The absorption coefficient for a 0.4-&mgr;m-thick GaN layer grown on a polished sapphire substrate was determined from transmission measurements at room temperature. A strong, well defined exciton peak for theAandBexcitons was obtained. TheA,B, andCexcitonic features are clearly defined at 77 K. At room temperature, an energy gapEg=3.452±0.001 eVand an exciton binding energyExA,B=20.4±0.5 meVfor theAandBexcitons andExC=23.5±0.5 meVfor theCexciton were determined by analysis of the absorption coefficient. From this measured absorption coefficient, together with the detailed balance approach of van Roosbroek and Shockley, the radiative constantB=1.1×10−8 cm3/swas obtained. ©1997 American Institute of Physics.