I've used ordinary thru-beam photo sensors in the past. Use a PE with a relay contact, and put an array in series. You may have to stagger the array depending on the sensing diameter and the hole you need to detect.
Alternately, Sunx has a sensor with a six inch long sensing area to detect down to 1/8 inch. These have frequency settings so they could be placed end to end without problem.
I've also seen some (old) home-made attempts with four foot light fixtures and a PE array, but the light source often was damaged by the leading steel edge.
Thanks for the info. Does Sunx have a web-site that you know of, I'd like to contact them, the only problem I see is that I need to cover an area 62" wide and the width of the product varies down to 28". Of course, the level 2 system knows when a width change is going to occur.
Sounds like a really good idea though, especially the part about the different frequencies, this would allow you to detect in which zone the hole were seen. Just out of curiosity, what was the thickness of the product you used? I am looking at detecting holes down to 1/8" in material that is up to .3" thick.
There is Harris Instruments that make a line of products called ScanLine, try an intertnet search they shouldn't be hard to find. It's exactly what you are looking for.
Depending on speed, any machine vision setup with a light source behind the product should work. You could have a long working distance to get around dirt and curl problems.
Remember, many years ago, (>25), product made by Binks, (Spray Paint Equipment Manufacturer), - "Pin Hole Detector".
Consisted of 3 fluorescent tubes above sheet steel, (One on each phase pair of a 3 phase supply so that light would be continuous - and also seem to remember that supply was at 400Hz), and a fiber optic bar bundle beneath sheet feeding photo detector arrays for holes in any of five zones across sheet. Was capable of detecting holes down to a few mils in dia at a fairly high speed sheet speed. (But don't remember specs). When hole detected, paint spray nozzle marked edge of sheet with color corresponding to the zone hole was found in. Suggest checking with Binks, they may still be in that business.
Tony Firth, Electrical Eng., Quester Technology Inc., Fremont,CA