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Chromostereopsis

from Michael’s Visual Phenomena & Optical Illusions

German

What to see

Observe the flickering rings of different colour. What would you say: do they lie in a plane or is one color ring more distant, the other one closer?
[If the flicker annoys you, use the Stop/Start button.]

Most people see the red ring in front, the blue one more distant. This seeming depth effect is called “chromostereopsis”.

What to do

• Close one eye: For most people, the depth effect vanishes.
• Use the preset “≈ Equiluminance”. The red becomes darker, and now should have about the same brightness as the blue one. For most observers, the depth effect is reduced. We thus learn that part of the strong effect is simply the difference in luminance, not just the difference in color.
• Choose the preset “No colour”. This makes the same point as above, I certainly still see some depth effect.
• Switch to “White Background +…”. For me, at least, the effect reverses with a bright background.
• The preset “Heterochromatic flicker fusion” is for specialists who want to adjust the equiluminance setting (use the luminance slider at the bottom of the control that pops up when you select the colour badges.)

Comments

This is a well known phenomenon, occasionally employed in art (example in Wikipedia). The first scientific description was by Einthoven (1885); he even uses actual colour (rare & complicated then) for demonstration, namely the word “BLAU” next to “ROT”. The present configuration was adapted from Akiyoshi Kitaoka.

The mechanisms behind chromostereopsis are quite multi-varied and intricate. Certainly chromatic aberration plays a role, but there’s more (e.g. luminance). I won’t even cite many sources on this, the Wikipedia page makes a good job here, and Faubert (1994) has a nice review. Also this is one of the illusions with very marked differences between different people (so don’t get mad at each other, neither over this effect nor over anything else :).

Sources

Einthoven W (1885) Stereoscopie durch Farbendifferenz. Graefe’s Archiv für Ophthalmologie 31:211–238

Wikipedia: Chromostereopsis

Faubert J (1994) Seeing depth in colour: More than just what meets the eyes. Vision Research 34:1165–1186