# Proper use of Lux Meter?



## FlashlightOCD (Sep 5, 2003)

I just got an LM631 Lux Meter today, and was wondering what the proper technique for using it is.

This is what I did:

1) Set the meter in a relatively dim corner and set it to peak mode.
2) Waited a few seconds for it to settle on a peak (30).
3) Hit it with the center beam of my E2e from about one meter away (1880).
4) Subtracted 30 from 1880 to get 1850.

Did I do that correctly? Does 1850 Lux seem plausible for E2e with fresh Batteries?


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## PaulW (Sep 5, 2003)

Yes, that is about right -- a good approximation. But if you had measured the distance at one meter exactly, your accuracy would have been maximum, equal to the accuracy of the meter. A 10% error in distance will create a 20% error in light intensity.

Your ambient light level is less than 1 per cent. If it's less than, say, 10 per cent, your subtraction method should be valid. It might be valid at any ambient level, but I'm not sure. Perhaps Doug S will chime in. He's a smart cookie on stuff like this.

850 lux sounds about right for am E2e. Believe it or not, although I have an E2e, I have not measured the intensity of its beam. But I did measure my A2 at 2400.

When I first got my meter, I tried the peak mode. It worked all right, but I enjoy seeing the reading vary as I move the light to maximize the reading. It helps me find that "sweet spot." . . . just a personal preference.

Paul


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## Doug S (Sep 5, 2003)

Paul is spot on with his emphasis on the sensitivity of the distance error. Your method of subtracting out the ambient should work fine of any ambient level. I have no idea what a reasonable measurement would be for your light. To the best of my knowledge, I have never even laid eyes upon a Surefire product. I can hear people gasping in disbelief but it is true.


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## JohnK (Sep 5, 2003)

As an optometrist that has done industrial work, I don't really understand the value of the Lux meters (with flash lights). They measure a brightness value at a "point", which says absolutely nothing about the "surround", or "lumens", or really anything.

I could forsee a manufacturer making a reflector that pops a zillion "lux" at a tiny point, then goes to heck every where else.

I think the only scenerio where this measurement is useful, is in real world situations with DIFFUSE lighting; where perhaps an assembly line is fairly uniformly lighted by fluroscents, etc., and judgements can be made as to the proper brightness/illumination level.

With tightly focused lights, I find the measurements almost useless; viv a vis a LUX reading for the SL TL-2 LED at far higher readings than the TL-3 LED, or the SF L4. I like the TL-2 LED, and TL-3 LED, I own them both, but feel that these reading are at best misleading (Quickbeam, whom I respect a great deal, is the first to explain this, when he gives the readings), and at worst, totally misrepresent what we are trying to find out. 

"Lux" measurements give you nothing except the often noted example of a Laser brightness at ONE POINT, and absolutely NOTHING else.
NO other information, zip, nada !

Gotta be a better way.

I suppose reviewers astute comments, added to the "Lux" stuff will have to make the difference.


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## Saaby (Sep 5, 2003)

They're still fun toys though. Flashaholics and toys go hand in hand. Flashlight related toys are even better.

I know nothing on the topic, so take my advice for what it's worth--but I would say that the most important thing with your readings is consistancy. Always measure all your lights exactly the same way, otherwise comparing 1 flashlight to another based on your numbers is useless.


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## Roy (Sep 5, 2003)

Is the distance measurment to the front of the flashlight (the lens) or to the source of the light


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## PaulW (Sep 5, 2003)

Roy, that's a good question. I had never asked myself that. I always measure the distance from the lens of the flashlight to the sensor of the LM 631. I don't know whether that's appropriate or not. 

I use a yard stick to which I have taped an 8-1/2 by 11 sheet of paper folded around the stick. It's cut so that the overall length is exactly one meter. It provides the consistency that Saaby properly recommends.

Paul


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## Roy (Sep 5, 2003)

The distance problem is not so bad at one meter, but I don't have the room for a one meter test range, so I'm using a distance of one foot to measure Foot Candle. Thus, one Fc at one foot equals one Lux at one meter (or so I've been told).


