Makes a lot of sense. I had kind of wondered about that myself when measuring from 5 meters, a 4inch spot on the wall that came from a 2inch lens.
But I foresee difficulty changing the longtime “standard” way people measure/calculate lux/cd. Though the fact that the new “correct” method results in better numbers should help make people want to adopt it. 
Ummmm..... okay....
If it's broke I can fix it... From now on if it's math I'm sending it to you...
Hello Endermann,
Very nice research and good explanation 
My math is not as bad as I make it sound. However I do use a phone app for the calculations. It's an app I use for work mostly that happens to include lighting calculations lol.
Most flashlights are not that highly collimated so the lux measurements are “accurate enough”, and you can’t get sued for it because the true numbers are actually higher, the numbers on the box are an underestimate.
If everyone agreed to measure lux at 10m or something standard like that then it would be easier to compare between flashlights, but that’s not gonna happen
Hahaha and I’m not even a math major xD
Thanks though
Thank you
My mistake even using the app has always been either the room was not totally dark or I had my distance wrong. And the angle... well it's always wrong
I don’t turn off the lights when taking my measurements, when I set the luxmeter range to x100 the tiny 50lux from my room lights become 0 and do not affect the 50000 lux from my light
So for me it’s an insignificant decimal place that doesn’t even get considered.
I see your point bu i somewhat remember having the exact same number when measuring my B158 at 5 and 100 meters using the simplified widespread method of calculation, maybe i should try it another time.
I was under the impression that the common wisdom among us was that 1 or 2 meters readings may not be accurate and that around 5 meters was kind of the minimum distance to take readings.
I have to say that this possible effect has caused me some confusion regarding my understanding of throw and how to predict it, so I did some measurements to see what is actually happening.
My test light was a UF-1504, with 62mm diameter lens and XPL HI V2 1A. I put in a 8x7135 driver to keep the output more constant with time. I measured the lux at three different distances measured from the lens.
dlux__cd
6.05m7230_264.6Kcd
8.34m3840_267.1Kcd
10.29m2530__267.9Kcd
These measurements are consistent with measuring the distance right from the lens and not at some point behind the lens.
Plugging the 10.29m measurement (the beam size at this distance was 36cm) into the calculator you linked (after converting units), results in:
divergence distance behind aperture: 2.14m
candlepower: 390.9Kcd
The calculator results are not consistent with my measurements and how they vary with distance. There are certainly some things in your explanation of the effect that make sense, but actual measurements tell a different story.
A different effect could have caused your conflicting results when you measured the lux at 1m and 2m. At small distances like 1m, the lens might not be filled with the image of the LED, from the point of view of the lux meter. Moving farther back to where the lens is filled would then result in a larger throw number.
I’m pretty sure the reason people don’t measure at 1m is because a luxmeter can only do like 20000 or 200000 lux, and at 1m an aspheric flashlight is above that by a lot.
That’s interesting. Is the entire lens actually being used to focus the light?
I usually check by making sure there is led light spilling off the sides of my lens, and using a water sprayer to put water in the air so I can make sure the beam is filling the lens.
Your results still show that the farther you measure at the higher the cd you get. Maybe try measuring at 50 or 100m?
I will need to do more tests with different lights to double check.
I really can not follow your story very well, I think that you are simplifying the direction that the rays coming out of a flashlight as if they start from a point source which certainly is not the case, flashlight optics and hotspot size/brightness have everything to do with the led not being a point source.
Hotspot brightness has to do with the fact that flashlight optics are imaging a led at a distance, so it more or less follows the lens maker’s formula creating an image of the led at a distance, smaller led or longer focal distance of the lens=smaller image, . Even reflectors do something similar although the way they ‘image’ is way more complicated.
One consequence is that if the image is created at twice the distance from the lens, the image is twice as large (4 times the surface area) and is thus 4 times less bright.
And my aspheric lights follow that simple: twice the distance from the lens= 1/4 the brightness pretty accurately, even when starting from 1 meter.
Reflector lights need more distance to form their uniform hotspot (they are not focusable like zoomies), but my usual 7 meters indoors is enough, and the simple relation between distance and brightness is followed.
But the lens is not producing a point source of light…
That’s why applying the inverse law of light assuming that the lens is at r=0 is incorrect.
If you had an ideal light source behind a lens, the lens would collimate the rays and the intensity would not change by distance, which would be infinite candlepower.
[quote=Enderman]
No, ever since i started this hobby back when flashlight where much less bright people pretty much agreed on the wisdom that the closer the measuring distance, the less accurate, one or two meters readings have always been considered irrelevant and met with encouragment to measure from farther as far as i can remember, absolutly nothing to do with luxmeter capabilities.
Yes, the entire lens is filled with the image of the LED for the distances I measured. I check by looking into the beam at different distances using sun viewing glasses.
I think it is because of the reason I mentioned above; at close distances the lens is not filled with the image of the LED. You cannot tell this by spraying water; you have to actually look into the beam at the lens.
There are ANSI standards, developed for and by the lighting industry by specialists in the field. There is probably a reason they don’t use if’s and assuming in their definitions. The primary objective is to establish parameters of comparison, to create an equal and repeatable playing field for the purpose of recreative ability in determining the value of one light versus another as suited to a particular task. At the end of the day, deviation from the accepted standard is disqualified summarily as non applicable to the established criteria.
So, in simple terms, if we don’t all use the same test, our results cannot be compared and as such, the tests themselves are disqualified.
Don’t sell us, sell ANSI, then we will adapt accordingly.
An Enderman’s primary purpose is to create chaos and confusion.
it is irrelevant where the imagine source of the flashlight is
you get only higher lumens if you go to a negative distance towards the spot where the light should for a focal point,
but that is physically impossible
if the light has 20mrad divergence it doesnt matter where you put a ideal luxmeter as long as the beam is uniform in brightness,
as a reflector light is not uniform in brightness at close distances, so you have to go further away,
if you are too close you measure too low lumens in the donut hole or too high lumens in the ring where the brightness is concentrated
on a reflector light the spill also affects the measured lux if you are very close
the next factor is that your measurement would need to be taken on an indefinate small point in the middle of the beam,
so the bigger the spot is the less you get a false reading as your luxmeter has a quite big spot to catch light at low distances
It does though…
If you take a measurement at 10m you would think at 20m it would be 1/4 brightness, right?
But if the convergence point behind the flashlight is 100 000m behind, then the difference between 10m and 20m is nearly 0 because 100 010^2 and 100 020^2 are almost the same number.
Unlike 10^2 and 20^2 where the second number is 4x bigger.
Hopefully the 100000m exaggeration is enough for you to picture what is happening here.
Lol I need you to crunch some numbers for me. Then in about a week there will be a tiny upgrade to 1 of my sk68 and 1 sk98 lights. I'm curious what of any difference will be.
I have some higher output cells coming. I'm told I should notice a difference.
If you are up to it, my light meter is built into my phone app. Just let me know at what distance or whatever you may need.
Let me know if you are up to it. I will be typing up a review so it may take me a few minutes to respond. And if you don't feel up to it that's olay too. I know all about being tired. I'm very good at it.
The more the better, because the margin of error when calculating lux and measuring the spot size is lower.
Although it should be in the tens of meters, not hundreds, because then the lux values are too low to be read by the meter