| 摘要 |
In a microfilm web handling apparatus wherein a photocell senses position markers corresponding to image frames along the microfilm web to enable positioning of a given image frame within a viewing station, a reference photocell provides a reference signal which is compared with a detected signal from the position sensing photocell to produce a position signal which is processed via logic networks to drive a film transport mechanism to thereby selectively position the microfilm web. Preferably, three photocells are provided, two of which sense position markers adjacent one edge of the microfilm web, while the third provides the reference signal. Each photocell has an effective sensing diameter which is not greater than one-half the dimension of a position marker parallel to the direction of travel of the web. In such an embodiment, the output of the reference photocell is compared with the detected signal from each of the two position indicating photocells.
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