Patent ID: 7728977

Claim:
An open path gas detector comprising: a transmitter unit having a radiation transmitter; optics configured to shape radiation emitted by the transmitter into a beam for transmission along a path, at least a partial alignment shadow is formed in the beam; a communications signal detector; and a receiving unit having a detector configured to detect the beam of radiation from the transmitter unit; and a communications transmitter configured to send a data signal to the transmitter unit at a communications wavelength that is different from a transmitted wavelength, the data signal containing data concerning the intensity of radiation detected by the detector, and where the optics is arranged to direct the data signal towards the communications signal detector; where the transmitter unit includes a signal generator configured to generate a composite signal having at least two different modulation frequency components; a driver arranged to drive the radiation transmitter with the composite signal to generate radiation that changes wavelength in accordance with the composite signal; wherein the signal generator is so configured that at least one of the following: a) the sum of any two modulation frequency components in the composite signal or harmonics thereof and b) the difference between any two modulation frequency components in the composite signal or harmonics thereof is neither at the fundamental frequency of any of the modulation frequency components of the composite signal nor at a harmonic thereof.