Easy to Understand Lumens Vs Lux Explanation

casnova

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Thanks, in advance, for any info.
Very interesting thanks for sharing. :thumbsup:










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etc

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Right. Good point.

Point is, you can have 800 unfocused, very floody lumens that are almost useless unless for a very specific application.

Or focused 120 lumens that are more useful in almost all applications.

Case in point, I have a Malkoff M60L, the original, 5+ year old device, "Low". It's just such a nice balance of thrown and high lux that I consistently found it more useful than even more recent, higher-powered units such as Malkoff M91. The latter had considerably more lumens (was it 800 or something to that effect) but it felt like holding a household lightbulb in your hand. All flood, no throw. Useless beyond room distance. Ended up selling M91. Still use M60L.
M60L has higher lux than the higher powered M61.
 

TEEJ

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Right. Good point.

Point is, you can have 800 unfocused, very floody lumens that are almost useless unless for a very specific application.

Or focused 120 lumens that are more useful in almost all applications.

Case in point, I have a Malkoff M60L, the original, 5+ year old device, "Low". It's just such a nice balance of thrown and high lux that I consistently found it more useful than even more recent, higher-powered units such as Malkoff M91. The latter had considerably more lumens (was it 800 or something to that effect) but it felt like holding a household lightbulb in your hand. All flood, no throw. Useless beyond room distance. Ended up selling M91. Still use M60L.
M60L has higher lux than the higher powered M61.

Its the opposite for most people actually, in that most EDC lights at least are used most for closer illumination, such as task lighting...and too tightly focused beams make a glaring hot spot surrounded by darkness, and are far less useful.

Its like any tool of course, if its a Phillips headed screw....that's the best driver to use.

:D
 

Draglid

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this was easy for a newcomer of the flashlight world to understand , thanks !
 

KBobAries

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Oops. Hit the wrong button and it posted before I was finished.

I have the concept in general and I suppose by the 10th time I read the OP it will completely sink in. Is it ALWAYS the lux number ONLY that determines throw?

A couple examples pulled from the web. One light is 2350 lumens / 22K lux and the other is 3550 lumens / 23K lux. For the moment, disregarding the human eye & inverse square law about perceiving brightness. Both of the lights are listed as "great throwers." Is 1200 lumen difference for a 1K gain in lux a result of LED efficiency? Beam angle?
 
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KeepingItLight

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A couple examples pulled from the web. One light is 2350 lumens / 22K lux and the other is 3550 lumens / 23K lux. For the moment, disregarding the human eye & inverse square law about perceiving brightness. Both of the lights are listed as "great throwers." Is 1200 lumen difference for a 1K gain in lux a result of LED efficiency? Beam angle?

The two flashlights you describe are both "general purpose" lights. For the most part, they are flooders. Due to the large number of lumens they output, they also gain some throw.

Note, however, that even a single 18650 flashlight like my ThorFire C8 can get more throw than they have. And it is only a 900-lumen flashlight.

Assuming the numbers are OTF (out-the-front), ANSI FL 1 lumen ratings, then the difference between the two flashlights you cited is most likely due to beam angle or a difference in spill/hot-spot brightness.

(BTW, I don't mean to nitpick, but just in case you do not already know, the proper term is "candela," rather than "lux." Without getting bogged down in too much detail, we can say that candela is the maximum lux that can be found, i.e., measured, at any position that is exactly 1 meter from the emitter. Usually, this means dead center in the hot spot, but not always.)
 
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KBobAries

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...For the most part, they are flooders. Due to the large number of lumens they output, they also gain some throw...

...Assuming the numbers are OTF (out-the-front), ANSI FL 1 lumen ratings, then the difference between the two flashlights you cited is most likely due to beam angle or a difference in spill/hot-spot brightness.

(BTW, I don't mean to nitpick, but just in case you do not already know, the proper term is "candela," rather than "lux." Without getting bogged down in too much detail, we can say that candela is the maximum lux that can be found, i.e., measured, at any position that is exactly 1 meter from the emitter. Usually, this means dead center in the hot spot, but not always.)

