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This answer explains that the device labeled "B" in the photo below is a "Ultraviolet Multifilter Rotating Shadowband Radiometer."

What is a Rotating Shadowband Radiometer, and how does it work?

Mauna Loa Observatory

Screenshot from https://vimeo.com/315754123

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A rotating band shadow meter will give you the diffuse radiance as well as direct radiance. When the band is in the lower location the sensor will measure all the radiance from the sun both direct and indirect. when the band is raised it blocks the direct sun into the sensor giving an indirect radiance measurement. The total radiance - the indirect radiance will give you direct radiance. In this case the meter is setup to capture the UV portion of the spectrum. Multifilter suggests that it can record radiance for bands of UV radiance so it would be tracking radiance on two or more ranges wavelengths. The model described in this link supports six distinct frequencies though none are in the UV band.

The band rotates above the sensor occluding the light from the sun, when the radiance is at its lowest during the band rotating you are, typically, capturing the point where the direct sunlight is being filtered by the band. When the band is not occluding the sun you would be capturing the highest radiance. The sampling rate would be setup in the datalogger or computer to allow radiance values at specific time intervals.

A short video of one in operation can be seen in this tweet.

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