Test Results - Well, this was interesting! The intensity of light from a point source is inversely proportional to the square of the distance from the source, so for a similar light meter setting the flashlights must produce four times the power when the distance is doubled.  That makes the M4 with the MN60 bulb almost nine times as powerful as the U2 and the AEX20 is a retina searing eight or nine times more powerful than the M4!

Weird Results - Switching the Surefire M4 MN60 bulb to the high output MN61 bulb actually REDUCED the output even though the power consumption increased from 15.6 watts to 23.5 watts.  I replaced the cells with a fresh set and reran the test.  I also soldered a very thin wire to the positive contact of the first cell and ran it along the cell stack and out the back so I could measure actual voltage under load. With the MN61 bulb installed the new cells dropped from 12.26 volts to 8.9 volts under load. That is a very clear indication that the SF123A cells don't do well with that heavy a load.  I don't think the M4 is capable of putting out 350 lumens unless it's connected to a car battery.

Voltage and Current Measurements - I measured current by removing the tail caps and measuring from the negative battery terminal to the housing using an Agilent 34401 6-1/2 digit Multimeter.  Measuring voltage is much more difficult since you don't have access to the bulb or LED while the flashlight is on.  To get around this problem I removed the batteries and loaded them down to the same current I had previously measured using the Triton 2 battery charger in discharge mode.  I captured the voltage under load using the Agilent Multimeter in Min/Max mode.  I was surprised to see how much voltage drop there was under load.  Next time I do this I'll use the thin wire down the side trick instead of the indirect method I cooked up.

Late news - I just received the new Fenix TK10. It's almost an inch shorter than the Surefire U2, consumes less power, cost less, is very high quality, and it's definitely brighter. It's spec'd to be 225 lumens which would make it as bright as the M4 with the MN60 bulb, but it isn't nearly that bright. I reran my earlier test and it came in at 14 feet. It is noticeably brighter than the U2 Ultra. (Click here to see comparison beam shots.) Fenix uses bulb lumens and Surefire uses lumens out the front of the flashlight. The losses inside a small reflector and lens assembly would reduce that 225 lumens quite a bit. Also, the Cree Q5 LED that's used in the TK10 is rated at 225 lumens at a current draw of 1000 mA and the flashlight only drew 890 mA. In spite of all this the TK10 is a very bright flashlight!

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