Development of an inexpensive handheld LED-based Sun photometer for the GLOBE program

Sun photometers that use light-emitting diode (LED) detectors in place of o ptical interference filters and photo diodes have significant potential adv antages, including low cost, durability, and long-term optical stability. H owever, their relatively wide spectral response bandwidth poses some challe nges in calibration and interpretation. Analysis of LED-based Sun photomete rs developed for the Global Learning and Observations to Benefit the Enviro nment (GLOBE) program has demonstrated that such instruments can, in fact, be calibrated using the standard Langley plot method and that their perform ance can be described in terms of effective response wavelengths. Several G LOBE Sun photometers have been calibrated at Mauna Loa Observatory on two s eparate occasions. The derived extraterrestrial constants are essentially t he same in spite of the fact that the calibrations were performed under sig nificantly different atmospheric conditions. These reference instruments ha ve been used to transfer calibrations to other optically and electronically identical Sun photometers, thereby making it possible to establish a large network of inexpensive LED-based Sun photometers. Data collected by studen ts at a GLOBE high school near NASA's Goddard Space Flight Center (GSFC) an d compared against aerosol optical thickness measurements from Sun photomet ers at GSFC demonstrate both that students can reliably make the required m easurements and that LED-based Sun photometers give results that compare fa vorably with conventional filter-based instruments, even though their optic al properties are significantly different.