Quote:
Originally Posted by Top_oz
I can't find any literature that indicates white is any better or worse than black paint. All non-specialty paints seem to be around the same emissivity and are 'black' in the IR spectrum (black in this context should not be associated with visible colour).
There are some high and low emissivity coatings out there but I'd suspect this is not used on amateur gear. My guess is that colour choice is down to aesthetics (marketing).
If emissivity is really a concern, the lowest emitters are polished metallic surfaces and metallised films. Not too far behind are aluminised paints.
|
Emissivity and
Albedo are two different properties of a material or surface. Both important, along with many other factors, in radiant heat transfer.
Emissivity is the proportion of usually long wave / Infra Red thermal energy
emitted by an object compared with a thermally "Black Body" at the same temperature and is used in the
Stefan Boltzman Law to account for the heat transfer performance of thermally "grey" bodies. As a second order complication the emissivity is also a function of wavelength, angle, shape, etc...
Albedo on the other hand relates to the reflectiveness of a surface and is the proportion of incident solar radiation
reflected from a surface. Albedo is sometimes more generally referred to as reflectivity. Albedo is defined as between Zero (0= Not reflective at all) to 1 (= Perfect reflector). As an example snow typically has an albedo of 0.9, whereas asphalt is around 0.1 and white and black paints/surfaces follow similar widely different values.
Emissivity and Albedo are 2 important parameters in radiant heat transfer. One you can think of as emission from the surface, the other reflection from the surface. Whilst the emissivity of white and black (say painted) surfaces may not be that different, their albedo or reflectivity
in the solar radiation wavelength range(~250-2500nm) is very different and will result in more solar radiation being reflected from the white surface object. If you have your scope outside in the sun, either solar imaging or setting up in preparation for nightime imaging, there's no need for it to bake in the sun so much if you go for a lighter/white colour. Note that albedo / reflectivity is effected by dirt and dust on surfaces.
Best
JA