| 摘要 |
A beam of radiation is scanned across a sheet 13 and is either reflected therefrom or transmitted therethrough. Faults in the sheet 13 deflect, scatter or alternate the beam which is then passed to a retroreflector screen 24 where it forms an image 15. Analysis of the image 15 is carried out via the scanning means 22 to determine the faults in the sheet 13.
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