Abstract: Apparatus for monitoring a web of material for faults which affect the remission and/or reflection of light incident on the material. Light from two light sources, one emitting light of a narrow spectral band width and one emitting light of a broad spectral band width, are projected onto the web in the form of two at least overlapping light spots and so that the light spot of the former source is relatively smaller than the light spot of the latter source. Light remitted and/or reflected by the web is concentrated on light receiving rods for transmission to photo-multipliers mounted to end faces of the rods. The light from the respective sources is suitably filtered so that each photo-multiplier receives only light from one source. In an alternative embodiment the remitted light is transmitted to a single photo-multiplier which simultaneously receives light from both sources.