# Emissive heatsink coatings



## JohnR66 (May 26, 2010)

This is a recreation of an experiment someone performed here a while ago (sorry can't find the link). It is not always possible to convective cool the heatsink well to keep the power LEDs cool and happy. Emissive coatings on the heatsink can play an important roll in getting rid of heat. It is something that many people don't think about when heatsinking their power LEDs.

I attached two power resisters to two aluminum plates of the same size. One was a "control specimen" to check repeatability. I powered them up and let the temperature stabilize for a while and took readings. Next I painted one plate with black latex paint and retested.







Results: (Deg F)
Control: 1st run 136.6, 2nd run 135.5
bare plate 1st run 139.8, 2nd run (painted black) 132.6

Coating the heatsink resulted in 7.2 Deg cooler operation!

Since my heatsinks were vertical with perhaps better convective cooling, the difference was not as dramatic, however, coating with a better blackbody radiator still made quite a difference.

The difference in temp between the test and control pieces is probably due to thermal contact of the resistor to the plate. I used double sided tape (ATG). The large clamps are just to hold them up.


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## kingofwylietx (May 26, 2010)

Your experiment does bring up a few questions. 

How long did you allow for temps to stabilize?

Did you coat both sides of the aluminum? 

What did you use to measure the temperature? Hopefully not an IR thermometer...


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## SemiMan (May 26, 2010)

You painted your heat sink with an insulator .... which could quite likely have resulted in a lower temperature measurement as less heat was transferred into the probe.

Semiman


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## blasterman (May 26, 2010)

I see this discussion brought up from time, and still don't quite get the science around it. Well, I hear the science, and it still doesn't make sense. I've also been in over 20 corporate sever farms and have yet to note a coated heatsink on tens of millions of dollars of mission critical blade servers, routers, switches, etc.

IBM for instance has spent ghastly sums of money on thermal management research in data centers, but I don't see anything noting how painting heat sinks on a $750,000 iSeries improves thermal dissipation. Same for Cisco, Sun....where are these painted heatsinks? Computer forums are full of geeks that have painted their fans black only to watch their thermal readings climb and not fall.

Assuming for instance this does actually work (and I'm not saying it doesn't), wouldn't the 'black-body' effect work even better if you just dumped the heatsink and emitter in a bucket of black latex paint? Hell, why are we even bothering with a metal heatsink at all? Just paint the back of the emitter and see how well it works.


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## greg_in_canada (May 26, 2010)

SemiMan said:


> You painted your heat sink with an insulator .... which could quite likely have resulted in a lower temperature measurement as less heat was transferred into the probe.
> 
> Semiman



Agreed. You also need to measure the resistor before and after temperature to show that it is cooler, not just the heatsink.

Greg


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## saabluster (May 27, 2010)

blasterman said:


> I see this discussion brought up from time, and still don't quite get the science around it. Well, I hear the science, and it still doesn't make sense. I've also been in over 20 corporate sever farms and have yet to note a coated heatsink on tens of millions of dollars of mission critical blade servers, routers, switches, etc.
> 
> IBM for instance has spent ghastly sums of money on thermal management research in data centers, but I don't see anything noting how painting heat sinks on a $750,000 iSeries improves thermal dissipation. Same for Cisco, Sun....where are these painted heatsinks? Computer forums are full of geeks that have painted their fans black only to watch their thermal readings climb and not fall.
> 
> Assuming for instance this does actually work (and I'm not saying it doesn't), wouldn't the 'black-body' effect work even better if you just dumped the heatsink and emitter in a bucket of black latex paint? Hell, why are we even bothering with a metal heatsink at all? Just paint the back of the emitter and see how well it works.



Emissive coatings do indeed help with a heatsink that uses natural convection. The heatsinks designed into computers also come with fans to force air across them. In this case a coating can be a hindrance. If you are designing a heatsink as a radiator it should be designed far different than a normal pin or fin heatsink as those would tend to trap the radiant heat. There is merit in considering a more radiant based heatsink vs a convective heatsink for flashlights since there rarely is much in the way of forced air blowing over a flashlight.


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## saabluster (May 27, 2010)

JohnR66 said:


> This is a recreation of an experiment someone performed here a while ago (sorry can't find the link).



How about this one? I spent hundreds of hours reading his work and have committed it to memory.


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## carrot (May 27, 2010)

saabluster said:


> How about this one? I spent hundreds of hours reading his work and have committed it to memory.


That's the one I was thinking of as well. Thanks Saabluster.


