Abstract: An optical range simulator device (10) for testing the ranging function of a laser rangefinder (14) receives output pulses from the rangefinder (14) and delivers these to an optical fibre delay line (11,12). The output of the delay line (11,12) and the input of the fibre delay line (11,12) are interconnected by an optical bridging means (15,17,22) which collects at least a portion of each pulse delivered by the output of the delay line (11,12) and recirculates that pulse portion to the input end of the delay line, the remaining portion of each pulse at the output end of the delay line (11,12) being returned to the rangefinder (14) so that for each output pulse received from the rangefinder (14) a simulator device (10) delivers a series of successively delayed return pulses to the rangefinder (14), these delayed return pulses being representative of ranges successively augmented by the range distance represented by the delay line (11,12).