1. Higher Physics
Unit 3
Irradiance

2. Irradiance 1
Irradiance is a measure of the ‘brightness’ of a light source (or any other wave).
Consider a 100 W tungsten filament light bulb (25% efficiency).
The lamp emits 25 J of light energy in all directions each second.
The irradiance of the light falling on a surface is a measure of the amount of energy per second per square metre.

3. Irradiance 2 We have the equation - I = P A where –
I – irradiance (W m-2)
P - power (W)
A - area (m2)

4. Irradiance 3
As the light radiates out from the source the energy is spread out over a larger area.
This causes a reduction in the irradiance as the distance from the source increases.
How can we find a relationship for the irradiance of a light source, I , at a distance, d ?
lamp
Light sensor
distance, d
Interface PC

5. Irradiance 4
By measuring the irradiance, I, at a range of distances, d, we get the following graph - I
(Wm-2)
This seems to show an inverse proportion relationship, but the situation is more complex.
d (m)

6. Irradiance 5
Redrawing the graph for I against 1/d 2, we get the following - I
(Wm-2)
According to this graph, I is directly proportional to 1/d2.
i.e. I = k d2
1/d 2 (m -2)

7. Irradiance 6