You're thinking of
Optical Time-Domain Eavesdropping Risks of CRT Displays by Markus G. Kuhn in Proceedings of the 2002 IEEE Symposium on Security and Privacy, Berkeley, California, 12–15 May 2002.
Available here: http://www.cl.cam.ac.uk/~mgk25/ieee02-optical.pdf
Pretty amazing that this actually works, just using a high-speed photo-sensor – and doesn't even require line of sight, a reflection off a wall is enough. Unfortunately, I don't think it will work on LCD screens.
