A model based approach to intensity normalization for terrestrial laser scanners

Terrestrial Laser Scanners (TLS) measure distances and record angles to objects in the surrounding environment. In addition, a TLS also measures the amount of reflected light (intensity). The intensity value provided by the TLS is a function of the distance to the object, angle of incidence, the object's reflectance properties and internal parameters of the laser scanner. In some cases it is possible to obtain the reflectance properties of an object from the intensity value, distance and angle of incidence. The object reflectance provides information that can be used to improve the speed and accuracy in segmentation or registration algorithms. In order to model the relationship between intensity and reflectance it is necessary to calibrate the model for the specific laser scanner using reflectance targets. These targets have surfaces calibrated to a specific reflectance value. The targets are expensive and it is a time consuming task to calibrate the model in this manner. Alternatively, this paper presents a method to determine the 'normalized intensity' using a similar model but without using the calibrated reflectance targets. Instead, overlapping registered scans are used to determine the model parameters. The normalized intensity can be used in the same way that object reflectance is used in segmentation and registration algorithms. The only difference is that it is not a true reflectance value. The scans used to test the method were taken with a 3DLS-K2 TLS. It is shown that using this method produces normalized intensity values that are independent of distance, instead providing a value related to the actual object's reflectance.