DIOPTRY
\dɪˈɒptɹi], \dɪˈɒptɹi], \d_ɪ__ˈɒ_p_t_ɹ_i]\
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A refractive unit, being the amount of refraction produced by a lens or other refracting medium having a principal focal distance of one metre; the unit used in measuring glasses and the refractive states of the eye. Symbol, D. A lens of 1 D has a focal distance of one metre, of 2 D (or of twice the refractive power), a focal distance of 0.5 metre, etc. Prism d. (symbol, PD), a unit of prismatic deviation, being a deflection of one centimetre produced at a distance of one metre.
By Alexander Duane