spot 9

blelb spot 9

Experiment with the color assimilation effect

pull the curtain

control the translucency of the curtain

Button C1
repeatedly click to change color 1

Button C2
repeatedly click to change color 2

Button C3
repeatedly click to change the color 3


Colour illusions in Spot 09

Experimental procedure for Spot 09

When the curtain is open, the scene on stage is homogenous, flanked by two black stripes at the edges. The centre button is used to select the colour for the stage. The two-part slatted curtain in the foreground can be closed again by means of the left slider. The left and right buttons can be used to change the colour of the two sections. The curtain slats grow longer like a telescope when the curtain is closed and finally mesh together without overlapping. Apart from black and white, only three colours are involved, which are layered horizontally in different combinations and which conjure up special harmonies of colour of the kind we have been aware of in painting since the discoveries of the pointillists (Georges Seurat, Paul Signac). Coloured dots are replaced here by lines. The right slider adjusts the scale (width of the slats), which has a considerable effect on the play of colours.

Colour illusions with different degrees of scaling

With coarser scaling, the Bezold effect dominates (see Spot 04 and Spot 08), while with finer scaling it is the assimilation effect [1]. The relevant literature frequently identifies these colour illusions [2] or sometimes even confuses them. The Bezold effect is also called the «confetti effect» by a number of authors.

Bezold effect versus assimilation effect

When playing with this spot, you can see for yourself that many shades of colour that are produced by coarse scaling with the Bezold effect are not present with fine scaling with the assimilation effect, and vice versa. If you successively increase your distance from the picture when observing the Bezold effect, the illusion automatically changes into the assimilation effect. If you increase he distance with a finely scaled picture, you soon go beyond the critical distance at which the slatted structure vanishes completely. The lights from the various colour elements then overlap one another on the retina. When this happens, we talk of an optical or proportionate colour mix [3]. Good examples of this are to be found on the colourful wings of butterflies, which are transformed under the microscope into structured entities with tegulae in a variety of colours.


[1] Helson H (1963)
Studies of anomalous contrast and assimilation
Journal of the Optical Society of America 53-1 179- 184
[2] Backhaus W G K, Kliegel R, Werner J S (1998)
Color Vision
Berlin, New York: de Gruyter
[3] Goldstein E B (1997)
Heidelberg, Berlin, Oxford: Spektrum, Akad. Verl.