What is burn time

If this has already been discussed somewhere else, could someone please point me in the right direction.

I see a lot of use of the terms "run time", and "burn time", and "usable light". As it relates to LED lights, is there a generally accepted definition of these terms. It seems to me that the terms would be a function of the type and purpose of the light.

I personally, would tend to use different definitions for an L4 and an Arc AAA. I would replace the batteries on an L4 way before it gets as dim as an Arc AAA when it's fresh.

Also I would have different standards for a light that I use for every day walking around the house in the dark (call me crazy) than I would for a light that I don't use often, but need to be as bright as possible whenever I do use it.

The common definition is used when evaluating lights. That definition is the time it takes for the light to dim to 1/2 the initial brightness.

It works simply because it's arbitrary. A lithium battery has a fairly flat discharge curve, so the light does not dim much as it's used, then Bam!, down it goes. An alkaline slopes gently (after the initial few minutes) from bright to dim. At some point it also hits that 1/2 brightness point.

Most flashlight batteries are pretty well exhausted when they hit that point.

Another common run-time definition is the time until it drops out of regulation. Measuring the runtime on some lights (like the arc4) is problematical, in that they change the brightness level as the battrey is depleted, so the light dims in steps.

I figure, like you, that the run time is based on how I would use it. I've had my arc AAA get pretty dim before realizing that I needed a new battery. I change the batteries sooner on my big lights because, well, they are supposed to be big lights! /ubbthreads/images/graemlins/smile.gif

Daniel

Gadget_lover,
Thanks for that excellent and most informative reply.

What has been your experience when applying that definition against manufactures claims (i.e. some good, most good, certain ones always good, certain ones always bad).

Just yesterday, I tested a Pelican M6 LED on my computerised battery destroying satanic robot death machine (a charting DMM, a computer, some wires, a solar cell, and a couple of pieces of computer software), and got just a tick under 2 hours to half-intensity. Yet the box reads "40 hours" of battery life, and the instructional material inside reads that the "brightness will noticeably decline after 4 hours of operation".
So is it 2 hours, 4 hours, or 40 hours? /ubbthreads/images/graemlins/confused.gif
My equipment says 2 hours. After 3 hours, the light is at about 12% intensity. /ubbthreads/images/graemlins/jpshakehead.gif

GadgetLover/Daniel
RE"That definition is the time it takes for the light to dim to 1/2 the initial brightness." Thanks for the excellent definition and elaboration.It seems this relates to a measurable duration of a meaurable metric (lumens) irrespective of context- since a usability meausure or 'run-time' would simply correlate to the minimum amount of light remaining that is useful for illumination in total blackness.Correct?

Here is my take on the subject!

When starting with a good set of batteries, a LED normally doesn't go OUT, it just gets very dim!

In the case of "The LED Museum"'s M6, he measured the runtime at 2 hours to 50% of max brightness. The next 50% step could very well be at 4 hours (Runtime at 25% of max brightness). As to "usefull" amount, some folks say that if you can read text with the light one inch away from the print, then that is usefull light.

The reason for the 50% value......I've been told that most folks can't notice a change in brightness untill it drops by 50%. 50% of max brightness is a value that is easy to measure. Be aware though, that 50% can be very missleading applied to a direct drive light. Such a light can decrease VERY slowly.....look at the runtime plot for the Inretech AA Minimag drop-in module.

Long Long time ago, I wanted to develop a "score" for flashlights/etc - the number would be

XX:YY

XX = Initial amount of light (in lumens/etc)
YY = Number of minutes of light before light < 50% of Initial

Got a lot of people upset, since they all wanted to measure it differently, and I wanted a "one for all" - good or bad, thats it...

So there..

I hate to say, but it appears that logistics are part of the problem since most of us each have different equipment , or ways to measure light output.
The closest I could come was to do a
"this light versus a 60 watt household bulb",
Or "this light versus a dot approved car headlight"
This is the reason I did my pics with common light sources we all see everyday.

Something like this:
http://www.geocities.com/cpf_3rd_shift/trilight3_2.html?1079808438270
or,
http://www.geocities.com/cpf_3rd_shift/Trilight_red_orange_take_2.html?1081544915620

Back to burn time:
Logistics are the problem again since we each use many different batteries.
Batteries with mah ratings such as, good name brand rechargeables that keep a near steady output through most of thier discharge driving a milliampere or ampere rated light, can lend a good clue as to the actual full brightness runtime.
A well regulated light source running on any battery type with a ma or amp rating can also lend a clue as to what the full strength light output runtime will be.

Some lights are still quite strong at half brightness and can be used quite a bit further still.
Like this one for instance:
http://www.geocities.com/cpf_3rd_shift/Trilight3_3dcase_vs_4dcase.html?1081707898010

While others are already so lame at 50 percent that the half brightness value just won't do for some of us on those units.

edit:
BTw; The pics above feature a couple of Inretech products.
They were kicking some serious butt. /ubbthreads/images/graemlins/buttrock.gif

/ubbthreads/images/graemlins/confused.gif
I don't understand your point.......50% of max brightness is 50% of max brightness. Given that the max brightness will be different, half of it is still 50%.

Remember that runtime is the TIME it takes to get to 50% of max brightness.

It's just human nature for many of us to want to know how long the light will run at full, or near full strength before it starts to peter out.
Put another way, some lights will still turn heads at less than half brightness, while others which were marginal to begin with, are only useful at or near full brightness.

[ QUOTE ]
JimH said:
Gadget_lover,
Thanks for that excellent and most informative reply.

What has been your experience when applying that definition against manufactures claims (i.e. some good, most good, certain ones always good, certain ones always bad).

