Higher Physics
Unit 3
Irradiance
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.
Irradiance 2
We have the equation -
I= P
A
where –
I
P
A
–
-
irradiance (W m-2)
power
(W)
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 ?
PC
distance, d
lamp
Light
sensor
Interface
Irradiance 4
By measuring the irradiance, I, at a range of
distances, d, we get the following graph I
This seems to show an
inverse proportion
relationship, but the
situation is more
complex.
(Wm-2)
d (m)
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)
Irradiance 6