0 a unit of measurement of the power of a lens, used for measuring a person's eyesight:
A typical short-sighted person in Britain is -4.5 dioptres.
We adopted four dioptres because the professional advice was that that was a sensible cut-off point.
There is some fraction of a dioptre difference in need of correction.
We took this into account in arriving at the limit of 4 dioptres for reading glasses.
It seems too from the literature of one company operating in this country that 4 dioptres is the ceiling.
The main difference between this amendment and my original one is that this amendment mentions four dioptres where my amendment mentioned five dioptres.
I also understand that statistical surveys have shown that it is normal for adults to be slightly longsighted up to a power of about 1 dioptre.
The individual's refractive power is then read off a linear dioptre scale.