Measuring throw: Why do we use lux @ 1m?

MrNaz

Enlightened
Joined
Jul 20, 2006
Messages
244
Location
Melbourne, Australia
Hi all,
When measuring throw, it seems to me that 1m is not really long enough to separate total output from directed output, as the beam has not really had enough distance to spread.

Without doing any geometry or calculation, I would have thought that in order to really separate the men from the boys when it comes to throw, you'd need to do lux @ 5m or even lux @10m to really get a feel for which lights out throw others.

Thoughts?
- Naz.
 

thedoc007

Flashlight Enthusiast
Joined
Feb 16, 2013
Messages
3,632
Location
Michigan, USA
When measuring throw, it seems to me that 1m is not really long enough to separate total output from directed output, as the beam has not really had enough distance to spread.

You are correct that one meter is not far enough away to get the max intensity correctly measured, for real throwers. It isn't because the beam hasn't spread, though. It is because it is not focused for short range use.

Without doing any geometry or calculation, I would have thought that in order to really separate the men from the boys when it comes to throw, you'd need to do lux @ 5m or even lux @10m to really get a feel for which lights out throw others.

Or even more, I have heard people who claim that fifteen meters is more appropriate. To be honest, though, just because the lux is standardized into units at one meter, doesn't mean that you can't easily measure at ten meters, and adjust the numbers. If you know the beam intensity at any given distance, you know it at every given distance. It was always going to be arbitrary, and virtually everyone knows how far a meter is, and can easily test it themselves if they wish to do so. There are lights that can't even make it out to ten meters (a Fauxton, for example), which would mean their beam intensity would be virtually zero, if you took the measurement at ten meters. Standardizing at one meter allows pretty much every light to be comparable with one number.
 

WalkIntoTheLight

Flashlight Enthusiast
Joined
Jun 18, 2014
Messages
3,967
Location
Canada
To be honest, though, just because the lux is standardized into units at one meter, doesn't mean that you can't easily measure at ten meters, and adjust the numbers. If you know the beam intensity at any given distance, you know it at every given distance.

I don't think that's necessarily true. You can't just run an inverse-square law and apply it to the brightness of the light at a certain distance. It all depends on the geometry of the reflector, size of the LED, focal length, etc. A perfectly parabolic reflector with a pin-point emitter focused at infinity would have tremendous throw without the need for very many lumens. How close the design comes to theoretical limits greatly influences what kind of intensity drop-off curve it has.

IMO, the OP has a good point. 1 metre is not a useful distance. 100 metres seems more appropriate.
 
Last edited:

thedoc007

Flashlight Enthusiast
Joined
Feb 16, 2013
Messages
3,632
Location
Michigan, USA
I don't think that's necessarily true. You can't just run an inverse-square law and apply it to the brightness of the light at a certain distance. It all depends on the geometry of the reflector, size of the LED, focal length, etc. A perfectly parabolic reflector with a pin-point emitter focused at infinity would have tremendous throw without the need for very many lumens. How close the design comes to theoretical limits greatly influences what kind of intensity drop-off curve it has.

IMO, the OP has a good point. 1 metre is not a useful distance. 100 metres seems more appropriate.

True that you can't EXACTLY determine at any given distance, given only one point. But you can get a very good estimate. Certainly enough to compare one light to another.

100 meters would not be practical. Think about how much more difficult that would make it to test. Also, candela doesn't measure just throwers. It is a useful tool for any type of beam, including flooders. And like I said before, the best thing about it is that you can compare pretty much every light to any other light. Measuring at 100 meters would not allow that.
 

WalkIntoTheLight

Flashlight Enthusiast
Joined
Jun 18, 2014
Messages
3,967
Location
Canada
100 meters would not be practical. Think about how much more difficult that would make it to test.

I don't understand why it would be difficult to test. Manufacturers must surely have access to a dark area, even just outside at night, if not a warehouse. They'd only have to test a few units to see what the design outputs.

Also, candela doesn't measure just throwers. It is a useful tool for any type of beam, including flooders.

It's less useful for flooders. Most of the lights I've bought which are floody don't even list candela, they only list lumens. Throwers seem more (rightly) concerned about candela. Floody lights all seem to compete on lumens.

And like I said before, the best thing about it is that you can compare pretty much every light to any other light. Measuring at 100 meters would not allow that.

That's true, it would eliminate floody lights. It would make a more useful comparison for throwers, which I think is the more important category of lights for this kind of measurement. Most people buying a thrower want to know how it really performs at 100 metres or more.
 

