Intensity = Light output divided by distance squared

One lumen is equal to the amount of light emitted by one candle that falls
on one square foot of surface one foot away.

One foot-candle equals the amount of light falls on one square foot or
surface located on foot away from one candle.

What this means is that the relationship between distance and intensity is
built into the calculation and determination of the two standards standards.
A lamp that is rated at 1000 lumens means that at *one foot* distance from
the lamp surface the intensity is 1000 lumens (not at the lamp surface....my
bad). It decreases at an inverse square from one foot. At two feet the
lumens per square foot is 250 or 1/4 less. The initial distance is fixed at
one foot, it is part of the calculation. It is wrong to think of intensity
as decreasing in the same relationship if an arbitrary distance is used.
One yard form the light source is still 1/9th less intense.

"Al" wrote in message news:j3BKi.6941$f%1.2933@trnddc01...
it always falls off at the same rate. The difference is a florescent tube
is long and narrow, so the 'point' of light is really many points of
light. But the rate of decrease remains the same from each point source of
light no matter what type of lamp produces it. It is an inverse square;
so three feet is 9 times less intense and two feet is 4 times less
intense...than the intensity at the source, and four feet is...?

...and gravity propagates forward at the speed of light. If the sun
disappeared now, the earth would behave as if it had a gravitational body
holding it in orbit for how long?

...and the efficacy of earth made O-Rings and foam insulation on
interstellar space craft can be summed up best by what Simpson-esc buzz
word?

"Duncan" wrote in message
news:c8AKi.90$x%6.65@pd7urf2no...
I still falls off at the same rate.
"
HOWEVER, from a large or diffuse source such as a fluorescent tube,
the falloff is less drastic because the tube is, in effect, a large
number of overlapping point sources. The handy rule for such a source
is that the intensity falls off as the inverse of the distance, i.e.
twice as far ,1/2 as intense, three times as far, 1/3, etc.

J. Del Col