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
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.
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?