diopter noun. [dʌɪ'ɒptǝ] Also dioptra [-trǝ], pl. -trae [-tri:]. In sense 3 also dioptre. L16.
[French dioptre from Latin dioptra from Greek.]
1. = ALIDADE. L16.

2. An ancient optical instrument for measuring angles, altitudes, etc. E17.

3. A unit for expressing the power of a lens, equal to the reciprocal of its focal length in metres. L19.

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