Originally posted by Doug S:
You are going to look at the die only and not the slug its mounted on? Dimensions of the die are on the order of 1mm. I would be surprised if your equipment can do this. If your equipment is good enough to see only the die plus slug, you could use the approximation that the measured temperature is the slug temperature [this approximation works since the slug area is large relative to the die area in the view of the instrument] and then use the die to slug thermal resistance to calculate the die temperature [15C/W in the case of the 1W emitters]
If you really must use IR measurement methods, a better way to "calibrate" your technique would be to take a "Star" configuration device and measure the backside under the emitter, calculate the die temp using the Luxeon datasheet thermal resistance, and then compare to your IR measurement of the front of housing. Measurements should be made after thermal steady state has been reached.
For other discussion of thermal measurement methods, see the link below:
http://www.candlepowerforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=14;t=000376# 000019
<font size="2" face="Verdana, Arial">Thank you Doug. I followed the link and read through the thread you started. Excellent reading all.
I agree a direct thermocouple connection to the die and if that is not possible, to the slug is ideal.
Of course, this may not always be possible at the time it matters most, and that is when the LED is in the flashlight.
So even though the pyrometer is not as accurate, it is more accurate than using a thermocouple against the outside of the flashlight or against one of the heatsink or bulkheads inside the flashlight that can be exposed while the light is in operation.
OK, here we have a luxeon emitter being held tightly against a tempurature probe. Also, the probe is liberally gooped in heatsink compound to facilitate transfer. I believe this is one of the ideas you suggested.
Notice that the thermocouple indicates 10C less than the pyrometer.
You could say this is due to the inaccuracies of the IR method or the heat gradient across the part or both.
Either way, if I keep the pyrometer reading low, the LED is probally doing even better.
Needless to say, the original question was about if a 5W emitter would be ok in a Maglite running at 6v. The answer was, it depends on your heat sink. Well, how do you test your heatsink? Now, if you can get your thermocouple inside the light to measure the slug temp, then excellent. Otherwise, the pyrometer may be a better option. Since our lights are typically quite small, the pyrometer works.
I invite anyone to produce a better method of course since we always want to advance the art.
Peter Gransee
