questions about measuring OTFlumens/watt

[djozz]

I am no battery-expert and not really into flashlight measurements (ok, a little), I am messing with my smaller flashlights (I do not own any of those high power throwers) and a lux-meter (which is not calibrated, but gives very consistent readings) and I have a few questions that hopefully one of you with a bit more experience than I can answer:

I am trying to do a measurement of the OTF lumens of my flashlights. I do not have a IS, so to do this I use the ceiling bounce lux-reading that is often done instead, and relate this to a flashlight with known output. In my case this is the Thrunite Ti (stated as 60 OTF lumens on a fresh Duracell alkaline).

Question 1: how far am I wrong when I calculate the output in lumens of my flashlights like this? Is it less than 10% (which is very ok to me), or is there anything much more unreliable about this method that I don't know of?

Assuming that the OTF-output measurements are reasonably correct like this, I combined this with the tailcap current draw of the lights, and assumed that a freshly charged Li-ion keeps at about 4V under load (also at more than 2A) and a fresh alkaline 1.5V (ok, I could have measured that, but, well, I didn't), in this way I calculated the OTFlumens/watt of the lights.

Question 2: Do I assume correctly that the voltage of fresh batteries are maintained well under load (of say 2A).

The OTFlumens/watt figures I came up with are quite sobering I am afraid, most of my lights (most are cheap and modded ) calculate round 40 lumens/watt, the best one actually is the Thrunite Ti on alkaline with 55 lumens/watt.

Question 3: Are these figures remotely consistent with measurements done by others? I did a (ok, quick) search and I was not able to find a complete set of measurents on any light to do the calculation (that is OTFlumens together with battery current draw/voltage measurements)

Lots of questions, hoping for answers

[djozz]

ok, with some fiddling I just did a measurement on a 14500 trustfire flame in a flashlight: when the battery is delivering 1.4A current, the voltage drops from 4.02V to 3.88V. So the lumens/watt figures I calculated will be slightly better. Other questions remain...

[Slewflash]

I’ll give some concise answers:

1. I don’t think anybody knows, it entirely depends on the variance of the light and how honest the manufacturer is.
2. If it’s an alkaline, no. Alkalines sag almost straight away compared with NiMHs or LiIons, so use those during your testing.
3. I do not calculate this so I can’t collaborate my results.

Edit: punctuation.

[djozz]

@Slewflash

I give some concise answers: 1. I don’t think anybody knows, it entirely depends on the variance of the light and how honest the manufacturer is. 2. If it’s an alkaline, no. Alkalines sag almost straight away compared with NiMHs or LiIons, so use those during your testing. 3. I do not calculate this so I can’t collaborate my results.

Thanks Slewflash for some answers. So I am getting into a bee's nest of uncertainties I begin to understand . And into the world of educated guesses I am afraid. Still curious about the questions though...

[Slewflash]

@djozz

@Slewflash

I give some concise answers: 1. I don’t think anybody knows, it entirely depends on the variance of the light and how honest the manufacturer is. 2. If it’s an alkaline, no. Alkalines sag almost straight away compared with NiMHs or LiIons, so use those during your testing. 3. I do not calculate this so I can’t collaborate my results.

Thanks Slewflash for some answers. So I am getting into a bee’s nest of uncertainties I begin to understand . And into the world of educated guesses I am afraid. Still curious about the questions though…

I think with time, when more people become interested with the workings of a light and wish to compare more features, more numbers will come out making it easier to compare.

[djozz]

There is not too much response on these questions yet, but then they may not be very easy to answer.

I think the the out-the-front-lumens/watt figure (=measure of efficiency) of a flashlight is a very significant feature of a flashlight, but it is hardly spoken of, I only hear qualitative stuff like: 'this driver (or led) is very efficient', or 'the runtime is surprisingly good', but always without the numbers that give the overall efficiency of the light.

There can be three reasons for that:

1) who cares about efficiency, here at BLF we are interested in OTF-blast and frying eggs on the body of our lights.

2) we really want to know, but it is so difficult to figure it out with all the uncertainties in every measuring step that we decided just to ignore it

3) don't do that calculation please, because we'd rather not know how bad the efficiency of our lights is

So which one is it?

[djozz]

@leaftye

I think it's more like #2. The second #2. It's so easy for people to poke holes in the methods that it may not be worth the aggravation for testers to publish their findings if they even bother to get OTF readings.

Yes, but say I am only interested in a OTFlumens/watt accuracy of 10% (anything more accurate is not that usefull anyway). It can not be that hard to get to that accuracy without too sophisticated measuring, and it gives very usefull information about the flashlight and if the manufacturer has done its work well (example: high resistance clickies will directly influence this parameter).

[texaspyro]

Sees all, be’s all, knows all, tells all, does all, saws all: Any interest in a LED/Battery analyzer device?

And you might be rather surprised what it takes to make those measurements with any kind of accuracy.

[djozz]

@texaspyro

Sees all, be’s all, knows all, tells all, does all, saws all: Any interest in a LED/Battery analyzer device? And you might be rather surprised what it takes to make those measurements with any kind of accuracy.

I had seen the beginning of your thread, but stopped following it. It is amazing what you found out about leds once you started measuring all those parameters accurately. Keep up all that work! and when the equipment is finished I hope that a) a lot of people will buy the equipment so that that kind of measurements will be done by many, and b) a reference chart will be build-up with extensive measurements of all known/popular flaslights to be consulted by everyone .