Abstract: An apparatus for measuring the optical transmissive power of the atmosphere, having particular application to measuring visibility at airports, in which luminous flux from a source passes to a remote first detector through the atmosphere, and flux from the source also passes to a second detector through a fibre optic arrangement. The output of the second detector is compared with a reference signal and the difference signal from the comparator is fed to a variable gain amplifier which controls the light source to remove errors in the output of the first detector which would otherwise occur by virtue of aging of the detectors and/or by virtue of changes in the luminosity of the source. Also described is an arrangement for compensating for accumulation of dirt on the detectors and/or the luminous flux source without cleaning off the dirt.