ANY COATING of paint will REDUCE the emissivity of the overall device.
Even if the paint is highly emissive on its surface after drying, the paint SLOWS the migration of heat from the box though its thickness ever so slightly.
The best emissivity you can get is to grit blast the box exterior so that it gains a matte finish. This is more emissive that the smooth finish, and does not act like a blanket, like paint does.
Better still is to ATTACH the heat source side of the box to a THICK, "cold plate", which is a heat sink that relies on mass not fins.
A blanket is a blanket. A layer is a layer. The heat will migrate OUT of the box SLOWER if it is painted.
The only difference is if the right IR paint is put onto it AT the right coating thickness. Then, and ONLY then, will the painted box dissipate its source faster than the unpainted box.
And as has been stated, that is true only if the box is not sitting out in direct sunlight, which MAY be able to keep it from giving up its heat as fast due to infusing more additional heat into the box than the box is able to radiate out.
So, there are SEVERAL factors that determine whether or not painting will slow or speed heat dispersion from a surface, and it has nothing to do with "the first mm of air".
The starting point. Except, asshole, the box is die cast, so it is very likely Zinc or magnesium. You lose, again.
You are a goddamned idiot. The paint numbers alone are way off, and too ambiguous. That list is from the '40s. Just like your two neurons.
The emissivity of the surface tells nothing about the migration through the paint layer.
You also left out SEVERAL factors. Mainly because you are too stupid to know enough to even consider them.
Note how there is no "matte" finish numbers.Convenient that you only posted listings that would be favorable to your pathetic pointlessness as well.
I'd say that the folks you got that layman's chart from are about as stupid as you are.
Just so you know, dipshit, there are many different types of paint, and there are many different types of plastic, and they all act differently from each other.
You are also so goddamned stupid that you give a blanket definition for "polished", which indicates clearly that you have no clue what the term "surface quality" means, much less how it plays into industry and physics.
A #4 polish on Copper is going to yield a much higher number than a #8 mirror polish.
The mere fact that you are lost on these particulars puts you firmly in the Johnny Doesn't Know Shit bandwagon.
"plastic paint"? The list was made by total retards. Probably Larkin or one of his butt buddies.
Matte metal has much lower thermal IR emissivity than paint does. Matte metal is a diffuse reflector rather than a specular one, still mostly reflecting - even in thermal IR. If it reflects most incoming thermal IR, it is a poor thermal IR emitter.
That's why many aluminum heatsinks are anodized - and not many are blasted to have a matte surface.
Also consider what the thermal resistance of a coat of paint is:
Most plastics have heat conductivity around .2-.3 W/m-K. Most paint coatings are probably similar, especially if the vehicle in the paint is an organic solvent (the solute is probably a polymer or something similar). Supposing .2 W/m-K for dried spray paint, .2 millimeter thickness, a single square centimeter of this coat of paint has heat conductance of .1 watt per degree of temperature difference across the paint coating. So if the heatsink is dissipating .1 watt per square centimeter, the paint's thermal resistance would make the heatsink 1 degree C warmer than otherwise. Make that half a degree C if you achieve a thinner coating of .1 mm. The improved thermal IR emissivity would make an unfinned heatsink surface with this much heat loading cooler by a few degrees. The improved thermal IR emissivity outweighs the added thermal resistance.
What's wrong with a plate of same area but just enough thickness to have good heat conductance? Adding mass to it will not reduce the temperature rise, only increase the amount of time it takes for that temperature rise to occur.
The manufacturer of my non contact thermometer makes the blanket statement that organic compounds generally have high thermal IR emissivity, to the point of reading reasonably accurately. One exception I found is ink coatings (very thin) and very thin polyethylene films, fraction of a tenth of a millimeter thick.
Coloring a bare metal heatsink with a Sharpie increases its thermal IR emissivity noticeably, but not nearly as much as coating it with something thicker than that like a usual coat of paint or a layer of masking tape or electrical tape.
What's interesting is that thin/cheap black garbage bags are almost transparent at thermal wavelengths. So you can use them to make airtight windows into enclosures and scope what's going on inside. Or else buy a few kilobucks of quartz window from Flir.
I like black whiteboard marker. It has high emissivity but is easy to remove.
Kapton tape is good, too. Here's a piece of copperclad with a dab of Kapton tape:
Total bullshit. Matte surfaces... ALL matte surfaces will be MORE emissive than a polished surface. There are far more exit vectors on the rough surface. So abrasive polish or grit blast will both yield a greater emissivity than the mirror polished face.
It isn't about reflection, it is about emission. You are in the wrong mindset. If reflection is what is desired, such as a mirror focusing the image onto a 2mm bolometer spot, then a gold mirror is what you want. Note that the emissivity is nearly zero. Reflection and emission are two entirely different things. Thinking about a polished surface... if it reflects surface light (and it does), it will also reflect the IR back INSIDE the media when it encounters the surface polish from within. THAT is the very nature of emissivity. Scratches (roughing the surface polish) creates a greater number of exit vectors for the IR that do not reflect it directly BACK IN. That is the whole point. The abrasions create micro-ridges.
The very definition of the term refers to surface quality as a MAIN factor.
The sheet I posted states it right at the start, and it references the CRC Handbook. You gonna trash them too now?
All non-silicone non-metal-containing paints, varnishes and laquers listed there achieve at least .77. Except for shiny shellac, glossy varnish, laquers not noted to be flat, and white/near-white radiator paints, they all achieve .9-.98. Even clear silicone coating achieves .66-.82.
The highest number shown there for oxidized zinc is .28.
Because if it is not enough additional mass, it will "settle in" at a higher than desired temperature. You want a LOT of mass there so it can SOAK the heat as soon as it is presented to it and for as long as it is presented. If the mass is too small, the heat starts to move like you like it, but begins to heat up the cold plate sink faster than it is getting carried off to the point that it will rise up to some overall "running temp". If that temp is acceptable, you use it. If not, you increase the mass or modify the coupling or spread so that it always has a place to go at such a rate as to keep things below your target point.
Then, there is that "the lower the better" thing.
For the same reason, you WANT a large mass INSIDE the box against the source device and made such that the heat gets spread over a larger area of the thin cross section box wall. THEN, you can maximize the exodus by way of exterior conduction. Without the spreader inside, the source device will run away or operate at a higher temp than one likes, even though the overall temp 'appears' ok. The thin box wall is the reason. The heat will move across it fast, but moves along it much slower, and bottlenecked from the start. Bonding (or as part of the casting) a large block of media to the box wall allows the source device heat to move in two planes, and spread its heat far better to the box wall and over a much larger area making for a more efficient evacuation.
Roughened metal has higher emissivity than polished metal, but the emissivity is still low when roughened if it's very low when polished.
Absorption and emissivity of any material at any given wavelength are equal. Otherwise, an object surrounded by a blackbody radiator surface can achieve and maintain a different temperature from the temperature of the surrounding blackbody radiator surface.
My Raytek cite for metals mentions values for some metals both polished and roughened. Although polished has less emissivity than roughened, roughened metals are still rather low when polished is very low.
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