A question on thermal radiation

[cfung]

It is common knowledge that black colored surface absorbs light and heat the best. Then it seems to me the next logical leap would to think that black colored surfaces would radiate heat most efficiently.

If my assumption is correct, then why do most heating devices, such as oil heater or common home heater, have silverish finish instead of black?

Thank you for your time reading this.

 

[marcusl]

It is common knowledge that black colored surface absorbs light and heat the best. Then it seems to me the next logical leap would to think that black colored surfaces would radiate heat most efficiently.

Very good, your intuition about absorptivity and emissivity (as they are called) is correct.

If my assumption is correct, then why do most heating devices, such as oil heater or common home heater, have silverish finish instead of black?

Am not sure what kind of heater you are referring to. The kind with heating coils that glow orange are usually backed by a polished metal reflector that projects the infrared radiation forward.

A better example is to consider the heat sinks on electronic devices--they are nearly always black for maximum radiative cooling. The only exceptions tend to be the the integrated heat sink/fan units on big computer chips. The color doesn't matter in this case because cooling is achieved via forced air flow.

 

[Topher925]

Its not always about being the most efficient. Cost comes into play a lot as well. Back anodizing or powder coating costs money which a manufacturer may omit due to keep costs down.

Also, while black does transfer radiant heat the best, it doesn't always conduct it well. In other words, a black powder coating may transfer heat from the air to itself well, but not from itself to the material its been coated to.

 

[Andy Resnick]

It is common knowledge that black colored surface absorbs light and heat the best. Then it seems to me the next logical leap would to think that black colored surfaces would radiate heat most efficiently.

If my assumption is correct, then why do most heating devices, such as oil heater or common home heater, have silverish finish instead of black?

Thank you for your time reading this.

You are correct, absorption = emission, so a black object would be a better emitter than a reflective object- with the caveat that I mean "black" and "reflecting" within the waveband of interest (8-12 microns for room temperature, 3-5 microns for hot objects).

Radiators are not generally painted black purely for decorative reasons. In addition, those 'decorative' enclosures also act to decrease the efficiency of the radiator. AFAIK, heat fins on circuit boards and in non-visible places (i.e. backs of amplifiers) are indeed black.

 

[cfung]

Cool, millions of thanks guys, the answers are reasonably convincing and exactly resolve a thought that's been giving me a headache