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# Thread: light meter measurement: incident light and the inverse square law

1. Originally Posted by Tim Gray
You treat the line source as a line or a collection of points, there's no real difference. Actually the math is quite easy. I'm not sure what you are trying to prove with this kind of statement.

If you really insist on calling a line a collection of points, then go right ahead. Most of us, casually and mathematically, have a name for a collection of points: a line. As was stated before, inverse square law stems from essentially geometric arguments.
What that statement is trying to show (if it is trying to show anything) is that this inverse square thing is not such a strange and exotic thing.
There's no real difference indeed, whether point or line, except that you don't calculate the effect of a change in distance from subject to one point light source, but to many. Each behaving in the 'inverse square way'.

Why make it clear that this is not an exotic phenomenon, but on the contrary quite 'the norm'?
Because despite it being so obvious that the OP's hunch about the light fall off was spot on, it gave rise to some serious doubt, and hard work explaining that there is nothing strange going on.

2. Originally Posted by Q.G.
There's no real difference indeed, whether point or line, except that you don't calculate the effect of a change in distance from subject to one point light source, but to many. Each behaving in the 'inverse square way'.
All I can say it's much easier to think of a line source as a line mathematically in this case and not as a collection of points. One can do that problem, but it's a heck of a lot easier to look for the symmetry in the problem.

3. Originally Posted by Tim Gray
All I can say it's much easier to think of a line source as a line mathematically in this case and not as a collection of points. One can do that problem, but it's a heck of a lot easier to look for the symmetry in the problem.
It hides the fact that this inverse square thing is at work then too.

And i'm not going to do the math.

4. Alright you win. Look at a big line source and think, "That's a collection of points, all behaving the inverse square law." What does that tell you about how the light falls off with respect to distance? I'd rather look at it and say it's a line source and it falls off as 1/r.

It's geometry. That's it. Inverse square law is for a point source (hint - it's not just for light). A collection of points is a line. It doesn't hide anything about that the 'inverse square thing is at work'. It's not a point source. A collection of points is not a point.

Again, the math is pretty dang easy. If you can write a fraction, you've done the math. I'm out. Hope I didn't annoy anyone being too pedantic.

5. You're not one for representing things the way that makes most sense in a given context then?

6. Originally Posted by Q.G.
There's no real difference indeed, whether point or line, except that you don't calculate the effect of a change in distance from subject to one point light source, but to many. Each behaving in the 'inverse square way'.
Not in any radiometric sense. As Tim stated, a point source, a line source, and an extended source are all different. The inverse square law is for point sources only. An area is not a collection of point sources not does it behave the same. This is well recognized in astrophotography where extended objects and stars are effected differently based on f-number and entrance pupil.

7. As stated before and from the Focal Encyclopedia of Photography:

Point source E = I/d squared
Line source E = I/d
Extended source E is invariant with d

8. Originally Posted by Hikari
Not in any radiometric sense. As Tim stated, a point source, a line source, and an extended source are all different.
Of course they are.
Where did you get the idea that only Tim thinks or says so?

Originally Posted by Hikari
The inverse square law is for point sources only. An area is not a collection of point sources not does it behave the same.
Why does this happen all the time..?!

Yes, it is, and it does.

I'm not going to tackle the, judging by how hard it has turned out to be to explain something very simple, no doubt unsurmountable problems that would be encountered explaining that to you.

9. Originally Posted by Q.G.
Of course they are.
Where did you get the idea that only Tim thinks or says so?

Why does this happen all the time..?!

Yes, it is, and it does.

I'm not going to tackle the, judging by how hard it has turned out to be to explain something very simple, no doubt unsurmountable problems that would be encountered explaining that to you.