What Are Spherical and Aspherical Glasses?
1. Refractive index
The refractive index of the lens depends on the material used for the lens.
In the case of the same center thickness, lenses of the same material of the same degree have a higher refractive index and thinner lens edges; when the brand is the same, the level of refractive index means the difference in materials used, and the price of the lens can also be judged , The higher the refractive index, the higher the price, and vice versa.
2. The reason why the wear resistance coefficient should be mentioned is that the resin lens is not very wear-resistant when it first enters the market, and basically wears the lens for half a year. However, with the continuous iterative update of technology, the surface hardness of ordinary resin lenses is 2-3H. After hardening, the hardness reaches 4-5H, and now the hardness of resin lenses has exceeded 6-7H, and then the surface is plated. Adding a dura mater solves the disadvantage that the lens is not wear-resistant. In terms of wear resistance, both Essilor and ZEISS have launched their most confident diamond crystal film layer and diamond cubic platinum film, respectively, which shows that they attach importance to the wear resistance coefficient.
Although the hardness is still incomparable with that of glass flakes, most glass flakes have been withdrawn from the market. The only remaining ones are mainly high refractive index, such as 1.8 and 1.9.
By the way, talk about PC tablets.
Lightness, high toughness and not easy to break are the obvious advantages of PC tablets. However, the maturity of the PC film brand has yet to be confirmed, and it is recommended to choose products from major brand lens manufacturers.
3. Spherical and aspherical lens A part of a lens with a convex or concave shape cut from the sphere is called a spherical lens. Because spherical lenses are unavoidable, aberrations (especially astigmatic aberrations, like bending aberrations) and other phenomena occur, so lenses that correct the shortcomings of spherical lenses, namely aspherical surfaces (usually single-sided aspherical lenses), appear.
Aspherical lens: Refers to the rotating surface with the same non-circular cross-section (such as parabola) on all meridians. The biggest advantage is that there is no object distortion in the edge field of view, and the lens edge is about 1/3 thinner than the ordinary spherical lens. Therefore, the price will be relatively higher. In addition, there are double-sided aspherical lenses on the market, the field of vision is clearer, the lenses with the same refractive index are lighter and thinner, and the imaging effect is better. Of course, the price is also very expensive. 4. Abbe number Abbe number is the dispersion coefficient, which is an important index to measure the imaging clarity of the lens. It is usually expressed by the reciprocal of the dispersion coefficient, also known as Abbe number. The larger the Abbe number, the smaller the dispersion. Conversely, the smaller the Abbe number, the larger the dispersion and the poorer the definition of imaging. The Abbe number of the lens has a certain relationship with the refractive index. The greater the refractive index of the material, the greater the dispersion, that is, the lower the Abbe number.
Therefore, as far as this single index is concerned, the lower the refractive index, the better. However, it is necessary to judge based on the comprehensive situation of your own vision when fitting. In addition, the physicist Ernst Karl Abbe, the inventor of the lens Abbe number, is one of the founders of Zeiss, so Zeiss lenses have always paid special attention to the control of Abbe number. At the same refractive index, the dispersion of Zeiss lenses is far lower For other brands, the wearing comfort is greatly improved.
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