Novel geometry for a high speed outdoor laser scanning system

Today, the phase shifting technique is still the fastest and most accurate measurement method for laser scanners in the meter and some 10 meters range. A major handicap for outdoor applications such as railway measurement systems is the influence of optical cross talk caused by pollution of the optical surfaces. This effect is closely linked to the multi target response of phase shifting systems which is analyzed quantitatively. It is shown that optical cross talk is a major cause for nonlinearities when dark objects are measured. To overcome this disadvantage a sealed cover with light traps was designed for a 350 degree scanner to protect the most critical surface of the scan mirror and to absorb the stray light from the polluted outer surfaces. The sampling rate of this system can be doubled without loss of measurement accuracy by using a new scan geometry with two fibre coupled measurement heads on each end of a hollow scan axis with a double sided scan mirror.