Thanks for the reply. For the first part of the quote, if I'm following you correctly, the gain in throw is due to power. Even though it's a flood reflector enough light is being forced out the front and it has to go somewhere. Granted, if a thrower is wanted one would change the reflector. As is, the main gain is in having the area under flood be brighter and the increased throw is an incidental effect due to physics.

Regarding the second part of the quote. Not sure if the numbers are OTF. They came from 2 different lights from the same manufacturer. I used them just as an example to confirm my guess about beam angle.

As to the third part of the quote, I didn't take it as nitpicking :) Always glad to learn something. My inference is that this is similar to resistance & ohms. Lux is the topic & candela the numerical value.

Dan
 

KeepingItLight

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If I'm following you correctly, the gain in throw is due to power. Even though it's a flood reflector enough light is being forced out the front and it has to go somewhere.

You got it!


Regarding the second part of the quote. Not sure if the numbers are OTF. They came from 2 different lights from the same manufacturer. I used them just as an example to confirm my guess about beam angle.

Assuming that the manufacturer is consistent in its procedure for measuring lumens, we ought to be able to compare outputs of two different models it makes.

Beam angle is one factor. If two flashlights output the same number of lumens, and one of them has a beam that is wider than the the other, then the beam intensity of the wide one would be dimmer than that of the narrow beam.

Beam angle might not be the only reason two flashlights have different intensities. It is possible that they could use different optics that produced identical beam angles, but which divided the light between hot spot and spill in dissimilar ways. One could have a brighter spill and dimmer hot spot than the other. It would throw less.

I am not an expert on reflector design. It is easy, however, to imagine that this could be accomplished by some combination of reflector and/or lens. One flashlight, for instance, might use a traditional reflector; the other, an aspheric lens.


As to the third part of the quote, I didn't take it as nitpicking :) Always glad to learn something. My inference is that this is similar to resistance & ohms. Lux is the topic & candela the numerical value.

This is not quite right. Lux and candela are both measurable quantities.

Lux has units of lumens per square meter. It is a measure of the number of lumens that hit or pass through a 2-dimensional surface in space. In general, each point on the surface will have a different lux.

On a smooth surface, there is a tangent plane at every point. Lux measures the number of lumens that hit or cross through the surface (at that point) perpendicular to the tangent plane. The calculation requires constructing a unit normal vector to the tangent plane, and taking the dot product with the vector representing the light beam. The result is to ignore everything except the lumens that are perpendicular.

When the surface is a sphere of radius 1 meter, centered at the emitter of a flashlight, then the flashlight's beam intensity, measured in candela, is the highest lux measured anywhere on the sphere. Usually, but not always, the highest lux will be found in the center of the hot spot. If a beam has artifacts, however, the brightest part may not be in the center.

A more complete definition is more complicated, and gets involved with "solid angles," and such, but this hand-waving explanation is good enough for flashlights.

Sorry if this sound too technical! You can learn more at Wikipedia. But be forewarned, it gets pretty messy, very fast!
 
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CivilGear

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Thanks for sharing you explanation :) I always like to collect metaphors to help explain lumens vs lux vs candles.
 

KeepingItLight

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Thanks! I'm glad you liked it.

It's one of the more technical explanations in this thread, but I think it gives a fairly accurate description of the difference between lux and candela, especially as the latter term applies to flashlights.

There is an unstated assumption that TEEJ has made throughout this thread. All of his targets are facing directly toward the flashlight, so that his flashlight beams strike them head on, i.e., perpendicularly.

If, for instance, the target is the side of a building, the building must be facing the flashlight. If it is not, then you must use the dot product, as described above, in order to determine the lux on target.

This makes sense.

Suppose, for instance, that the flashlight beam is a circle 10 meters in diameter when it strikes the building. If the building is facing the flashlight, then a section 10 meters wide will be lit by the flashlight. If, however, the building is angled to the flashlight, a much longer section may be lit up. For purposes of discussion, let's say that a 20-meter section gets lit.

The lumens have not changed, but the area being lit up has changed. Because the area is larger, the number of lumens per unit area will be less, i.e., the lux will be less.

