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Dioptres (dpt) Meaning and values
Anyone who is myopic and hyperopic knows the dioptre value that the eye specialist writes on their prescription for spectacles. Using this value, the optician can individually adjust the strength of the lenses to the refractive error. But only very few people know what exactly dioptres are.
So what are dioptres?
Dioptre is the unit of refractive power. The human eye focuses incoming light and forms images on as small an area as possible on the retina. This ability of the eye is called the refractive power, which is expressed in units of dioptres.
The dioptre owes its name to the French eye specialist Ferdinand Monoyer. Since there was no unit for the "strength" of lenses in 1872, he simply invented the dioptre himself, and it is still used today to measure refractive error. The measured values indicate what strength the lenses must have to compensate for myopia and hyperopia. Irregular astigmatism can also affect the dioptre value.
The higher the dioptre, the more severe the refrective error. In the case of myopia the values are in the minus range, in hyperopia in the plus range. Using the values, the optician can custom-fit spectacle lenses and contact lenses. Depending on the dioptres, the lenses have to refract the light more or less strongly.
Why does the dioptre value change?
The cornea is responsible for the largest part of the natural refractive power, while the lens is responsible for only about a quarter. Due to the ciliary muscle and zonular fibres, the lens remains mobile, allowing it to change its refractive power and adapt to different conditions. This can sometimes result in a difference of up to 15 dioptres. This adaptation is called accommodation in medical language.
How many dioptres are normal?
A healthy eye normally has a refractive power of about 60 to 65 dioptres. At values of this magnitude, we perceive images as sharp. However, if there is a refractive error, the refractive power of the eye deviates from the norm. Then the dioptres provide information about how high the deviation of the respective eye is. Often, different values are measured for each eye, which can be compensated for by adjusting the spectacle lenses.
However, the value noted on the prescription does not indicate the actual dioptre value, but rather the strength the visual aid must have in order to return the value to normal. This value is calculated exactly from the measured values of the eyes and the respective distances at which sharp vision should be possible. In myopia, the refractive power is too high and must be reduced again by the power of the spectacle lenses. When correcting hyperopia, on the other hand, the reduced refractive power of the lens must be increased.
Incidentally, the dioptre value changes with increasing age – a completely natural process, because the elasticity of the lens gradually decreases, which also reduces its adaptability.
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How are dioptres measured?
The inventor of the dioptre, Ferdinand Monoyer, is also the creator of an eye test for measuring visual acuity. The so-called Monoyer scale is still used today by eye specialists and opticians. The dioptre value is measured in 0.25 increments. Sometimes the results vary by up to a quarter of a dioptre, depending on the patient's daily form; that is why this scaling has proved successful so far.
When is one myopia and when hyperopia?
The value measured by the eye specialist tells you whether you are myopic or hyperopic. If there is a minus in front of the dioptres, this indicates myopia. If there is a plus in front of the value, then you are hyperopic. Myopia is classified as mild, if the value is up to -3,00 dpt, as moderate if the value is -3,25 dpt up to -6,00 dpt and as severe with a value more than -6,00 dpt. In hyperopia, the values of the classification are a little bit different: it is classified as mild with a value up to +2,00 dpt, moderate with a value +2,25 dpt up to +5,00 dpt and severe with values more than +5,00 dpt
What is the refractive index?
In general, the higher the dioptre value, the thicker the lenses should be. However, this turns out to be very impractical at high values, as the lenses would not stay in the frame due to their thickness. The refractive index is the solution to this problem. This value depends on the dioptres, the material of the lenses and the type of glasses. It ensures that the glasses remain comparatively light and thin at higher values.
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