Easy to Understand Lumens Vs Lux Explanation

[njhart]

And it is appreciated! I fully admit, I'm far more of a knife guy (at least at the moment) than a light guy. I have sort of specific wants that I'm having a hard time picking out. As such rather than just blindly asking around, I decided that I should at least have a moderate knowledge of what I'm looking at if I expect to get what I want.

 

[japudjuha]

So with lumens being the total light output, does Lux roughly equate to candle power/ intensity of the light?
I get lumens, but the Lux/Candella that I don't fully 'get' just yet.
Thanks

 

[TEEJ]

So with lumens being the total light output, does Lux roughly equate to candle power/ intensity of the light?
I get lumens, but the Lux/Candella that I don't fully 'get' just yet.
Thanks

Lux is a measure of how bright the target looks to you. Its the lumens per square meter....so the more lumens you send into a square meter of target, the brighter the target looks to you.

The equivalent Lux at one meter is the candela (cd) rating for the light. This is a way of standardizing the specs.

So if a light puts 10k lux on a target one meter away, it's rated at 10k cd.


Examples:

If the beam angle is the same, for a given range, if I double the lumens it will double the lux (Because I am sending twice the lumens per square meter).

If I want to keep the lux the same at that range, I can make the beam angle wider, to spread the added lumens out over twice the area (So I end up with the same number of lumens per square meter).


The lux will decrease with range, so more distant targets are progressively dimmer.

The targets will look one quarter as bright at double the distance. (4X DIMMER at 2X distance)


That means that the 10k cd light in the above example, with 10,000 lux at one meter, would have 2,500 lux at 2 meters, and 625 lux at 4 meters, 6.25 lux on a target at 40 meters, and 1 lux at 100 meters, and, finally, 0.25 lux at 200 meters, and so forth.

The ANSI standard uses a 0.25 lux limit as defining a light's maximum range...so, if a light is advertised with a 200 meter range, that means it's rated at 10k cd, and so forth.

 

[Poppy]

Good job TEEJ :twothumbs

Something I learned from you is that if you take the square root of the published cd of the light, that will give you the distance in meters that it will produce 1 lux.
For example the square root of 10,000 cd = 100 meters
For me, 1 lux is much more usable light than 0.25 lux.

 

[Asad]

So with lumens being the total light output, does Lux roughly equate to candle power/ intensity of the light?
I get lumens, but the Lux/Candella that I don't fully 'get' just yet.
Thanks

You're right. Lux is the unit of beam intensity, which is the number of lumens per unit area. As the beam spreads the beam intensity decreases. Moreover, the beam intensity is not uniform all across, usually more intense in the centre (the hotspot).

 

[TEEJ]

Good job TEEJ :twothumbs

Something I learned from you is that if you take the square root of the published cd of the light, that will give you the distance in meters that it will produce 1 lux.
For example the square root of 10,000 cd = 100 meters
For me, 1 lux is much more usable light than 0.25 lux.

Thanks Poppy!

A lot of people find, especially for longer ranges, that 1 lux is a lot more useful than 0.25. ANd that's its simply the square root of the cd is just so darn convenient.

 

[TEEJ]

You're right. Lux is the unit of beam intensity, which is the number of lumens per unit area. As the beam spreads the beam intensity decreases. Moreover, the beam intensity is not uniform all across, usually more intense in the centre (the hotspot).

Correct, for most lights...the hot spot is the brightest central part of the beam, and the corona surrounds that like a donut. The unfocused light that is not in the corona or hot spot, that just spills out is, strangely, simply called the spill. Some people confuse the spill with the corona, so its always a good idea to separate them in discussions.

Some lights, such as those with aspheric lenses simply project a picture of the LED itself, so there's little or no spill or corona, just a die shaped hot spot.

 

[davyro]

brilliant someone at work was asking me something similar but you've explained it 10x better than i could

 

[TEEJ]

brilliant someone at work was asking me something similar but you've explained it 10x better than i could

Thanks! Its a VERY common question, and a tough concept for many to get their heads around, especially with all the advertising specs and so forth that muddy the waters.

Feel free to quote me as needed.

 

[phosphor]

Thank you TEEJ ! More than once I've come across a post of yours that has added meaning, depth, and understanding to these basic scientific principles.

 

[TEEJ]

Thank you TEEJ ! More than once I've come across a post of yours that has added meaning, depth, and understanding to these basic scientific principles.

That's great, as making these topics easier to get a handle on is my goal.

 

[Razzle]

Agreed, TEEJ.

This newb new nothing about handheld lights until I read some of your posts yesterday. I'm still in the dark, but it's becoming clearer!

 

[Cataract]

Agreed, TEEJ.

This newb new nothing about handheld lights until I read some of your posts yesterday. I'm still in the dark, but it's becoming clearer!

We're all here to learn and CPF is the best place to learn all about flashlights and lighting. Perhaps I personally learned nothing about the difference between lumen and lux (I do calibrate light meters as part of my job), but I did learn a simple and efficient way to explain it to others. This stuff is great for everyone!

 

[TEEJ]

LOL

That's why this is a simple to understand explanation...those who either need a way to explain it to others, or, just want to understand for the first time, can use it as a resource.

I'm glad so many have found it a valuable resource, that just makes me glow.

:thanks:

 

[Beefcake the Mighty]

Thanks. Makes sense.

 

[TEEJ]

Glad it helped.

 

[Poppy]

Hey TEEJ,
Congrats!
:thumbsup: Someone made this thread a sticky!

I was going to recommend it for the Unforgiven's "Threads of interest!" list.

 

[TEEJ]

Thanks guys!

 

[peterharvey73]

How is this?
Lumens vs Lux is the mechanical equivalent of Pressure = Force per Square Meter.

Lumens = Lux per Square Meter
Pressure = Force per Square Meter

So the Lux is like Force.
While he Lumen is like the Pressure.

I made it worse?

 

[Cataract]

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.