Foucault Light Source Light sources for Foucault testers may have a pin hole or a narrow slit as the exit aperture.
Slits provide increased brightness over pin holes.
The razor blade is obtained from disassembling a plastic throw away razor. Use caution when taking apart a razor. The razor is a thin strip of metal that cuts easily with scissors. Use your shop pair if you know what's good for you. I estimate the gap with dial calipers but you can use feeler gauges to set the approximate gap.