The name “camera lucida” (Latin for “light chamber”) is obviously intended to recall the much older drawing aid, the camera obscura (Latin for “dark chamber”). There is no optical similarity between the devices. The camera lucida is a light, portable device that does not require special lighting conditions. No image is projected by the camera lucida.
In the simplest form of camera lucida, the artist looks down at the drawing surface through a half-silvered mirror tilted at 45 degrees. This superimposes a direct view of the drawing surface beneath, and a reflected view of a scene horizontally in front of the artist. This design produces an inverted image which is right-left reversed when turned the right way up. Also, light is lost in the imperfect reflection. Wollaston’s design used a prism with four optical faces to produce two successive reflections (see illustration), thus producing an image that is not inverted or reversed.
In 2001, artist David Hockney’s book Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters was met with controversy. His argument, known as the Hockney-Falco thesis, is that great artists of the past, such as Ingres, Van Eyck, and Caravaggio did not work freehand but were guided by optical devices, specifically an arrangement using a concave mirror to project real images. His evidence is based entirely on the characteristics of the paintings themselves.
A couple art professors decided to kickstart their own NeoLucida recently:
(Hat tip to Todd.)