Abstract

The temporal modulation transfer function (De Lange curve), using a sinusoidally modulated stimulus, is an important investigative method in physiological as well as clinical studies (e.g., ref ¹) of the human visual system. Both mechanical and electrical techniques have been used to modulate the light sinusoidally. A common mechanical technique is to use a steady light source and cross polarizer system to modulate the light beam (e.g., ref ¹). By contrast, in electrical techniques the power supplying the light source is modulated. Commonly, the light source used is an LED because of its large frequency response and its short time constant. However, its luminance vs. supplied voltage is nonlinear. In order to overcome this nonlinearity different techniques have been used. Analog techniques use, for example, a feedback controller to correct the nonlinearity. This technique requires special sensors, is elaborate, and is unsatisfactory for a large range of modulation depths. Pulse width modulation (PWM) techniques supply the LED with pulses whose amplitude and frequency are constant and whose width is variable depending on the desired modulation depth of the stimulus (e.g., ref ²). This technique requires elaborate analog electrical circuits. It is not always satisfactory for very small modulation depths because the pulse widths may be smaller than the time constant of the LED, preventing it from following the supplied signal. In addition, varying the pulse width can produce fluctuations in the junction temperature of the LED causing variations in the LED’s luminance. One way to overcome this drawback is to use a pulse density modulation technique (PDM). With PDM the amplitude and pulse width are kept constant and the pulse density is varied with respect to the desired modulation depth of the signal (e.g., ref ³). However, this technique still requires elaborate analog circuits.

© 1992 Optical Society of America