| 摘要 |
A movable web which is both specular an transparent is checked for defects by beaming a sheet of radiation at the web; and such radiation has an angle of incidence, with respect to the web, which is sufficient to permit some radiation to be mirrored off the web, and some to be transmitted through the web. Respective arrays of photocells cooperate with the reflected and transmitted radiation, and the photocells of such arrays, are jigsawed and interlaced together by means of photocell fingers having edges which are orthogonal to the direction of travel for the web. Pairs of photocells cooperate with respective differential devices to cancel the effect of web vibration to give false alarms; and non-adjacent photocells cooperate with the respective differential devices to prevent system blind spots.
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