The effect of measurement spot size on the accuracy of laser radar devices in industrial metrology

Time-of-flight laser radar devices utilizing a single heterostructure semiconductor laser diode as a light source have a measurement spot diameter from a few millimetres to several centimetres depending on the optics and the required optical output power. Inhomogeneities in the reflectivity and surface profile of a target inside the area illuminated by the laser beam can be a source of error in distance measurement. If inhomogeneities, the intensity profile of the transmitter beam on the target surface and the shape of the received optical pulse in the time domain from a flat homogeneous surface are known, the shape of the received pulse from an non-homogeneous target surface can be calculated. As the measurement spot sweeps over the surface of the target its shape in the time domain varies, causing distance error. If the timing discrimination method used in a laser radar device is known, the distance error can be estimated by means of a numerical analysis. A simple method of estimating the error caused by the finite measurement spot size together with inhomogeneities of the target surface is presented here. A number of real measurement results obtained with a prototype of a commercial laser radar device are presented, together with simulation results. Both sets of results show that a spot size of several centimetres together with large variations in the reflectivity of the target surface can really cause an error of some centimetres in distance measurement.