MrNaz

Enlightened
Joined
Jul 20, 2006
Messages
244
Location
Melbourne, Australia
The main reason that I've brought this up is that I am looking for the ultimate thrower to mount on my scope. I want to be able to look into the scope and have a target lit up even if it's 200m+ away. I've bought throwers based on lux @ 1m ratings from reviews, but have found that the lux @ 1m rating are absolutely useless for throwers.

I think we need some new measure for rating a light's throw ability. If we're going to take throwy lights seriously, then I suggest we as a community get together and develop a test that can more accurately measure this so that people like me, looking for a good thrower, don't end up having to buy 10 lights before they find the one that can *really* throw.

My initial suggestion would be for lights that are supposed to be throwers, set some standard distance that any half way decent thrower should be able to make, say, 25m. We could call this rating the "True Lux @ 25m" or something like that.
 

TEEJ

Flashaholic
Joined
Jan 12, 2012
Messages
7,490
Location
NJ
Just to clarify:

1) They DON'T measure at 1 meter, they back calculate to 1 meter to make extrapolations easier. (They measure at 10 meters, do math to see what that would give at 1 meter, and give the cd as the lux at 1 meter)

The ANSI standard specifies 10 meters as the measurement distance.

In some cases, 10 meters is not enough either.

2) Using the inverse square law, you can MATHEMATICALLY extrapolate the lux at one meter to the lux at any other given distance. Again, the back calculated lux at one meter is used, NOT the actual lux at one meter.

3) Lumens are the measurement of a light's TOTAL output. This has zero to do with throw, other than that if you know the throw and the lumens, you can make guess about the potential beam shapes that might result. For example, a light with a throw of 1 mile and 1,000 lumens is going to have a small dowel shaped beam, and a light with a throw of 50 meters and 1000 lumens will have a floody beam, etc. You can't know the ACTUAL beam shape as to the proportions of the beam's lumen allocation to each range though, from that, only the concept of flood vs throw as a priority, etc.



So, sure, publishing specs is a good idea. The viewers of the specs just need to know how to interpret what they are given.


For example, the ANSI throw is measured at 10 meters. Some throwers might throw farther than their rated throw if they have beams that take longer to reach a representative brightness in their hotspot. Some definitely need over 15 - 20 meters for example. The 30 meter ANSI measurement range can be chosen then, etc.

My own preference would be to take lux readings at gradually increasing distances until the MAXIMUM cd was reached. (The LUX would keep dropping with distance of course, but, using the inverse square law, eventually, the lower lux at a longer distance would result in a higher cd)




So, to me, the light's cd is super useful, as I can then calculate the lux at any given distance. If they "under estimated the cd", say by 10 meters being too short for full beam formation, that's not as bad as OVER estimating it...as a light selected for a particular use would be able to AT LEAST reach an estimated distance and intensity.

So, the cd and the lumens give some apples and apples comparisons between lights, that while in no way perfect or complete, allow - for the first time - a way to COMPARE lights relative to potential performance attributes.


This is in sharp contrast to the "Candle Power Wars" waged on the shelves across the world, with lights claiming "1 Million CP! - 5 Million CP! - 15 Million CP! - etc, and the 15 million CP ones might be WEAKER than a 1 million CP version, simply because the "1 million CP" version exaggerated its CP less, and the "15 million CP" version exaggerated it more, etc.

So the 1 million CP light might have had, in reality, 50,000 cp, and the 15 million CP one might have had ~ 35,000 CP, and so forth, there was no correlation between advertised numbers and actual performance - it was like "eBay Lumens" and eBay mah", etc....100% advertising/0% performance.


Using the ANSI standard was therefore a real step in the right direction - as - at least for regions with jurisdiction, the crazy claims all but stopped. The other areas are still "Wild West" though.



------------


So if you are selecting a light for use at 200 meters, say for night shooting, depending upon the optics low light performance, and the shooter's degree of night vision/adaptation, most need ~ 1- 5 lux on target to aim well enough to hit the target at that range (A paper target). They might need closer to 15 lux on target to hit a low contrast target though, say a guy prone in dark clothing, etc.

If you know the light's cd, you can then calculate what the lux would BE at 200 meters.

You can IGNORE the PUBLISHED ranges in meters, that are BASED UPON the lights lux at 1 meter (cd), because those use 0.25 lux as the target intensity at the published range.

