Yeah, i had thought of that but the size of my disks are only about an inch round OD - Pretty hard to do something like that by hand. Perhaps there are microscopic holes in the toner still letting the light through. I might pop one under a microscope at work and have a look. I also read somewhere else that when printing these, it can be better to print them on a colour laser printer but using CMYK to print it and not just the black. The theory there is more toner on the surface and it's more likely to block the IR. I might also give that a try.
Great find - i looked all over for something 'pre-fabbed' that was not too exensive but didn't find anything. Had i come across them before i might have gone that way, however i've now built the 'sensors' so i'd like to persevere with them for the moment.
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