The standard set CS3 is used to check the correct function and to calibrate the scattered light sensor OS 500. It contains two roughness standards, relating to the lower and upper sensor capacity limit. The lower limit is known to be Aq ≈ 1.6, which is equivalent to Ra < 0.001 µm. It is comparable to a flat glass measurement for tactile roughness testers which is performed to measure inherent system noise. The set also contains a light trap, which is used for sensor calibration.