When you use a LED flashlight, you will feel it getting hot on the shell, especially the Aluminum flashlight. Is it normal?
Actually, the shell is hot means your flashlight may has a good heat dissipation. If it is not hot on its shell, it is smoldering, so that the LED is easily damaged.
When LED lamp is lighting, it converts electrical energy into light energy. During the conversion process, most of the electrical energy is converted into heat energy, and only a small part is converted into light energy. Therefore, you will feel the shell of the flashlight is hot when it is working, and the heat comes from high-power LEDs. If you feel the heat quickly, that means the internal components of the flashlight have a high thermal conductivity, and the heat is spread quickly and dissipated very well. This is a normal heat dissipation phenomenon.
Conversely, if you don’t feel the heat in the shell of your flashlight, it means that the heat is accumulated inside the LED, which is dangerous to the LED. The LED will deplete too quickly due to the high temperature, resulting in a sharp decrease in life. After using a flashlight with poor heat dissipation for a period of time, the core part of the LED will turn yellow and the brightness will drop to about 50%. This is because the temperature causes the LED to age too fast.
High quality flashlight is designed with corresponding heat dissipation and current control to ensure that the LED can work at a proper temperature.
I see the point that XTAR is making. The heat is generated inside, and if conductivity to the outside is poor, the surface of the flashlight does not get hot quickly, because the heat is trapped inside, where it will do damage. If instead, conductivity from inside to outside is excellent, the outside of the flashlight will feel hot quickly, because heat is being quickly conducted from the inside to the outside, where it can be dissipated.
Sinking depends on mass.
Dissipation depends on surface area.
Conductivity from sink to surface is dependent upon proper design.
If you're talking about dissipating heat to your skin, it's going to be way more efficient than to the air (at least for a relatively smooth surface). Compare holding a hot flashlight with hovering your hand very near above, but not touching, a hot flashlight.
Unless the flashlight has a way to shed that heat--either through your hand or to the air, just getting hot to the touch doesn't mean it's dissipating. It'll just heat up the body of the flashlight.
Now if it has some fins (a heat sink), you'll move heat out of the body more easily.
Semi-related: I built a thermal test rig for a device that happens to include an OSRAM OSTAR stage LED. The LED is not the primary heat source, but the unit needed to be thermally coupled to the ambient air for the test. I made a large aluminum plate with 4 large CPU heat sinks attached to keep the device chassis close to the air temp.
Perhaps another factor to consider is how much lag there is between heat generated and how soon the surface feels hot as an indicator of heat conductivity and heat sinking design. I’ve used three different FW3 flashlights in aluminum, titanium with a copper head, and full titanium. On turbo, the copper heats the quickest, followed by aluminum, but the full titanium has a definite lag before the heat reaches the surface. So feeling heat on the surface alone is not a good metric. Whatever heat is generated needs to be efficiently moved away from the source (LEDs) coupled with the medium for removal of heat (such as the hand, air).
It comes down to how fast you can get the heat away from the led into the lights body and into to the air, your hand, etc. Just because it’s hot doesn’t mean it’s moving heat away efficiently and the led could be overheating. There are so many factors like contact between mcpcb and pill/head, thermal paste, mcpcb type, etc. Some materials are better at that. Aluminum (depending on the alloy to some degree) has less thermal mass than copper or brass, but transfers heat quickly. It heats up fast, cools off fast. Copper has higher thermal mass, and transfers heat energy very well, better even than aluminum. This makes it an ideal heatsink material. Which is better? For higher power applications, and where weight is less of an issue like a powerful thrower, more copper than aluminum is ideal. For lower power and where weight is more of a consideration like for an edc light, aluminum/copper is more suitable. It’s always a tradeoff.
Aluminum holds more heat for the same mass. However, it is not as dense as copper. For the same volume copper holds more heat (volumetric heat capacity). Source
Some flashlights are offered in both aluminum and copper versions. the copper versions might be entirely copper (FW3C), or partially copper (Ti-Cu Emisar D4). The copper parts in these lights typically are the same shape and volume as the equivalent aluminum parts. Copper parts aren’t manufactured thinner than their aluminum versions for a number of reasons:
Strength - copper is quite soft and making copper parts thinner than same parts in aluminum would make for a delicate light.
Cost - why spend money redesigning a part to use less material in copper when you can just use the same schematic to make the part out of different metals?
Result: When it comes to flashlights, in virtually every case copper parts are quite a bit heavier than the same parts out of aluminum. And those copper parts can hold more heat. This usually means:
Weight - A light with copper parts is heavier than the same light made entirely from aluminum.
Heat Capacity - A copper head or light can hold more heat than an aluminum version. The copper version should take slightly longer to saturate with heat.
Conduction - since copper conducts better, the entire copper portion should heat up more evenly than an aluminum head. This might be good as it helps protect the electronics. But this can be bad as it may mean the exposed portion on the outside of the light heats up to dangerous-to-touch temperatures faster than an aluminum light.
Emissivity - this is a measure of how well the material radiates heat to surrounding atmosphere. Anodized Aluminum has excellent emissivity (especially dark-anodized). Tarnished copper is almost as good as anodized aluminum. However, polished copper or polished aluminum are much worse. Basically, if you get a copper light its ability to shed heat to atmosphere will improve as the copper tarnishes.
Given the same flashlight (ie. same mass), one made out of copper and the other made of aluminum, the copper would be significantly heaver and more expensive. The aluminum would be hotter to the touch faster whereas the copper would absorb more heat and delay the heat that is eventually felt.
Which is best? Depend on your definition of “best”. But given the topic, it seems to be the one that takes the heat away from the LED the fastest. Since most quality LED MCPC these days are made out of copper vs aluminum, it appears that copper is best. But since most flashlights are reasonably built, an aluminum flashlight with a copper MCPCB also seems to be a good compromise between costs and weight.