An optical test fibre is bent restorably and the bend illuminated to couple an optical signal into the test fibre. The optical signal coupled into the fibre by this method propagates through the optical fibre in both directions. If the bending point which is simultaneously the point of coupling is provided near a first end of the optical fibre, as the power of the optical signal leaving the first end is substantially one half of the power of the optical signal coupled into the fibre at the bend, the transmission loss of the optical fiber may be measured by comparing the optical powers at the respective ends of the fibre.