Abstract: A method of characterizing an optical system, wherein the optical system comprises an optical fibre having a proximal end and a distal end, wherein the optical fibre comprises a non-single-mode optical fibre, and comprising a reflector assembly comprising a stack of reflectors disposed at the distal end of the optical fibre, wherein the stack of reflectors is arranged to provide different reflector matrices in dependence on illumination wavelength. The method comprises projecting calibration patterns at a plurality of characterization wavelengths onto the proximal end, then obtaining, at the proximal end, data relating to reflected calibration patterns. An instantaneous transmission matrix of the optical fibre is then determined using the data relating to the reflected calibration patterns and the reflector matrices.