Definition
The refractive power of the eye is measured in the value diopter, which is abbreviated as dpt. The value of the refractive power indicates how far behind the lens the light is bundled and thus the image in the eye is brought into focus. From this it follows that dioptre is the reciprocal of the known unit of measurement, meter.
Measurement and calculation of diopters
Since dioptres are the unit of measurement for refractive values, it is also the unit of measurement for spectacle lens values. Internationally, dioptres are considered the value for lens systems, and they are calculated worldwide according to the laws of geometric optics. The normal-sighted eye has a maximum total refractive power of about 63 dpt and an eye length of 23.5 mm.
The cornea contributes the major part to the refractive value of the eye with approx. 43 dpt, while the lens is relatively low with a refractive value of 10 to 20 dpt (depending on the adjustment degree of the lens). The values cannot be directly added up because there are different optical media in the eye, so that the calculation of the total refractive power is done individually.
In a healthy eye, the refractive power is about 60 dpt at a long distance of the object to be seen, so that the focal length is about 16.6 mm. However, this ability of the eye to adapt is age-dependent and decreases steadily from about 25 years of age. In order to be able to see sharply even in the vicinity, the refractive power must increase.
The values on the eyeglass prescription
The prescription for a new pair of glasses contact lenses often contains the following: This means the following:
- Rdph-/+ 1,0 cyl -0,75 Axis 100° -0,75 Axis 100°
- Lsph -/+ 1,0 cyl -0,5 axis 72°
- R= right eye
- L= left eye
- Sph = spherical = basic visual impairment. A minus value stands for myopia, a plus value for hyperopia
- Cyl = cylinder = value for correction of astigmatism
- Axis = in this direction the glass must be fitted so that the values are in the right place and the astigmatism is well balanced
- The numerical values are always given in dioptres and a graduation takes place in steps of 0.25 dpt each.
