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 optical interference filters and photo diodes have significant potential advantages, including low cost, durability, and long-term optical stability. However, their relatively wide spectral response bandwidth poses some challenges in calibration and interpretation. Analysis of LED-based Sun photometers developed for the Global Learning and Observations to Benefit the Environment (GLOBE) program has demonstrated that such instruments can, in fact, be calibrated using the standard Langley plot method and that their performance can be described in terms of effective response wavelengths. Several GLOBE Sun photometers have been calibrated at Mauna Loa Observatory on two separate occasions. The derived extraterrestrial constants are essentially the same in spite of the fact that the calibrations were performed under significantly different atmospheric conditions. These reference instruments have 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 students at a GLOBE high school near NASA's Goddard Space Flight Center (GSFC) and compared against aerosol optical thickness measurements from Sun photometers at GSFC demonstrate both that students can reliably make the required measurements and that LED-based Sun photometers give results that compare favorably with conventional filter-based instruments, even though their optical properties are significantly different.