Lux to Lumens

[Bigwilly]

Candela.

 

[TeaSipper]

Ok, I looked up lux and candela and they seem to be analogous? I guess I won't get it.

 

[KeepingItLight]

Ok, I looked up lux and candela and they seem to be analogous? I guess I won't get it.

Lux measures lumens per square meter at a given distance. The ANSI FL 1 distance rating for a flashlight, for instance, gives the distance at which peak beam intensity is 0.25 lux.

Candela gives the number of lumens per square meter, i.e., the lux, at a distance of 1 meter. Measurements of candela are usually made by measuring lux at distances greater than 1 meter, and normalizing back to 1 meter. This is so that a flashlight beam can focus properly before the measurement is taken.

Note: Checking with Wikipedia, I see that candela has a more complex definition than the one given above. For flashlight usage, however, I think the one above will suffice.

 

[Bigwilly]

Lux measures lumens per square meter at a given distance. The ANSI FL 1 distance rating for a flashlight, for instance, gives the distance at which peak beam intensity is 0.25 lux.

Candela gives number of lumens per square meter, i.e., the lux, at a distance of 1 meter. Measurements of candela are usually made by measuring lux at distances greater than 1 meter, and normalizing back to 1 meter. This is so that a flashlight beam can focus properly before the measurement is taken.

Note: Checking with Wikipedia, I see that candela has a more complex definition than the one given above. For flashlight usage, however, I think the one above will suffice.

That pretty much sums it up.

 

[lumen aeternum]

"Lux measures lumens per square meter at a given distance. The ANSI FL 1 distance rating for a flashlight, for instance, gives the distance at which peak beam intensity is 0.25 lux."

So if I'm standing in the dark, illuminated by some far off light source and 0.25 lux is shining upon me -- what do I see? How functionally bright is that?

If I am using binoculars to see a target 500 yards away being so illuminated, and they are really good binoculars wrt transmission losses -- what do I see?

 

[lumen aeternum]

Uh oh. Now I want a "moonlight" level flashlight which has a distance sensor. I point the light at some object some distance away, adjust the brightness, and then the sensor maintains that lux as I move the light around, pointing it at things different distances away.

So I can point it at a tree over there, and then point it at my map, and then the ground in front of me -- and my eyes experience a constant level of brightness.

 

[Bigwilly]

Uh oh. Now I want a "moonlight" level flashlight which has a distance sensor. I point the light at some object some distance away, adjust the brightness, and then the sensor maintains that lux as I move the light around, pointing it at things different distances away.

So I can point it at a tree over there, and then point it at my map, and then the ground in front of me -- and my eyes experience a constant level of brightness.

Checkout the Nitecore Sens CR.

 

[KeepingItLight]

So if I'm standing in the dark, illuminated by some far off light source and 0.25 lux is shining upon me -- what do I see? How functionally bright is that?

If 0.25 lux is what shines on and around you, it will be roughly as bright as a full moon on a clear night.

You've found the weakness in my hand-waving explanation. Lux does not really require a distance. That comes in because we are flashlight people, shining our lights from a certain distance away.

If you check Wikipedia, you will see that the definition is more complicated than I described. Given a surface in space, lux measures how many lumens per square meter are striking that surface. In general, each point on the surface will have a different lux. One lux is equal to one lumen per square meter.

There are some vectors involved as well. In the flashlight world, we always assume our lights are striking a flat target that is perpendicular to the beam. If the target is not perpendicular to the beam, however, then it will receive less lumens per square meter, i.e., less lux!

This is easy to understand if you visualize a distant wall as the target. Suppose the hot spot of a flashlight has expanded to a diameter of 10 meters when it strikes the wall. If the wall is perpendicular to the beam, then all the lumens from the hot spot will hit along a 10-meter section.

Now suppose the wall is strongly angled to the beam. If the angle is large enough, it might be that the hot spot lights up a section of the wall that is 100 meters long. You have the same number of lumens, but this time they are spread over a 100-meter section. On average, the lux at any point where the hot spot hits must be 1/10 of what is was when the wall was perpendicular to the beam.

If I am using binoculars to see a target 500 yards away being so illuminated, and they are really good binoculars wrt transmission losses -- what do I see?

I think the answer here depends on the binocular. Those with wide lenses gather more light than those with narrow lenses.

 

[drmaxx]

"Lux measures lumens per square meter at a given distance."

It already was established that this is wrong. There is no given distance. It is just the light intensity at a given spot.

So if I'm standing in the dark, illuminated by some far off light source and 0.25 lux is shining upon me -- what do I see? How functionally bright is that?

The easy answer is: Go outside on a clear night with half a moon. There is probably about 0.25 lux around you.
The long answer is: If you have 0.25 lux at a specific spot and you are looking at it from a distance, then this spot acts as a light source that reflects the light back to you. What you see depends on how much is reflected and how far you are away = how much of this light reaches your eye. E.g. The dark bark of a tree might get 10 lux from a flashlight - but you won't see anything, because the light gets adsorbed and not reflected into your eyes.

If I am using binoculars to see a target 500 yards away being so illuminated, and they are really good binoculars wrt transmission losses -- what do I see?

The binoculars don't change how much light (energy in form of photons) reaches you - it just makes it visually bigger. There might be a biological effect, that if you have a bigger light source you will excite more light receptors in your eye and therefore make it 'brighter'.

 

[KeepingItLight]

If I am using binoculars to see a target 500 yards away being so illuminated, and they are really good binoculars wrt transmission losses -- what do I see?

I think the answer here depends on the binocular. Those with wide lenses gather more light than those with narrow lenses.

The binoculars don't change how much light (energy in form of photons) reaches you - it just makes it visually bigger. There might be a biological effect, that if you have a bigger light source you will excite more light receptors in your eye and therefore make it 'brighter'.

