Which lens is right for us? First of all, because the purpose of buying round tortoise shell glasses is to correct vision in medicine, it is necessary to use thermoset molding lenses. If due to the relationship of movement, fear of breaking, must choose thermoplastic materials, of course, the absolute quality of imports may be better, but it is difficult to find in China, basically domestic, called space pieces, PC pieces. There are also some injection molding low-cost acrylic reading glasses, 10 yuan a pair on the morning market, the quality is worse. Secondly, from the impact performance indicators of the lens, to explore the choice of lenses now on the market to promote space film or PC lenses.

Which lens is right for us?

First of all, because the purpose of buying lenses is to correct vision in medicine, it is necessary to use thermoset molding lenses.

If due to the relationship of movement, fear of breaking, must choose thermoplastic materials, of course, the absolute quality of imports may be better, but it is difficult to find in China, basically domestic, called space pieces, PC pieces. There are also some injection molding low-cost acrylic reading glasses, 10 yuan a pair on the morning market, the quality is worse.

Secondly, from the impact performance index of the lens, to explore the choice of the lens

Now in the market to promote the space film or PC lens, again and again to claim its safety performance, said that the hammer can not break the lens; For the general consumer, a listen to feel is good, such a lens can not be used for a long time? In fact, any material is relative, this rise and fall. For example, if the glass is hard, the knife can't get in, but the hard material must be brittle, the wood is elastic, the hammer can hold it, but the knife can cut in very well. PC lens the same reason, toughness is very good, but the surface hardness is very small, the formed substrate, with gauze gently wipe, there will be a lot of scratches, you do not know? It is because the hardness of the surface is too small, the chemical hardening process of the surface is very demanding, and at present there is no very effective method in China to make the quality of the hardening reach the level of CR-39 resin lenses.

Just imagine, what kind of environment are our lenses used in? Do you need such strong impact resistance? If you put on your glasses and someone gave you a hammer, even if they were made of iron plates, would you keep your eyes? Will your frame hold up? Can your eye socket hold? It goes without saying, must be permanent disability! So we don't have to pay so much attention to the toughness of the PC, but to focus on the key quality indicators of the lens.

In the national standard, there are clear indicators for the impact resistance of the lens, and the detailed test content is described in ANSI Z80.1-1999 and FDA regulation 21 CFR 801.410. Simply put, let a 5/8 "diameter 16 gram steel ball fall freely on the lens under test from a height of 50 inches, if the lens does not break or no debris comes off the inner (rear) side of the lens, the lens passes the test.

Among thermosetting lenses, only lenses made of CR-39 material can pass this test.

Do you think about it, a steel ball with a diameter of 15.9mm, falling from a height of 1.3 meters, how big the impact is, usually we will encounter this kind of impact? So I personally think that it is enough to meet the requirements of the national standard, don't listen to the shop. Don't buy expensive, buy the right!

From this indicator, we believe that the CR-39 lens is the best choice, 1.56, 1.61, 1.67, 1.74 lenses are the second.

Third, the lens is optimized from the Abbe number (dispersion coefficient) and refractive index of the lens

There is a very important quality index for resin lenses - the dispersion coefficient. The dispersion coefficient is an important indicator to measure the clarity of the lens image, and is usually expressed by the reciprocal of the dispersion coefficient, also known as the Abbe number. The larger the Abbe number, the smaller the dispersion, and conversely, the smaller the Abbe number, the greater the dispersion, and the worse the clarity of its imaging. As the name implies, you also understand that the separation effect of a certain material on the light source (light wave is seven colors); It is ideal that a beam of light through the lens, no distortion, still focused on a point, but due to the different refractive index of the material and its own unique properties, there is a higher refractive index, the greater the effect on the color separation of light trend.

The larger the Abbe number of the lens, the better. This indicator is indicated on the regular lens packaging bag. However, for those that are not indicated, the indicator is generally too low and ashamed to indicate it.

The dispersion of CR-39 lens is the smallest, and the Abelian number is the largest, 56~58, which is the best lens material known at present. The Abel number of 1.56 lenses, which is quite large on the market, is 60% of CR-39, and the Abel number of 1.60 lenses and 1.74 lenses is lower.

In about a decade ago, when 1.56 lenses were just popular, Western countries were not allowed to import the cabinet, the reason is that the Abbe number is too low, not up to standard, which is harmful to the correction of vision. From this point of view, we believe that we should pay attention to the Abelian number of the lens and analyze it from a scientific point of view.

One of the highlights of high refractive index lenses is thin, but for the main consumer group with low myopia, lenses with myopia below 300 degrees, CR-39 and 1.56, 1.61 three lenses after loading, the thickness is not as big as you think, and there is almost no obvious difference.

To sum up, we believe that for the ladies and gentlemen with low myopia, especially the majority of primary and secondary school students, it is best to buy products with high abbe number, rather than buy "ultra-thin" lenses!