>>> Hi,
>>>
>>> I'm curious how infrared thermometers work.
>>> Here is my hypothesis: There must be some simple
>>> photodetector -- I looked in electronics catalogs and
>>> didn't find it-- that can sense the range of IR light
>>> (8 to 12 micrometers?) that is radiated heat.
>>> This is surely placed at the end of a tube and
>>> only IR light that enters is from whatever object
>>> is being measured. This is then converted to a voltage
>>> and read with a D to A converter by a microcontroller
>>> and displayed.
>>>
>>> More or less?
>>>
>>> Thanks.
>>>
>>
>> D to A? Don't you mean A to D?
>>
>> Anyway, it doesn't work by measuring the amplitude of the IR. If it did
>> then
>> the reading would be sensitive to the distance from the detector to the
>> source (and it's not).
>>
>> If you do some reading on "black body radiation" then you'll be able to
>> figure it out.
>>
>> Bob
>
> It _does_ measure the intensity of the IR. It's not sensitive to the
> distance from the detector because as you get farther away from the source
> each unit area of the source affects the bolometer as 1/distance^2, but
> the area that the bolometer "sees" goes up as distance^2, so it evens out.
>
> --
> Tim Wescott
> Control systems and communications consulting
>
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>
Tim,
That makes sense. It's certainly easier than locating the wavelength of the peak of the radiation spectrum.
There's a *pretty good* writeup in this pdf, but it doesn't go into the details of the thermal detector. However, it does say that if the target is smaller than "spot diameter" then the reading will be less accurate (which supports your answer).
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Thanks, Bob