two different physical colors for two test fields on different backgrounds that have the same appearance. For example, in the demo figure above the physical color of the small test square on the right was adjusted to give it approximately the same appearance as the test square on the left when they are on the green and magenta backgrounds. These same two (unequal) physical colors appear quite different when they are on white backgrounds (top figure).

While this approach can sometimes be effective, there are several reasons why it is not a universal solution:

• It can be very difficult to apply in dynamic graphics, e.g., moving maps, in which the symbols move from one background color to another. The physical color of each symbol would have to be changed to a different color when it moved to a new background.
• The appearance may not match if there are changes in viewing conditions, e.g., small differences in display equipment.
• Some combinations of symbol and background colors have no physical match. For symbol colors near the edge of the display gamut the contrast effect of the inducing background may push the physical test color required on the other background outside the gamut of the display or even outside the gamut of real physical lights. In the figure below the three small squares are all at R,G,B = (255, 0 0). The green background increases the saturation of the small red square through simultaneous contrast. There is no light within the gamut of the display which will achieve such high saturation on a gray background, so no adjustment of the color of the square on the gray background will produce an appearance match. Similarly, the red background reduces the saturation of the small red square through simultaneous contrast. No light within the gamut of the display will have as much saturation on the red background as the square on the gray background.
  
  

For these reasons it may not be desirable to offset the effects of simultaneous or successive contrast by adjusting the symbol (test field) color.

Color purity of the inducing field. Another approach to mitigating the effects of simultaneous or successive contrast is to reduce the color purity of the inducing field. The effect of the color purity of the inducing field is visible in the first demonstration above. The pale green and magenta backgrounds in the right panel have smaller simultaneous contrast effects on the small patches than do the saturated backgrounds, and they produce only very faint afterimages.

This is the preferred solution when the viewer will look back and forth among regions on one or more displays as it is the only way to reliably avoid successive contrast effects.

Spatial proximity. One might decrease the spatial proximity of the test and inducing fields. The effects of the inducing patches are largest when they are immediately adjacent to the test patch, and they fall off rapidly with increasing spatial separation of the fields. In the figure below simultaneous color contrast is reduced by placing a narrow neutral buffer around the test fields (The neutral buffers appear colored due to simultaneous contrast with the backgrounds).

Temporal proximity. The effects of inducing fields are largest when they are viewed immediately prior to viewing the test field. As the demonstration above shows, most of the aftereffect usually fades within several seconds, but in the majority of graphic applications this is long enough to be a serious problem. Viewers usually need to look back and forth among multiple information displays to do complicated tasks. Under some conditions successive contrast effects can be detected long after the inducing field is removed.

The only way to avoid these undesirable successive contrast effects is to use saturated colors sparingly and then only for small fields. Even small inducing fields can be a problem if they fall on a small test field, as, for example, when a brightly colored symbol is viewed foveally for a period of several seconds and then a second symbol is fixated.

Simultaneous and successive color contrast were among the first visual phenomena to be studied scientifically, and there is an enormous basic research literature. Reviews of the effects of these and other parameters are provided by Wyszecki & Stiles (1982) (Ref 3) and by Wyszecki (1986) (Ref 4).

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  Site Author: Larry Arend