# Test method used here for lux output of LED's



## Burnt_Retinas (May 17, 2003)

*Test method used here for lux output of LED\'s*

Can anyone tell me what the accepted test method is for measuring the lux output of LED powered torches? Distance from torch and room ambient lighting etc.

I'd like to know if we are all comparing apples with apples with lux readings.

Thanks for any input,

Chris


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## shiftd (May 17, 2003)

*Re: Test method used here for lux output of LED\'s*

The distance from the led to the meter must be 1 meter. The best ambient lighting is none at all. If the led is so focused, then a lux reading seldom useful, same with lux reading from incandescent bulbs.
If the leds are not focused tight, then you are probably (and hopefully) comparing apples with apples


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## Burnt_Retinas (May 17, 2003)

*Re: Test method used here for lux output of LED\'s*

Thanks for that. Now I have can see how I go with others. I thought >12k lux at 1 foot was too much.

There's still room for cheating here though as "not focussed tight" is rather subjective. Still, I can now see if I'm in the ballpark with other mods. At least it'll let me know if something's not quite right (which can happen in this game with LED placement, LED/reflector combo etc).

Thanks again,

Chris


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## shiftd (May 17, 2003)

*Re: Test method used here for lux output of LED\'s*

Oh, you can also measure in foot distance. The measurement is called Candela (Cd).


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## Jonathan (May 17, 2003)

*Re: Test method used here for lux output of LED\'s*

[ QUOTE ]
*shiftd said:*
Oh, you can also measure in foot distance. The measurement is called Candela (Cd). 

[/ QUOTE ]

Not quite. I strongly recommend using Russ Rowlett's Dictionary of Units of Measure if you have any questions about any measurement units. Go and look up the terms: candlepower, candela, lux, lumen, foot-candle, steradian

The lux is a unit of illumination or illuminance, the amound of light falling upon (or radiating from) a _surface_. It is measured in _lumen_ per _square meter_. If you shine a light at a surface, and place a lux meter on that surface, you will get a reading that tells you how much light falls on each unit of area of that surface. If you move the light further away, the reading will go down. But it will still be an accurate reading of the light falling on a unit area of that surface. Lux tells you about what is hitting the surface, _not_ what is coming out of the light.

If you take your light, and hold it 1 meter from your surface, and _then_ take your lux reading, then you are being told something about the light. This is because at 1 meter, a light source with an _intensity_ of 1 Candela will produce an illumination of 1 Lux. So if you have a lux meter, and you place your light at 1 meter distance, then the measurement in Lux will be numerically equal to the Candela output of that light source in the direction of the meter.

The foot-candle is a different unit of illumination. It measures the same sort of thing as the Lux, but in a different system of units. If you have a foot-candle meter, you can convert the measurement in foot-candles to Lux by multiplying by 10.76 {which happens to be (39.37/12)^2, because of the inverse square law}

If you have a foot-candle meter, and you place it 1 foot from a source, then the measurement in foot-candles will be numerically equal to the candlepower of the source. 

For historical reasons, 1 candlepower is pretty darn close to 1 Candela.

Finally, regarding questions about 'how tight the beam is', this is the reason for using two different measurements. You use Candela (or candlepower) to say how intense the light source is, meaning how much light gets beamed in a particular direction. You then use Lumen to say what the _total_ light output is in all directions. A low lumen source can have a very high Candela rating if it is _very_ tightly beamed. A high lumen source can have a low Candela rating if it is a broad flood light.

-Jon


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## shiftd (May 17, 2003)

*Re: Test method used here for lux output of LED\'s*

Oh yeah, sorry for the mistake. Man, it is very late here and i haven't sleep all night long. 
lux measures the same as ft candle. Candela is a different measure.

thanks for clearing that up


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## FalconFX (May 17, 2003)

*Re: Test method used here for lux output of LED\'s*

Lux numbers are pretty well accepted as a general indication of intensity of a beam. For measurement, I have a table (glass table at that) that's exactly one meter off the floor. I just placed my Meterman on the floor, and place the light right at the edge of the table, and get the lux readings. Of course, any light pollution you have is generally a no-no, so usually, I'd do this in the dead of night... 

Otherwise, I could take a meter ruler and take measurements in my restroom, which has no windows, and can then do it anytime...


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## pedalinbob (May 17, 2003)

*Re: Test method used here for lux output of LED\'s*

i have seen some people here giving what appeared to be inflated lux numbers...and i later found out they had the light 1" or 3" from the meter!

the standard seems to be to take the reading in lux with the light source 1 meter away from the sensor.

Bob


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## FalconFX (May 17, 2003)

*Re: Test method used here for lux output of LED\'s*

_i have seen some people here giving what appeared to be inflated lux numbers...and i later found out they had the light 1" or 3" from the meter!_

Well, of course when you see an Opalec reading 2,000 lux, you know it's "fuzzy math"... 
/ubbthreads/images/graemlins/tongue.gif


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## Doug Owen (May 17, 2003)

*Re: Test method used here for lux output of LED\'s*

[ QUOTE ]
*FalconFX said:*
_i have seen some people here giving what appeared to be inflated lux numbers...and i later found out they had the light 1" or 3" from the meter!_


Well, of course when you see an Opalec reading 2,000 lux, you know it's "fuzzy math"...
/ubbthreads/images/graemlins/tongue.gif 

[/ QUOTE ]

I work with a world class cynic that has a saying, 'the statistics can be tortured to say anything you want'.