As 0.25 lux is 100% useless (Unless your optics can let you take a good shot with a half moon as your only light source at 200 meters...) - you need to forget the PUBLISHED range, and, focus on the PUBLISHED or, the CALCULATED cd. (This means the lux at 1 M spec is not useless, its just not ready to USE w/o understanding what it represents, which is the throw to 0.25 lux)


So if the light's published range is ~ 200 meters, and, most shooters need AT LEAST 1 lux on target to hit it, you know that 1 lux is 4x the 0.25 lux they used.

To get 4x brighter illumination, you'd need to either get twice as close, or, get a light with 4x the cd....to have one lux at 200 meters instead of 0.25 lux at 200 meters.


Example: A light publishes a range of 200 meters. That means its got a rated 10 k cd (As that would get you 0.25 lux at 200 m): Even if they ONLY give a range in meters, you KNOW the cd - as they gave you the range to 0.25 lux.


If you REALLY need 1 lux at 200 m, you need a light with 4x the cd, or, 40 k cd.

If you REALLY need 15 lux at 200 M, then, you'd need a light rated at ~ 600 k cd (15 times 40 k)

And so forth.


So picking ONE range and illumination level (Such as "True Lux at 25 M", etc) would be about the same as having a back calculated lux at one meter. Except the math from 25 M is harder than using 1 M...so, the 1 M range is fine...as it allows plugging in the values to figure out what lux at what range will work for YOU.

:D
 
Last edited:

WalkIntoTheLight

Flashlight Enthusiast
Joined
Jun 18, 2014
Messages
3,967
Location
Canada
2) Using the inverse square law, you can MATHEMATICALLY extrapolate the lux at one meter to the lux at any other given distance. Again, the back calculated lux at one meter is used, NOT the actual lux at one meter.

I disagree with your statement. The inverse-square law is only accurate for calculating the brightness of a source of light without any reflector. As soon as you introduce a parabolic reflector, you're intentionally designing a light that will maintain its intensity over a long distance, and the inverse-square law no longer applies.

The whole point of a thrower is to project a column of light, not a spread-out beam. How good the design is at doing that will greatly effect how much true throw it has, not the lux measured at 1m or 10m. If you want to know what the lux is at 100m, you have to measure it, you can't calculate it based on the inverse-square law.

That said, I will agree with you that the inverse-square law often does apply, since a design often fails to project a column of light. But in something like a spotlight, (or a laser!), it doesn't.

P.S. Even if the design isn't able to project a column of light, the reflector design may be constructed to focus the light at a certain distance. At that optimal distance (which likely isn't 1 metre), the intensity will be brightest. After that point, perhaps it might follow approximately an inverse-square.
 
Last edited:

dss_777

Enlightened
Joined
Oct 31, 2004
Messages
900
I saw this post, and immediately thought of TEEJ. :)

Just to clarify:


[... really smart guy stuff]


:D



Be honest: did you pump your fist and say "YES!!" when you saw this question?

:D

Since I have nothing intelligent to add to the thread, I'll go back to my seat in the back of the room.
 

thedoc007

Flashlight Enthusiast
Joined
Feb 16, 2013
Messages
3,632
Location
Michigan, USA
I disagree with your statement. The inverse-square law is only accurate for calculating the brightness of a source of light without any reflector. As soon as you introduce a parabolic reflector, you're intentionally designing a light that will maintain its intensity over a long distance, and the inverse-square law no longer applies.

There is plenty of empirical evidence that it DOES apply. rdrfronty is one member here who has taken lux measurements both up close, and at ~600 meters, if I remember correctly. And the values are very close to what you would expect (extrapolating one point from the other), demonstrating that it does apply to a whole slew of throwers in real use.

As to why it would make it more difficult, you have to admit that testing in one meter is pretty easy. Anyone can easily measure that distance accurately, for one. 100 meters becomes more difficult...not too many people have the tools to measure it accurately. I also like the idea that anyone can test it out - we don't have to take the manufacturer's word for it. I assume the tests require complete darkness to be most accurate, and making a room dark is a hell of a lot easier than finding a 100 meter long area that is totally dark, or creating one in a warehouse. Ambient light can definitely change a reading, especially when you are talking about fractions of a lux.

Just about every light I have seen, by a decent manufacturer, has published a candela rating along with lumens. It gives you some idea of the beam profile instantly, and therefore is quite useful for flooders also. Just like every thrower is different, every flooder is different too. Just because you think they should be excluded, doesn't mean that makes sense to the rest of us.
 

