how does increasing light distance affect exposure time

[freedda]

I have moved my light from 29 inches to 39 inches away from my vacuum screen, or 1.5x the lenght.

If I have a 1 minute exposure time at 29 inches, how much do I have to increase it with light at 39 inchs away to get comparable exposure?

(If I apply the inverse square rule, I think I get an exposure of about 2 minutes, 20 seconds (assuming my math is correct), but I'm not sure this rule applies here: for one thing, since the light bulb is so big, I'm not sure its considered a point source at these distances)

Regards, David.

[squeegeethree]

I think your math is about right.