Why does it matter?

Thermal design is incredibly important in lighting and often overlooked.

There is often a misconception that LED’s “run cooler” which is true from an efficiency standpoint compared to halogen or HID bulbs. But they still generate a tremendous amount of heat, and how we shed that heat is crucial to product performance and reliability.

LED’s are also very temperature sensitive, if they begin to overheat, the efficiency drops off which increases the power draw. Every LED datasheet will show this. There is a happy temperature range that LED’s operate at, and our goal is to have the light working within this range at a bare minimum. Ideally as "cold" as possible.

This graph below is for the CREE XD16 LED used in Hangover. As you can see it's at 100% efficiency at 85°C and only gets better the colder it runs. But as it warms up the efficiency drops quickly.

Thermal Load Consideration

Now comes the engineering decisions and where we also set ourselves apart. It is easy to make a huge heatsink if temperature control is priority number one. However that adds weight and encroaches on the product dimensions we defined early on.

It is easy to make a small heatsink that looks sleek and slim, but it may be quickly overpowered by the power required by the LED’s. So how do we approach it?

Our first goal is to spread that thermal load out. We prefer to use multiple small LED’s running at lower power not only for optical beam control, but also thermal control. Instead of having one 20 watt heat source stuck deep inside the light, we can have five 4 watt sources spread out over the face of the light instead. This reduces the problem of thermal overload right away.

Direct-to-airflow design

The second objective is to create an effective thermal pathway for heat to go from the LED chip to the outside air to be extracted.

A lot of cheaper lights will use an extrusion with standoffs for the board to sit on, and sometimes don’t even have material behind the LED chip itself to pull heat away. This results in a light quickly overheating even when in motion.

High quality lights will have a direct thermal pathway, although admittedly this is very hard to determine unless you have a light torn down in front of you.

Magnesium & Thermodynamics

One of the other items that sets Outbound apart from typical light manufactures is our focus on magnesium as our heat sinking material.

Magnesium is an extremely lightweight metal that has good thermal properties. However when a lot of engineers look at a material suitable for heat sinks they gravitate straight to aluminum due to the fact the thermal conductivity is 33% higher than magnesium (96 W/mK vs 70 W/mK for diecast applications). Though thermal conductivity is only part of the thermal equation.

When we look at heat rejection for a lamp, we often are considering it moving within an air stream which means that the thermal bottleneck is not going to be how fast heat moves through a material (thermal conductivity) but rather how fast we can remove the heat via convection.

The only time that the rate of heat removal via convection in an air stream outpaces how fast the heat moves through the material is at extremely high speeds with forced direct airflow. Speeds that no biker will ever hit. So instead of focusing on the thermal conductivity of a material, we’ll focus on improving the weight of the light and increasing the surface area. This is why all our lights use magnesium die casting with diecast bodies that feature finned surfaces. We get the benefit of an extremely light body, a lot of surface area, and improved thermal characteristics.

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