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The sensor is in two parts in the same package. One is exposed to the outside world and one is not. The temperature of these is calculated from their known characteristics. Then about 80 pages of fine print math calculations are done (I am not kidding, I saw the printout of the code for a consumer/industrial model.) These calculations involve the black body radiation spectrum being a function of temperature.
As you would expect, the difference in temperature of the two sensors is very low which makes the accuracy problem serious. Also, any temperature gradients across the sensor, such as by heat generated by the internal electronics will cause errors.
This is why there are so few companies making them.
Differently for emissions of "cold light" which has a different physical explanation, all solids, liquids and gasses bodies will emit a radiation if heated.
In solid and liquid bodies the radiation spectrum is continuous, that is all waves length are represented in it, so waves length falling in the Infrared wave lenght are measured. The amount of Infrared radiation is proportional to the heat quantity and in turn to the temperature value of the body.
Of course, as has been said, the measurement is carry out for comparision between an irradiated heat amount of a reference body. The absolute reference is constituted from black bodies which have the property to absorbe radiations in proportional amount of what they can irradiate (and viceversa). In an environmental at a certain ambient temperature Ta, the amount of heat irradiated from a black body will be proportional to Q=k(T^4-Ta^4) where k is the irradiation constant of black body in the vacuum. (Stefan-Boltzmann law).
Just a different note, though a bit different from temperature measurements with IR sensors, but still related with body emission. If the irradiated spectrum in solids and liquids is continuous, the gasses or vapours have discontinuous spectrum and any of substances have their own wave length of emission. Because a gas can absorbs the same wave lenght it can irradiate, if we redirect throught a gas a radiation with continuous spectrum, the emerging radiation will have a spectrum presenting discontinuities, which indicates the wave length that particular substance in the gas may emit. Analysing these spectrum discontinuities, one can deduce the nature of the substances at vapour or gasses state. This is the principle of spectrography analysis.
FRAGANCE ... You don't know how much you have helped me ..
This tip is going to make possible a design that i have been working on.
It was getting too EXPENSIVE .. On top of that i deal a lot with MELEXIS
and i was not even aware that they had that chipset ..
Hmm, most probably the MELEXIS thermophile sensor is butt slow due to the exessive thermal timeconstant of the bolometer. The pyroelectric sensors should be much faster because they react to the derivative of the temperature, but this imply that for measuring the object temperature some have to either use a optical chopper or pann technicue. This things are really cheap and readily available in the IR security devices for couple of bucks.
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