Turner Whitted.
  <b>An improved illumination model for shaded display </b>.
  <i>Communications of the ACM</i>, 23(6):343-349, June 1980.
  <A HREF="http://vis.cs.brown.edu/results/bibtex/Whitted-1980-AII.bib">(bibtex:
  Whitted-1980-AII)</A>.<br />