# Easy to Understand Lumens Vs Lux Explanation

#### jorn

##### Flashlight Enthusiast
Why not: Lumens is the amount of light emitted from a light. Lux is how bright it is.

#### TEEJ

##### Flashaholic
Why not: Lumens is the amount of light emitted from a light. Lux is how bright it is.

Mostly because "how bright it is" is typically interpreted to mean the lumens...and those needing the explanation would not be able to differentiate...and, they needed a way to differentiate lumens from lux in meaning.

Add to that - that lux is how bright it looks on a target, not how bright the light is anyway.

#### TEEJ

##### Flashaholic
Not completely bad, but a little confused or confusing I'd say. This one would be simpler:

Lumens = total amount of pressure inside a closed system
Lux = specific pressure on the square inch which has the most pressure is applied.

Lux is like the force of the flow of water going forward
Lumens is like the total amount of pressure inside the whole system, including side-wall pressure that barely gets the flow going anywhere.

Pressure is harder to visualize though....especially for those with no plumbing experience, etc.

I want the explanation to be something that can be visualized...a mental picture. Using yet ANOTHER concept instead, is, IMHO, not as clear.

#### AbbyY

##### Newly Enlightened
Thanks a lot TEEJ for this useful information!

If I correctly understand, a value of 250.000 cd will be traduced in a 500 m distance (using square root rule - 1 lux, instead the inverse square meter rule - 0.25 ANSI standard). That means i.e. SR95S UT beam (250.000 cd) could reach 500 m as effective range (what our eyes can clearly see) vs ANSI range?

I have also noticed that if you double the value of the square root you get the value based on ANSI standard (500x2 = 1.000 m for SR95S UT).

#### TEEJ

##### Flashaholic
Thanks a lot TEEJ for this useful information!

If I correctly understand, a value of 250.000 cd will be traduced in a 500 m distance (using square root rule - 1 lux, instead the inverse square meter rule - 0.25 ANSI standard). That means i.e. SR95S UT beam (250.000 cd) could reach 500 m as effective range (what our eyes can clearly see) vs ANSI range?

I have also noticed that if you double the value of the square root you get the value based on ANSI standard (500x2 = 1.000 m for SR95S UT).

250,000 cd would yield 1 lux at 500 M and 0.25 lux at 1,000 M, as predicted.

Because its 1/4 as bright (lux) at two times the distance, whatever range its at 1 lux, will be a 1/4 lux at double that distance.

I would caveat the "effective range" and "what our eyes can clearly see" phrases you mentioned though.

Essentially, the lux needed to resolve details INCREASES WITH DISTANCE.

Testing performed to calibrate rifle scopes for night use revealed for example that at 200 meters, 1 lux WOULD be enough to shoot a 3' x 2' white paper target, for some people, even a bit less, but others might need closer to 5 lux, but, for everyone, closer to 15 lux was needed to even FIND a rusty ~ 3' steel target.

The contrast of the target and its background CHANGES the needed lux dramatically.

The farther away the target, the more you depend on your fovea to resolve small details (Farther away, things LOOK smaller, etc...) and your fovea has TERRIBLE night vision, and, requires a LOT of light to work.

So there is no one "Minimum Lux Level" that will always be "enough"....and that one lux figure is simply a common rule of thumb that is handy to work with....not a real hard figure.

:wave:

This is one of the reasons why someone might want a light that has a claimed 1,000 meter range, even though their friends chide that you can't see things 1,000 m away in daylight, so who needs a light with that kind of range, etc.

IE: That SR95 UT with its claimed 1,000 M (ANSI) range is probably NOT going to be USED to see things 1,000 M away....but, it CAN be used to see a low contrast target 100 M away...in practice.

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#### AbbyY

##### Newly Enlightened
Thank you TEEJ. A very good explanation!

#### experimentjon

##### Newly Enlightened
I've been into flashlights for a few years now...good to finally know the difference between lumens and lux. Great analogy. I also like the alcohol analogy too.

