# Easy to Understand Lumens Vs Lux Explanation

#### TEEJ

##### Flashaholic
Here is a concrete example of what I am talking about. You don't have to buy an Acebeam K70 to carry out this experiment, but it will be more fun if you do!

So get your light set up for throw, and wander down field with your favorite book on the physics of light. Have an assistant sit with the flashlight, and turn it on and off as you move away. Use a cell phone to coordinate with your assistant.

Find the shortest distance at which there is not enough light to read by. With the K70, that should be about 1300 meters! When you do this, hold the book exactly perpendicular to the flashlight beam. Now, take a couple of steps back towards to the light. Move just enough so that you can read, and no more.

Okay, you have found the lowest lux at which you can read. Now tilt the book back 60 degrees. As the cosine of 60 degrees is 1/2, this will cut the lux in half. Voilà! You can no longer read. Even though you are still the same 1300 meters away from your K70 flashlight, the lux on target has changed.

This is proof that the lux on target depends on the angle at which your flashlight beam strikes the target.

I understand what you are saying, but, lux is what bounces back to your eyes....the lumens per square meter.

If the surface is angled, less is reflected back to your eyes, so the flashlight sent out the same amount and quality of light, but, essentially, less was reflected back to you.

If comparing lights, the differences in how far you can see, at the maximum range, will therefore be proportional to the beam's cd.

This THREAD is to explain the difference between lux and lumens...and to allow readers to gain a feel for what they represent.

If we go off on tangents (pun intended), we dilute the light we are trying to cast on the subject.

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Fun with pun.

#### dreze888

##### Newly Enlightened
Thanks for the in depth info. Learning as I go.

1 :devil:

#### vettex2

##### Newly Enlightened
I appreciate the explanation !

#### Herb987

##### Newly Enlightened
Re: Integrating sphereLux to lumens, final conversion confusion

Hi TEEj,

I would like to get your advice on calculating the lumens using an Integrating Sphere.

My issue is "not having enough area inside the sphere" in order to satisfy the quote in the next paragraph:

In your post you wrote: "So if the lux is the lumens per square meter (m2), and the interior surface area of the sphere is 1 m2, then a reading of 1 lux means that there must have been 1 lumen to create that reading."

===========================================

My equipment for measuring light is an Extech HD450.

===========================================

I was thinking of purchasing two 12" Smoothfoam Half Ball's from Hobby Lobby.
http://www.hobbylobby.com/Crafts-Hobbies/Basic-Crafts/Styrofoam/12"-Smoothfoam-Half-Ball/p/110834

and following the instructions here:

Let's ASSUME:
-- the INNER diameter of the above domes is 12" and
-- that I purchased 2 of them and glued them together to form a sphere.

I would then calculate the inner surface area as:
Area = 4 pi r^2

I would convert the diameter from 12" to a radius of 6".

I would convert the radius of 6" to meters ...

6 /39.37 = 0.1524 meters ... or ...

6*0.0254 = 0.1524 meters

and then plug the values into the formula for the area:

Area = 4 (3.14) (0.1524)^2
Area = 0.291716 square meters

===> At this point I have calculated that I do not have at least 1 square meter of surface area inside the sphere so I would not be able to have a 1 to 1 match when reading the lux on the meter and saying "the lux reading on the meter is the same as the lumens" because I do not have 1 square meter of surface area.

If I multiply the area I calculated above (0.291716 square meters) by 3.427 I would get an answer of 1 square meter.

Does that mean that since the area inside the sphere is less than 1 square meter that I would need to:
-- divide my lux reading by 3.427 -or-
-- multiply my lux by 0.291716
in order to get the lumens since I have less than 1 square meter?

Thanks.
H

(I removed some of the quoted original post that I'm replying to - Herb)
...

So if the lux is the lumens per square meter (m2), and the interior surface area of the sphere is 1 m2, then a reading of 1 lux means that there must have been 1 lumen to create that reading.

If the sphere is larger for example, then the surface area inside it might be 10 m2, so a reading of 1 lux would have required 10 lumens to achieve, or, the original 1 lumen source would have yielded 0.1 lumens as a result, and so forth.

#### lamp flashlight

##### Newly Enlightened
Fun with pun.

Thanks for the posting and have a good night

#### friskyplatypus

##### Newly Enlightened
I think this was the easiest explanation I've ever seen. Made it so that I understood even after having looked at multiple different explanations when buying flashlights.

#### bulrush

##### Newly Enlightened
This really helps. Thanks.

#### toasterz12

##### Newly Enlightened
If I'm understanding this right, wouldn't the lux change depending on how far the flashlight was from the target and/or whether or not the flashlight was focused? If so, how far away is lux measured?

#### MC408

##### Enlightened
Great info. New to the forum and learning right away!

#### TEEJ

##### Flashaholic
If I'm understanding this right, wouldn't the lux change depending on how far the flashlight was from the target and/or whether or not the flashlight was focused? If so, how far away is lux measured?

Exactly.

Lux can be MEASURED at any distance at all...as long as its far enough away for the beam to have fully formed....if the lux at that distance is to be used to calculate the lux at OTHER distances.

The cd of the flashlight is back calculated to represent the lux at 1 meter though.

#### ven

##### Flashaholic
:twothumbs:twothumbs hope all is well TEEJ, good to see you

#### UrbanExplorer

##### Newly Enlightened
I am by no means a specialist yet but I'm working towards it haha

Lumens is used to quantify the rate at which light flows from a source. For example it is like saying a hose puts out 5 gallons of water a minute. A flashlight might give off 1 lumen of light per second. Therefore although this is not a common practice you can technically convert lumens to a photon count if you take into account the wavelengths. One lumen of your average white light gives off 10^15 photons per second.

As for lux it is also a rate of the flow of light. The only difference is that it has more strict parameters. Luminous flux (lumens) include ALL light regardless of direction or spread or area in which it covers. Illuminance (lux) just tells you how much light is supplied to a specific surface per second, per square meter.
So let's say we have a beam of light that's circular and has 200lumens. The circle is 2 square meters. This means the luminous flux is 200 lumens but the spot it makes on the wall is 100 lumens per square meter. This can be changed by affecting the distance of the light from the wall or the focus of the light.

Hope this helps!

#### Landonb

##### Newly Enlightened
Good information here ,new to the addiction
need to read couple more times to fully sink in
thx

#### convolutionx

##### Newly Enlightened
So laser points would have high lux due to small area of output, but low lumen due to just general less photons coming out of the flashlight?

#### Keitho

##### Enlightened
So laser points would have high lux due to small area of output, but low lumen due to just general less photons coming out of the flashlight?
Yes, spot on. One way to think of it is to imagine the amount of power that the battery is producing. A laser pointer is bright enough to blind someone, even if it is inside a pen and is only powered by a tiny watch battery, because that tiny amount of power is focused into such a small area (and hopefully that area isn't on your retina). A huge 4x D cell maglight would have a hard time blinding someone, even at pretty close range, even though it is making a lot more lumens and consuming a lot more power--all those lumens are going all over the place.

High lux = "I'm blind"
High lumens = "crap, my huge battery is already dead" and "crap, my flashlight is too hot to hold"

#### lumen aeternum

##### Enlightened
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.

How big is the target? Is it 1m^2 or an infinite wall, at 1m distance?

Wondering how to glean hotspot vs corona information.

#### Sabbs1

##### Newly Enlightened
This was incredibly helpful for a new member, thank you. Excellent explanation!