The point is that the proof is in the pudding. (Talking about throwers...)
If you know a light's lux at any given distance, as long as that distance is REPRESENTATIVE of the light's beam strength at the hottest part of the hot spot, you can calculate the lux at any other distance. Is it ABSOLUTE? No, of course, not, and batch to batch variations in reflectors, LED, which cells, etc, will quickly negate what a TESTED light might produce as far as specs are concerned...but, its a very reliable way to predict performance.
So regardless of how close to a point source the initial beam was (A laser beam? Aspherical?) etc, whatever you measure at a given distance (Again, as long as that distance/measurement gets a REPRESENTATIVE lux value), if you measure that beam again at say twice as far away...the lux WILL be 4x LOWER.
If measured at HALF the distance (And, again THAT distance is past where the beam is already fully formed...) the lux will be 4x HIGHER, and so forth.
IE: The inverse Square Law APPLIES to flashlight beams.
And, yes, the earlier clarification that the "throw to one meter" is not measured at one meter, its merely a MATHEMATICAL value that can then be plugged in, after its back calculated form a measurement at an appropriately longer distance.
So if the cd measured at 10 m was 1,000, the cd of the same light MEASURED at 1 m might have been only 600. But, that light, produced illumination on distant targets that correlated to a cd of 1,000, not a cd of 600...so 1,000 cd was the REPRESENTATIVE measurement distance.
The cd is a CONSTRUCT used to plug in as a variable, so that its easier to calculate the lux on targets of OTHER distances. It ONLY is representative though of distances where the beam is already formed, and, as pointed out, the cd of a thrower, ironically, tends to grossly OVER ESTIMATE the throw to SHORTER distances, as, at shorter distances, the cd is NOT REPRESENTATIVE.
(And that's why there are ANSI measurement options, with the assumption that its in their best interest to choose the one that's going to give the highest throw numbers...by using a representative distance)
For example, very early in my career, I MEASURED lux AT one meter, and, it was fine for small lights with low cd and short beam formation distances...but, I would always get LOW cd numbers if I measured some throwier lights at one meter.
Moving the meter further from the light would then get a more representative lux reading...with the PURPOSE of getting a more representative lux reading being the ability to then extrapolate the lux at OTHER distances.
So, eventually, I found that different beams needed different measurement distances to find the REPRESENTATIVE measurement distance, the one that allowed accurate extrapolation to other distances.
This was analogous (In that distances used for measurements can be too short to be representative) to measurements I was taking of magnetic and electric fields, radio waves, etc...in that there were near field and far field effects, etc...so the concept of needing longer distances to have extrapolatable results was not unknown.
If you know a light's lux at any given distance, as long as that distance is REPRESENTATIVE of the light's beam strength at the hottest part of the hot spot, you can calculate the lux at any other distance. Is it ABSOLUTE? No, of course, not, and batch to batch variations in reflectors, LED, which cells, etc, will quickly negate what a TESTED light might produce as far as specs are concerned...but, its a very reliable way to predict performance.
So regardless of how close to a point source the initial beam was (A laser beam? Aspherical?) etc, whatever you measure at a given distance (Again, as long as that distance/measurement gets a REPRESENTATIVE lux value), if you measure that beam again at say twice as far away...the lux WILL be 4x LOWER.
If measured at HALF the distance (And, again THAT distance is past where the beam is already fully formed...) the lux will be 4x HIGHER, and so forth.
IE: The inverse Square Law APPLIES to flashlight beams.
And, yes, the earlier clarification that the "throw to one meter" is not measured at one meter, its merely a MATHEMATICAL value that can then be plugged in, after its back calculated form a measurement at an appropriately longer distance.
So if the cd measured at 10 m was 1,000, the cd of the same light MEASURED at 1 m might have been only 600. But, that light, produced illumination on distant targets that correlated to a cd of 1,000, not a cd of 600...so 1,000 cd was the REPRESENTATIVE measurement distance.
The cd is a CONSTRUCT used to plug in as a variable, so that its easier to calculate the lux on targets of OTHER distances. It ONLY is representative though of distances where the beam is already formed, and, as pointed out, the cd of a thrower, ironically, tends to grossly OVER ESTIMATE the throw to SHORTER distances, as, at shorter distances, the cd is NOT REPRESENTATIVE.
(And that's why there are ANSI measurement options, with the assumption that its in their best interest to choose the one that's going to give the highest throw numbers...by using a representative distance)
For example, very early in my career, I MEASURED lux AT one meter, and, it was fine for small lights with low cd and short beam formation distances...but, I would always get LOW cd numbers if I measured some throwier lights at one meter.
Moving the meter further from the light would then get a more representative lux reading...with the PURPOSE of getting a more representative lux reading being the ability to then extrapolate the lux at OTHER distances.
So, eventually, I found that different beams needed different measurement distances to find the REPRESENTATIVE measurement distance, the one that allowed accurate extrapolation to other distances.
This was analogous (In that distances used for measurements can be too short to be representative) to measurements I was taking of magnetic and electric fields, radio waves, etc...in that there were near field and far field effects, etc...so the concept of needing longer distances to have extrapolatable results was not unknown.
Last edited:
