by Jim W. » Thu Jan 25, 2007 3:15 am
The seminal work on diamond-shaped holes seems to have been done by one A.O. Hopkins, who was an active experimenter in the mechanical-TV age, and even became a member of the NBTVA shortly prior to his passing, in the '90s, if memory serves.
Although I am not a maths-whiz, his argument for this shape was quite convincing to me, enough so that I created a diamond-hole disc of about 12 inches in diameter (304.8mm to the rest of you!). At the time (1980s) circuit boards and print publications were still created with paste-ups and huge cameras. I was in a position to make the pattern of the disc at four times normal size, then have it shot-down to the proper diameter onto high contrast litho film. The negative was stuck-down to a vinyl phonograph record, which had been pre-drilled with oversize holes in approximate positions.
Even using every means at my disposal to position the holes properly, and with the advantage of the reduction to lessen layout slop, I was so disappointed with the precision of the scanning lines that I quit the project in disgust, never even trying to create a viewable image.
Since then I have become enamored of "bead disc" scanners, which are poor cousins to the lens disc. A local plastics emporium has quite perfect quarter-inch styrene beads available on the cheap; these can be glued into quarter-inch holes drilled or punched into a plastic or other disc. A single-LED light source placed well beyond the focal length of the plastic sphere shows up at a greatly reduced size, but every bit as bright as the (shrunken) original. What's more, with simple masking the shape of the scanning dot can be made anything you wish... square, diamond, etc. A bit of trial-and-error even corrects for the barrel distortion that the spherical lens imparts to anything but a round aperture. The main problem, of course, is drilling quarter-inch holes with sufficient precision, but a drilling jig should help here. 32 plastic beads cost only a few US dollars, and can be mailed in a JiffyBag® easily if anyone is interested in experimenting with this.
Furthermore, I do have somewhere an offprint of A.O. Hopkins' articles on diamond-scanning, published in a TV journal from the electronic period. I believe that I could locate this, scan it, and make it available for posting, if someone has a suitable site.
Many thanks to Andrew for setting up this forum! It's a great place to kick around ideas, and gives more instant gratification than the NBTVA Journal with its limited annual issues. Having said that, I do feel that it is very important to chronicle any new developments for publication in the Journal; many members depend on the printed edition.