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Why is a current produced when shining light into LED

  1. May 6, 2009 #1
    Right. Here's how it is, in the past I measured approximate laser power output using diffused LEDs of similar colour wavelength as the laser I was measuring. A multimeter across the terminals showed a current which was proportional to the power of the laser.

    I once asked my physics teacher but I got an explanation way over my head. (It didn't sound simply like the photo electric effect ...or at least it didn't sound like it).

    A few years on. I'm asking the same question...

    Cheers.
     
  2. jcsd
  3. May 6, 2009 #2

    mgb_phys

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    It's similair to the photo-electric effect. the photon gives the electron enough energy to get over the potential barrier in the diode.
    An LED and a solar panel are the same thing, both (more or less) work both ways round.

    It's sometimes a problem in surprising places. CCDs chips emit light from their amplifiers when you read them which is a real pain in astronomy applications and memory chips are light sensitive when they aren't encased in black plastic.
     
  4. May 6, 2009 #3
    Ok that makes more sense... But of what consequence does the colour of the LED have in relation to the incident light source colour/wavelength.

    Would shining a laser into a non diffused coloured LED (like those ultra bright ones) have still created a current?
     
  5. May 7, 2009 #4
    You would be Amazed at how many things work in reverse. They often dont work very well though,( low efficiency). Here are only a few things that work "backwards":

    1. DC motor can become generator

    2. Speaker can become transducer

    3. Black painted surfaces absorb heat, but also "radiate" heat better.

    I'm sure others can chime in and add to the list

    P.S. L.E.D.'s more or less,(not usually thought of for rectifier applications), is a DC device,(anode and cathode). I once saw a technician construct a piece of AC equipment. He used a L.E.D. as a power indicator on the 110VAC side. All he needed to do was to add an appropriate current limiting resistor.

    I have seen L.E.D.'s used as power indicators on many DC applications, I found it interesting he had the L.E.D. "self rectifying" in a AC application. Obviously the "Diode" part of L.E.D was doing it's part
     
    Last edited: May 7, 2009
  6. May 7, 2009 #5

    mgb_phys

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    The chemistry of the LED is tuned so that a particular energy is needed to move an electron across the junction - this gives a particular energy (colour) photon.
    That's why red (low energy) LEDs were invented first and why it has been so hard to make (high energy) blue ones.

    Diffuse is just the surface finish on the plastic case - the LED is the same. It would be less efficient because the diffuse surface would scatter most of the light.
    Note the colour of the LED doesn't come from the colour of the plastic - it's intrinsic to the chemistry, the only reason for making red LEDs out of red plastic is aesthetic.
     
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