AbstractSince the first infra-red distancers were exhibited in July 1968 at the International Society of Photogrammetry Congress in Lausanne, the electronic distancer has ceased to be regarded by the practising surveyor as a mysterious instrument which could only be used by specialists. There is no doubt that modern electronic distancers can nowadays be used economically for day to day survey work [1]. Since 1968, the number of distancers available for ranges up to 1000 m has increased. Most of these instruments use an infra-red carrier wave with a wavelength of about 0.9μm, i.e. that portion of the infra-red spectrum next to visible light. The chief reason for using infra-red as a carrier wave is the ease with which the GaAs diode—which is normally used as the source of radiation—can be modulated. Unlike incandescent or gas-discharge lamps, the GaAs-diode can be modulated directly in the high-frequency range by varying its supply current. Other advantages of infra-red radiation with regard to absorption and atlnospheric scatter are, however, c,f subordinate importance. The 0.9μm wavelength is near that of visible light and, except in haze where it has a slight advantage, its extinction characteristics (i.e. loss of range in practical terms) in mist and fog are hardly different from those of visible light. It is true that the infra-red portion of the spectrum (wavelengths from about 0.7μm up to about 1mm) includes certain wavelengths with good penetration characteristics in certain types of mist [2, 3], but they are not yet used for distance measuring instruments.