#### Poppy

##### Flashaholic
A couple of weeks ago I discovered something that I *think* may be pertinent to this discussion.

It appears that cd or lux is additive. In other words if you point a 5K cd light at an object you'll get 1 lux at 70 yards
If you point another 5K cd light at the same object, I believe that you'll get 2 lux at 70 yards.

#### TEEJ

##### Flashaholic
Essentially, as the lux is the lumens per square meter, if you add lumens to an area, the lux goes up proportionally. The lux doesn't know if one LED, a triple LED, three flashlights each with an LED, a candle and a mirror and magnifying glass and a glow stick, etc, SENT the lumens, the target just bounces back the ones that hit it...which is the lux you see.

The thing to keep in mind that this DOESN'T mean that if I have a flashlight with one LED, and it has say 10k cd, that if I add two more LED to the same flashlight it will now have 30k cd....but if I shine two more of that flashlight at my target, I might get 3x the lux on my target.

This is akin to a chandelier. If you have one with three 60 watt bulbs, another one with six 60 watt bulbs would make the room look brighter (More lux), even though each individual bulb has the same cd.

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#### Power Up

##### Newly Enlightened
So would it mean that the amount of Lumens is directly proportional to the Lux in a mathematical equation?

#### TEEJ

##### Flashaholic
So would it mean that the amount of Lumens is directly proportional to the Lux in a mathematical equation?

It might depend on the equation, but, its going to be directly proportional in relationship. Again, if the area's size the beam is ON is the same, but I have more lumens hitting it, I will have more lumens per square meter, which, = more lux. (Lux = lumens per square meter)

If I hit the same area with double the lumens, I will have double the lux, and so forth.

This is why two light bulbs in the same room's lamp will make the illuminated objects brighter even though the two bulbs have the same lumen output as each other...whatever lux I had with one bulb will essentially be doubled if that surface is hit by the lumens from the second bulb too.

(Its also why a multi-LED light can often have more throw than you'd think, as its dramatically increasing the lumen output, and in turn the resultant lux....despite the lumens form each individual LED being less focused due to the reflector focus efficiency compromises made to HAVE a head with multiple emitters, and why those reflectors are much larger, proportionally, to compensate, etc)

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#### degarb

##### Flashlight Enthusiast
Yeah, I hate it when someone describes a light as, "bright". I think it is the job of the moderator to get the poster to better define their term.

I think sellers, marketing folk, love the term "bright". Caveat Emptor.

I don't think it is a hard thing to understand, as long as it is not described in terms of fractions or inverse square law. The lux meter is a great tool to show people why a flashlight can be brighter to the Eye's Cones than a fixed light-even if the fixed light is 500 times the lumen output. I like also that photographer's lights (that I have purchased) are rated in lux at a distance, rather than lumens: meaning how bright the subject is illuminated is all that matters.

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#### TEEJ

##### Flashaholic
Yeah, I hate it when someone describes a light as, "bright". I think it is the job of the moderator to get the poster to better define their term.

I think sellers, marketing folk, love the term "bright". Caveat Emptor.

I don't think it is a hard thing to understand, as long as it is not described in terms of fractions or inverse square law. The lux meter is a great tool to show people why a flashlight can be brighter to the Eye's Cones than a fixed light-even if the fixed light is 500 times the lumen output. I like also that photographer's lights (that I have purchased) are rated in lux at a distance, rather than lumens: meaning how bright the subject is illuminated is all that matters.

"Bright" is one of those words that can have different meanings, and should probably be replaced, albeit replacing words is easier said than done.

#### degarb

##### Flashlight Enthusiast
"Bright" is one of those words that can have different meanings, and should probably be replaced, albeit replacing words is easier said than done.

I tried to coin the terms "Luxier and Lumier" ☺ I should relent, and we can go with "throwier" and "greater overall output".

