Single Channel, Sinusoidally Modulated Signal Generator, with Variable Temporal Contrast

Abstract

Frequently in visual science experiments a stimulus is needed for which the luminance varies sinusoidally with time2, 4, 5,6, 8 set of 2 linear polarizers, one of which rotates at a constant angular velocity, is a very simple and straightforward method for producing such a stimulus, although this method only gives 100% modulation. In order to vary the amplitude of modulation, while maintaining a constant mean, previous workers have used two-channel optical systems5, split beams3,4, or used extensive electronics6, 8. However, a simple modification of the single channel polaroid method allows independent control of amplitude and mean intensity. Figure 1 shows in more detail what happens to the light as it passes through the separate components. Light passes through polarizer P and subsequently passes through quarter wavelength plate L. Because of the bireffingent properties of L, the beam of light leaving L will be linearly, elliptically, or circularly polarized, depending on the angle α between the plane of polarization of P and the fast axis of L. In the following section it will be shown that the light leaving rotating analyzer A is modulated sinusoidally in intensity, with fixed mean and amplitude depending on α. The mean may be decreased by using ND-filters.