Measurement methods and test procedures: Cutoff Wavelength. This part of IEC 60793 establishes uniform requirements for measuring the Cutoff Wavelength of optical fiber, thereby assisting in the inspection of fibers and cables for commercial purposes. Theoretical cutoff wavelength is the shortest wavelength at which only the fundamental mode can propagate in a single mode fiber, as computed from the refractive index profile of the fiber. In optical fibers, the change from multimode to single mode behaviour does not occur at an isolated wavelength but rather smoothly over a range of wavelengths. For purposes of determining fiber performance in a telecommunications network, theoretical cutoff wavelength is less useful than the lower value actually measured when the fiber is deployed.