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    • 13 December 2008 at 12:01 am #31038
      Anonymous

        In thermal images, windows sometimes look bright, as one might expect since walls and roofs are better insulated. But sometimes windows look extremely dark. I can speculate about why that might be, but does anybody know? Better yet, can anybody point me to an authoritative reference, preferably available online?

        Thanks, Dave

        PS There doesn't seem to be a topic category that fits exactly for this question, so feel free to move it if you think it belongs elsewhere.

      • 23 January 2009 at 6:14 pm #35711
        Peter Warm
        Participant

          Dave,

          IR photos show up the amount of radiation, whihc is dependany upon
          a) the termperature difference ( to the forth power) and
          b) the emissivity of the material

          most building materials have an emmisivity of around 0.95
          glass, howeer, can be lower, which can give misleading visual images.

          how this helps!
          peter

        • 23 January 2009 at 10:35 pm #35712
          Tom Foster
          Participant

            the amount of radiation, whihc is dependany upon
            a) the termperature difference ( to the forth power)

            Isn't it the difference between two fourth powers, not the fourth power of the difference?
            i.e. the radiation emitted by the higher temp object (fourth power of abs temp of the higher object) minus the radiation emitted by the lower temp object (fourth power of abs temp of the lower object). Over limited temp range e.g. 263 to 303K, that makes the nett radiation close to proportional (power of 1.1?) to the temp difference, as in radiator output is proportional to delta t.

            Wouldn't the measured thing be one of those objects, and the sensor be the other object?

          • 25 January 2009 at 12:32 pm #35713
            Anonymous

              I think there are other factors at work as well.

              Glass transmits some radiation and it reflects some radiation as well. And its properties are sensitive to angular effects – obvious for reflection, for example. Oh, and it's strongly frequency sensitive, of course.

              So I think there are lots of possibilities to speculate about, too many for me! Which is why I was wondering if somebody could point me to an established, accepted account of the various phonomena and how they interact in this situation?

              Thanks, Dave

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