Light-emitting diodes (LED), which are designed as quasi-monochromatic light sources, can also function as spectrally selective photodiodes. This provides a new kind of photosynthetically active radiation (PAR) sensor that is inexpensive and has much better stability over time than interference filters used in some PAR sensors. The action spectrum of photosynthesis in green plants has principle peaks in the blue and red regions. LED with response peaks in the UV-A (380 nm) and red (620 nm) regions have been used to measure PAR at or near solar noon in an ongoing study begun on 30 April 1996. The sum of the signals from the two LED is highly correlated with measurements by a calibrated filterless PAR sensor (Apogee QSO; Logan, Utah) from 13 September 1997 to 16 January 2002 (r2 = 0.97). The sum of the LED signals is also highly correlated with measurements by a calibrated filter PAR sensor (LI-COR LI-190SA; Lincoln, Nebraska) from 20 April 1998 to 16 January 2002 (r2 = 0.97). Thus, pairs of spectrally selective LED can function as PAR detectors in economical PAR radiometers. The separate 380 and 620 nm responses also permit an assessment of the differential impact of aerosol events on blue and red PAR and phototropic radiation.