# Easy to Understand Lumens Vs Lux Explanation

#### fixit7

##### Newly Enlightened
So it looks like lux is variable as it varies with the object it is striking. The lux reflected off a mirror would be higher than from reflection off a black cloth.

Whereas lumens is good indicator of how much light a light is giving out.

#### Bullzeyebill

##### Flashaholic
Actually. the light reflected back from a medium would show the perceived brightness of that object. Go up to a black wall and point a light meter at the incoming light and you will get a lux reading no different than shining the light at a white wall, if the distance is the same.

Bill

#### brttina

##### Newly Enlightened
Thanks for the sharing.:twothumbs

#### TEEJ

##### Flashaholic
The "target" is a lux meter.

The meter is calibrated to the size of its sensor, so, the lux (Lumens per square meter) is calculated from whatever hits the sensor of known size.

The actual UNITS are expressed in terms of lumens per square meter (Lux).

The throw unit (cd) is expressed in terms of lumens per square meter at one meter distance.... even though the actual throw is basically never that number at that distance... as that number is merely useful for calculating the lux on target at long ranges, where a thrower shines, so to speak.

#### TEEJ

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

Wondering how to glean hotspot vs corona information.
The "target" is a lux meter.

The meter is calibrated to the size of its sensor, so, the lux (Lumens per square meter) is calculated from whatever hits the sensor of known size.

The actual UNITS are expressed in terms of lumens per square meter (Lux).

The throw unit (cd) is expressed in terms of lumens per square meter at one meter distance.... even though the actual throw is basically never that number at that distance... as that number is merely useful for calculating the lux on target at long ranges, where a thrower shines, so to speak.

As for corona and hot spot, spill, etc.... its information that is typically not available.

If looking for throw, assume there is no corona or spill, and concentrate upon the cd. The more cd, the more distance you can see to.

If comparing two lights, for example, the one with the higher cd throws further.

After that, look at the lumens.

As a rule of thumb, if they have the same cd, but one has more lumens, the hot spot is USUALLY larger (same brightness, but a larger diameter beam spot).

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#### lednight

##### Newly Enlightened
Thank you for this in-depth conversation about Lux, Lumens and cd, wow! I started reading all the replies, but did not see my own question, for a smaller size light with good throw, should we look for a aspheric lense to minimize corona (wasted light?) Would this make a big difference in keeping the light concentrated?

#### Dave111

##### Newly Enlightened
Thanks a ton! Always had been scratching my head between those two terms.

#### 37Blenman

##### Newly Enlightened
Thanks for the info, quite interesting!!

#### dmichael

##### Newly Enlightened
Very interesting. Thanks!!

#### msmith

##### Newly Enlightened
Much appreciated - was trying to figure this exact conversion out while shopping for my new lights! Thanks! :twothumbs

#### desert.snake

##### Flashlight Enthusiast
Since this topic has already surfaced, I remembered one old explanation, in fact the same as the first few messages here, but with live pictures for better understanding

#### chillinn

##### Flashlight Enthusiast
Appreciate the explanation of Lux, massively, as I was never quite sure of their relation to Lumens. But I have pulled two of TEEJ's quotes (and one twice) from separate comments because they contradict each other, but also something in the first post I disagree with and most know it not to be true. I'll start with that, then go back to what I'll call the hotspot conjecture.

but you can see more with more lumens

This is generally false. To illustrate why, some absurdity: imagine having all the lumens, every single one. If TEEJ's statement was true, you'd see more than anyone else ever, but in practice you wouldn't be able to see anything at all, and I think you'd probably never see anything again. What I have learned from CPF is that you'll see a lot more with a dimmer and warmer HiCRI light than with a much brighter much cooler and lower CRI light. The irony is the popularity of bright, extremely cool color temperature lights, because the user actually sees less, and less and less as the blue in the cool white slowly permanently blinds them. This has been argued ad infinitum elsewhere in many CPF threads.

Humans suck at judging brightness in of itself...but you can see more with more lumens, but, most of the added vision is to the sides in a broad pool of light...and less is added to a central hotspot.

In real life, the beams tend to have three main parts though: A hot spot, the brightest part in the center, the corona, a donut shaped ring of somewhat dimmer light around the hot spot, and spill, the light that was sent out past the reflector bowl without being focused.

The second half of the quote from the first post and the quote from a subsequent post are contradictory. The second quote explains reflectors quite nicely, but the issue I have is the insinuation from the first quote that most of the light that comes out of a reflectored light is the spill even though the hot spot is brighter. But assuming the second quote is true, and I think it is, and we follow the logic and geometry, the spill couldn't have more light than the hotspot.

The light coming off a filament or emitter goes in all directions. But because the filament or emitter is surrounded on all sides laterally and most of it's bottom and the sides of above it, and all light bounced off the reflector contributes to the hotspot, and the spill is only the light that happens to miss the reflector on one side, namely, in some direction not parallel (or else it may contribute to the hotspot or otherwise be negligible) but definitely out the front, then the hot spot contains most of the light that the filament or emitter produces because of the larger surface area of the 3D reflector compared to the 2D lens.

I'm terrible at explaining things, but I hope that makes sense, that the hot spot represents most of the light produced, except for the minority of light that misses the reflector and becomes spill.

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