Wise man....

Doug Owen


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## The_LED_Museum (May 17, 2003)

*Re: Test method used here for lux output of LED\'s*

I tend to measure in mcd (millicandelas) with a Meterman LM631 or a Tektronix J16, and place the source 12" away from the sensor, and do it at night with all the lights off.
If there's any light pollution being shown on the meter (such as from a computer boob tube or from streetlights outside), I'll just subtract it from the LED or flashlight I'm measuring.

Crude, but effective and decently accurate.
The ProMetric setup I use depends on the light meter; it does not measure on its own.

A wide-angle source will always read lower than a focused source, even if the light-emitting chip or filament is the same.

Flashlight makers that rate their product in Lumens probably have an integrating sphere to do that measurement with - a device that few, if any of us CPF members have.


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## Quickbeam (May 17, 2003)

*Re: Test method used here for lux output of LED\'s*

Likewise, I measure all of my lights with a Lux meter at one meter distance. So the measurements are directly comparable (numerically) with foot-candles measured at one foot (essentially, Craigs's and Brock's measurements.)


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## Burnt_Retinas (May 18, 2003)

*Re: Test method used here for lux output of LED\'s*

Seems there are a few way's we measure the light output of our torches. This is great for relative measurements with out own mods, but perhaps it's worth considering a CPF 'standard test method' so we can know where we stand and push the technology to it's limits in meeting our own needs for the technology ie a bloody bright torch with the best beam pattern possible, reasonable run-time etc.

I noticed the candela vs lux 'issue' came up again. I've read the definitions and to be honest it bends the mind a bit trying to work a direct relationship out based on singular definitions alone. I originally wanted to know how many 9000mcd 5mm LED's of a given degree of pattern it would take to match the lux output of an LS LED. I did however come across the following site which puts it in such simple terms that it actually made sense. It even came with example calculations. Perhaps it may help others like myself that are less knowledgeable on the subject to grasp the difference. Check out the "design considerations" section.

http://dcwi.com/~nybarra/LED.html

Thanks for the info too.

Chris


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## UK Owl (May 18, 2003)

*Re: Test method used here for lux output of LED\'s*

Remember the saying ...

There are lies, damn lies and statistics.

The only way to compare like for like is to put them both through the same test on CALIBRATED equiptment. 

In other words send them to someone who can compare like for like and publish unbiased results, for example the LED museum.

The better way of stating light output would be to express in terms of lumens per watt, as do the manufacturers.

I can't help but think this 'my torch is brighter than yours' is a little childish. What matters is unit is reliable, and if it gives enough light for its intended use. If you want a small EDC for map reading etc... and want a very long battery life, a unit with a single Nichia will suit your needs adequately. A person wishing for a more powerful light source for activities such as caving will want more than this.

One other thing, why all this fuss over units that use non-standard batteries. My priority in a flashlight is availability and cost of consumables (batteries). A torch may be fantastic, but if I can not pick up the batteries (eg AAA, AA or PP3) cheaply at any corner shop I will not be interested.


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## Doug Owen (May 18, 2003)

*Re: Test method used here for lux output of LED\'s*

[ QUOTE ]
*UK Owl said:*
What matters is unit is reliable, and if it gives enough light for its intended use. .


[/ QUOTE ]

Spot on!

A highly subjective measure, to be sure.

I submit that there is another objective measure we can easily use 'how long will the battery last'. 

I also fully agree, that a really neat light with an 'unobtainable' battery is pretty useless.

Doug Owen


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## Burnt_Retinas (May 19, 2003)

*Re: Test method used here for lux output of LED\'s*

I agree too, bar the 'unobtainable' battery. Don't know where that one sneaked in.

I'll take it that unless a test method is given in a post regarding "look what I got from my torch" etc then it is to be taken as a purely subjective opinion of the torches output relative to that persons needs, not a factual statement of the torches capability or specification relative to anything else, unless of course there is a back to back-to-back beam shot shoot-out (as occurs often) etc. where a relative mesurement is perfectly satisfactory.

I wasn't sure if that's how it works here at CPF, hence my asking if there is a standard test method used here. Excuse my ignorance.

Chris


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## UK Owl (May 19, 2003)

*Re: Test method used here for lux output of LED\'s*

[ QUOTE ]
*Burnt_Retinas said:*
I agree too, bar the 'unobtainable' battery. Don't know where that one sneaked in.


[/ QUOTE ]

What I was on about here are the 123 batteries. I use them in a 35mm camera I have and find the typical shop price to be around £6 (approx $9 US), but you can not get them at your corner shop and will have to go out of your way to get them. The few corner shops that sell them tend to add another £2 or so to the price.

A search on the net for UK mail order suppliers, typically quotes prices of £4 + P&P.

Compare this to AA cells bought locally:

20 (sometimes 30) AA Zinc Carbon £1 - Hyper Value
4 AA Alkaline £0.50 - Hyper Value
4 AA Quality Alkaline e.g. panasonic £1.00 - Poundland
4 AA High Quality e.g. Duracell £2.00 - Typical market price

Budget PP3's are typically 2 for £1, Quality ones £1, and high quality £2 each.

Note that you can pay a bit more at your corner shop but not a lot more. You only tend to have to pay top prices when you go to the large 'chain' shops or supermarkets. 

Sorry to everyone if I went a little off topic with this and my previous post.


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