1. The problem statement, all variables and given/known data I am having a hard time discerning the difference between radiant intensity and irradiance. I searched PF and found this distinction, but am still confused. Radiance is the power from the source per area into a certain solid angle. Irradiance is the power onto a surface per area 2. Relevant equations Radiant intensity (Ie) is the output power per steradian, specified in mW/sr. Irradiance (E) is the power per unit area at a given distance from a light source, specified in mW/cm2. 3. The attempt at a solution Perhaps my confusion lies in the definition of a steradian. I know that a radian is an 2-dimensional angular measurement representing the angle "cut out" of a circle when a length of its subtended arc is equal to the radius. A steradian cuts out an area of a sphere equal to r2. So, it appears that radiant intensity and irradiance both measure an amount of electromagnetic power per given area. Where is the difference in the two terms?