COMPACT LIGHT-OMITTING-DIODE SUN PHOTOMETER FOR ATMOSPHERIC OPTICAL DEPTH MEASUREMENTS

A new compact light-emitting diode (LED) sun photometer, in which a LE D is used as a spectrally selective photodetector as well as a nonline ar feedback element in the operational amplifier, has been developed. The output voltage that is proportional to the logarithm of the incide nt solar intensity permits the direct measurement of atmospheric optic al depths in selected spectral bands. Measurements made over Ahmedabad , India, show good agreement, within a few percent, of optical depths derived with a LED as a photodetector in a linear mode and with a LED as both a photodetector and a feedback element in an operational ampli fier in log mode. The optical depths are also found to compare well wi th those obtained simultaneously with a conventional filter photometer