# Radiant Intensity vs. Irradiance

Gold Member

## 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 (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?

## Answers and Replies

Let a source of light give 1mW/sr.
The Irradiance at 1cm away is 1mW/cm2
At 2 cm away it is 0.25mW/cm2
At 10cm away it is 0.01mW/cm2

This is because area subtended by solid angle = solid angle in steradians x distance squared.

Gold Member
Let a source of light give 1mW/sr.
The Irradiance at 1cm away is 1mW/cm2
At 2 cm away it is 0.25mW/cm2
At 10cm away it is 0.01mW/cm2

This is because area subtended by solid angle = solid angle in steradians x distance squared.

Apelling,

Your example helps make perfect sense of the subject. Thank you!