[/ QUOTE ]


Well, I don't usually double check them against the manufacturer's claims. I tend to look at run time plots posted here or I do some measurements and calculate it myself. I know that the manufacturers are frequently optomistic and probably use the best batteries they can find.

The 1/2 brightness metric can be used even if you are looking at someone else's graphs. If it starts at 700 lux and falls to 350 in 25 minutes, that's the same information you get if it starts at 70 lumens and falls to 35 lumens in 25 minutes.

Where it gets deceptive is when you have direct drive for the first few minutes until the battery voltage drops enough to put it in regulation. A 3 cell light may do this, as may a freshly charged set of NiMH batteries. When this happens you can see very bright for a few minutes falling to a lower level (but still bright) that's steady for many hours. I think my Pelican Sabrelight does this.

The only manufacturer that I explicitly trust for runtime estimates is ARC. Surefire is also pretty close.

Anytime the runtime claim greatly exceed the amphours / (wattage of LED/battery voltage) I know that they are using the "able to read newsprint at 1 inch in a dark room standard".

/ubbthreads/images/graemlins/smile.gif

If the runtime claims exactly equals the amphours / (wattage of LED/battery voltage) I know they did not really test it, because there are inefficiencies in the converters. They could be underdriving the LED, but you can usually tell from the lumens claims.

A reasonable formula for maximum run time is:

(Battery amphours / (wattage of LED/battery voltage) ) * converter efficiency.

Example; 1 watt luxeon with a 123a battery and 65% efficient.
(1.3 ah / (1 watt / 3 volts) ) * .65 efficient = 2.5351 hours

That only works, of course, if you actually know the amphours of the battery at the current that's being drawn. I use this formula only to see if the claims are reasonable.


Sorry about pontificating.


I do have a lightwave 4000 that was left "on" in my attic for a week and was still bright enough to use to see my way out.

Daniel

There is also another factor to consider:
What will the light be used for?
Many security personnel like myself really want to know what the runtime is at, or near full brightness.

An icecream machine repairman would be content with a halfcocked 50 percent lit minilight to find a screw that he dropped on the floor behind the machine.
This is another reason why the 50 percent value doesn't work for everyone.

INRETECH's score is a perfect answer to 3rd's concern about a light's usefulness at 50% brightness. That method of scoring helps a user to determine suitability of a particular light.

For example: if 30 lumens is just enough to be useful for your needs, then a 30 lumen flashlight better have a long runtime to half that brightness if you're going to consider using it.

Like Roy said ,too, R-T to 50% makes sense for a standard rating and having a conservative standard makes sense.

Gadget, I liked the way you put it: " they're supposed to be big lights." I, too, keep the cells fresh in my little "BIG" lights. Like you, I find that for less demanding needs, the useful R-T can be considerably longer than T/50% for some lights, but XX:YY is a good benchmark for important tasks.

Anyone know where we can find a good lumen measurement device that doesn't cost an arm and a leg?
I saw Gadget Lover's on another post elsewhere on cpf and it did look cool.
I think we can bring a few more manufacturers to be a bit more honest if more of us had good measuring equipment that doesn't cost an arm and a leg, and post our findings of those light products here.

This is another one of those things that varies by definition from person to person. Burn time could be said until it stops making light, so it appears to run a long time, but useable burn time may only be a short fraction of useable light, or light close to the intensitty of the purpose of the light. Example, a flashlight may be a far thrower, and may still put out plenty of useable light for upclose/medium work when diming, but the task of this light being a far thrower is no longer feasable. So do you rate it at the point where it no longer is a far reacher, or the point when you can't use it at any range?

Also add to this mix that LED's unlike incandecents, will glow much longer at a dimmer and whiter level than incadecents which will usauly not even light anymore... So you can have a highpower bulb, and a high power led, both ran down bats, the bulb won't light but the led will light like a low-power led...

So there's no real standard unfortunatly that every manufacturor goes by.... Once again depends on marketing and how conservitive it is... my 2 cents!

I agree with IsaacHayes.
That was what I was trying to communicate in other ways.

This is why I normally try to post with a link to some pics and let the cpf members judge for themselves.
Most of the lights pictured on my links are running on near steady discharge nimhs to maintain advertised brightness or close to it for maximum amount of time.
Any regulated light I have in a pic may have any type battery since it doesn't really matter there.

The standard should apply to all products, and be used without caring "how its done" inside; since the customer doesn't care - all he/she is looking for is:

1) How bright will it be when I turn it on
2) How long will it last

A customer came to our booth once with a small 5w flashlight, I thought it was going to burn holes in paper it was soo bright, then I asked him - how long does it run; he said "4 minutes"; a simple scoring system done without regard to internal "guts" would allow the customer to quick guage the product to see if it meets his/her needs

One person might want SUPER_BRIGHT for a short time, and another person might want longer operation - as I said long time ago, there is no "magic bullet" here

Operating_time X Brightness -> Battery Weight

"Our new Whiz-bang flashlight runs for 24 days on a single set of batteries !"

"Whats that small cord running up your arm mister ?"

Ok, how's this;
Inretech Trilight red/orange in a mgg 2d case with Rad Shack super capacity Nicad 4500mah D cells.(Measured last night)
Runtime is: 9 hours at full brightness, 2 more hours to 50 percent brightness for a total of 11 hours of advertisable runtime with these batteries.
edit: the light was allowing 600ma of current early on and likely, averaged 500ma throught the discharge cycle.
Starting at 07:00 this morning central, I'm now running a pair of Supreme Power 9000mah nimhs and will post my findings on those sometime tomorrow.
I have to go to work tonight for a 16 hour double shift.
The light will be coming with me to work and left on until it has petered out past 50 percent.