Higher Physics

Unit 3Irradiance

Irradiance 1Irradiance 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.

Irradiance 2

We have the equation -

I = P A

where –I – irradiance (W m-2)P - power (W)A - area (m2)

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 ?

lampLight

sensordistance, d

Interface

PC

Irradiance 4

By measuring the irradiance, I, at a range of distances, d, we get the following graph -I

(Wm-

2)

d (m)

This seems to show an inverse proportion relationship, but the situation is more complex.

Irradiance 5

Redrawing the graph for I against 1/d 2, we get the following -I

(Wm-

2)

1/d 2 (m -

2)

According to this graph, I is directly proportional to 1/d2.i.e. I = k

d2

Irradiance 6