Material classification and characteristics of spectacle lenses (part 1)

(1) Natural materials
   Generally speaking, the main component of crystal lens and crystal is silicon dioxide, which is divided into two types: colorless and brown. The high price of crystal lenses is mainly due to the scarcity of materials. Moreover, it is difficult to polish them into lenses. The characteristics of crystal lenses are :
Advantages: hard, not easy to wear, not easy to damp (fog is not easy to retain on the surface), thermal expansion coefficient is small (not good heat absorption)
Disadvantages: almost no UV protection, easy to cause visual fatigue, uneven density, easy to contain impurities, double folding phenomenon (seeing objects have ghosting)
There is a saying among folks that crystal lenses can be eye-catching. This is mainly due to the poor heat absorption of crystal, especially in summer, it feels cool when you wear it. In fact, this kind of statement is unscientific. Crystal lenses (especially dark brown ones), because they are not protected against ultraviolet rays, the effect is the same. Wearing inferior sunglasses is not only not protective to the eyes, but it is harmful. (For example: the road in summer, the sunny day after snow, the specific concept of ultraviolet rays)
UV tips
Ultraviolet light is an electromagnetic wave with a wavelength from 200nm to 380nm. Because it has a shorter wavelength than the shortest violet light in visible light, it is called ultraviolet light.
Ultraviolet rays have a bactericidal effect, but they are harmful to the eyes. Ultraviolet rays are generally divided into three categories: uvc200-280nm uvb280-315nm uva315-380nm. Human eyes absorb more ultraviolet rays and easily cause various eye diseases, such as photo-induced keratitis and cataracts. And retinal degeneration.

          Ozone layer Cornea Lens Retina

uvc 99.9% 0.1% ---- ----

uvb ---- 70% 30% ----

uva ---- 35.5% 63% 1.5%

(2) Glass materials
1. Ordinary optical glass lens
Crown glass is generally used. The main components are silicon dioxide and sodium titanium silicate. The refractive index is about 1.523, the light transmittance is 90-92%, and the specific gravity is 2.54. It can absorb ultraviolet rays below 320nm wavelength, and the Abbe number is about 59, with good uniformity. The optical stability is relatively high. The disadvantage is that it is fragile and heavy. According to the different chemical components added to the glass, the lens is white and red, commonly known as optical white film or light white film, white film; Kesai film or red piece.
2. High refractive index glass lens
    In other words, ultra-thin sheets are commonly used, and compounds such as titanium oxide are usually added to the glass to make the lens thinner and lighter. It is suitable for making high-brightness lenses. Its refractive index is about 1.7, and the Abbe number is about It is 40 and the specific gravity is 2.99. The disadvantage is that the dispersion is large, the rainbow effect is easy to produce, and the nature is relatively soft.
(3) Resin materials
    Resin lenses are lenses made of organic substances with optical properties, also known as plastic lenses. Thermoplastic lenses (acrylic) were made for the first time in 1940. In 1942, the Pittsburgh Flat Glass Company invented the aircraft for the Columbia Space Agency. Zakura material CR-39 (C stands for Columbia Space Agency, R stands for Resin resin) In 1954, French Essilor company used CR-39 to make sunglasses lenses. In 1956, Essilor successfully trial-produced CR-39 for ophthalmic use. There are power lenses.

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