Which of these 2 lights is doing a better job at removing heat from its circuitry?

Here's a thread I started about 1.5 years ago after doing a little test between 2 single 18650 lights of about the same size and with similar brightness specs. They are the Eagletac G25C2 and the Sunwayman T20CS. Both lights are roughly the same size and the both are advertised at around 770L(620-650 OTF). I put both of them on turbo, set each on its tail so the head is pointed to the ceiling, and let them run for a while. I noticed that the Eagletac G25C2 seemed to maintain a significantly lower temperature on the exterior of its head and body, whereas the Sunwayman T20CS seemed significantly hotter. It did get to a point to where the Eagletac leveled out and wouldnt get hotter and I am pretty sure the Sunwayman had reached its limit and wasnt getting hotter either.

At that time I assumed this meant that the Eagletac light was staying cooler because its thicker, more dense aluminum body was doing a better job of conducting its heat away from the core to the exterior and into the surrounding air. But a few people said the opposite was true. They stated that because the Sunwayman T20CS was getting significantly hotter, that meant that it was doing a better job, and the heat I was feeling was proof that it was pulling heat from its core outside to its exterior. They implied that the Eagletac was cooler because much of its internal heat was stuck inside the light, not being radiated outward to its body as efficiently as with the Sunwayman.

To me, both ideas had merit, and I couldnt decide which was correct and I dont have any way to reliably determine which is more accurate. Below is my original thread. I thought I'd ask a few people here to consider this conundrum and decide which light is likely doing better...........

http://www.candlepowerforums.com/vb...at-heat-retention-with-your-lights&highlight=

The cooler light is doing a better job (or is generating less heat to begin with). You want your light to heat up quickly, as that indicates a good thermal path, but at steady state, the cooler one is shedding more heat overall, which means radiating more efficiently to the outside. I'm finishing up an LED flashlight heat dissipation simulation that I'm writing in R. I'll present results as soon as it is ready for primetime.

There's no brightline rule that the cooler light is always doing a better job at managing heat. There are lots of factors involved:

1. Head construction - Does the light use an internal pill? And if so what is it made from? Some lights can get hot fast because they have better thermal transfer from the LED to the body. Take for example a light with a one-piece head and no pill: The head may heat up very fast because there's a direct thermal path from the outside of the head to the star. This is a much better heat transfer mechanism than a light with a separate pill. Also, some lights use brass pills, which don't transfer heat as well as aluminum. Also, does the light use a hollow pill where the star is attached only at the edges rather than sitting on a platform? Hollow pills are much worse at transferring heat.

2. Type of star - most lights use aluminum stars with a dielectric layer under the center of the LED. For superior heat transfer a light should use a copper star with direct metal under the central heatsink bondpad, such as Noctigon, Sinkpad, or the ones from Illumination Supply. Note that even high quality production lights do not tend to have direct copper stars. If you want one you may have to install it yourself (which means you will need a soldering iron and some solder paste)

3. Body construction - Does the light have a separate head or is the head and body in one piece? Heat transfers better when it's going through fewer joins. This is one reason why Zebralights are known for their excellent thermal management: The head and body is one piece and there's no internal pill. Heat is transferred all the way through the body tube instead of concentrating in the head. Another factor is what material the body is made from. Most high-powered LED lights are made from aluminum. Some are made from copper (better for heat transfer) or Titanium and Stainless Steel (worse for heat transfer). Even the finish may make a difference as a black colored light radiates heat better than a shiny silver colored one.

4. Mass - In general, more metal in the light means more mass to absorb heat, so the light will probably get hotter slower. This doesn't necessarily mean the more massive light has better thermal management however.

5. Surface area - More external surface area, especially around the head, means it's easier to radiate the heat away from the light. This is why many lights have heatsink fins around the head. Having great internal thermal management doesn't mean much if there's insufficient surface area to radiate the heat away from the light.

6. Thermal grease - All good quality lights should come with thermal grease under the star, but some budget manufacturers occasionally ship with insufficient or no grease. Also, the correct amount of thermal grease or compound should be used. Too much and it will hurt rather than help.

7. Current to the emitter - The more current a light draws the brighter it will probably be and the more heat it will produce. It's hard to compare the 2 lights the original poster mentioned without knowing if they are drawing the same current.

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So with so many different factors how do you test which light has better thermal management? Ultimately the only real way to test is to turn both lights on max and then check both their temperature and their lumen output over time. If a light maintains the same output even after running awhile then it probably has good thermal management. If another light finds its output diminishing then it could have poor thermal management.

I don't think anyone could really say which one is dissipating the best. I would say use a thermal temperature gun to measure the heat in different areas around the LED possibly with the lens removed.

If the ET warms up quicker initially and the SWM has a slower build to a higher temperature, I think it'd be safe to assume the ET is doing a better job. Is this what happens?

It's hard to say. The ET warming up more quickly could mean that it's pulling heat from the LED faster, or it could just mean that it has less aluminum in the head so it warms up faster. A thicker block of aluminum will take longer to warm up than a smaller one. Both could be pulling heat from the LED identically and the thiner walled ET would still get warmer faster.
Neither of those are showing which gets rid of heat to the surrounding air faster. A larger chunk of metal will hold heat longer than a smaller one too, so just seeing how long it takes to cool after turning it off won't be a good test either. Unless the heavier, thicker, head cools the same or faster of course as that would mean it's dumping it's heat faster... which would take more surface area. Larger head, cooling fins, or active cooling.