NewBie
*Retired*
For some time now, there has been some mis-information that folks here on CPF have been mislead on.
It has been stated that the various coatings on flashlights have no effect on the temperature of the flashlights.
So, I ordered some 0.750" diameter AL 6063 rod stock to prepare an actual test to demonstrate the difference.
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 that are matched to within 0.04V of each other.
I took the rod, and cut two pieces, each to 1.735" long.
I bored holes into the side of each piece, 0.375" deep, for K-type 42 guage thermocouples.
Both pieces were polished up a little, but not nearly anywhere near a perfect mirror finish.
One was coated with black paint, the other was left bare.
The contenders:
Each were handled to get fingerprints and some finger print oil on the surface, much like a flashlight in the real world. If you look at the polished one, please don't lift my fingerprints!
The LEDs were wired in series, each is getting 1000mA on the nose.
The bored holes were filled with thermal paste, to get a more accurate reading.
The LED on the Black one is 40mV higher than the other, so the temperature would technically be slightly higher, if everything was equal- due to the higher power in the LED.
Both LEDs were directly mounted with Artic Alumina thermally conductive epoxy, cured under pressure, over night.
After 20 minutes, when things stabilized, the temperatures for the bare one:
And then I transferred the same thermocouple and measured and photographed the black one:
As you can see, the temperatures are obviously 17.4 degrees C cooler for the black one. Which also works out to a 63.32 degrees F difference.
No surprises here.
.
I then set each down on the surface, to help reduce the added cooling of chimney effect, let things cool and repeated the same test.
First the polished one:
Next the black one:
As you can obviously see, there is a 20C degree difference between the two now. This works out to a 68 degree F difference.
After 1 hour, the temperatures still read the same, so they had thermally stabilized.
Ambient temperatures in both cases remained at 20.6C.
.
So, in short, using a coating with a higher emissivity has a pretty drastic effect, reducing temperatures by 65 degrees Fahrenheit in the example shown.
This reduction of temperature would cause the die to run cooler, making it more efficient, and producing more light out of the flashlight.
If I had fully polished the bare Aluminum to a mirror finish, the bare one would have performed even worse in comparision.
If the surface area was greater (larger, such as is more common flashlights) than what is found on this 0.750" diameter 1.735" long rod , the temperature differential would be even greater, since the semi-polished aluminum has a very low thermal emissivity ~5%, as compared to the ~95% emissivity of black paint, and emissivity depends heavily on surface area.
Regular anodized Aluminum has an emissivity of ~77.6%
Hard Anodize, Type III, has an emissivity of 83.5% to 85.6% depending on the color, for typical specimens, and this will vary a bit for the quality and thickness of the coating.
Keep in mind, that if the light was tiny, like the Fenix P1, or smaller yet like the CR2 Ion, the cooling provided by a hand would be dominant, but it would still heat up much the same when set down on a surface.
The effects in a Flashlight would be even worse, since the whole flashlight is not a solid core of aluminum, and there are thermal resistances in the path, which would cause the area near the LED, and especially the LED die to run much hotter.
If the flashlight had heavy deep knurling, the surface area would also be greater, which would benefit the black painted one even further than the polished one. I've seen the effect in a thermal camera many times before.
I hope you had as much fun reading over this as I did doing the demonstration.
.
It has been stated that the various coatings on flashlights have no effect on the temperature of the flashlights.
So, I ordered some 0.750" diameter AL 6063 rod stock to prepare an actual test to demonstrate the difference.
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 that are matched to within 0.04V of each other.
I took the rod, and cut two pieces, each to 1.735" long.
I bored holes into the side of each piece, 0.375" deep, for K-type 42 guage thermocouples.
Both pieces were polished up a little, but not nearly anywhere near a perfect mirror finish.
One was coated with black paint, the other was left bare.
The contenders:
Each were handled to get fingerprints and some finger print oil on the surface, much like a flashlight in the real world. If you look at the polished one, please don't lift my fingerprints!
The LEDs were wired in series, each is getting 1000mA on the nose.
The bored holes were filled with thermal paste, to get a more accurate reading.
The LED on the Black one is 40mV higher than the other, so the temperature would technically be slightly higher, if everything was equal- due to the higher power in the LED.
Both LEDs were directly mounted with Artic Alumina thermally conductive epoxy, cured under pressure, over night.
After 20 minutes, when things stabilized, the temperatures for the bare one:
And then I transferred the same thermocouple and measured and photographed the black one:
As you can see, the temperatures are obviously 17.4 degrees C cooler for the black one. Which also works out to a 63.32 degrees F difference.
No surprises here.
.
I then set each down on the surface, to help reduce the added cooling of chimney effect, let things cool and repeated the same test.
First the polished one:
Next the black one:
As you can obviously see, there is a 20C degree difference between the two now. This works out to a 68 degree F difference.
After 1 hour, the temperatures still read the same, so they had thermally stabilized.
Ambient temperatures in both cases remained at 20.6C.
.
So, in short, using a coating with a higher emissivity has a pretty drastic effect, reducing temperatures by 65 degrees Fahrenheit in the example shown.
This reduction of temperature would cause the die to run cooler, making it more efficient, and producing more light out of the flashlight.
If I had fully polished the bare Aluminum to a mirror finish, the bare one would have performed even worse in comparision.
If the surface area was greater (larger, such as is more common flashlights) than what is found on this 0.750" diameter 1.735" long rod , the temperature differential would be even greater, since the semi-polished aluminum has a very low thermal emissivity ~5%, as compared to the ~95% emissivity of black paint, and emissivity depends heavily on surface area.
Regular anodized Aluminum has an emissivity of ~77.6%
Hard Anodize, Type III, has an emissivity of 83.5% to 85.6% depending on the color, for typical specimens, and this will vary a bit for the quality and thickness of the coating.
Keep in mind, that if the light was tiny, like the Fenix P1, or smaller yet like the CR2 Ion, the cooling provided by a hand would be dominant, but it would still heat up much the same when set down on a surface.
The effects in a Flashlight would be even worse, since the whole flashlight is not a solid core of aluminum, and there are thermal resistances in the path, which would cause the area near the LED, and especially the LED die to run much hotter.
If the flashlight had heavy deep knurling, the surface area would also be greater, which would benefit the black painted one even further than the polished one. I've seen the effect in a thermal camera many times before.
I hope you had as much fun reading over this as I did doing the demonstration.
.
Last edited:
Should be good for some extra free lumens no? 