So, is "BrightGuy" really LuxierGuy , or just LumierGuy.com ? Its been a mystery for years.

#### TEEJ

##### Flashaholic
I tried to coin the terms "Luxier and Lumier" ☺ I should relent, and we can go with "throwier" and "greater overall output".

So, is "BrightGuy" really LuxierGuy , or just LumierGuy.com ? Its been a mystery for years.

LOL

We HAVE lumens and lux, but, few know how to USE them in a sentence, hence this very thread.

I have noticed over an over again that the average person considers the amount of glare/pupil stop down to be how they judge "brightness". If my 131 lumen LED maglight is shined next to my 900 L SC600, the giant area flooded by the 900 L is "not as bright" as the teeny dot made by the 131 L mag. IE: Glare = Bright, for the guy off the street...in practice.

As the average person is used to an average off the shelf light, and those lights HAVE TO concentrate the little lumen output they have as much as they can just to create enough lux to resolve anything/reach anything...the average person thinks of a flashlight beam a making a small spot of light....that's "Normal".

If the light is floody, to them, that's a "Lantern".

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#### thedoc007

##### Flashlight Enthusiast
I really don't have a problem with "bright". I simply take it as "overall output", or lumens. "Focused" works for me to describe thrower lights, in a word. Virtually everyone understands what focused means, and how it makes a given output seem more intense, albeit over a smaller area.

It isn't perfect either, but it allows a quick summary to tell a non-flashaholic something useful. Personally I always look for a candela rating, which can give you a pretty good idea of what the beam will be useful for in a real world application. But you can't expect everyone to know what a candela rating means. Even some people on this forum have trouble with it, so the general public is hopeless...

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#### TEEJ

##### Flashaholic
I really don't have a problem with "bright". I simply take it as "overall output", or lumens. "Focused" works for me to describe thrower lights, in a word. Virtually everyone understands what focused means, and how it makes a given output seem more intense, albeit over a smaller area.

It isn't perfect either, but it allows a quick summary to tell a non-flashaholic something useful. Personally I always look for a candela rating, which can give you a pretty good idea of what the beam will be useful in a real world application. But you can't expect everyone to know what a candela rating means. Even some people on this forum have trouble with it, so the general public is hopeless...

Correct, except those people perceive glare (throw) as brightness, hence the lack of cross usage...as you think of bright as lumens, but, the great unwashed would be more inline with it meaning lux, even if they know neither term.

I see post after post from Newbs that say things like "I want the brightest light!" and "How many lumens do I need to see a fish 3' under water?" and "Is 1,000 lumens enough to let me shoot a coyote 200 m away?" etc.

Lux is not on the radar, and candle power is the gazillion thingies that the 55 watt halogen spot lights for \$29.99 advertise as having millions of...so they are USED TO seeing lights on shelves advertised as having 15 MILLION Candlepower!!!!, next to the smaller ones with a measly 10 million cp, and so forth. That's their world as they know it...ours is new.

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#### Power Up

##### Newly Enlightened
It might depend on the equation, but, its going to be directly proportional in relationship. Again, if the area's size the beam is ON is the same, but I have more lumens hitting it, I will have more lumens per square meter, which, = more lux. (Lux = lumens per square meter)

If I hit the same area with double the lumens, I will have double the lux, and so forth.

This is why two light bulbs in the same room's lamp will make the illuminated objects brighter even though the two bulbs have the same lumen output as each other...whatever lux I had with one bulb will essentially be doubled if that surface is hit by the lumens from the second bulb too.

(Its also why a multi-LED light can often have more throw than you'd think, as its dramatically increasing the lumen output, and in turn the resultant lux....despite the lumens form each individual LED being less focused due to the reflector focus efficiency compromises made to HAVE a head with multiple emitters, and why those reflectors are much larger, proportionally, to compensate, etc)

Got it loud and clear, thank you for enlightening a noob