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Old Dec 19th 2013, 06:28 AM   #1
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LED and Ambient light

I am setting up a basic experiment with the set up shown below in an attachment.

Between the monochromatic LED and the monochromatic photo-diode shown in the diagram there is a blockage. The aim of the experiment is to detect a blockage or lack thereof by comparing the intensity transmitted by the LED and the intensity received by the photo-diode.

So I have had results as expected but the diodes have been sealed in a container and not exposed to ambient light.

My question is this:
If ambient light was allowed to pass through the blockage with the monochromatic light from the LED, how would they interact? The ambient light would be a flat wave front as if generated by a source an infinite distance away.

I am having a bit of a brain freeze with this and I do not know if I am thinking about it the wrong way but I was thinking that the fraction of the ambient light with the same wavelength as the LED would interfere with LED light and increase its intensity by a fixed amount, but this does not include the idea of constructive and destructive interference.


Any and all help with his would be much appreciated.
If anyone knows of any good sources of information such as websites that might cover this topic it would also be appreciated.
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LED and Ambient light-photodiode-final-circuit-diagram.png  
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Old Dec 19th 2013, 09:21 AM   #2
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Assuming the ambient light is not coherent (i.e., it's not from a laser but rather is ordinary ambient light such as from the sun or a light bulb) then all you need worry about is the added intensity at the wavelength of interest. You do not need to worry about wave pattern interference issues causing increased or decreased intensity, as neither the LED nor the ambient light is coherent.
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Old Dec 19th 2013, 10:57 AM   #3
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Wink Sensitive Problem

The only issue I can see could be the performance of the photo diode.
If it has a non-linear response to light intensity, then the sensitivity of your measurement may be affected.
I believe that some devices of this type have a saturation point beyond which any increase in intensity will not produce an increase in output.
If the added ambient light takes you too close to this point, then sensitivity might suffer.

I note that you mention that it is a mono-chromatic photo-diode,
thus only a restricted frequency band of the ambient light will be involved.
I would guess that as long as your LED is bright relative to the ambient light,
there should be no problem.
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Old Dec 19th 2013, 01:13 PM   #4
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Originally Posted by ChipB View Post
all you need worry about is the added intensity at the wavelength of interest.
So would the intensity of the combined ambient light (of the relative wavelength) and LED light be proportional to the summation of the two amplitudes squared?

Thank you both for the help
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Old Dec 20th 2013, 06:04 AM   #5
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Yes - since the power of a light wave is proportional to the square of the magnitide of the electromagnetic wave, then yes - you add the squares of the amplitudes. But I'm wondering why you're asking this - do you have data on the amplitudes of the electromagnetic wave for the LED and ambient light? Usually the intensity of these sorts of things are measured in lumens/square meter, or lux.
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Old Dec 20th 2013, 10:27 AM   #6
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Brill that makes my life so much easier. Thank you.

I have the details for the LED (in Lux) as for the ambient light I hope to find that out by comparing the intensity of the LED measured by the photodiode in a black box compared to the LED result exposed to ambient light.
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Old Jan 11th 2014, 01:44 AM   #7
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I wish to encourage any off-angle photons to bend away from the center line and become absorbed by the inner walls of the tube. I wish the angles reaching the other end to be as selective as possible,
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