LED Sun Photometry

The history of sun photometry—the practice of measuring atmospheric aerosols and water vapor by detecting sunlight—traces back at least a century. Traditional photometers detect sunlight using optical interference filters and broadband silicon photodiodes. But while silicon photodiodes are very stable and have low temperature sensitivity, the filters are expensive and can be subject to unpredictable drift or degradation that can severely affect the quality of measurements.

In 1989, I devised a filterless method for measuring sunlight by using light-emitting diodes (LEDs) as spectrally selective photodiodes—a design that greatly improves the long-term stability of sun photometers. The first LED sun photometer, which was assembled from components that cost less than $50, exhibited minimal calibration drift over two decades.

The device incorporates two near-IR LEDs, one of which detects the 940-nm water vapor absorption band and the second of which detects a nearby reference band. Since February 1990, the ratio of the signals from these LEDs has provided the total column abundance of water vapor over my South Texas site at solar noon to within 3 percent of the mean of 3.5 years of more recent measurements made by the nearest GPS receiver in the National Oceanic and Atmospheric Administration’s (NOAA) GPS Integrated Precipitable Water project. Measurements of the atmosphere’s aerosol optical density (AOD) can be made with similar accuracy.

Here, I provide a history of handheld sun photometry; recount the development of LED sun photometry and radiometry; and provide some resources for scientists, educators, students and science enthusiasts who are interested in working on their own sun photometry projects.

Frederick Volz introduces the hand-held sun photometer

A century ago, scientists at the Smithsonian Institution’s Astrophysical Observatory (APO) demonstrated a new way to measure direct sunlight with enough accuracy to assess aerosols, ozone and water vapor using simple spectrometers. The APO also showed that the mean intensity of solar radiation at the top of Earth’s atmosphere (the solar constant, Io) could be inferred from a series of a dozen or so measurements of direct sunlight (I) at Earth’s surface during a clear morning.

Although the APO’s solar instruments could be transported in carrying cases, they were not designed for hand-held operation. It wasn’t until the mid-1950s that Frederick Volz pioneered hand-held sun photometry. In 1957, he developed a simple photometer that measured AOD with a selenium detector and a green-transmitting 500-nm filter. The photocurrent produced by the device was displayed on a small analog meter.

Volz subsequently improved his instrument by adding filter channels and replacing the selenium cell with a silicon photodiode and amplifier. Volz’s photometers were used in various atmospheric haze studies and networks. Unfortunately, filter drift and the lack of calibrations contaminated the data from the networks.

Volz also developed a photometer that had a filter with a passband over the 940-nm water vapor absorption band and a nearby band that was unaffected by water vapor. This instrument could be used to measure the total water vapor in a column through the atmosphere. In May 1965, the meteorologist Robert A. McCormick visited Hawaii’s fledgling Mauna Loa Observatory (MLO) and gave MLO Director Howard Ellis a Volz photometer to calibrate in the pristine air of that alpine site, 3,417 meters (11,210 feet) over the Pacific Ocean. Ellis still recalls rising at sunrise to calibrate the instrument using the century-old Langley method, in which direct sun readings are taken at frequent intervals over a wide range of air masses as the sun rises in the eastern sky.

Volz photometers also served as models for Glenn Shaw, an atmospheric scientist working at the University of Alaska’s Geophysical Institute in the mid-1970s. (Today he is a professor emeritus.) Shaw developed his own hand-held and automated instruments and traveled to Mauna Loa many times to calibrate them. In 1976, he measured aerosols from the eruption of the Alaskan volcano Mt. Augustine. And in 1979, he became the first to photograph and measure the AOD of Asian dust clouds drifting over Hawaii on their way to North America.

In 2008, I worked with Glenn’s son Joseph, who directs the Optical Technology Center at Montana State University, to calibrate several of his father’s sun photometers during a pleasant morning at MLO. (View an elapsed time movie of the occasion.)

Light-emitting diode (LED) sun photometers

My own work on sun photometers traces back to my days as a high school senior in 1962. Observing that an electromagnetic speaker can double as a microphone, I wondered whether semiconductor light detectors could emit light as well. To test the hypothesis, I connected a cadmium sulfide photocell to the terminals of a high-voltage automobile ignition coil.

As one might expect, the surface of the cell glowed brightly when power was applied to the coil. Most of the glow was due to the high voltage discharge. However, there were distinct points of bright green light emitted by the CdS, seemingly proving the hypothesis, since the green wavelength of the emitted light approximated the peak detection wavelength of the photoresistor. (This finding would not have surprised a semiconductor physicist, but I wasn’t aware of the details at the time.)

While studying government and history at Texas A&M University in 1966, I repeated my semiconductor experiment by applying square wave pulses of a few volts to a silicon solar cell. The cell emitted pulses of near-IR that were received by a second solar cell connected to the input of an audio amplifier; this generated a tone with a frequency that matched the pulse rate applied to the solar cell used as a source.

After LEDs and laser diodes became widely available in the early 1970s, I assumed that they might also double as light emitters and detectors. Simple experiments showed that audio frequency signals could be transmitted bidirectionally between two LEDs or laser diodes. These experiments worked best when LEDs and lasers with the same emission wavelength were used as dual-purpose emitter-detector pairs.

Soon, I was transmitting voice bidirectionally through free space using a single GaAs:Si near-IR LED as a dual-function emitter-detector at each end of the link. I also sent voice and digital signals bidirectionally through optical fibers using a single GaAsP red LED at each end of a fiber. I published these results in various electronics books and magazines for hobbyists.

While preparing to take over “The Amateur Scientist” column in Scientific American magazine in 1989, I developed several handheld sun photometers for measuring direct UV, the ozone layer and visible and near-IR wavelengths of sunlight, including two homemade TOPS (total ozone portable spectrometer) instruments that were placed in daily service on February 4, 1990. These instruments used GaP photodiodes and narrow-bandpass interference filters.