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light and distance

Posted by colokid Z5/Co (My Page) on
Tue, Mar 3, 09 at 10:58

Is it true that the light from a tube fluorescent diminishes in proportion to the distance (line source) Twice the distance, half the light. While a CFL (point source) diminishes as the square of distance? Twice the distance, one fourth the light. Would a good reflector change this?

Follow-Up Postings:

RE: light and distance

AFAIK, it makes no difference what the type of light is. They all follow the laws of physics.

A reflector will, um, reflect light toward a source, but it still will follow the same laws.


RE: light and distance

Boy, if light ever quits following the laws of physics we are up a creek. It is absolutely correct that with only a point source of light the intensity diminishes with the square root of the distances - twice the distance is 1/22 the light, three times the distance is 1/32 the light, and so on. However, the light falloff will be VERY different if the source has a reflector OR is not a point source, such as a straight fluorescent lamp.

If a reflector is re-directing the light (reinforcing it in one direction) then the falloff will be less, sometimes MUCH less. For instance, if your point source of light is in the focal point of a perfect parabolic reflector, the falloff will be negligible for a VERY, VERY long distance. We would hardly be able to drive at night if our headlights were not mostly parabolic.

For a linear souce (flourescent) the falloff is near linear for at least several feet near the lamp - twice the distance will result in about half of the light. If you add a good reflector to a fluorescent, then the falloff ratio is LESS than half the distance - a far cry from the point source squared law.

Several years ago I used AutoCAD to design some reflectors, and I also did some calculations of light falloff of a flourescent lamp by dividing the lamp into a series of small point sources in a row. The overall light emitted below a series of point sources DOES NOT follow the square inverse law. I also analyzed few of the so-called "specially designed" reflectors touted on grow sites by tracing reflection angles and areas covered. I would say they had no design whatsoever - they just bent the metal to fit the parts they had available.


RE: light and distance

Thanks Zink. That is what I was after. I am seeing this in actual practice. With my light meter it seems that light from a T12 does not fall off near as fast as does the light from a CFL. I like CFLs but they need to be close (compared to tube). Will think about good reflectors for them.
Kenny, who is still waiting for his first tomato under light to ripen.

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