Some Demos

Here's just a bunch of demonstrations of various color effects.


Simultaneous and Successive Contrast


Decrease in perceived saturation with continued stimulation. Place a white piece of paper over the right half of the yellow rectangle. Stare at the fixation cross for about 30 seconds. Then quickly remove the white mask while continuing to stare at the fixation cross. Note the relative saturation of the left and right halves of the yellow rectangle.


Bidwell's Disk

When this one works, it's really cool. This quote is from Color Vision by Leo Hurvich.

"In the usual afterimage situation, the primary stimulus is first seen and it is then followed by, say, an afterimage. Thus stimulation by a long-wavelength red-appearing stimulus leads to a complementary green after-image. The BIDWELL PULSATIVE AFTERIMAGE is then of particular interest because the stimulus arrangement is such that, unlike the situation just described, a complementary afterimage is continually seen, whereas the primary stimulus that generates it is never seen. The stimulus setup is shown below. If the sector disc is rotated so that, say, a red 650-nm stimulus is exposed through the cutout 30-degree sector for about 0.05 second, followed by the white portion of the disc (which is illuminated with a broad-band achromatic source) and then the black portion, only a green afterimage is seen. If the observer is kept in ignorance of the nature of the primary stimulus, he could well think the light source itself was a midspectral green one, with a wavelength peak of, say, 530 nm."

I've tried to make a Flash demonstration of Bidwell's Disk. It works a bit on a CRT with the room light's off, especially if it a CRT that is run at 75 Hz. Click here to give it a try.


Simple Demos

These next two figures show successive and simultaneous contrast at the same time.

Green focal afterimage is seen after viewing a small reddish stimulus on a white surround. The afterimage of the white surround is seen as pinkish



Red focal afterimage is seen after viewing a small white stimulus on a red surround. The afterimage of the red surround is seen as greenish.



In simultaneous contrast, the complementary color is induced into adjacent color areas. In assimilation, the color appears to spread. Roll the mouse over the words Hide Lines to remove the stripes that are causing the assimilation


The McCollough Effect

















The McCollough Effect. To demonstrate it, gaze at the pattern for about four minutes, from your usual reading distance. Do not let your eye rest on any point for very long and try to look as frequently at red as at green areas. Resist the temptation to tilt your head to either side; the picture should be perpendicular to your line of sight.

After about four minutes, rest your eyes for a few moments to allow any conventional afterimages to dissipate. Now look at the black and white by moving the mouse over the words, "Show Test Pattern." You should see illusory colors that vary according to the orientation of the tilted lines; those tilted to the right appear pink and those tilted to the left appear pale green. Do not expect the illusory hues to be very strong; the effect is scientifically, rather than phenomenally, striking. Tilt your head, 90 degrees to one side; the apparent colors should exchange position.

One of the most curious aspects of the effect is its persistence, particularly if you restrict the amount of your exposure to the black and white test pattern. Try testing yourself again after half an hour, or adapt to the colored pattern before you go to bed and test yourself next morning. To obtain the basic effect, some people will need to look at the colored pattern for a little longer than four minutes, some a little less. The size of the effect should increase with viewing duration.


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Retinal Imaging