| 摘要 |
The block is shaped like double concave lens, and the angle between the mainly vertical surfaces can be up to 180 deg . The block (100) is located in a holder, and both are completely enclosed by a housing (1), so that holder and block can rotate about the vertical central axis of the lens, and the block emerges from the housing. The edge connecting the vertical surfaces is curved, pref. with a radius of curvature of less th1n 3mm, pref. less than 1mm.
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