Twist structures can be distinctly different over the whole seal seat circumference. The local dependency is characterized by varying portions of roughness and waviness. Consequential the periodicity of the twist structure is disturbed. However, the partitions of the seal seat surface with dominating twist structures cause leakage. In this case a 100%-inspection of the surface over the circumference is recommended. Especially long wave twist structures (DP 200μm) are difficult to prove. The disturbed periodicity leads to a non-continuous stripe pattern. Above a period length of 200μm the spacing between the diffraction lines is very small and it is difficult to resolve it with human eye.

The pictures recorded by the video camera are magnified by a factor 4 and displayed on a screen. That leads to a clearly improved resolution of long wave twist structures (200μm - 500μm). The STT R100 NV is principally designed for the stationary 100%-inspection of seal seat surfaces close to the manufacturing process and is connected to a video camera and a screen. The Video camera can also be readout by a PC via a standard TV tuner card. The disturbed diffraction lines pattern can be compacted to standing lines, while rotating the shaft between centers or in the lathe chuck, which indicates the presence of twist.