How come heat sinks are cold by the touch?

[voko]

Others have already pointed out that coloring it black increases the emissivity.

In the visible part of the spectrum. How important is that for a typical home electronics heat sink?

Note: whether it is "black" in IR is far from evident from its visible color.

 

[rogerk8]

Consider physicists precious conservation of energy.

If you then look att a problem like a gravitational sling shot you may solve it all by considering it all to be within that energy system.

It is excatly the same when it comes to details like emissivity (or Boltzmann Law) in a system.

If you do two tests at two different temperatures the emissivity constant vanishes from the equation due to being much less than infinitesimally dependent on the temperature change.

More often than not you may even see it like quite large changes do not affect the constant at all (my guess).

So all you have to do is to calculate relatively.

Hence

$$P_s\propto T^4...[W/m^2]$$

Where Ps i the surface power density

Roger