A destructive technique for determining certain optical fibre transmission characteristics, such as attenuation and bandwidth, by (a) performing the desired measurements on a long length of the fibre under test, (b) cutting the fibre under test at a point near the launching end, (c) repeating the measurements on the short length of fiber, and (d) subtracting the results obtained on the short length to determine the results for the residual long length. Note 1: The cut should not be made less than 1 metre from the launch end. However, cutting the fibre so close to the launch end (in a multimode fiber) will introduce errors in the measurements because at that point, modal equilibrium conditions have not been established. The errors so introduced will result in conservative results (i.e., higher transmission losses and lower bandwidths) than would be realised under equilibrium conditions. Note 2: Several characteristics may be determined using the same test fiber. Note 3: A variation of the cutback technique is the substitution method, in which measurements are made on a full length of fiber, and then on a short length of fibre having the same characteristics (core size, numerical aperture,) with the results from the short length being subtracted to give the results for the full length.