Lux measuremtns distance

[sagar]

What is ideal distance to measure illuminance (lux) of light source.
Means distance from source to area which is illuminated.

 

[DIWdiver]

You want to be far enough away that any beam distortions have settled out (10-20 times the diameter of the optic is probably fine), far enough away that the size of the detector is less than the size of artifacts in the beam, and close enough that stray light is massively overpowered by the light you are trying to measure. Also, in a range where your meter is accurate. That's a pretty wide range. If you have a good dark spot to work, probably 1-10 meters for smaller lights, somewhat farther for larger heads. For real throwers, you can go even farther, for wide floods maybe a bit closer.

 

[sagar]

Thank you sir but i saw most of the cases 1 meter distance is used.

 

[TEEJ]

Thank you sir but i saw most of the cases 1 meter distance is used.

1 meter, 2 meters, 10 meters, 30 meters...are all used.

The "Correct" distance is the distance at which the beam is REPRESENTATIVE of what it will be farther away.


IE: The Inverse square law dictates the lux at a given distance, but, for longer throwers, the beam is not fully formed at closer ranges, so, to be representative, measurements need to be taken far enough away that the beam HAS fully formed.


That means that while the lux drops with distance, the cd is the back calculated lux at 1 meter, NOT actually the lux at 1 meter. This is so you can use the lux at one meter to PREDICT the lux at any other given distance via the inverse square law.

If the beam needs 30 meters to form, and you measure AT 1 meter, you will get a lower cd than if you measured it at 30 meters.




If you want to KNOW the best distance, you would need to MEASURE the lux at various distances, and calculate the cd from those measurements. There will be a point (distance) of diminishing return, where the cd stops getting larger, and remains constant with additional distance....and, that is the distance you need to measure that light at.


If in doubt, measure as far away as you can, because, as long as you are far ENOUGH away, the cd will remain the same....and be representative of the light's throw.



If you know the lux at that distance, you can calculate the lux at 1 meter (A mathematical value, and NOT ACTUALLY the lux at one meter) which is the light's cd.





So, lets say you run out 100 meters from the flashlight with your light meter, and yell back to your buddy to turn the light on, and aim it at your meter....and you get 1 lux on your meter at 100 meters range, that means your light has a 10,000 cd (10k cd).

If you go twice as close (50 m), it should be 4x brighter, or 4 lux.


If you get 10 m away, and get 100 lux, that's STILL a cd of 10 k the entire time.

If you get only 2 M away, and the lux is at LESS THAN 2,500 then 2 m is too CLOSE to represent the beam, as if REPRESENTATIVE, you should get 2,500 lux at 2 meters.


So, when the inverse square law doesn't predict the beam's longer range measurements, the closer ranges are too close.