Physics of an infrared thermometer The thing about infrared thermometers that bugs me is how can y

lifretatox8n

lifretatox8n

Answered question

2022-04-12

Physics of an infrared thermometer
The thing about infrared thermometers that bugs me is how can you get the same temperature reading regardless of the distance to the object. Shouldn't there be a difference when measuring from two different standing points since energy flux density decreases with 1 d i s t a n c e 2 and infrared thermometers work by focusing IR light on a thermopile, which then results in decreased (when measuring from further away) absorbed energy and therefore lower temperature and finally lower voltage across thermopile. Is there something I am getting wrong about this, or do IR thermometers make use of some other physics law like Wien's displacement law, by somehow measuring λ p e a k to determine the temperature?

Answer & Explanation

bamenyab4mxn

bamenyab4mxn

Beginner2022-04-13Added 16 answers

I believe the basic answer is that, within limits, as you move away from an extended source, the IR sensor can collect flux from a greater amount of the surface. Your 1 d i s t a n c e 2 formula only holds for a point source.
In the limit of an infinite flat surface of uniform temperature, the thermometer would receive the same power regardless of distance from the surface, as can be seen by considering the area in its cone of view (Field of View).

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