The lens prism combination is the three prism, an optical combinatorial mirror with low convex lens attached to the bottom, and the experts in the working group on myopia prevention and control in China. Is the visual virtual objects near objects into distant vision, eye ciliary muscle relaxation, can well prevent the occurrence and development of myopia. But it is a pair of small glasses that are worn outside the frame glasses and worn only in close reading and writing. The prism reduces the eye's polymerization force and relaxes the rectus muscles. The lens can reduce eye accommodation, the occurrence and development of ciliary muscle relaxation, can better prevent myopia.
A thin lens - a lens that is large compared to the thickness of a central part and the radius of curvature of the two sides. In the early days, the camera was only equipped with a lens of a convex lens, so it was called "single lens". With the development of science and technology, the modern lens has a number of convex concave lenses with different forms and functions to make up a lens called "compound lens". The concave lens in the compound lens corrections the effects of a variety of aberrations.
Optical glass has high transparency, purity, colorless, uniform texture and good refraction ability, so it is the main raw material for lens production. Due to the chemical composition and the refractive index of different optical glass, there are:
1. flint glass - adding lead oxide to the glass composition to increase the refractive index (1.8804)
2. - crown glass in the glass composition made with sodium oxide and calcium oxide to reduce the refractive index (refraction barium crown glass rate was 1.7055)
3. lanthanum crown glass, which is a variety found in recent years, has excellent characteristics of high refractive index and low dispersion rate. It provides conditions for creating large aperture advanced lenses.