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## LEDninja (May 27, 2010)

Panasonic claims they uses a similar technology in their light bulbs.
http://techon.nikkeibp.co.jp/english/NEWS_EN/20090911/175144/


Article on Panasonic light bulbs said:


> To enhance luminance efficiency, the company made improvements to the heat dissipation structure. Specifically, to reduce the heat resistance of the LED package and the chassis, it closely attached the substrate mounted with the package to the aluminum chassis and applied alumite treatment to the surface of the chassis. The alumite treatment improved the emissivity by four to five times, according to the company.
> 
> Many of the chassis of competitors' products are equipped with fins to enhance radiation performance, but Panasonic's LED light bulbs have a flat surface.
> 
> ...


I remember using an electrical cabinet supplier's software to calculate air conditioner requirements for some outdoor cabinets. We (including the suppliers rep) were surprised stainless steel required the most cooling. We were expecting the stainless to reflect the most sun. The manufacturer's tech support pointed out the shiny stainless has the lowest emissive qualities and white painted boxes can get rid of more heat by radiation. NEMA 4X (IPX8) air conditioners are expensive.
Note Panasonic was able to get 4X emissivity without painting the heatsink black. See pictures in the article.


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## AnAppleSnail (May 27, 2010)

It makes sense when you consider the airflow around fins in dead air. T(air) - T(fin) will be nearly zero because without airflow the air will heat up and sit. The coatings allow radiation of heat, which I think is based on the temperature of the whole 'outside' - flashlights are unlikely to affect that! Note that my recall of thermodynamics may be spotty.


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## JohnR66 (May 27, 2010)

Let clear a few things here...

Blasterman: A high surface area heatsink (fins) with forced air over the surface is vastly superior for cooling. This is a simple science experiment. Let's not insinuate, okay?

Latex paint (even white) is a good known blackbody radiator. It is all I had to coat with. It is not an ideal heat sink coating. Anodizing is also good. If the paint were insulating, the heatsink would get warmer and warmer until emission would balance the heat gain.

I coated both sides of the aluminum except were the resistor is mounted. Waited 5-10 minutes until temps stabilized. Did not use IR thermometer since these are dependent on the coating!

Wish I had a high vacuum chamber that would certainly show the effects of IR emission cooling.

Point is if I were to use aluminum bar as a heatsink for my power LEDs where convective cooling was limited, I would absolutely use a high emission coating.


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## purduephotog (May 27, 2010)

JohnR66 said:


> Let clear a few things here...
> 
> Blasterman: A high surface area heatsink (fins) with forced air over the surface is vastly superior for cooling. This is a simple science experiment. Let's not insinuate, okay?
> 
> ...



'Tis a good experiment.

One thing I've learned is that you have to insulate your emitter, too, so that the heat path is well known.

We use black epoxy to attach thermal probes- not red, not white, not grey, but black. It matters.

So a well insulated heat source with only one path out- the back of the coated plate- in passive radiation conditions should show the results you've seen.

Good work. We tested this once for Chem Engineering- pretty elaborate using mirrored vacuum setups- but the same results.


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## fyrstormer (May 27, 2010)

saabluster said:


> Emissive coatings do indeed help with a heatsink that uses natural convection. The heatsinks designed into computers also come with fans to force air across them. In this case a coating can be a hindrance. If you are designing a heatsink as a radiator it should be designed far different than a normal pin or fin heatsink as those would tend to trap the radiant heat. There is merit in considering a more radiant based heatsink vs a convective heatsink for flashlights since there rarely is much in the way of forced air blowing over a flashlight.


And yet, the brightness and color with which a black-body "glows" depends on its temperature, not on its original color. If this is true in the visible spectrum, I see no reason why it wouldn't also be true in the infra-red spectrum.

What are some "good" emissive coatings, and what are the properties that make them work?


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## JohnR66 (May 27, 2010)

saabluster said:


> How about this one? I spent hundreds of hours reading his work and have committed it to memory.



That's the one! His results were amazing. Hard to believe the paint made that much difference. My results were not nearly this drastic probably since I have a larger surface area.

I'd encourage everyone to look at that thread.


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## saabluster (May 27, 2010)

fyrstormer said:


> And yet, the brightness and color with which a black-body "glows" depends on its temperature, not on its original color. If this is true in the visible spectrum, I see no reason why it wouldn't also be true in the infra-red spectrum.
> 
> What are some "good" emissive coatings, and what are the properties that make them work?


