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HexBright is the open source light, fully programmable and powerful with 500 lumens of brightness.

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Can the brightness/runtime be improved?

So ThinkGeek has this flashlight:

Like the Hexbright, it's using the CREE XM-L U2 LED and a similar 18650 battery (2600mAh instead of 2400mAh); however, it states a max brightness of 860 lumens on the highest setting, and yet it also has nearly twice the runtime (1h45m). The 550 lumen setting states a runtime of 2h vs. the 1h for the Hexbright's 500 lumens.

If these two flashlights are using the same LED and power source, can the Hexbright be improved to increase the brightness to the level of this one from TG, and/or have a better runtime?

Interestingly, the lower brightness levels show similar runtimes to the Hexbright, so it seems the increased performance is only on the top end, unless their numbers are wrong.

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Brightness can be improved by changing the LED's driving current. You'll want to replace a resistor on the board for this. Note that this does negatively impact runtime.

I don't have the light you mention - it looks like a fine light and maybe they'll add the ability to program it at some point - so I can't tell you whether their claims are accurate. I can throw some math at the problem, though.

If it's using a CREE XM-L U2 then the most efficient one is about 300lm@700mA. Set this at 100%. The datasheet notes that if you want to go up in lumens to 860lm (860lm/300lm = 280%) you'd have to use about 2400mA drive current. If you have a 2400mAh battery that means with 100% efficiency, and ignoring the peskiness of the battery's voltage droppingn over time and such, you'd get 1 hour. The claim of 1h45m on a 2600mAh battery while keeping a constant 860lm output is bold, to say the least - which casts doubt on any other figures provided.

There is room for improvement in the Flex, of course, but most of that requires more invasive component changes :)

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In general, it seems that Christian's performance claims are conservative. He's an engineer, not a marketer :).

His battery life claims have a safety margin of about 15%, and I'm pretty sure his lumen claims are conservative, too.


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Christian has previously stated that the one hour of runtime at maximum brightness (500 lumens) is for a continuous light level and does not include any time once the brightness begins to degrade due to the battery level. That means the flashlight with the included 2400 mAh battery SHOULD last for one hour on maximum AND STILL BE 500 lumens.

Flashlights with battery life levels measured with ANSI FL1 are nothing like this. The ANSI FL1 standard measures the amount of time from 30 seconds after the flashlight has been turned on until the beam has degraded to 10% brightness.

I don't have any data nor do I have a way to measure to ANSI FL1 standards, but I have a feeling the HexBright would be more around 1:20 or 1:30 maximum setting battery life if it were measured to 10% of original brightness per ANSI FL1.

You must keep in mind the HexBright has a lot of additional electronics that most flashlights do not that also reduce battery life.

Christian has also mentioned he intended the LED to be replacable when more efficient LEDs come out. The HexBright is current limited to 1.6 amps, so you will not be able to increase brightness without replacing the LED with one more efficient. As for increasing battery life, well, I put an Orbtronic 3400 mAh battery into mine and per calculations I should have 85 or so minutes of 500 lumen brightness on maximum (not tested).

This shows what I was referring to. The specific flashlight you linked from Thinkgeek only shines the advertised 860 lumens for, well, almost no time at all. After approximately 7 minutes it drops to ~650 lumens. After 21 minutes, it drops to ~465 lumens and stays there until it begins to degrade. As you can see by the various degredation graphs of the different batteries that once it begins to degrade, it can still last a long time increasing an already inflated runtime. If this flashlight were to stay at 860 lumens for the entire duration, you can be assured the runtime would be MUCH SHORTER.

This second graph is on the high setting, ~515 lumens (similar to the advertised HexBright maximum setting). The different batteries get 55, 84, and 96 minutes before any degredation occurs. That is data that IS directly comparable to the HexBright's advertised 60 minutes on high (Although the 2600 and 3000 mAh batteries are a higher rated capacity than the included 2400 mAh with the hexbright).

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Nice catch on the voltage regulation issue! As to extra power draw of electronics, they only draw about 5 mA on the hexbright, so it barely affects runtime:


Good point, I really had no idea what the power draw of the electronics was. Still, based on the graph from the other flashlight, the HexBright is right in line as should all other flashlights with the same LED.


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There are changes that could improve the run time, but they require a hardware change (although a fairly simple one). Even so, about the best you could improve it is in the 10% to 25% range, depending on the brightness level you are testing at. Most other lights that I have seen do not use a buck/boost converter, which is why the light fades as the battery runs down. The Hexbright will maintain a steady level all the way. The buck/boost is a benefit if you are going for longer life at lower light levels, while still giving you the opportunity to use a bright setting on a low battery.

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Caleb Martin will be eternally grateful.
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