dioptre

Non-SI unit

Name	Symbol	Quantity	SI units
dioptre	D	optical refractive power	m^-1
The dioptre, symbol D, is a non-SI unit of measurement of the optical power of a lens or curved mirror. The dioptre is not accepted for use with the SI. One dioptre is equal to the reciprocal of the focal length measured in metres.
Definition			c^-1 Δν_Cs
$1 \mspace{4mu} \text{D} \mspace{6mu} = \dfrac{299 \mspace{4mu} 792 \mspace{4mu} 458}{9 \mspace{4mu} 192 \mspace{4mu} 631 \mspace{4mu} 770} \mspace{6mu} \dfrac{\Delta \nu _{Cs}}{c}\\ \\ \\ 1 \mspace{4mu} \text{D} \mspace{6mu} \approx 3.261 \mspace{4mu} 225 \mspace{4mu} 571 \mspace{4mu} 749 \mspace{4mu} 405 \mspace{4mu} 557 \times10^{-2} \mspace{6mu} c^{-1} \mspace{4mu} \Delta \nu _{Cs}$

$1 \ \text{D} = 1 \ \text{m}^{-1}$

Although the dioptre is based on the metre, it is not included in the SI. The unit for optical power should therefore be specified explicitly as the reciprocal metre, m⁻¹.