Spectral and emission characteristics of LED and its application to LED-based sun-photometry

In recent years, light-emitting diodes (LEDs) have found applications in fields like space science instrumentation in addition to its use in illumination. Sun photometry is one of the techniques for measuring aerosol optical depth. Photometers generally consisting of an interference filter and a photo-detector, measures the intensity of radiation in the wavelength band of the interference filter. LED alone replaces both the interference filter and the detector and works as a spectrally selective photo-detector. The spectral response (extinction spectra) of LED is required to calculate the aerosol optical depth. In general, it is assumed that the emission spectra and spectral response should be same. It has been found experimentally that the emission spectra and spectral response are different. The peak wavelength at which the maximum emission occurs is found to be higher than the peak of the spectral response curve. The FWHM of both are also found to be different. A typical example of Langley plot obtained from the LED-based Sun photometer is shown and optical depth obtained with this is compared with the conventional Sun photometer.