An array of sensitive pixel elements is configured to generate signals representative of sensed light that is associated with writing being done by a user, and an optical system concentrates light from a light source across a section of the array of sensitive pixel elements, the signals being useful...http://www.google.de/patents/US7755026?utm_source=gb-gplus-sharePatent US7755026 - Generating signals representative of sensed light that is associated with writing being done by a user