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## PaulW (Sep 5, 2003)

Yes, at one foot the difference between bulb and lens is significant. It seems reasonable that if you're measuring the light intensity from the flashlight you'd logically have to measure from the lens. The reflector, right up to the lens, is an important part of the light. If you can extend your range to two feet, you can use the FC scale and then multiply the result by four (2 squared). That will give you foot-candles, which equal lux.

It sounds like you're measuring in a closet. In my house, I can draw the drapes of my living room at night, and my meter measures less than 1 lux of ambient light. So I have lots of room.

Paul


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## Roy (Sep 5, 2003)

I'm using the top of my computer desk. My test range is used primairly to do runtime plots which can take 30-40 hours. The multimeter is plugged into one of the serial ports on the 'puter, so I can't get very far away. The light sensor is mounted on the wall next to the desk (7" from the edge of the desk to the wall".


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## FlashlightOCD (Sep 6, 2003)

Thanks for all the input.

My approximate 1 meter was measured by a yardstick and then I held the flashlight about 3" behind the yard stick which should be pretty close to a meter. I doubt I was off more than +/-5%.

John K,

I know that the Lux is only mesuring the brightness at a point and has nothing to do with Lumens, but I think the Lux reading at the hotspot of the beam at least gives some indication of the "throw" power for the light.


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## keithhr (Sep 6, 2003)

Since I got mine today, after getting no readings, I was ready to put it up on bst until I tried taking off the protective cap (which wasn't mentioned in the manual I think). It would help if somone would describe test methodology, where to place the meter and flashlight (hod the meter or hold the flashlight?),what type of backround and other factors which would effect test. I'm sure it would only take a few lines of explanation to help us beginners get more accurate tests from the beginning.


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## The_LED_Museum (Sep 6, 2003)

When I use the LM631, first I take the black protective cap off its sensor and stick it someplace I can find. Then I turn the instrument on (to FC mode), turn its backlight on, and set it by my computer keyboard so the sensor faces right. Then I put a ruler down in front of it, and orient it so the sensor is at exactly zero on the ruler. Then I note any reading on the meter face and remember it or write it somewhere. Then I take whatever I'm measuring and place it so the LED(s) are exactly at the 12" mark on the ruler, and move it around slighly until the highest number is shown on the meter. I subtract the ambient light reading from this, and type the final result on the light's web page or in an open CPF message. Then I turn the light meter off, put the protective cap back on its sensor, hang it up nearby, and put the ruler away.

If the light uses bare LEDs, I put them at the 12" mark. For LS lights, I place the front of the acrylic optic at the 12" mark. For incandescent lights, I place the front face of the reflector at the 12" mark.


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## Quickbeam (Sep 8, 2003)

Here's my procedure:

Turn on meter

have a beer

Turn on flashlight

have a beer

Put batteries in flashlight and try to turn it on again

have a beer

Fix the battery polarity and blind self by looking at light when turning it on

have a beer

Position meter in dark area so it reads "0"

have a beer

Step on meter, fall on face, drop flashlight causing batteries to fall out again.

Kick meter across floor, throw flashlight after it, slip and fall on the batteries.

Finish the 6-pack and make up the numbers... /ubbthreads/images/graemlins/twakfl.gif

OK, OK, that's not the REAL procedure... /ubbthreads/images/graemlins/tongue.gif

I just get the meter to "0" in a dark area, set the flashlight 1 meter from the sensor and move the beam around until I get the maximum reading.


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## Slick (Sep 8, 2003)

[ QUOTE ]
*JohnK said:*
"Lux" measurements give you nothing except the often noted example of a Laser brightness at ONE POINT, and absolutely NOTHING else.
NO other information, zip, nada !

Gotta be a better way.


[/ QUOTE ]

I agree with some of your statements regarding the limitations of a lux meter, but I find mine to be VERY useful. Consider the following..

For example #1, lets say that I have several emitters from a reel that I want to sort - simple, I just use the same optic and see which ones do the best.

#2 - I'm testing lens materials to determine the most efficient materials. I use the *same* emitter with a constant power supply, and just try the different lenses.

#3 - testing collimators, I'd also use the same emitter & power supply, but shine the output at the ceiling with each combo. Measuring the light reflected from the ceiling give me a pretty good idea of which optic works best.

