| 摘要 |
The section (20) is placed between a beam source (1) and screen (30). Light (L) illuminates one side with approximately parallel rays (rather than diffuse illumination), which reach the screen. The image is registered by a camera (50, 51). Test instrumentation (60, 70, 80) detects brightness variation. The spacing (a) is small, up to 1.5 cm. It is not a multiple of the transparent section thickness. The screen is uniformly-translucent, being especially matt. The camera is on the first side of the section, and angled with respect to the illumination. Visible, optionally coherent light is used. Various defects, e.g. measurable contrast variation, causes difference in brightness. A single inhomogeneity detected by an evaluation unit for the camera, results in classification of the sheet as defective. A variant arrangement locating the camera behind the screen, is also described. An Independent claim is included for corresponding apparatus.
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