I used 10 24" tubes spaced about 20mm apart, which used to illuminate a 50x60cm area very well. At first it the contact frame was around 120mm from he tubes, but then I made a vacuum frame that brought it to around 90mm. Still very even across the bulbs, but the ends of 20" long prints have to be burned in slightly, even with 24" long bulbs. I might make an extension to mount the vacuum drawer in to get the extra distance back.