The beamlines slits of Shanghai Synchrotron Radiation Facility(SSRF) are required to have a repeatability of better than 1 μm.Before the slits installation

the off-line and/or on-line repeatability measurements must be conducted.A machine vision measuring system based on high resolution CCD and adjustable high magnification lens was used in this regard.A multi-level filtering method was used to treat the imaging data.After image binarization

the imaging noises were depressed effectively by using of algebraic mean filtering

statistics median filtering

and the least square filtering.Using the subtracted image between the images before and after slit movement

an average displacement of slit blades could be obtained

and the repeatability of slit could be measured

with a resolution of 0.3 μm of the measurement system.The experimental results show that this measurement system meets the requirements for non-contact measurements to the repeatability of slits.