From this example, you can see that the angle at which a light beam strikes a surface, i.e., a target, plays a role in determining lux!

The definition of a flashlight's beam intensity, as measured in candela, is designed to eliminate this dependence.

When you model a flashlight as a point source, and place a sphere of radius 1 meter around it, all the stuff about tangent planes, normal vectors, and dot products goes away. That's because every light beam is perpendicular to the surface of the sphere, no matter where it strikes it.
 
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TEEJ

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Thanks! I'm glad you liked it.

It's one of the more technical explanations in this thread, but I think it gives a fairly accurate description of the difference between lux and candela, especially as the latter term applies to flashlights.

There is an unstated assumption that TEEJ has made throughout this thread. All of his targets are facing directly toward the flashlight, so that his flashlight beams strike them head on, i.e., perpendicularly.

If, for instance, the target is the side of a building, the building must be facing the flashlight. If it is not, then you must use the dot product, as described above, in order to determine the lux on target.

This makes sense.

Suppose, for instance, that the flashlight beam is a circle 10 meters in diameter when it strikes the building. If the building is facing the flashlight, then a section 10 meters wide will be lit by the flashlight. If, however, the building is angled to the flashlight, a much longer section may be lit up. For purposes of discussion, let's say that a 20-meter section gets lit.

The lumens have not changed, but the area being lit up has changed. Because the area is larger, the number of lumens per unit area will be less, i.e., the lux will be less.

From this example, you can see that the angle at which a light beam strikes a surface, i.e., a target, plays a role in determining lux!

The definition of a flashlight's beam intensity, as measured in candela, is designed to eliminate this dependence.

When you model a flashlight as a point source, and place a sphere of radius 1 meter around it, all the stuff about tangent planes, normal vectors, and dot products goes away. That's because every light beam is perpendicular to the surface of the sphere, no matter where it strikes it.


To clarify - This is incorrect regarding the beam spread on a target.

Remember that the light being emitted at the target has no idea of the target, etc.

So, if the target is a mouse, the beam is not more concentrated at its hot spot than if the target is an elephant, or a circus tent, etc.

The beam angle, for the most part, dictates the way the lumens are concentrated...and, if there's a target or targets out there for the lumens to hit and bounce back as lux...that in no way changes the output of the flashlight.


So, again - The candela (cd) is the lux at 1 meter........and, this is a CALCULATED value, for lights with more throw...as the beam is not typically fully formed AT 1 meter. IE: It might be MEASURED at 20 meters, or 3, etc, and back calculated to what it would be, mathematically, at 1 meter.

The entire reason for getting the cd is to allow calculation of the lux at OTHER ranges.


If a light provides the spec for lux at 1 meter, or, if not, the "range" of the light, say in meters, or feet, you can back calculate to find what cd would produce that spec..as the "range" is always to 0.25 lux on the target at that distance.


So, as far as raw throw goes, the higher the cd, the farther the throw.

If two lights have the same cd, but one has a higher lumen output, it will mean that the higher lumen output is part of a floodier beam (wider beam angle).



Keep in mind that the lux = the lumens per square meter.

So, if a flashlight puts a one square meter spot of light on a target 1 meter away, with an output of 1 lumen, it will also have an out put of 1 lux.

If the beam angle stays the same...but it puts out 2 lumens, the intensity will rise to 2 lux, because BOTH lumens are on the same square meter.

If I take the 1 lumen flashlight, and concentrate that 1 lumen onto a 1/2 square meter circle, we know have a beam with 2 lux, as we put 1 lumen on 1/2 a square meter.

If there is a mouse in that circle of light, we would see the entire mouse lit up by that 2 lux beam.

If there was an elephant in that circle of light, we would see 1/2 square meter of grey hide lit up by the 2 lux beam...and so forth.


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.



If being used as a thrower though, the only part of the beam REACHING the target is the hot spot, and the corona and spill fell off in intensity enough to now be invisible/not usable at the light's maximum ranges.


