On my newest project I want to have the the heat sink anodized black. What I am wondering is if it will affect the heat dissipation at all? I can very easily have the part machined all the way, have it anodized, and then finish machine the part where only the emitter will sit. Do you think this is an optimal solution or does it really matter?
Most flashlights are anodized black, and their exterior performs the function of heat sink, without issue. Heatsinking is mostly about mass, a sub-millimeter coating of even a specifically insulating material won't make much difference..
Its not so much that the "Insulation" of the anodizing on the flashlight head/body that impacts heat dissipation, its the emissivity of the color change from bare aluminum.
Black radiates more heat than say silver or white. A flat black would radiate more heat than a shiny black.
So while mass acts as a heat sink, being black helps that heat to be radiated from the mass, once the mass has absorbed the heat.
This allows the mass to absorb more heat as it loses the previously absorbed heat, and so forth.
Most people are not thrilled with the appearance of a flat black finish, so a shinier black is typically used for aesthetic purposes. If its not about aesthetic though, flat black is going to radiate away more heat.
Flat black will ABSORB more radiated heat as well, but most heat sinks work by conduction rather than radiation...on the side facing the heat....so the primary benefit would be better conduction for a sink on the SOURCE of the heat, where bare metal would be a better conductor.
So, ideally, you have bare metal ON (Where they touch) the source of the heat you need to conduct away....and flat black on the OTHER SIDE to radiate it away from the sink's opposite/heated side.
Here is an interesting thread about thermal conductivity. Post #116 got me curious so I checked out Newbie's page on emissivity. Anodizing and other surface treatments can make a significant difference. But since you asked about the heatsink in particular, if you increase the emissivity of a part inside the light probably won't make a significant difference because that air is trapped. But if you finned the flashlight head to increase its surface area and then anodized that surface would make a big difference.