LED Flashlight Thermal Management
One important engineering factor in LED flashlight design is thermal management. Every Light Emitting Diode (LED) generates heat during operation. This heat, unless transferred away, results in increased temperature which causes the LED to produce less light. To maintain steady light output, it is imperative to rapidly transfer the heat away from the LED.
The study of Heat Transfer is a science unto itself and this brief article is not about to discuss the intricacies of heat conduction, convection, and radiation. Nonetheless, a basic understanding of these phenomena is important in understanding the critical performance advantages of a quality flashlight over the common trinket variety. A true high-quality flashlight will produce a near-constant level of light output over an extended period of time (see our blog post on the FL1 Standard). Very few LED flashlights on the market achieve this goal.
Maintaining constant high-output demands an efficient mechanism to evacuate heat from the LED. The heat must “go” somewhere and, in a quality flashlight, it first goes into a metal-core circuit board which supports the LED itself. Since the mass of this board is very low, it will get hot quickly unless the heat is immediately conducted away from it. This is the point where well-engineered flashlights truly shine (pun intended). Whereas most flashlights create thermal “hot spots” (as exhibited by models marked with “Caution: Hot Surface” warnings), a flashlight of good design will heat up nearly isothermally (all parts maintain the same temperature). Isothermal conditions maximize the heat capacity of all materials while keeping operating temperature at a minimum. To achieve this, an unbroken “thermal circuit” must be created and materials possessing excellent heat conductivity and heat capacity are essential. Metals exhibit the best thermal properties in this regard and specially engineered bonding agents should be used at critical interfaces within the thermal circuit to maximize conduction. Plastics have very poor thermal conductivity and heat capacity which explains why the best flashlights are made from metal, generally an aluminum alloy. As discussed in our blog post on Aluminum Alloys, 6061-T6 Aluminum is an optimum material for flashlights. Heat finally arrives at the outer surfaces of the flashlight and is then transferred to an external source. A human hand, grasping the flashlight, provides a path of heat conduction as heat is absorbed into the body. Heat is also convected to the air and radiated away to the environment.
The LED flashlight that produces a steady high output of light for a long period of time while maintaining reasonable temperatures on all surfaces achieves these ends through thoughtful engineering and quality construction. When the best performance is required, it is worth seeking out such a flashlight, rare as it may be.