Well there is this although it is not sold as an emissive coating. There was some product I was looking for at one time I think called 3M Black Velvet but I never could track any down. Without going and searching for it I do also remember reading that there are also white emissive coatings that match the performance of black so it is not really the color that is key. Although I understand some of what impacts a surface to radiate I don't know the mechanism specifically that makes some so superior to others.


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## blasterman (May 27, 2010)

> Blasterman: A high surface area heatsink (fins) with forced air over the surface is vastly superior for cooling. This is a simple science experiment. Let's not insinuate, okay?


 
What I'm 'insinuating' is that big technological companies have spent hundreds of millions of dollars trying to milk a few percents better thermal performance from equipment far more sensitive to heat than anything we screw with here. Not like the LED lighting industry is having issues with heat or anything either and the engineering departments at Phillips, GE, and the entire manufacturing sector of China would love to know about this. Where are the the references to black paint in the passive thermal guides from Bridgelux, Cree, etc. that I've read through?

Also, everthing I've read on the topic from a pure science stanpoint indicates that black-body radiators only have better performance inside a purely radiative evironment - aka, vacuum. While in the presence of air or liquid in a gravity well, convection is the main source of thermal xfer. This can be clearly demonstrated by running a passive heat-sink upside down vs vertically and noting the rather obvious differential in temp.

The term 'black body radiator' also *doesn't mean* 'painted with black paint'. If refers to an object that is physically a different color than an objects that is shiny. Most paints are polymers, and polymers are *insulators*. Black paint is a polymer that has a tiny bit of black pigment or carbon in it. Black latex paint is not the same stuff in the Looney tunes cartoons that they paint on walls to make a pefect black hole they walk through. However, this tends to be the assumption with these threads. 

So, in order for this to work with paint on a heatsink in your living room the coating could only be a few molecules thick, and ideally on a simple heatsink such as a simple cube or sphere that has little convective performance. 

Otherwise, the only real experiment I've ever seen that confirmed the black body effect was two spheres in a vacuum, and the black one had some high tech surface treatment that made it near black and didn't use black colored paint.


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## AnAppleSnail (May 27, 2010)

Not looking to start or continue pointed words, but there are two ways to reduce conductive cooling - wrap a heatsink with hot unmoving air, or put it in vacuum (clearly more effective). Which situation will we encounter? Vacuum, or a light wrapped in unmoving air, perhaps in a gloved hand? It seems like the next logical step is to wrap each of these in a tight-fitting sock and compare the temperatures.


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## Kestrel (May 27, 2010)

JohnR66 said:


> I'd encourage everyone to look at that thread.


:bow: NewBie


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## bshanahan14rulz (May 27, 2010)

Great experiment!

Here's some more ideas if you need them.

*same resistor, just one plate, but half of the plate dipped.

*Anodized but not necessarily sealed, instead of painted for the black


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## fyrstormer (May 27, 2010)

blasterman said:


> What I'm 'insinuating' is that big technological companies have spent hundreds of millions of dollars trying to milk a few percents better thermal performance from equipment far more sensitive to heat than anything we screw with here. Not like the LED lighting industry is having issues with heat or anything either and the engineering departments at Phillips, GE, and the entire manufacturing sector of China would love to know about this. Where are the the references to black paint in the passive thermal guides from Bridgelux, Cree, etc. that I've read through?
> 
> Also, everthing I've read on the topic from a pure science stanpoint indicates that black-body radiators only have better performance inside a purely radiative evironment - aka, vacuum. While in the presence of air or liquid in a gravity well, convection is the main source of thermal xfer. This can be clearly demonstrated by running a passive heat-sink upside down vs vertically and noting the rather obvious differential in temp.
> 
> ...


Not quite. "Black body radiator" means a solid object that doesn't emit radiation unless heated. A "black body radiator" can be any color with any degree of internal conductivity and surface reflectivity, though for experimental purposes the "black body radiators" they use are actually black, so as to minimize the amount of _reflected_ radiation that would skew emission measurements. Anyone who's ever had the unfortunate experience of touching a glowing-hot piece of metal that was bathed in bright light, thus obscuring its emissive radiation, knows how reflected radiation can screw up such measurements.

You yourself are a (non-ideal) black body radiator, hence why you are visible on a far-infrared camera even in complete darkness. You're just not hot enough to emit radiation in the _visible_ spectrum, and you reflect most of the external radiation impacting you, which I'm sure we all agree are good things.