WalkIntoTheLight

Flashlight Enthusiast
Joined
Jun 18, 2014
Messages
3,967
Location
Canada
There is plenty of empirical evidence that it DOES apply. rdrfronty is one member here who has taken lux measurements both up close, and at ~600 meters, if I remember correctly. And the values are very close to what you would expect (extrapolating one point from the other), demonstrating that it does apply to a whole slew of throwers in real use.

I'll admit I was talking more theoretical, and that in practice the inverse-square is usually good enough. Still, on really good throwers, like a laser or a WW2 search-light, it doesn't work too well. But yes, that's more theoretical. I'll take your word for it that it applies to the other 99% of lights.

As to why it would make it more difficult, you have to admit that testing in one meter is pretty easy. ... Ambient light can definitely change a reading.

You're letting manufacturers get away with being very lazy! ;-)

Just about every light I have seen, by a decent manufacturer, has published a candela rating along with lumens. It gives you some idea of the beam profile instantly, and therefore is quite useful for flooders also.

Of the last 3 manufacturers I've purchased from, only 4sevens gives an indirect idea of the candela. The Quarks (tac and pro) list a distance, which I assume is the distance where lux drops off to 0.25? 4sevens don't list anything other than lumens for their mule lights, at least not on the box, maybe they do on their website.

Neither Zebralight or L3 Illuminations show any kind of candela or lux or distance on their packaging or manuals. I don't think they list anything on their website specs either. Okay, the L3 Illuminations is a cheap light, so perhaps they wouldn't be expected to list it. But Zebralights aren't cheap. I think they don't list it because they're floody lights and it wouldn't look good.

I don't have packaging for most of the other brands I've purchased. But IIRC, Maglite does list candella or something like it.

Just like every thrower is different, every flooder is different too. Just because you think they should be excluded, doesn't mean that makes sense to the rest of us.

Don't get me wrong. I don't think they should be excluded. More info is always better! I just think it doesn't really apply to their intended use. When people buy floody lights, they care more about lumens than candella.
 
Last edited:

Poppy

Flashaholic
Joined
Dec 20, 2012
Messages
8,359
Location
Northern New Jersey
I saw this post, and immediately thought of TEEJ. :)


Be honest: did you pump your fist and say "YES!!" when you saw this question?

:D

LOL... growing up, my Dad used to give my younger brothers the choice, do you want a beating?, OR do you want me to talk to you!? Invariably, they would take the beating. LOL... the pain didn't last as long :)

Yes..' I always try to read TEEJ's responses, I can often learn from them. For example, I didn't know that the ANSI specs call for the lux measurement to be taken at 10 meters and calculated back. I knew that was sometimes done, but not routinely.

Hey TEEJ... I'll bet you didn't get one of these :nana: in a while, eh?
 

MrNaz

Enlightened
Joined
Jul 20, 2006
Messages
244
Location
Melbourne, Australia
Thanks for that illuminating (hur hur) post. However, all that rests upon a fundamentally flawed assumption in the measurement process. Namely:

2) Using the inverse square law, you can MATHEMATICALLY extrapolate the lux at one meter to the lux at any other given distance. Again, the back calculated lux at one meter is used, NOT the actual lux at one meter.

This is ONLY true if flashlights were ideal point sources. They are not, and so using the inverse square law is an invalid assumption that leads to deeply flawed conclusions, especially when it is applied to measurements taken at short distances.

There is no need to argue over this point; I challenge anyone to demonstrate that the inverse square law holds true in even an approximate way for a light by measuring lux at one meter intervals from 1m to 10m. I'm willing to bet $100 that there is not a single flashlight that will even approximate the inverse square law over that distance range.
 

thedoc007

Flashlight Enthusiast
Joined
Feb 16, 2013
Messages
3,632
Location
Michigan, USA
Me and my big mouth. I just realized someone will take the reflector out of an XR-E and prove me wrong :ohgeez:

Not only that, but we were talking about the reasons to use 100 vs. 15 vs. 10 meters. For throwers, we can agree that one meter is not a good distance. But in real world use, over the distances that actually matter, the inverse square estimate works just fine. You can argue the technical point all day, but having the info standardized is still a good thing. And it works well enough.
 

880arm

Flashlight Enthusiast
Joined
Aug 29, 2011
Messages
1,752
Location
Wildlands of Western Kentucky
There is a lot of good information in this thread with a touch of misinformation and misunderstanding thrown in. To the OP, you are correct when you say measuring lux @ 1m will not give a representative measure of the output from some (probably most) lights. However, the number in the specifications does not necessarily mean what you think as it is a relative value and not an absolute. This is an important distinction.

