Each sensor contains a light-emitting diode (LED) and a phototransistor, in package widths ranging from 6 mm down to 3 mm (surface-mount). Package width determines resolution of particle size. For example, an array with 5-mm resolution would contain 200 sensors in a 1-m section. Pulsing the sensor LEDís in sequence produces maximum light with minimum power. The rate of rereading a segment determines resolution of speed estimates. Pulse trains are stored in first-in, first-out (FIFO) memory modules on each array section, and transmitted to a streamside computer for processing and recording.
A major technical problem in this design is determining the optimum on-off threshold level in the sensor amplifier circuit. Other problems to be solved include shielding from ambient sunlight, and finding a window material that resists abrasion by sediment. If these obstacles can be overcome, calibration remains a challenge, since the array only samples particles within a few millimeters of the bed surface. The video system sketched in a separate abstract offers some hope in calibration. One other encouraging note: correlations between reflective sensors gave estimates of speed profiles in snow avalanches (Dent et al. In press).
Dent J., In press. Annals of Glaciology, May 1997 symposium.