So anyway, I guess there really are coatings that can cause the surfaces of otherwise highly-conductive objects to glow brighter in the infrared spectrum than they otherwise would given their surface temperature, but the results posted here also suggest that cooling through conduction and convection far outstrip the capacity of everyday flashlight materials to cool themselves through radiation -- at least at temperatures low enough to avoid burning the user. Now, as for avoiding the _absorption_ of heat from the outside environment, the surface treatment is of paramount importance, as anyone who's ever touched something with a black or polished metal surface well knows.


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## bshanahan14rulz (May 27, 2010)

fyrstormer said:


> Now, as for avoiding the _absorption_ of heat from the outside environment, the surface treatment is of paramount importance, as anyone who's ever touched something with a black or polished metal surface well knows.



+1! I just about burned my hand the other day on my flashlights! I gotta remember not to leave them out. I was almost positive that the second I picked it up it was gonna' explode.


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## Bullzeyebill (May 27, 2010)

Kestrel said:


> :bow: NewBie



+1. I miss him too.

Bill


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## JohnR66 (May 31, 2010)

I just found these interesting videos on the subject.
pt1
http://www.youtube.com/user/infraredtraining#p/u/3/QHszoA5Cy1I
pt2
http://www.youtube.com/user/infraredtraining#p/u/3/QHszoA5Cy1I


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## tay (May 31, 2010)

Maybe I am nitpicking, but I noticed some errors in the linked thread in regards to fahrenheit/celsius conversion. 

Fahrenheit scale is 1.8 times Celsius scale plus 32.

change in temperature between two objects in fahrenheit scale is 1.8 times change in temperature Celsius scale. 32 is not added, because it is a differential measurement. 

20 degrees C is 68 degrees F. 

But if one object is hotter than the other by 20 degrees C, it is 36 degrees hotter in F. 

Otherwise, very informative thread by the misnomer Newbie, who seems to be anything but.


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## LukeA (Jun 1, 2010)

tay said:


> Maybe I am nitpicking, but I noticed some errors in the linked thread in regards to fahrenheit/celsius conversion.
> 
> Fahrenheit scale is 1.8 times Celsius scale plus 32.
> 
> ...



https://www.candlepowerforums.com/posts/1715793&postcount=22


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## tay (Jun 1, 2010)

LukeA said:


> https://www.candlepowerforums.com/posts/1715793&postcount=22



Ah, thanks. Glad I'm not the only CPFer who gets bothered by those things.


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## ICUDoc (Jun 1, 2010)

Bullzeyebill said:


> +1. I miss him too.
> 
> Bill



Yup. Pm me if you can tell me what happened to him?


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## znomit (Jun 1, 2010)

ICUDoc said:


> Yup. Pm me if you can tell me what happened to him?



+1 :sigh:


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## JCD (Jun 1, 2010)

saabluster said:


> How about this one? I spent hundreds of hours reading his work and have committed it to memory.



Thanks for the link. There were some factors in the experiment not taken into consideration, such as possible differences in surface topography after one of the samples was painted, which could (and likely would) affect convection based heat transfer. Without performing the experiment in a vacuum to eliminate heat transfer via convection, we can't draw logical conclusions from the results.


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## LukeA (Jun 1, 2010)

JCD said:


> Thanks for the link. There were some factors in the experiment not taken into consideration, such as possible differences in surface topography after one of the samples was painted, which could (and likely would) affect convection based heat transfer. Without performing the experiment in a vacuum to eliminate heat transfer via convection, we can't draw logical conclusions from the results.



We can draw the conclusion that, given sufficiently equal substrates in air (and I think we can agree that that is true for this experiment), a black-painted sample will be cooler than one that is uncoated. For casual flashlight hobbyists, I think that conclusion, the one we can draw, is by far the most important one and I think it's debatable whether or not any further conclusions are truly necessary.


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## arek98 (Jun 1, 2010)

This is pretty good paint for heatsinks. I bought it at local Pepboys store. Some time ago I did under cabinet lights for my kitchen and used it to paint bottom side of flat Al bars I used for heatsinks (I left side facing cabinets bare).
It's engine paint, so designed to be used on Al. It makes thin, matte and pretty tough coat.


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## JCD (Jun 1, 2010)

LukeA said:


> We can draw the conclusion that, given sufficiently equal substrates in air (and I think we can agree that that is true for this experiment), a black-painted sample will be cooler than one that is uncoated. For casual flashlight hobbyists, I think that conclusion, the one we can draw, is by far the most important one and I think it's debatable whether or not any further conclusions are truly necessary.



I don't believe we can draw that conclusion unless we _know_ that the objects have identical boundary layers. The experiment, as presented, offers no indication that the boundary layers are (or are not) identical.


