[Stoves] radiant heat capture, total heat measurement

[Crispin Pemberton-Pigott]

Dear Andrew

 

>>Because glass is a pretty good absorber of IR it is also a pretty good
emitter. 

 

>Surely glass absorbs the higher energy spectrum we can't see in the UV band


 

Yes. And it gets warm touch it and see).

 

>...and passes visible light and higher frequency infra red from hot bodies,
like the sun, 

 

Most of it, the rest makes it warmer (absorbs).

 

>but absorbs the lower energy infra red from cooler bodies like earth and
our bodies.

 

Yes, and because it is warm, and active in the IR, it also emits IR but with
a low emissivity. In other words if you know the emissivity, you can read
the temperature with an IR gun.

 

But more to the point I was saying that at a lower (non 90°) angle, it
starts to reflect radiation from the top of the surface. Look at glass at a
low angle and it looks like a mirror.

 

The point is that when reflecting heat, if the incident angle is past a
critical value, it reflects pretty much all of it so the issues Kevin
mentioned about the mirroring on the back don’t come into play.

 

Paul’s question was about reflecting the heat. So the principles are the
reflectivity, surface finish, incident angle and emissivity. While a stove
may be good at sending IR radiation towards the pot, pots are not all that
good at picking it up, actually. Stainless steel pots are quite reflective
and do better picking up heat by convection.

 

Regards

Crispin

 

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