| 摘要 |
PROBLEM TO BE SOLVED: To detect that a stack of a sheet material reached a specified height by giving an indication that the stack surface reached a critical distance from a light source when a detector detects that an illuminated region reached a specified size. SOLUTION: An LED light source 60 illuminates a topmost sheet region on a stack surface 40 to form an illuminated region A. A light from the light source 60 disperses to form a cone of diffused light. With decrease or increase of the distance between the light source 60 and stack surface 40, the size of the illuminated region A reduces or increases. A light detector 70 monitors a region Z of the surface 40 to monitor the size change of the region A. The quantity of a light illuminating the region Z varies with the distance between the light source 60 and the surface 40 and also varies with the change of the size of the region A. The region Z is so set, related to the region A, that the light illuminating the region backs to the axis 80 of a dispersed beam with decrease of the distance between the light source 60 and surface 40. A detector 50 indicates the arrival of the surface 40 at a critical distanceα.
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