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## Yoda4561 (Jun 1, 2010)

Were this the subject of a technical paper or engineering an expensive corporate product, I'd agree. However that's overthinking things at this level. 

We're at "hot pan in oven burns hands, gloves make hands not burn, answer: wear gloves when grabbing pans in hot ovens" right now. I've seen amateur studies with air cooling and black coated heatsinks, basically as soon as you throw any signifigant amount of airflow at the problem minor variations in surface finish and color become non-factors. For passively cooled applications like diy home lighting the quick and easy improvement isn't something that should be overlooked due to lack of a research lab to confirm your results


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## mudman cj (Jun 1, 2010)

JCD said:


> I don't believe we can draw that conclusion unless we _know_ that the objects have identical boundary layers. The experiment, as presented, offers no indication that the boundary layers are (or are not) identical.



I understand your point about boundary layers and have a modification to the experiment to present for discussion. What if one were to add reflective paint to the previously un-coated specimen? If the paints were made from the same polymers and applied to the same thickness, then the emissivity should be the primary difference left - unless I am missing something. :thinking:


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## JCD (Jun 1, 2010)

mudman cj said:


> What if one were to add reflective paint to the previously un-coated specimen? If the paints were made from the same polymers and applied to the same thickness, then the emissivity should be the primary difference left - unless I am missing something. :thinking:



At a microscopic level, the surface topographies of painted objects might still be different enough to affect their relative rates of convection. It also might be difficult to ensure identical thicknesses. Also, the emissivity of the paint might be independent of its color or how shiny its finish is.

I think the simplest solution would be to carry out the experiment in a vacuum chamber, eliminating any possibility of convection affecting the results. I would also suggest leaving the bottom side of the objects with identical finishes to ensure identical conduction with the surface upon which they rest.

In the end, our flashlights are typically held in the hand, often while walking, offering heat transfer via convection and/or conduction, so improving radiative heat transfer seems much less important than with stationary lighting.

The original experiment was a great idea. However, it is the nature of science to refine such experiments in order to eliminate undesired factors the can affect results so that we can have more confidence in our conclusions.


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## saabluster (Jun 1, 2010)

JCD said:


> At a microscopic level, the surface topographies of painted objects might still be different enough to affect their relative rates of convection. It also might be difficult to ensure identical thicknesses. Also, the emissivity of the paint might be independent of its color or how shiny its finish is.
> 
> I think the simplest solution would be to carry out the experiment in a vacuum chamber, eliminating any possibility of convection affecting the results. I would also suggest leaving the bottom side of the objects with identical finishes to ensure identical conduction with the surface upon which they rest.
> 
> ...


Do you use your flashlights in a vacuum? The test he presented is perfect for its intent. There are no significant or relevant improvements to be made to it.


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## Chauncey Gardner (Jun 1, 2010)

tay said:


> Maybe I am nitpicking, but I noticed some errors in the linked thread in regards to fahrenheit/celsius conversion.
> 
> Fahrenheit scale is 1.8 times Celsius scale plus 32.
> 
> ...


 

It was a good thread. Subscribed for achival.

A copper light painted with an emissive coating would seem the most efficient body if you really wanted to squeeze every possible bit of performance out of a host.

Like a hotrodded Maratac aaa Cu...


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## JCD (Jun 1, 2010)

saabluster said:


> Do you use your flashlights in a vacuum? The test he presented is perfect for its intent. There are no significant or relevant improvements to be made to it.



Obviously I don't use my light in a vacuum. The reason we do experiments in the first place is to be able to tightly control environmental conditions in order to isolate the specific factors we wish to observe.

If we want to draw any conclusions about improved _radiative_ heat transfer due to the the coating, we have to eliminate the possibility of different levels of _convective_ and _conductive_ heat transfer between the samples. The experiment, as performed, does not do that.


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## LukeA (Jun 2, 2010)

JCD said:


> ...The experiment, as performed, does not do that.



And it never sought to. The purpose, as stated by NewBie, is as follows:


NewBie said:


> It has been stated that the various coatings on flashlights have no effect on the temperature of the flashlights.
> ...
> The reason, is contrary to one of my old posts, where I publically calculated the difference, and how much more cooling a high emissivity coating has as compared to a low emissivity polished surface- some members still claimed I was wrong.
> 
> To do a real world test, in order to show the reality of things, I matched up two blue LEDs...



The purpose of the experiment was to demonstrate that a painted piece of aluminum with an LED attached, representing a flashlight, would reach a lower equilibrium temperature under identical, low-airflow conditions simulating the typical use of a flashlight than would an uncoated but otherwise identical sample with a sufficiently similar LED. The experiment did not find or seek to find the individual magnitudes of the conductive, convective, and radiative heat transfer rates, valid scientific pursuits though those endeavors may be.


