↑ Watt, Alan H.; Watt, Mark (1992). Advanced Animation and Rendering Techniques: Theory and Practice. Addison-Wesley Professional. pp. 21–26. ISBN978-0-201-54412-1.
↑ Bui Tuong Phong, Illumination of Computer-Generated Images, Department of Computer Science, University of Utah, UTEC-CSc-73-129, July 1973.