# diopter

Physical unit
Unit name diopter
Unit symbol ${\ displaystyle \ mathrm {dpt}}$
Physical quantity (s) Refractive power
Formula symbol ${\ displaystyle D}$
dimension ${\ displaystyle {\ mathsf {L ^ {- 1}}}}$
In SI units ${\ displaystyle \ mathrm {1 \, dpt = 1 \; {\ frac {1} {m}}}}$
Derived from meter

Diopter ( ancient Greek διά dia , German , pass ' and ὄψις opsis , German , seeing' , plural: diopter), the unit of measure for the refractive power (rarely also: refractive index ) of optical systems and represents the reciprocal of the length unit meters represents . Your unit symbol in Germany is dpt . ${\ displaystyle 1 \ \ mathrm {dpt} = 1 \ \ mathrm {m ^ {- 1}}}$

## Relation to the focal length

The refractive power is the reciprocal of the focal length : ${\ displaystyle D}$ ${\ displaystyle f}$

${\ displaystyle D = {\ frac {1} {f}}}$

Convex lenses have positive power and concave lenses have negative power.

## Legal status

The unit diopter is used as a legal unit in the EU and Switzerland to indicate the refractive power of optical systems, especially in ophthalmic optics . The optical system can be an optical lens , a curved mirror or an eye .

No international unit symbol is specified for the diopter. The symbol "dpt" is listed in the German Unit Ordinance, but does not appear in the underlying directive 80/181 / EEC .

## Human eye

At the suggestion of the French ophthalmologist Ferdinand Monoyer , diopter was introduced into ophthalmic optics in 1872 . The refractive power of the normally sighted, healthy human eye in the accommodationless state is about 59 to 60 D (corresponding to a focal length of about 16.6 mm) and can be enlarged to adapt to smaller viewing distances; this ability to adapt is age-dependent and already declines in youth.

Converging lenses are used to correct farsightedness , diverging lenses correcting myopia and toric lenses to correct a astigmatism . The refractive power is also the parameter of a spectacle lens or an eyepiece with diopter compensation . The larger it is, the stronger the correction of the ametropia .

The following rule of thumb is used to estimate the strength of reading glasses required for presbyopia :

Reciprocal value of the distance (in m) at which you want to read your newspaper
minus
Reciprocal value of the distance (in m) at which one can still see clearly
gives the refractive power of the reading glasses .

Example: reading distance ⅓ m ≙ 3.0 dpt; minimum visual range: ½ m ≙ 2.0 dpt; so reading glasses of 3.0 dpt - 2.0 dpt = 1.0 dpt (i.e. a converging lens) are required.