Spherical target-based calibration of terrestrial laser scanner intensity. Application to colour information computation

Terrestrial Laser Scanning (TLS) acquires for each scanned point its position and an intensity. Intensity of TLS is based on a measure of the electronic signal strength which is obtained by converting and amplifying the backscattered optical power of the emitted signal. In this paper, theoretical radar equation is established and particular attention is given to the necessary assumptions. A low-cost intensity calibration method is proposed that provides measurements insensitive to range and incidence angle. This process is data driven without simplifying assumption about radiometrical properties of the spherical calibration target. For the commercial device considered, the laser wavelength is 532 nm. It enables us to measure green levels. Intensity is interpreted for each point of 3D clouds in terms of green level in sRGB colour space viewed under CIE standard illuminant D65. Applications are realized on a data set acquired at Amiens cathedral. There is no more shading or multi-exposure effects compared with high-resolution camera colourisation.