The quality of an optical signal degrades due to physical layer impairments as it propagates from a transmitter to a receiver. As a result, the signal quality at the receiver of a lightpath may not be sufficiently high, leading to increased call blocking. Consequently, an all-optical network’s routing and wavelength assignment algorithm must verify the quality of the lightpath before accepting it. In this paper, analytical expressions for the total blocking probability are derived for first-fit wavelength assignment for networks suffering from transmission impairments. The new technique effectively predicts the performance of wavelength selection techniques that consider either a single candidate channel or all channels for quality of transmission compliance. The analysis is also applicable to first-fit algorithms with different static channel orderings.