The refractive indexn, extinction coefficientk, and optical band gapEoptof nitrogen incorporated amorphous carbon films &agr;‐C:N have been investigated by spectroscopic ellipsometry. The effect of the incorporated nitrogen on the bonding configuration of carbon atoms has been discussed in term of the optical behavior of the films. The results show that nitrogen incorporation into the films stabilizes the tetrahedral bonds of carbon atoms, resulting in lower values of bothnandk. However, if the nitrogen incorporation is accomplished by means of excessive N+ion bombardment, the formed &agr;‐C:N films possess zero gap and are more ‘‘graphitelike.’’ ©1996 American Institute of Physics.