How does IR (infrared) intensity measured

  1. Can a lux meter measure infrared intensity like it does for the visible light? I read from somewhere that it cannot. If you have an IR illuminator on in the dark and hold up your 'regular' lux meter to it, it will likely register 0 - an unhelpful and misleading result.

    I'm just wondering if there is a filter inside the lux meter that only passes a certain amount of wavelength, and if I can change some of its circuitry in order to have a reading for IR.

    I am looking for a cheap way to measure IR intensity. Or, if there is another way to measure infrared intensity (like a relationship between terminal voltage/current and temperature, temperature vs. illumination, etc.) please let me know (:
  2. jcsd
  3. Thermometer ?
  4. Mech_Engineer

    Mech_Engineer 2,347
    Science Advisor
    Gold Member

    Lux and Lumens are photopic units weighted based on the response curve of the human eye. The human eye is not sensitive to infrared, therefore you cannot define a lumen for infrared wavelengths.

    However, you can define infrared incident energy in radiometric terms such as power per area, e.g. W/m^2. There are instruments that can measure incident infrared energy as well, but you have to be careful in defining the wavelength band you're interested in, and the expected energy levels.
    Last edited: May 8, 2012
  5. Bobbywhy

    Bobbywhy 1,864
    Gold Member

    Your lux meter is meant to measure visible light. It has a light-sensitive detector which, when light strikes it, generates a photocurrent that gets amplified and sent to the (calibrated) meter readout.

    Since Infrared energy is just a lower frequency your lux meter may respond to it, but the calibration would not be correct. You would need to find the "spectral response" curve of the lux meter's sensor, and if it does detect the IR band you are interested in, then you would need to re-calibrate the readout to conform to the responsitivity curve. All-in-all, it's not a very useful thing to attempt because it's complicated and may not even work.

    Why not just test your lux meter? Place it in the IR beam and see if it reads anything at all. Better yet, get a new meter that is designed to measure IR intensity.
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