Thermal Detection And Infrared Cameras

[RayN]

Thermal detection is used in infrared cameras to detect objects not visible in the dark.

For an object to be detected by an infrared camera, its surface has to have good absorbance of infrared or good reflectivity? Also, how does emmisivity relate to absorbance and reflectivity?

Edited April 21, 2018 by RayN

[exchemist]

Thermal detection is used in infrared cameras to detect objects not visible in the dark.

For an object to be detected by an infrared camera, its surface has to have good absorbance of infrared or good reflectivity? Also, how does emmisivity relate to absorbance and reflectivity?

The IR camera detects thermal IR radiation, which is emitted by all objects, with a spectral distribution determined by the temperature of the object. It does not depend on reflecting or absorbing incident radiation. 

 

The intensity of emission will depend on the emissivity of the object. (A perfect "black body" being defined to have an emissivity of 1). 

 

The absorptivity is equal to the emissivity (Kirchhoff's Law) , so a good emitter (e.g. a matt black surface) will be a good absorber and a poor emitter (e.g. a shiny metallic surface) will be a poor absorber. Since when incident radiation is not absorbed it is reflected, you can see that reflectivity is linked to absorptivity, which is linked to emissivity.  

 

Here is a link to the Wiki entry on Kirchhoff's Law, which describes all this: https://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation

Edited April 22, 2018 by exchemist