Irradiation Illusions

The concept of irradiation illusions was first proposed by Helmholtz, more than a century ago.  Irradiation of light from the light and dark regions on a painting tend to differ.  It is suggested that bright light tends to “spill over” a little into the dark regions of an image on the retina.  The effect is most marked when bright light is shone onto the image. 

Fig. 66 – An irradiation illusion.  The distance between the
dark discs is equal to the disc diameter. 

Fig. 67 – The star illusion.  The distance from the star-tip to
the lozenge-tip is equal to the length of the lozenge.

In Fig. 66, the dark discs are each surrounded by a large white area.  The light area therefore encroaches into each of the dark discs, making it appear smaller than it actually is. 

In Fig. 67, in addition to the irradiation effect are the mechanisms of convergence and divergence.  Convergence makes each lozenge appear shorter than its true length.  Divergence makes the space between the star-tip and lozenge-tip appear much bigger than it is.  Irradiation “spills” the light over into the dark regions, “increasing” the separation.  The overall effect is very convincing indeed.