Instrumentation--characteristics of a light emitting diode--transistor photoplethysmograph.

A reflective transducer which combines a gallium arsenide infrared emitting diode and silicon phototransistor has been adapted for use us a photoelectric plethysmograph. This device is inexpensive, compact, and easily applied to the skin surface. The phototransistor responds in a linear fashion to the range of light intensities generally obtained from normal subjects, and the phototransistor output is relatively independent of both temperature and prior light exposure. The light source possesses a narrow spectral distribution in the infrared region. These characteristics suggest that the LKD-transistor photoplethysmograph is superior to the miniature tungsten lump-photoconductive cell combination generally employed in photoplethysmography.