Can there be different IR emissivity & IR absorptivity

[shivadevrao]

At a particular wavelength (In IR range), & at particular temperature, can anybody or coating has different IR emissivity & IR absorptivity?

 

[engnr_arsalan]

yes there can be bodies or surfaces that may have different absorptivity and emissivity, like in flat plate collectors they use selective absorbers which have high absorptivity and low emissivity in order to absorb maximum solar energy. the case about which u r talking is a gray surface having equal emissivity and absorptivity.

 

[shivadevrao]

Thanks for the reply,

I understand that certain coatings have high solar absorptivity & low IR emissivity, my question is are there any surfaces or coatings which have high IR emissivity & low IR absorptivity.

 

[engnr_arsalan]

u should read about ir bulbs used for curing of powder coating

 

[sophiecentaur]

At a particular wavelength (In IR range), & at particular temperature, can anybody or coating has different IR emissivity & IR absorptivity?

yes there can be bodies or surfaces that may have different absorptivity and emissivity, like in flat plate collectors they use selective absorbers which have high absorptivity and low emissivity in order to absorb maximum solar energy. the case about which u r talking is a gray surface having equal emissivity and absorptivity.

The above answer does not strictly address the above question. The wavelength specific absorptivity is the same as the emissivity (this is Kirchoff's law of thermal radiation). However, Kirchoff does not address what happens with a wide range of incident and emitted radiation frequencies (i.e. a practical case).

 

[Andy Resnick]

At a particular wavelength (In IR range), & at particular temperature, can anybody or coating has different IR emissivity & IR absorptivity?

No, otherwise energy would not be conserved and you could violate the second law of thermodynamics.

 

[technician]

if the 'absorptivity' were greater than the 'emissivity' the object would warm up spontaneously.
if the 'absorptivity' were less than the'emissivity' the object would cool down spontaneously.