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Wavelength and resistance of an LDR

  1. Apr 1, 2007 #1
    1. The problem statement, all variables and given/known data

    Light Dependent Resistors appear to have a spectral response; that is, at different wavelengths of light, the LDR will become less resistant than at others. For example, a cadmium sulphide LDR has a high response at 550 as shown at http://www.biltek.tubitak.gov.tr/gelisim/elektronik/dosyalar/25/LDR_NSL19_M51.pdf on page 2. This may adjust the response to light at a singe given light intensity.
    I am trying to measure the change in light intensity from a singular light source as it varies with wavelength, so as you can imagine the above factor causes me a problem. Is/are there either a) a kind of LDR that as a completely uniform "spectral response" or b) other kinds of resistors which have different spectral responses that could be used to compensate for this one or c) another way of compensating for the problem?

    2. Relevant equations

    Not Applicable

    3. The attempt at a solution
    Other kinds of LDR which have the same response at different wavelengths
    an LDR with a uniform spectral response
    some kind of scale which adjusts for the problem
    Last edited by a moderator: Apr 22, 2017
  2. jcsd
  3. Apr 2, 2007 #2


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    Gold Member

    I think you have to use the data on the LDR to correct your readings for different wavelengths. You are given the relationship between resistance and Lumens, so that is a simple calculation. Then to correct for wavelength, divide the calculated intensity by the percentage response for that wavelength.
  4. Apr 2, 2007 #3
    I think this is reasoable; what I'm confused is whether your light source has a spectral shift versus intensity and thats part of the problem. Maybe if I had a bigger picture of what you're trying to do here, could be of more help. When i worked with light as a grad student, monochromatic filters and neutral density filters were essential tools of the trade.
  5. Apr 2, 2007 #4
    Thanks Mentz114, that was really helpful, and I think may have fixed my problem. :D

    Just for completeness, in response to denverdoc, the light source has many wavelengths, the task is to find the intensity of each individual wavelength.
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