#4 - testing circuits. I use the same emitter and can play around with differing numbers of turns on the core to see what difference there is in output.

These are just a few of the many ways I use my LM631. I usually follow Quickbeam's method, except that I start by cracking open the brew first /ubbthreads/images/graemlins/smile.gif

My point is that a lux meter is a very useful tool - but not necessarily in the way most people would guess.


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## Quickbeam (Sep 9, 2003)

[ QUOTE ]
These are just a few of the many ways I use my LM631. I usually follow Quickbeam's method, except that I start by cracking open the brew first 

[/ QUOTE ]

Oh, THAT'S what I'm doing wrong! /ubbthreads/images/graemlins/ohgeez.gif


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## pedalinbob (Sep 9, 2003)

to quickbeam's method: LOL!!!!!

i actually laughed out loud!

thanks for the laugh, quickbeam (and the EXCELLENT website--my non-flashaholic wife was even impressed)!!!

Bob


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## Doug S (Sep 9, 2003)

Should I be drinking domestic if measuring in CP and imports if measuring in Lux? If so, my measurements are all hosed since I have been measuring in Lux but drinking [very cheap!] domestic.


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## brightnorm (Sep 10, 2003)

[ QUOTE ]
*JohnK said:*
As an optometrist that has done industrial work, I don't really understand the value of the Lux meters (with flash lights). They measure a brightness value at a "point", which says absolutely nothing about the "surround", or "lumens", or really anything.

I could forsee a manufacturer making a reflector that pops a zillion "lux" at a tiny point, then goes to heck every where else.

I think the only scenerio where this measurement is useful, is in real world situations with DIFFUSE lighting; where perhaps an assembly line is fairly uniformly lighted by fluroscents, etc., and judgements can be made as to the proper brightness/illumination level.

With tightly focused lights, I find the measurements almost useless; viv a vis a LUX reading for the SL TL-2 LED at far higher readings than the TL-3 LED, or the SF L4. I like the TL-2 LED, and TL-3 LED, I own them both, but feel that these reading are at best misleading (Quickbeam, whom I respect a great deal, is the first to explain this, when he gives the readings), and at worst, totally misrepresent what we are trying to find out. 

"Lux" measurements give you nothing except the often noted example of a Laser brightness at ONE POINT, and absolutely NOTHING else.
NO other information, zip, nada !

Gotta be a better way.

I suppose reviewers astute comments, added to the "Lux" stuff will have to make the difference.



[/ QUOTE ]

John,

That's exactly why I use indirect "total output" measurements and rate lights only in relation to each other, rather than to a specific standard. I have several methods of indirect measurement and find that though they may give different absolute figures, the proportional relationship between lights stays almost exactly the same, within 1%-2%, so I feel comfortable that my readings tell me what I need to know as far as total output is concerned.
I also agree that an astute eyeball description can be as useful, or sometimes even more useful than a "number".

Brightnorm


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## Luter (Jan 4, 2015)

Measuring Lumens with lux meater is tricky business, there is however better technique to find out the lumens your flashlight throwing out.
And all you need is Amp meter, all you have to do is just measure the current your flashlight using in giving mode, check the table of manufacture of the led according of current and lumens related to it and you will no how much lumens your flashlight throwing, I found like that many cheap flashlight are not correctly labeled for example SKYRAY 6x CREE XM-L T6 labeled as have 9600 lumens is nowhere near, it has less then 1/3 of that. In manufacture specification of cree xml t6 stated that at 700mA throw 280 lumens, now there are 6 of them so if the flashlight taking current of 4.3 Amps = 4300 mA it means 4300/6 = 716.666 so 1 led takes 716mA that means 280lumens x 6 = 1680lumens only! of course you have to be sure that you use proper battery which can give you enough current and it is charged to full voltage as the current flow drop with drop of voltage. xml t6 with forward voltage of 3.35V will take through the led 3000mA but with forward voltage 3.1V will take only half of that current. all this specification are on manufacturer of led, and most of the used Led are made by Cree and they have full downloadable .pdf specification sheet for their leds. I hope this will help to make the measuring lumens of your flashlight more easier if is that what you trying to do.


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