How to Choose the Refractive Index of Lenses for Different Degrees of Myopia?
First of all, you should know that it’s enough that the refractive index of the lens is suitable, not the higher the better.
The 1.56, 1.61, 1.67, 1.71, and 1.74 on the lens are the refractive index of the lens.
For myopia glasses, under the same other parameters, the higher the refractive index of the lens, the thinner the edge thickness of the lens. But it does not mean that when choosing lenses, choosing as high a refractive index as possible.
Refractive index refers to the ratio of the propagation rate of light in vacuum to the rate of propagation in the medium. With the same power, the higher the refractive index is, the stronger the ability to refract incident light and the thinner the edge of the lens are.
However, glasses are used for vision correction, the first thing is to ensure the overall visual comfort after wearing. Therefore, in addition to the thickness of the lens, the clarity and light transmittance of the corresponding lens should also be considered. Generally, the higher the refractive index is, the thinner the lens is, but abbe number will be lower.
The Abbe number is an index used to express the dispersion ability of a transparent medium. Generally speaking, the greater the refractive index of the medium is, the more severe the dispersion is, and the smaller the Abbe number is; conversely, the smaller the refractive index of the medium is, the less the dispersion is, and the greater the Abbe number is.
Simply put, the abbe number refers to the dispersion phenomenon, which will reduce the visual clarity.
The higher the refractive index of the lens is, the lower the Abbe number and the higher the dispersion are, so it is not that the higher the refractive index is, the better is.
Therefore, the refractive index is not the only factor that determines the thickness of the lens. It is necessary to comprehensively consider the shape, size, and interpupillary distance of the frame to affect the actual effect.
Refractive index selection reference:
(For reference only, not mechanical comparison)
1.56: 0 -400° (-4.00D)
1.60: 300°(-3.00D)—600°(-6.00D)
1.67: 400°(-4.00D)—800°(-8.00D)
1.74: above 800°(-8.00D)
