An Electro - optical EDM instrument, set up on one station, emits a continuous-wave carrier beam of light. It is generated in the transmitter and modulated by an electronic shutter before entering the aiming optics. The modulator chops the beam into wavelengths that are proportional to the modulating frequency, where the wavelength is given by
in which l = wavelength;
V= velocity of light through the atmosphere, m/s;
f = Modulating frequency, Hz (cycles per second).
The modulated light is transmitted to the reflector, placed at the other end of the line. The reflector, acting as a mirror, reflects the light beam back to the receiver, where the incoming light is converted to an electrical signal. A phase comparison is made between the projected and reflected pulses. Then the amount by which the transmitted and received signals are out of phase get measured electronically and registered in a meter by getting converted to an equivalent distance.
In using an Electro - optical EDM, a clear line of sight is required and thus required inter-visibility between the stations.