Objective diameter: The diameter of the objective lens determines how much light can be gathered to form an image. This number directly affects performance. When magnification and quality is equal, the larger the second binocular number, the brighter the image as well as the sharper the image. An 8×40, then, will produce a brighter and sharper image than an 8×25, even though both enlarge the image an identical eight times.

Source: Wikipedia

Using a wide objective lens is akin to opening the aperture wide in a camera. Doing so lets in more light, which is then focused to create an image. Obviously, more light means more photons, despite the fact that the magnification remains unchanged.

 

[Fireclaw18]

Lumens is more like "all the photons," while lux is the concentration of them at a single point.

If you shined a light into a lux meter and panned the light side to side, you'd see different lux readings in the hotspot, spill, etc.

To get lumens, you must perform math and have a device called an "integrating sphere." The sphere must be "calibrated," but this doesn't really involve changing the sphere but coming up with the calibration math. It's kind of a bumpy road to get started.

Or, for a much rougher calculation of lumens, you could try the "integrating bathroom" instead of an integrating sphere.

 

[lumen aeternum]

"Lux measures lumens per square meter at a given distance."

It already was established that this is wrong. There is no given distance. It is just the light intensity at a given spot.

IIRC, If it is a point source and you want to use that intensity to calculate the lux at other distances, then you need the distance.

 

[Bigwilly]

Or, for a much rougher calculation of lumens, you could try the "integrating bathroom" instead of an integrating sphere.

What do you use as a sensor or what device measures the lumens?

 

[reppans]

Eyeball, smartphone app, manual camera, photog light meter, lux meter, etc (I've used them all), but as many have mentioned above, they all require calibration from a known/trusted output.

 

[Bigwilly]

Eyeball, smartphone app, manual camera, photog light meter, lux meter, etc (I've used them all), but as many have mentioned above, they all require calibration from a known/trusted output.

What's the cheapest somewhat accurate way to measure lumens? I have a lux meter app on my phone that I have calibrated to be somewhat accurate.

 

[more_vampires]

Uh oh. Now I want a "moonlight" level flashlight which has a distance sensor. I point the light at some object some distance away, adjust the brightness, and then the sensor maintains that lux as I move the light around, pointing it at things different distances away.

So I can point it at a tree over there, and then point it at my map, and then the ground in front of me -- and my eyes experience a constant level of brightness.

Surefire tried that with their Intellibeam, but CPF reports that it's flaky and kind of a lemon at this time. Maybe SF will clean up the problems, maybe the NLA it. Who knows?

What's the cheapest somewhat accurate way to measure lumens? I have a lux meter app on my phone that I have calibrated to be somewhat accurate.

http://www.candlepowerforums.com/vb/printthread.php?t=200334&pp=30&page=1

 

[reppans]

What's the cheapest somewhat accurate way to measure lumens? I have a lux meter app on my phone that I have calibrated to be somewhat accurate.

I personally like the [URL="https://farm8.staticflickr.com/7310/11524977296_39c8145b17_z.jpg]$5 BLF lightbox[/URL]. It's plug and play.

If you can, add ~$30 for a BLF favorite lux meter (LX1330B). With 1.5" PVC piping, my conversion factor comes out to a nice even 0.1x . It pegs my HDS 325 at both max and min, and also closely matches all tested modes (moonlight through max) for my 2 matching lights with ti-force (again, only reviewer on CPF to claim true ANSI and use laboratory tested calibration lights... he's about a third lower than SB in the <500 lm range I care about).

Note that it is best to calibrate from lower modes since max tends to be much more dependent upon sample, and of course battery age and state of charge. Well except for HDS, which are individually calibrated and with stone flat output/runtime curves.

 

[MAD777]

I personally like the [URL="https://farm8.staticflickr.com/7310/11524977296_39c8145b17_z.jpg]$5 BLF lightbox[/URL]. It's plug and play.

That's the first time I've seen that approach. I may have to have some fun with it. I guess I would need the inside diameter to match my largest reflector.

Thanks for the tip.

 

[Bigwilly]

I personally like the [URL="https://farm8.staticflickr.com/7310/11524977296_39c8145b17_z.jpg]$5 BLF lightbox[/URL]. It's plug and play.

If you can, add ~$30 for a BLF favorite lux meter (LX1330B). With 1.5" PVC piping, my conversion factor comes out to a nice even 0.1x . It pegs my HDS 325 at both max and min, and also closely matches all tested modes (moonlight through max) for my 2 matching lights with ti-force (again, only reviewer on CPF to claim true ANSI and use laboratory tested calibration lights... he's about a third lower than SB in the <500 lm range I care about).

Note that it is best to calibrate from lower modes since max tends to be much more dependent upon sample, and of course battery age and state of charge. Well except for HDS, which are individually calibrated and with stone flat output/runtime curves.

Thank you very much. I'll look into this.

 

[Bigwilly]

I personally like the [URL="https://farm8.staticflickr.com/7310/11524977296_39c8145b17_z.jpg]$5 BLF lightbox[/URL]. It's plug and play.

If you can, add ~$30 for a BLF favorite lux meter (LX1330B). With 1.5" PVC piping, my conversion factor comes out to a nice even 0.1x . It pegs my HDS 325 at both max and min, and also closely matches all tested modes (moonlight through max) for my 2 matching lights with ti-force (again, only reviewer on CPF to claim true ANSI and use laboratory tested calibration lights... he's about a third lower than SB in the <500 lm range I care about).

Note that it is best to calibrate from lower modes since max tends to be much more dependent upon sample, and of course battery age and state of charge. Well except for HDS, which are individually calibrated and with stone flat output/runtime curves.

It looks like your using an app on an Iphone? Does it measure lumens?