Irradiance of a Solar Eclipse Using Spectroradiometers

The solar eclipse in 2024 was a fascinating moment for those lucky enough to be in its path. People across the country looked up at the sky to watch the moon pass by the sun and get a glimpse of their position in the universe. It is astronomical events like this that inspire future scientists. During this eclipse, Spectral Evolution conducted an experiment measuring the direct and global horizontal irradiance. To do this, we deployed two of our state-of-the-art Naturaspec’s™ from our remote sensing series of spectroradiometers and our dual field of view DARWin™ software.

Measurements were taken at Woodsom Farm Park in Amesbury, Massachusetts, from 2:16 to 4:39 p.m., encompassing the entire solar eclipse in this area. The eclipse totality occurred at 3:29, and the percentage coverage at totality was 94.2%. Continuous measurement of the eclipse was made easy by the DARWin software due to its automated measurement at time interval. The instruments were set to collect measurements of direct irradiance and global horizontal irradiance in W/m² every 5 minutes. One Naturaspec was equipped with a Gershun tube. This is a 1-degree optic designed to point directly at the sun to measure direct irradiance. The other Naturaspec had a cosine diffuser set on a level field above the rest of the equipment to measure total horizontal irradiance. 1 minute before each scan, the Gershun tube was adjusted to keep it on the moving sun. The cosine diffuser did not need to be adjusted due to its 180-degree field of view. Since these instruments are spectroradiometers, this means that they have a radiometric calibration that allows them to calculate energy. The data was then analyzed, and measurements of when the Gershun tube missed the sun or a cloud blocked the view were filtered out. Direct energy measurements are critical for not only astronomers but also for determining the energy balance in an environment.

The weather was consistently clear throughout the experiment, apart from a few cirrus clouds after 4 p.m. As seen in the figures below, the measurements for irradiance came out as expected, with the lowest irradiance measurements coinciding with the maximum of the eclipse. The total global irradiance followed a very clear parabolic curve, except for the latter half. The total irradiance was slightly reduced after the eclipse due to the time of day as well as a few scattered clouds during this sampling period. This sampling event was fast and easy to implement due to the streamlined DARWin software and the easily equipped accessories.

Similar studies can be conducted in a wide variety of settings. The Dual DARWin software has the functionality to operate two instruments in many ways. It can take a simultaneous ratio measurement where one instrument measures a reference panel while the other faces the target. This is incredibly useful for field measurements when the weather is rapidly changing due to cloud coverage. It can also take ratios between the two instruments, simultaneous direct radiometric measurements, radiometric difference measurements, and customizable independent measurements.

There is a wide range of applications in environmental remote sensing, climate science, the solar industry, and much more. The Naturaspec has incredible sensitivity and spectral resolution of 2.7nm @700nm, 5.5nm@1500nm, 5.8nm@2100nm. This is currently the highest-resolution field spectroradiometer on the market. The capabilities of this instrument, combined with the ability to network and automate their sampling, make them an incredible tool for energy and environmental studies. Consider Spectral Evolution’s remote sensing series for your next irradiance study.