A method for identifying contamination upon a lens of a stereoscopic camera is disclosed. The stereoscopic camera is arranged such that it has the same capturing area over time, and is provided with a first camera providing first images of said capturing area and a second camera providing second images of said capturing area. The first and second images are divided into at least one evaluation area correspondently located in respective image. A traffic surveillance system is also disclosed where contamination upon a lens of a stereoscopic camera is identified according to said method.