Not sure if this has been posted before, maybe it will help someone.

Sorry for the bad math on the first revision , I have corrected and double checked this one.

I just created a Excel quick calculator and removed the confusing image.

Shouldn't the 75 on the bottom in the equation be 150? (2h=2*75=150) Or did I miss something?

Also, again I ask why the .5 in the equation above becomes 2 when you plug the numbers in?

I wasn't going to mention after the same-thing was in other-thread...I figured someone would catch-that...<g>
[The above is the sort of mistaken-proof that comes from calculator-use...]

Whoops!, guess I need to revise that picture, I was trying to get it done fast at work.

Bump for update.

Worked out fine when measuring one of my ladles Subg, nice work.

Nice work! I'm going to double check my reflector tonight.

Thanks, it was fun.

Don't want to appear over critical, but .....

Your equation is correct for the center of the sphere, i.e. the radius, but isn't the focal length actually half the radius?

Regardless, it's a good job, and if I'm correct then an easy mod. An excellent addition to the LL tools provided by community members.

If someone would be kind enough to confirm or deny my point please.

I never said it was the focal length, I was trying to point out the "focal point of the arc center" (where to place you lamp arc) I probably shouldn't have used the word focal at all to avoid confusion.
Thanks for the props.

OK, well you got me there. But is the focal length different from the focal point? Do you place the lamp arc at the focal length or at the focal point as you describe, (i.e. center of sphere)? Because now I'm really confused. I'd better go read some threads and check my own reflector dimensions and setup!!

OK, well I'm getting the impression that I misunderstood about reflectors. The focal length, (R/2), doesn't really come into play in our use, does it? Because it is quoted in the specifications for the pro-reflector I took it as the point at which the arc should be. I'm wrong aren't I?

Any way, I'm off to pull my reflector out and measure it.

the focal point is at r/2 but the lamp should be placed in the center of the sphere.
If you place the lamp @ the focal point you only get a very small light cone ( in fact it's the smallest possible) and you get a sharp image of the arc.
When placed in the center of the sphere you get an arc image with same size but mirrored overlaying the arc.

Yeah, I realise this now. I took a better look at the pro-reflector diagram and not just the cursory glance I had given it before. My bad.