SPIE Proceedings [SPIE Sixth International Symposium on Instrumentation and Control Technology: Signal Analysis, Measurement Theory, Photo-Electronic technology, and Artificial Intelligence - Beijing, China (Friday 13 October 2006)] Sixth International Symposium on Instrumentation and Control Technology: Signal Analysis, Measurement Theory, Photo-Electronic Technology, and Artificial Intelligence - The separation and assessment methods of the helix feature line of helicoidal surfaces