One of the least understood, and most important aspects of quality high power LED lighting is thermal management. While most consumers tend to compare the LED drive current, that is only one part of the light output equation. Poor thermal management can lead to reduced light output and premature aging and degradation of the LED from excessive heating.

When the Lambda Illuminator Pill was first built, Luxeon LED drive currents were specified at 350mA. The original Illuminator circuit pushed that spec a little bit, supplying around 420mA for a little extra brightness. Thermal management was in the form of the copper cladding on the circuit board. The copper cladding, while very thin, still supplied an acceptable thermal path from the Luxeon emitter to the flashlight body. While not ideal, it did provide enough heatsinking to keep the Luxeon die under its 100C maximum temperature under normal usage. 

However, times have changed. New improved Luxeon LEDs with much higher current capabilities are now available. For example, the Luxeon III has a maximum drive current of 1000mA, almost three times that of the original Luxeon LEDs. The increase in drive current makes thermal management even more important and critical to quality lighting designs. This increased need for thermal management is why the Lambda MiniPro series of drop in modules all make use of an integrated  aluminum heatsink. Without the heatsink, the increase in drive current results in only a limited and temporary increase in brightness.

To illustrate this point, refer to the following test charts:

The test light with Luxeon on PCB  is charted with the green line, and the test light with Luxeon on heatsink is charted with the blue line. The test are 10 minutes long. The left axis is Lux (10m = 100Lux).

Not much difference at 100mA, for all practical purposes no difference in output is evident.

At 200mA a small difference in light output is noted.

At 300 mA the difference in light output is even more noticeable. 

At 400mA the PCB begins to fall even farther behind the heatsink in brightness as the Luxeon is starting to get hot.

At 500mA the test unit with PCB is loosing nearly 10% of initial brightness due to heating of the Luxeon.

After a few minutes at 600mA the PCB unit only produces slightly more light than the heatsink unit does at 500mA.

At 700mA the PCB unit only produces about 83% of initial brightness after 10  minutes. The Heatsink unit produces about 93% of initial brightness; 10% brighter than the PCB test unit.

At 800mA the difference in brightness between the two test units becomes very noticeable; after 10 minutes the PCB unit only produces as much light as the heatsink unit did at 600mA.

At 900mA the difference in light output expands even more. Initial brightness is almost 1200Lux but the PCB has decreased below 900Lux after just 10 minutes.

After 10 minutes at 1000mA the PCB unit really doesn't produce any more light than it did at 700mA.

At 1100mA the PCB unit begins actually producing less light than it did at 700mA.

At 1200mA the thermal effects on the PCB unit really begin to take a toll; after 10 minutes it is only producing as much light as it did on 400mA!

It is evident from the test results that using the copper cladding on the circuit board is only effective to about 400mA, marginally effective at 500mA, and starts becoming ineffective at 600mA. At currents above 600mA, the circuit board copper cladding is basically useless as a heatsink as the Luxeon overheats so much after a few minutes the light output actually drops below the light output of a properly heatsinked Luxeon driven at less current.

So it's clear from the results, for currents over 500mA a heatsink is a must or that extra power just turns to heat and the light output actually goes down.


To perform these tests, two test modules were constructed. Neither module contained any electronics, but served only to host the test Luxeon on a module of the same size/shape that a drop in module would need to be to function properly in a MiniMag flashlight. A series of four springs were used to simulate the spring pressure a module would normally see when installed in a flashlight to ensure good mechanical/thermal contact. A lab power supply was used to control drive current with less than 1% variation during testing. 

Test Notes:

1) Heatsink used was a MiniPro Flat Top heatsink, 0.070 inch thick (~1/16 inch).
2) All test runs were started after the test unit returned to room temperature (70F).
3) Test flashlight was mounted in static position throughout all test.
4) Drive currents were maintained with less that 1% variation.
5) Test Luxeons (BIN TY0J) where mounted with Artic Silver thermal epoxy.
6) Test light was a MiniMag with reflector modified for use with high dome Luxeon.
7) Lux measurements taken at one meter from source