Wedge Control Device

The photograph below shows the device used to control wedge and determine the thickness of the lens.  The lens rests on two ball bearing tipped posts (these are on the right and just barely visible through the fine ground glass) and a horizontal steel rod (immediately below the dial gauge).  Wedge was controlled to within the limit of reproducible measurement in the dial gauge (~±0.0002").  The position of the dial gauge over the support rod is adjusted such that its contact point with the lens matches its contact point with the dial gauge (this can only be done to within ±0.0005" at best).  With this arrangement, and with knowledge of the sagitas of both sides of the lens (to within ±.0001") and the position of the dial gauge with respect to the center of the lens (to within ±0.003"), it is possible to determine the center thickness to within ±0.002".  At the end of fine grinding the thickness measured in this way was 0.749 ± 0.002".  Later, when the central plug in the lens was removed the thickness was measured directly with an "Etalon" micrometer to be 0.7490 ± 0.0001".

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This page was last updated on August 26, 1999.  Send questions and comments to: