How much can an LED vary in brightness?

burpee

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I'm more than a little confused. I was shopping for a budget 1x18650 XM-L flashlight and discovered dozens of models that had various lumen ratings. Some were listed as low as 300 while others boasted as much as 800.

What I want to know is: Doesn't the manufacturer have an official standard of what voltage and current range a given emitter will tolerate -which I would assume produce an optimal possible lumen level?

Can anyone explain what they know about emitter ratings? For instance is the "max" for an XM-L emitter something like 3.0A at 4.0V?

Other than the heat sinking - what else goes into limiting the power of some of these flashlights? Why aren't all XM-L's about 800 lumen?

I humbly await a deluge of data.

Sincerely,
Burp Pee
Saint Louis, MO
 

MikeAusC

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. . . . is the "max" for an XM-L emitter something like 3.0A at 4.0V? Other than the heat sinking - what else goes into limiting the power of some of these flashlights? . . . . .

The Driver is a significant cost at higher currents. So that they can advertise the latest LED, they might advertise an XM-L - but only drive it at 2 amps - because they have lots of that driver still in stock.
 

Moonshadow

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Heatsinking is certainly a factor - at higher drive currents the LED generates a lot of heat, which needs to be carried away and this adds to the cost of the head (more metal, better thermal bonding, cooling fins etc.).

There's also a tradeoff between output and runtime, and different manufacturers might choose different priorities here.

Finally, it's worth bearing in mind that there might be a little exaggeration going on - especially at the budget end of the market !
 

Th232

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Also note that Cree doesn't bin according to Vf, so one LED that has a Vf of 3.5 volts at 2.8 A will be more efficient than one that has a Vf of 3.8 volts at that same amperage. While both LEDs (assuming they're the same flux bin) will create the same amount of light if they're being driven at the same current, the one with a Vf of 3.8 V will create more heat, leading to thermal issues and possibly reducing the output as the temperature of the LED increases.

There are also two ways that output is advertised, one is emitter lumens, i.e. the whole amount of light that the LED produces. The other is out the front (OTF) lumens, indicating how much actually escapes the torch, taking into account what has been absorbed due to the reflector/optics/window/other elements.
 
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burpee

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While both LEDs (assuming they're the same flux bin) will create the same amount of light if they're being driven at the same current, the one with a Vf of 3.5 V will create less heat, leading to thermal issues and possibly reducing the output as the temperature of the LED increases.
So there seems to be a sort of "matrix" of conditions that ultimately affect what a flash light will produce. You're saying its a combination of driver/LED paring along with the resulting physical environment that determines both efficiency and brightness? (heat@current?)

So now I wonder just how much a given brand or quality of a battery cell and the ambient temperature the light is operated in affect its optimal brightness/run time.

I'm assuming heat and cell quality could or would be an issue affecting max current in a single-cell carrier flashlight configurations? Or am I clueless here too?

Thanks, maybe I'll just have to buy and try.
 

tre

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Many factors.

1) battery type. You can buy a small light that uses a single primary cr123 battery. A single cr123 can't produce enough amperage to run an XML at 800 lumens. likewise for a single or even 2 AA cell light.
2) size. A small light does not have enough metal to shed the heat. The larger the output of the XML, the larger the heat sink that is needed. A smallish light will burn up the XML emitter (and your hand) when run at that output.
3) lies! You need to make sure the light is rated using the ANSI/FL1 standard. Many cheap lights (untrafire, uniquefire, and other xxxfire brands) are not measure according to the ANSI/FL1 standard. Some are rated using emitter lumens (which is much higher than what you actually get out the front of the light) and some lumen ratings are pulled from thin air (most ebay lights are famous for massive high lumen claims that are nothing more than a lie).
4) driver. You need a driver that can fully power the emitter. That means sustained wattage - enough to power the xml.
 

yellow

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As has been already typed:

the MAX of the XM-L is about 2.8 A, then one could calculate these ~800 lumen.

When less is typed - f.e. 300 lm - then the driver used was built for a normal single-emitter light and they used the XM-L just for whatever reason
(but the beam itself will then be about double the size of the one coming fom an XP-G and thus will be less bright --> it illuminates about 4 times the surface with just a bit of more lumen than the XP-G)

reason for using "less" power: RUNTIME
1 hour for full power to an XM-L vs. 2,5 hours for the light with about 800-900 mA to led.
Thats an argument!
(the 300 lm from an XP-G (or underdriven XM-L) is way brighter than the general public will even think of, so ...)


and then there is simply useless info, which for cheapo offers is often a high chance to get.
:rolleyes:
 

Th232

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Ugh, brain fart. For anyone who didn't notice it, my earlier post should have said that the one with the higher Vf will create more heat, leading to a reduced efficiency.

Less heat will not create more thermal issues.:banghead:
 
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