Method and apparatus for inspecting apertured mask sheets.

摘要

An on-line method of inspecting an apertured mask sheet (10) to be formed into a shadow mask for a colour cathode ray tube is disclosed. In one of several embodiments described the mask sheet (10) is linearly scanned by a laser spot (32) transverse to the direction of advancement (30) of the mask sheet. The peak values of the grey levels of the laser light transmitted by the mask sheet are detected and the data is convoluted electronically to form a series of convolution windows (35). The grey levels of the successively formed convolution windows (35) are compared to at least one reference value and an output is produced at least in those cases where an error in the mask aperture size is detected. Other methods of scanning are disclosed which enable the mechanical tolerances of the apparatus to be less stringent. In the case of a low resolution, oversized spot an accurate assessment of the aperture shapes can be made by comparing alternate grey values with its respective reference value.