Near-field and far-field measutrements

[RadLight]

Could anyone give a good definition of near-field and far-field photometric measurements and their use.

 

[IRstuff]

Far-field is when a source can be treated as a point source.

for lighting, usually 5 to 10 times the maximum dimension of the source is often used as the transition from near to far-field

TTFN

 

[RadLight]

With near-field there are apparently two types of measurements viz. Appliction-distance photometry and Luminance-field photometry. The first is still treated as a point source. I think this is what is confusing to me. Do you have experience with these measurements and calculations and does the inverse square rule, for illumination, have meaning here.

 

[peebee]

I'm not familiar with the terms "Appliction-distance photometry" and "Luminance-field photometry"

But if it's point source, then light falls off with the cube of the distance, as it dissipates spherically.

If your close to a fluorescent tube, though, the light would dissipate cylindrically. That would lead to inverse-square instead.

If you had a large planar source, the light would remain constant in near-field.

However, as you get further away from any source, it will eventually look like a point and therefore the light will eventually start dissipating inversely to the cube of the distance again.

Make sense? Hope that helps.

 

[RadLight]

Hello peebee,
Thanks for your reply. The terms I refer to are in the 9 th edition of the IESNA reference under the heading "Near Field" or from the index on near field.
I have not heard that illumination (Lux) was proportional to the inverse cube but certainly to the inverse square of the distance for a point source. What light measurements are you refering to for a sphere and cylinder? Are you refering to the surface area of a cylinder and sphere?