| 摘要 |
A measuring head for a densitometer comprises a light source and an optical system for forming a spot of light of defined size on the surface of an object under test. The optical system guides the reflected or transmitted light, depending on the type of densitometer, from the area of the spot to a photoelectric measuring transducer. The light intensity distribution across the spot as seen by the transducer is weighted to have a bell-shaped curve with its maximum at the center of the spot. The weighting may be done in the optical path from the spot area to the transducer or, as described, in the path between the source and spot area by means of a filter comprised of a plano-convex transparent lens and a plano-concave lens of neutral grey glass. The lenses have the same index of refraction and the same curvatures and are joined to form a plane parallel plate having a maximum transmissivity on its axis, the transmissivity decreasing radially from the axis. The head finds particular use for measuring raster-like material such as offset printing.
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