Air Mass is the measure of how far light travels through the Earth’s atmosphere. One air mass, or AM1, is the thickness of the Earth’s atmosphere. Air mass zero (AM0) describes solar irradiance in space, where it is unaffected by the atmosphere. The power density of AM1.5 light is about 1,000W/m2; the power density of AM0 light is about 1,360W/m2, which is considered to be the solar constant.