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## JCD (Jun 2, 2010)

LukeA said:


> And it never sought to. The purpose, as stated by NewBie, is as follows:
> 
> The purpose of the experiment was to demonstrate that a painted piece of aluminum with an LED attached, representing a flashlight, would reach a lower equilibrium temperature under identical, low-airflow conditions simulating the typical use of a flashlight than would an uncoated but otherwise identical sample with a sufficiently similar LED. The experiment did not find or seek to find the individual magnitudes of the conductive, convective, and radiative heat transfer rates, valid scientific pursuits though those endeavors may be.



Fair enough. But, the fact remains, we cannot draw any conclusions about the effectiveness of emissive coatings on flashlights from the experiment, except the specific application of the specific coating Newbie applied for his experiment, but we don't have any information about why he saw the results he did.


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## JohnR66 (Jun 2, 2010)

Experiment 2: In this test, I took an aluminum bar and mounted two XR-E P4 LEDs. Bar was laid flat on a plywood board (horizontal). LEDs driven at 1 amp. Temperature was taken with bar unpainted after about 1/2 hour to stabilize. Next, bar was panted and test rerun. I masked off the LEDs and location where the temp probe was.

Results @ 77 Deg F ambient
Bare 123 Deg F
Painted 118 Deg F

Conclusion: The painted aluminum bar was cooler, but not by as much as I expected. Why? This is due to the wooden board. Cooling by radiation relies on a cooler reservoir (delta T) to radiate into. The insulating board prevents this, so nearly 1/2" the surface is rendered less effective at losing heat. I chose this layout because it is often how I see power LEDs used here on CPF in fixed lighting DIY projects - LEDs mounted to an aluminum bar that is mounted against something (usually wood) often in a horizontal position where convective cooling is inefficient.

I have seen enough evidence that I would suggest coating heatsinks with poor convective cooling with a high emissive coating (such as anodizing or painting) to help reduce the temperature. I would have used white spray paint since data seems to indicate that it is just as emissive as black. I only had this brown shade available.


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## LukeA (Jun 2, 2010)

JCD said:


> Fair enough. But, the fact remains, we cannot draw any conclusions about the effectiveness of emissive coatings on flashlights from the experiment, except the specific application of the specific coating Newbie applied for his experiment, but we don't have any information about why he saw the results he did.



And the fact remains that no one else who's replied to this thread besides you cares about the means to this end beyond "in still air and all else equal, paint = cooler."


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## carrot (Jun 2, 2010)

JCD said:


> Fair enough. But, the fact remains, we cannot draw any conclusions about the effectiveness of emissive coatings on flashlights from the experiment, except the specific application of the specific coating Newbie applied for his experiment, but we don't have any information about why he saw the results he did.


Just curious, what are you trying to prove?


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## JCD (Jun 2, 2010)

LukeA said:


> And the fact remains that no one else who's replied to this thread besides you cares about the means to this end beyond "in still air and all else equal, paint = cooler."



Yet, most people don't typically use their flashlights in still air, without convective and/or conductive heat transfer, so we really don't have any more real world information than we started with.

All we have is evidence to suggest that "emissive coatings enhance cooling" is a reasonable hypothesis worthy of further testing. No more, no less.


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## JCD (Jun 2, 2010)

carrot said:


> Just curious, what are you trying to prove?



???

I'm not trying to prove anything. I'm merely pointing out that the experiment presented doesn't prove what some think it proves.

Any good scientific experiment includes critical analysis that includes questioning what sort of different things could have caused the results seen. Then, additional experiments are designed to eliminate those possibilities.

There is nothing wrong with the experiment that Newbie performed. However, it is only a start in determining whether emissive coatings are effective or not. It shows that it is possible that they are effective, but it doesn't show that they are, only that it is a hypothesis worthy of further testing.

Unfortunately, in today's society, use of the scientific method to obtain logical, scientifically valid information tends to meet vehement opposition, as been demonstrated in this thread.


