Chromostereopsis

from Michael’s   Visual Phenomena & Optical Illusions

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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

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 Arhiv 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