So, 2 lights COULD have beam patterns that produced the same sized hot spot and cd, but one had more lumens, but the added lumens were "wasted" in spill and corona.......if you would rather see more at the max range.....or "spent" illuminating proximal stuff such as where you were walking, while looking way off in the distance, etc.



So, if shopping for "A Thrower"....the first question is how far you want to see what at.

That set's the cd you need......the range the light will hit what lux at can be calculated.



The second question is how large an area you want to light up at a time...at what distance.

The wider the beam angle, the wider the beam is, and, it spreads with distance. (Think of the light pattern sent out as an ice cream cone you are holding at the cone's tip...with the fat end, with the ice cream, as what you are lighting up your targets with).

Generally, for any given cd, if the cd is the same for two flashlights, but there are more lumens in the beam of one of them...a larger area will be lit up off in the distance....but you won't see FARTHER, just more of what's at the same max distance.


There are obviously other thrower issues to consider, albeit this thread's part is relevant here.

:D
 
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KeepingItLight

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Thanks, TEEJ, for all the work you have done explaining these topics. I don't disagree with your analogies. I think they do a good job explaining the concepts.

Thanks, as well, for explaining that a flashlight beam must achieve focus before you can measure its highest lux. As you say, that means candela must usually be found by measuring lux at a distance farther than 1 meter, and then calculating back to 1 meter.

I think, however, you have misinterpreted my use of the word "angle." Your reply speaks about "beam angle." Beam angle is the "width" of a beam as measured at the flashlight head using a protractor. Beam angle is not what my post describes.

The point is a subtle one.

The angle in my post is the angle at which individual light rays strike the surface of a target. When they arrive exactly perpendicular to a flat target, then your description of the lux that falls on the target is correct. When they arrive at a different angle, one that is not perpendicular, then the lux is reduced! By this, I do not mean that flashlight candela is reduced. Obviously, the flashlight has not changed. What I mean is that the lux at each individual point of the target is reduced.

Your discussions of lux on target, in this thread and others, implicitly assume that each individual light ray strikes the surface precisely perpendicular.

You have said, for instance, that at the beam distance given by the ANSI FL 1 rating, the most brightly lit point on a target will have 0.25 lux falling on it. That is only true, however, when the target is facing the flashlight, so that its surface is perpendicular to the incoming light rays. When those rays are not perpendicular, the lux on target is reduced.

This is hard to understand, and seems to violate common sense.

That's why I cooked up my analogy of the building. It is easy to see that shining a flashlight along the side of a building causes the illuminated section to stretch out. A beam that makes a small circular target on the front of a building, will stretch into a larger oval when it is shined along the side. The number of lumens striking the target has not changed. The area being illuminated, however, has changed. It is larger. Same lumens; larger area. By definition, that means the lux on any point of the target has been reduced!

Now, using the ideas of calculus, shrink that building down to an infinitesimal size. The same result holds. When the surface is angled with respect to the incoming light rays, lux is reduced.

Here are a couple of relevant Wikipedia quotes.

The cosine that is mentioned comes from dotting the unit normal vector of the target surface with a vector in the direction of an incoming light ray.

Illuminance

Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (measured in lumens) as a measure of the total "amount" of visible light present, and the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance (lux) is inversely proportional to area when the luminous flux (lumens) is held constant.

One lux is equal to one lumen per square metre:

1 lx = 1 lm/m2​ = 1 cd·sr/m2​

The illuminance provided by a light source, on a surface perpendicular to the direction to the source, is a measure of the strength of that source as perceived from that location.

[Emphasis added.]

The illumination provided on a surface by a point source equals the number of lux just described times the cosine of the angle between a ray coming from the source and a normal to the surface. The illumination provided by a light source that covers a large solid angle is proportional to the cosine of the angle between the surface normal and a sort of barycentre of the light source, so long as all of the source is above the plane of the surface. The number of lux falling on the surface equals this cosine times a number (in lux) that characterizes the source from the point of view in question.

[Emphasis added.]

This gets a little hairy!

That's why I used the analogy of the face of a building. Hopefully, it helps put a little light on the subject!
 

KeepingItLight

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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.
 
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