Fechner’s Colors and Behnam’s Top

Fechner colors refer to the creation of illusory or subjective colors through an alternating pattern of black and white stimulus. Fechner reported the discovery of this illusion in an 1838 paper, and his name has been given to these illusory colors. The discovery was accidental as Fechner at the time was involved in the creation of disks to create different shades of gray [1]. From the beginning, a spinning disk was the means for creating these illusory colors. There were several studies throughout the nineteenth century of this phenomenon with spinning disks including studies by such a luminary as Helmholtz. Helmholtz even proposed one of the early explanations for these colors. Each of the early studies had used individually designed disks to create the illusory colors. In 1894, Behnam developed what has become the near-standard disk pattern of producing these illusory colors that he called the “artificial spectrum top” and has come to bear his name [2]. This disk is called usually either Behnam’s top or Behnam’s disk. Figure 1 shows some examples of a Behnam’s top. Variations in the disk include having thicker single lines or thinner three parallel lines. To generate these illusory colors, it is important that the flicker rate of the stimulus is not too slow or too fast. The flicker is noticeable and the stimulus is well short of the critical fusion frequency. The rate of alternation of the black and white areas is not terribly fast with effective stimuli frequencies going from 3 Hz on up with optimal rotational frequencies for the spinning disk of about 4–6 Hz [3, 4]. The direction of the spinning seems to affect the colors experienced [10].