I have used a Si CCD detector setup to measure the electroluminescence of a LED. The y axis unit is given as counts/sec. I want to extract the output power density of the LED. Is there a way to convert this counts/sec to some other standard units like A/W or # photons/m2/s ?
Hi Ramesh, You need some information about the spectrum of the LED output. Also you really need an integrating sphere to collect ALL the light from the LED, otherwise you will have to estimate the how much of the LED light is actually hitting your CCD.
Correct the counts per second from your CCD for its efficiency.. ie it may only count 90% of photons landing on it. Ideally then distribute the total photon counts over your LED spectrum.. or assume it is a symmetrical function. Multiply it up to cover the total light from the LED, and then use E = hf (Energy of 1 photon (eV) = planks constant times frequency of the light) to work out the energy from it. You can probably find more detailed maths of it all if you search for "Planks constant" on the web.
Dear Ramesh,
your LED is emitting divergent light,
so for getting the photon flux density [ph/(m²*s)] at least in the direction of the detector please
a) divide your cts/s value by the solid angle omega set up by the detector (or the detector pixels)
(omega = ad/(rd)2 ; ad = detector (pixel) area and rd = distance between LED and detector (pixel) )
and
b) divide by the net LED output area aL.
So your photon flux density = y / (omega*aL) *).
Now you have your photon flux density in photons per second, per solid angle, and per area of your LED.
Please handle the units m, mm, and µm properly.
Phil is right:
please pay attention to the detection efficiency , the wave length distribution and the angular dependence when you will evaluate integral values.
*) when operating very off axis you have to consider appropriate angular functions
Dear Ramesh,
The problem can be solved when you have the following information:
- How many photons in one count of the detector output.
So you need to calibrate your detector using a known source.
- If this number is known then you can easily get the received photons/ s/ wavelength interval. You can convert this in watt/wavelength interval by simply multiplying the photon flux by Eph the photon energy= hc/lambda.
Best wishes
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