Most reputable manufacturers state their product claims based upon testing in accordance with ANSI FL 1-2009 which outlines specific methods to be used in lab conditions. With regard to peak beam intensity, the manufacturer has a choice of three distances to use for the test - 2m, 10m, or 30m -so long as the distance is at least 10 times the largest dimension of the light's lens. A light measuring device is used to identify and record the highest indicated values (between 30 seconds to 2 minutes after turning on the device) from three separate samples. These are averaged and plugged into the following calculation:

Avg. Measured Intensity x (distance)^2 = Peak beam intensity

Naturally it would be in the manufacturer's best interest to take the readings at a time and distance that was most beneficial to them. They are not required to report either of these testing parameters as part of the performance claim. The result of this calculation is then used to calculate the Max Beam Distance using the Inverse Square Law as mentioned in the other posts.

In no way is the calculated Peak Beam Intensity meant to be used as an absolute value or accurate reflection of what the given light's measurement would be at 1m. Instead it is intended to provide a reasonable basis of comparison between lights with differing beam profiles and output levels. Without this basis, most consumers would fail to recognize the considerable difference between a light rated at 300 lux (at 10m) and one rated at 30 lux (at 100m).

As the standard was written specifically for hand-held/portable flashlights, spotlights and headlamps I think the results, when applied correctly, are relevant and useful. There will always be extreme cases that become statistical outliers but standards are typically written for the norm, not the exception. This is not a defense of the standard as I'm not sure I agree with every part of it, but until a better alternative comes along it's the best we've got. Like most standards I expect it to evolve over time, albeit slowly.

One important part of FL 1 that isn't often discussed is that a manufacturer making any claim based on FL 1, must base all other performance claims for that product upon the requirements of the standard. For example, they do not have the option of doing their runtime testing according to FL 1 and then using their own method (or test distance) for determining peak beam intensity.
 

thedoc007

Flashlight Enthusiast
Joined
Feb 16, 2013
Messages
3,632
Location
Michigan, USA
In no way is the calculated Peak Beam Intensity meant to be used as an absolute value or accurate reflection of what the given light's measurement would be at 1m. Instead it is intended to provide a reasonable basis of comparison between lights with differing beam profiles and output levels.

This was my essential point from the beginning, but you said it far better. Thanks for that summary - I learned something today!
 

Timothybil

Flashlight Enthusiast
Joined
Nov 9, 2007
Messages
3,662
Location
The great state of Misery (Missouri)
Thanks for that illuminating (hur hur) post. However, all that rests upon a fundamentally flawed assumption in the measurement process. Namely:



This is ONLY true if flashlights were ideal point sources. They are not, and so using the inverse square law is an invalid assumption that leads to deeply flawed conclusions, especially when it is applied to measurements taken at short distances.

There is no need to argue over this point; I challenge anyone to demonstrate that the inverse square law holds true in even an approximate way for a light by measuring lux at one meter intervals from 1m to 10m. I'm willing to bet $100 that there is not a single flashlight that will even approximate the inverse square law over that distance range.

You are close, but missing the point here (point, get it, point). Yes, the inverse square law defines what happens to light from a theoretical point source of no size. In and of itself, that does not take into account illumination sources larger than a point, reflectors, lenses, coherent light and all that jazz. But, if I have an led die giving off n lumens evenly across the surface, the lumens from any point on that surface will obey the inverse-square law. If there are p molecules emitting photons on that surface, then one had p point sources, and the photons from each point source will obey the inverse square law. And unless one is talking about an extremely short focal length parabolic reflector, the vast majority of those photons will not even touch the reflector, As a rule of thumb, the shorter the focal length of the reflector, the less spill, because a greater percentage of the photons will be reflecting off of the reflector and being redirected forwards. The net result is that the deeper the reflector, the more using the inverse square law to calculate the lux for a given distance will tend to under-represent the actual lux available at that distance. But nothing is perfect, and it still gives us basis for doing an apples to somewhat apples comparison. I agree that I wish all vendors provided ANSI info for their lights. Streamlight and Nitecore do, but sadly, Surefire doesn't. Since those are the three I have experience with, they are the ones that I look to for my lighting needs.
 

Poppy

Flashaholic
Joined
Dec 20, 2012
Messages
8,359
Location
Northern New Jersey
I agree that I wish all vendors provided ANSI info for their lights. Streamlight and Nitecore do, but sadly, Surefire doesn't. Since those are the three I have experience with, they are the ones that I look to for my lighting needs.

When Surefire states their beam distance in meters, I take that number, divide it in half, then square the product. That should give an approximate cd.
 
Top