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## Colt357TW (Jun 2, 2010)

fyrstormer said:


> Not quite. "Black body radiator" means a solid object that doesn't emit radiation unless heated. A "black body radiator" can be any color with any degree of internal conductivity and surface reflectivity, though for experimental purposes the "black body radiators" they use are actually black, so as to minimize the amount of _reflected_ radiation that would skew emission measurements. Anyone who's ever had the unfortunate experience of touching a glowing-hot piece of metal that was bathed in bright light, thus obscuring its emissive radiation, knows how reflected radiation can screw up such measurements.
> 
> You yourself are a (non-ideal) black body radiator, hence why you are visible on a far-infrared camera even in complete darkness. You're just not hot enough to emit radiation in the _visible_ spectrum, and you reflect most of the external radiation impacting you, which I'm sure we all agree are good things.
> 
> So anyway, I guess there really are coatings that can cause the surfaces of otherwise highly-conductive objects to glow brighter in the infrared spectrum than they otherwise would given their surface temperature, but the results posted here also suggest that cooling through conduction and convection far outstrip the capacity of everyday flashlight materials to cool themselves through radiation -- at least at temperatures low enough to avoid burning the user. Now, as for avoiding the _absorption_ of heat from the outside environment, the surface treatment is of paramount importance, as anyone who's ever touched something with a black or polished metal surface well knows.



Great experiment. Are you going to try a coating of hBN soon?


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## saabluster (Jun 3, 2010)

Colt357TW said:


> Great experiment. Are you going to try a coating of hBN soon?


I have some hbn. Does it improve the emissive characteristics of the surface?


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## hank (Oct 4, 2010)

Here's one I'd like to see tried, since most LEDs (Rebel, I just looked) are specced at 25C/77F.

Found just with Google; this is "preliminary performance data as described in Photonics Spectra and Space Engineering

DURACON HIGH EMISSIVITY BLACK COATING 
www.aboutmtc.com/DURACON4_MTC.pdf

emissivity greater than 0.95 ... over the 2-20μm band (IR)
excellent adhesion ...
withstood hypersonic ... up to Mach 16
stable up to ... 1000°F (>500°C)
excellent thermal diffusivity
tailorable electrical conductivity
... spin-coat, dip-coat, spray or paint ...


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## hank (Oct 4, 2010)

Huge(!) table here of various materials (coated metal, paint, etc) with very high emissivity numbers. 
http://www.energystar.gov/index.cfm?fuseaction=find_a_product.showProductGroup&pgw_code=RO
Most are white (reflect visible light). Some come in designer colors. Y
It's roofing, to throw off incident heat, but ought to work the same for heat from inside the surface. Some white painted/coated metal emits very well in the infrared.


EDIT -- and here's the page others have talked about somewhere at CPF, with far more info:
http://www.molalla.net/members/leeper/coatbar.htm

Short answer: anodized, aluminum has decent emissivity, from that site:
... 
Aluminum, highly polished 0.039
...
Aluminum, sand blasted, 0.21
Aluminum, anodized 0.776
Aluminum, Hard Anodize, Type III 0.835 to 0.856 <---***


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## MikeAusC (Oct 7, 2010)

blasterman said:


> I see this discussion brought up from time, and still don't quite get the science around it. Well, I hear the science, and it still doesn't make sense. I've also been in over 20 corporate sever farms and have yet to note a coated heatsink on tens of millions of dollars of mission critical blade servers, routers, switches, etc..


 
Unless things have changed in recent years, equipment in Server Farms uses moving air to cool equipment.

The convection cooling will swamp any increased cooling by improving heat radiation from heatsinks.

You don't have to get the science - it's easy enough to verify in experiments at home.


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## MikeAusC (Oct 7, 2010)

LEDninja said:


> I remember using an electrical cabinet supplier's software to calculate air conditioner requirements for some outdoor cabinets. We (including the suppliers rep) were surprised stainless steel required the most cooling. We were expecting the stainless to reflect the most sun. .


 
Stainless steel is also a woeful conductor of heat, so it will discourage cooling through the walls.

In rural Australia, equipment shelters usually have a freestanding wall on the Northern side to stop radiant heating on the cabinat, while allowing air cooling.


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## HarryN (Oct 7, 2010)

JCD said:


> ???
> 
> I'm not trying to prove anything. I'm merely pointing out that the experiment presented doesn't prove what some think it proves.
> 
> ...



Hi, I appreciate your interest in purist based scientific method and its application. As you might guess, many of the people that post are engineers, so the tendency is to use a combination of
- textbook sourced information
- what we learned in college
- what we experienced in our working lives
- and simple experiments

to draw basic conclusions.

I don't claim to have a sufficient background to design a seriously rigourous experiment which would stand up to a full scientific panel of experts, but I don't have any reason to doubt that the well tested science of emissive coatings and IR spectroscopy are all that far from the truth.

