The NeoLucida allows you to trace what you see. It is the first portable, authentic camera lucida in nearly a century.
[Philip Lelyveld comment: Two art instructors are developing a modern Camera Lucida - a prism/lens tool that allows you to "trace" what you see onto paper. The brief video describes the technology and history of it.
They nicely acknowledge David Hockney's book describing how the old masters may have made extensive use of technology like this.]
See the full story here: http://neolucida.com
NeoLucida (Long Edit) from STUDIO for Creative Inquiry on Vimeo.