What is the field of view of a sun photometer, and why is it important?
The equation that describes theoretically how to interpret sun photometer measurements requires that the instrument should see only direct light from the sun — that is, light that follows a straight line path from the sun to the light detector. This requirement can be met only approximately in practice because all sun photometers will see some scattered light from the sky around the sun. The cone of light a sun photometer’s detector sees is called its field of view, and it is desirable to have this cone as narrow as possible. The GLOBE sun photometer’s field of view is about 2.5 degrees, which GLOBE scientists have concluded is a reasonable compromise between the theoretical ideal and practical considerations in building a handheld instrument. The basic tradeoff is that the smaller the field of view, the harder the instrument is to point accurately at the sun. Very expensive sun photometers, with motors and electronics to align the detector with the sun, typically have fields of view o
