How NOT to Work Out the Efficiency of a
Light Bulb
By Melissa Dube, Joshua Garfinkel, Abbie Head ,Moiz Paracha
Our investigation
Method
We wanted to use a thermal imaging camera in order to find
out which type of light bulb was the most efficient. In order
to do this we decided upon 2 methods: one that involved
measuring the temperature to find out how much of the
energy the light bulb was giving out as radiation, and one
that involved measuring its change in temperature in order
to work out how much energy it wasted.
-Using the thermal camera, record the initial temperature
of the three different light bulbs. This allows us to locate
their change in temperature.
-At the same time of starting a stopwatch, turn on all
lamps in order to start the investigation.
-Record light bulb temperatures every 30 seconds until 11
minutes have been completed.
-Once light bulbs and lamps have cooled down to room
temperature, repeat this twice in the same way.
-Work out an average of results and use equation shown
within to work out the difference in efficiencies.
Theory
It is possible to work the power radiated by using Stefan
Boltzmann’s law which states:
Where L is power radiated , sigma is the Stefan Boltzmann’s
constant, A is the surface area and T is the temperature. In
doing this we model the bulb as a black body ( a perfect
emitter) and so assume the radiation it gives out is black
body radiation (BBR)
In order for us to calculate the energy used to heat up the
light bulb we need to use the formula:
Where m is the mass and Cp is the specific heat capacity at
constant pressure.
To work out the specific heat capacity we had to break the
light bulb and measure the masses of individual pieces
separately. We then researched the specific heat capacity of
various materials in order to come up with a value for the
specific heat capacity of a light bulb at 0.702 kJ/K kg.
Halogen
Bulb
Mercury Vapour
Bulb
Evaluation
80
In doing this experiment we discovered that using a
thermal imaging camera is definitely NOT a good way to
measure light bulbs efficiency.
70
60
Temperature / Degress Celcius
Incandescent
Bulb
50
40
30
20
10
0
0
5
10
15
20
25
Time / Minutes
Results
The results from one method (specific heat capacity)
showed that the light bulbs were incredibly efficient, with
the incandescent, halogen and mercury vapour bulbs having
efficiencies of 97.7%, 97.4% and 94.9% respectively. On the
other hand, the method using BBR gave efficiencies much
less than 1%, with values of 2.8*10-6 , 3.5*10-6 and 2.4*10-6
respectively.
Problems we encountered that invalidated our results
include: Light bulbs not being black bodies; heat from
light bulbs being dissipated to the surroundings; black
body radiation being a mixture of visible and non-visible
light; and the fact that finding out the surface area and
specific heat capacities of light bulbs is difficult to do with
any high degree of accuracy.
Thus we can safely say that if you want to discover the
efficiency of a light bulb, thermal imaging is not a good
way to go about it. If we were to repeat, some method of
measuring light intensity against power would be better.