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.
😀
