What does it mean to calibrate a sun photometer?
A sun photometer is considered to be calibrated if its extraterrestrial constant is known. This is the voltage you would measure with your sun photometer if there were no atmosphere between you and the sun. As an exercise, you could think about pointing your sun photometer at the sun from the open cargo bay of the Space Shuttle as it orbits Earth above the atmosphere. The voltage you measure would be your instrument’s extraterrestrial constant. This value depends primarily on the wavelength at which your sun photometer detects light and also on the distance between Earth and the sun. (This distance varies slightly because Earth follows a slightly elliptical, rather than a circular, path around the sun.) Note that if you really could use a sun photometer outside Earth’s atmosphere, you would not have to worry about limiting the field of view. Why? Because outside the atmosphere there are no air molecules or aerosols to scatter sunlight. Hence, your sun photometer will see only direct su
