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How do I asses the % of light transmission that passes through glass

  1. Sep 1, 2010 #1
    I am currently building a solar panel. I have sheets of glass that I collected, some is saftey glass and some are tempered (probably the same). I have read that Low Iron Tempered glass has 91% light transmission. How do I test the glass that I have for its light transmission?
  2. jcsd
  3. Sep 2, 2010 #2


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    Staff: Mentor

    Measure light intensity before entering glass, measure on the other side, compare.
  4. Sep 2, 2010 #3
    Thanks Borek. What instrument is best to use to measure the light?
  5. Sep 2, 2010 #4


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    Staff Emeritus
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    A photodiode.

    BTW, you DO know that different frequencies will be attenuated differently, don't you?

  6. Sep 2, 2010 #5


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    Staff: Mentor

    OK, Zz already addressed the problem, while I was busy with other things.

    In general device you are looking for is called a photometer. However, whole thing is not as easy as you seem to be thinking, as the result will depend on the wavelength - in other words, transmission for different colors will be different. (When we speak about wavelength and/or frequency, we speak about the same thing). So, before you will want to measure something, you have to know which part of the spectrum you are really interested in. Or, if you are thinking about solar panel used to heat the water, you will be interested in transmission of not only visible light, but also about infrared.
  7. Sep 2, 2010 #6
    Broadband detectors with very flat absorption spectra do exist for lasers: www.gentec-eo.com. You could simply focus sunlight on one.

    Perhaps they also exist specifically for sunlight.
    Last edited: Sep 2, 2010
  8. Sep 2, 2010 #7
    The easiest/fastest way to completely characterize the glass in the way that ZaperZ is talking about is to use a spectrometer. There are a lot of cheap (relative of course, ~3kish) compact prism spectrometers that do an amazing job of this. The one i use all the time is an Ocean Optics HR-4000. Since you said you're trying to build the solar panels yourself I'm going to assume that you don't want to spend that much on a spectrometer to do this. That said here's what you would do if you can get a hold of one.

    What you need:

    White light broadband source
    Your glass

    Generally you take a dark spectra and subtract it to get rid of some of the electronic noise. Then take a spectra without the glass in the way. Insert glass and take another spectra.

    Use the transmission calculation on the spectrometer to figure out what wavelengths are being absorbed and at what percentages. You could also do this with a grating spectrometer and step through the wavelengths but that will be even harder, although possibly cheaper since you could find one on ebay.

    The real question is do you really need to know the absorption coefficients for that glass at different wavelengths? I would say no. Because the responsivity of a photodiode changes with wavelength you won't get an accurate power reading from one because you don't know the spectral makeup of the light hitting it.

    However, as Dr Lots-o'watts said above a broadband detector (usually called a thermopile detector) will do a very good job of this because the responsivity changes very little with respect to wavelength. They are still pretty expensive (1kish new) but you may be able to buy the thermopile by itself somewhere and use an op-amp circuit to amplify if needed.
    Last edited: Sep 2, 2010
  9. Sep 3, 2010 #8
    When you are going to buy glass, take a small hand-held solar battery charger, or just a small solar cell hooked up to a meter. See if the glass blocks the power generation, or to what extent it does.

    My solar charger doesn't work at all in my car, because the windows block the specific wavelengths it needs, even though it doesn't appear dark to the eye. So it's best to test against the real use.
  10. Sep 4, 2010 #9
    Thansk JDlugosz that seems a simple and effective way. The glass is for a PV solar design and I have them ready to go.
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