A visual stimulator for orbiting light spots and equal energy annuli with continuously variable parameters

When a light spot moves at high angular velocity along a circular path on a screen, an annulus is perceived. This principle is used in a simple annulus generator consisting of a plane mirror that rotates about an axis making an angle α with the normal in the center point of the mirror. Angle α can be varied with a micrometer screw to change the diameter of the annular path (orbit) followed by the light spot that is projected on a screen via the rotating mirror. This results in variable diameter equal energy annuli. The diameter of the light spot can be changed to manipulate the width of the annulus. Finally, at low angular velocities an orbiting light spot is obtained. It is shown in the companion paper (Molenaar and van de Grind, 1979) that this slow orbiting of the light spot allows a precise study of the fine structure of receptive fields.