Perhaps you would be so kind as to offer up and conduct a more scientific method based experiment than what is posted here ? I assume that you are a professor somewhere, so you can have your students do the work for you.


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## MikeAusC (Oct 8, 2010)

JCD said:


> Unfortunately, in today's society, use of the scientific method to obtain logical, scientifically valid information tends to meet vehement opposition, as been demonstrated in this thread.


 
He did his best to ensure only one variable was changed.

Does he have to do a double-blind study with a statistically significant sample size to find things that make a practical difference ???


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## SemiMan (Oct 8, 2010)

JCD

A "real" scientist would be able to estimate the maximum amount of conduction and convection based on some realistic assumptions for the experiment in order to isolate the convective variable.

Of course a "good" engineer does something similar by looking at it and realizing that those two factors are not the significant variables in the experiment and are held somewhat constant hence convection is what is being changed most.

Semiman


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## MikeAusC (Oct 8, 2010)

To a Hammer, everthing looks like a Nail !

It helps to use the right tool for the right problem.


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## sigmo (Oct 22, 2010)

*Interesting thread.*



MikeAusC said:


> Unless things have changed in recent years, equipment in Server Farms uses moving air to cool equipment.
> 
> The convection cooling will swamp any increased cooling by improving heat radiation from heatsinks.
> 
> You don't have to get the science - it's easy enough to verify in experiments at home.



This is absolutely correct. And this is a key thing.

In any _enclosed_ electronic device, the thermal emissivity of the heatsinks within the enclosure is irrelevant because any heat "emitted" by said heatsinks will simply be absorbed by other items inside the enclosure, or perhaps by the enclosure itself. So you don't gain much by radiating heat away from one component only to have it absorbed by another and still be trapped inside the enclosure. I suppose you could argue that the heat radiated from the heat sink to the inner surface of the enclosure would then find an easy path out, but we're really on the edge of practicality with that because you can't know what lies on the other side of the enclosure, etc.

So forced air is used if a lot of heat needs to actually be removed from the equipment enclosure, and the heatsinks inside the gear are often simply bare, shiny aluminum (with its abysmal thermal emissivity). Sometimes you also see heat sinks mounted on the outsides of enclosures to assure that the heat is removed from the enclosure, but there are problems with this approach, too (mostly ruggedness issues).

The only time a heatsink benefits from higher emissivity is when you don't mind the fact that surrounding objects will be heated by that emitted IR energy. Usually, that's not the case for an enclosed system.

We designed the heat sinking for a number of very high power linear power supplies years ago using data from the heat sink manufacturer. I don't have that data at hand anymore, but even in an "infinite" space, the difference in thermal performance between a coated heatsink and a bare one was less than ten percent. And that was with nothing but natural convection, and, again, assumed that you did not mind that heat being absorbed by other objects in the area (thus the "infinite" space surrounding the heatsink under test - hardly a practical situation).

As soon as you started to use any forced air, the effects of the thermal emissivity became even less of a factor to the point that heatsinks with forced airflow really didn't benefit much at all from higher emissivity coatings.

BUT. The fact remains that many commercially made heat sinks came (and still come) coated. Often, they're anodized with a nice hard anodizing. That makes them "prettier", and acts as an electrical insulator which can be valuable. And it does increase the emissivity, too.

If you've ever drilled a good quality anodized heat sink, you may have noticed that little puff of "smoke" from the tip of the drill bit just as it begins to plunge into the sink. That's the anodizing being ground away by the drill tip. It's some very hard stuff, indeed! (Aluminum oxide - you know, sapphire  ). If you've drilled a lot of anodized heatsinks in a production environment, you may have also noticed that your drill bits don't last as long as you would have expected. Again, drilling through aluminum oxide takes its toll. So this is, actually, a consideration for manufacturers working with lots of heat sinks. If the higher emissivity is of no value, then non-anodized sinks are often preferred simply because they drill easier. Of course, other (softer) coatings change that calculation.

Anyhow, for a flashlight's body, it still makes some sense to me to have it anodized or coated with a proper high-emissivity - low thermal resistance coating for optimum performance. It may make little difference in some situations, but it'll help a bit in situations where there is low airflow and you don't mind radiated heat flowing into nearby objects or out into space.

But you still need to choose your coating wisely. A very small increase in thermal resistance can easily swamp any possible improvement you might have gained by the increased emissivity.

And, some extra surface area in the flashlight barrel would almost always yield greater benefit than altering the emissivity.

But, again, all things being equal, if you can increase the emissivity without adding thermal resistance, then go for it. Besides, a finished flashlight looks better than a bare aluminum one anyhow.


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