An improved version of the commonly employed C. C. Teng and H. I. Man (Appl. Phys. Lett.56, 1734 [1990]) measurement technique is illustrated by the characterization of films of a new high glass transition temperature polyurea. Measurements taken in the transmission mode are shown to be free from non‐negligible errors introduced by interference effects present in standard reflection geometry measurements. ©1996 American Institute of Physics.