- #1

JJBladester

Gold Member

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## Homework Statement

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

## Homework Equations

Radiant intensity (

*I*) is the output power per steradian, specified in mW/sr.

_{e}Irradiance (

*E*) is the power per unit area at a given distance from a light source, specified in mW/cm

^{2}.

## 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 r

^{2}.

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?