Absorbance measurements with light-emitting diodes as sources: Silicon photodiodes or light-emitting diodes as detectors?

Light-emitting diodes may also serve as light detectors, and the combination of two of these devices, one serving as light source, the other for detection, has been reported repeatedly for use in analytical photometry. A comparative study of the performance of light-emitting diodes in this role and that of a standard photodiode is reported herein. The spectral sensitivities of the light-emitting diodes were found to be as narrow as their emission bands, but shifted to shorter wavelengths, so that the spectral overlaps between emission and sensitivity of the same devices are very limited. The photocurrents of the light-emitting diodes were found to be about ten times lower than those of the photodiode. In the discharge mode (the time for discharge of the p/n-junction by the photocurrent is measured) as well as the photovoltaic mode, both of which had previously been reported for light-emitting diodes used as detectors in photometric devices, the performance of a light-emitting diode was on a level that is adequate for many analytical purposes, but the photodiode generally gave better precision and the signals showed faster settling times.