How to choose the material for your glasses lenses?

Different materials have different optical features. Current materials for lenses include ：

Dispersion and high refraction index material:

If the material has high index of refraction, the level of dispersion increases, we can see the lenses surrounded by colorful circles. Dispersion exists in all the optical glass and is more obvious in the high refraction index materials.

Glass and crystal lenses:

Although glass and crystal lenses are heavier than others, they are the most resistant materials. By going through heating or chemical processes, their resistance abilities are enhanced. Their advantages include stable refraction indices and long-lasting optical properties. But they can be broken easily.

1. Natural crystal lenses:
Made by quartz, which the main element is Sio2. Pure crystal has 99.99% of Sio2.

A) Colorless: high degree of Sio2.
B) Brownish: There are other elements apart from Sio2. The optical penetration is reduced and is more comfortable for the wearers.

Advantages: Harder and not easy to be scratched.

Disadvantages: Density is not well distributed, stripes and bubbles can occur to the ones that have impurities, which will cause double refraction and affect your vision.

2. Glass lenses:
There are two types of glass lenses-normal ones and high refraction index ones. The main difference between the two is that Tio2 and PbO are added in the high refraction index (HRI) lenses. There are 1.70,1.80 and 1.90, three types of HRI lenses (red or white).

Advantages: clearer and harder

Disadvantages: Abbe value is low and chromatic aberration is high, which can cause the peripheral vision reduced and peripheral lines curved.

High refraction index lenses:

The thin lenses can only made be using the high refraction index plastic material. The refraction index measures the ability of reflection and determines the feature and thickness of the lenses. No matter how the lenses are designed, the higher the refraction index is, the flatter and thinner the lenses will be. Furthermore, the special features of this type of lenses ensure its central thickness is reduced.

Plastic and thick rosin lenses:

Plastic and think rosin lenses are half of the weight of the normal glasses, but with stronger resistance. They are highly optical penetrable and safer than other materials. The only disadvantage is they get scratched easily.

Rosin lenses:

Rosin lenses include normal rosin lenses, medium to high refraction index rosin lenses, colored rosin lenses and photochromic rosin lenses.

Advantages:
1) Much lighter than normal glass lenses. Its specific gravity is 1.32, only half of glass lenses (2.5). They are more comfortable to be worn, especially for teenagers and highly shortsighted people.
2) Highly resistant and unbreakable. Even they are broken, the pieces are not sharp.
3) Better ultraviolation absorbency than glass lenses, especially the ones with anti-ultraviolation elements.
4) Low degree of heat conduction, which means it is not easy to get mist caused by water vapor.
5) Low dispersion, which provides clearer vision. Higher optical penetrable than glass lenses (before any coating).
6) Can be colored and suitable for many kinds of frames including self-rim and rimless frames.

Disadvantages:
1) Easy to be scratched.
2) Low refraction index, which means they are 1.2-1.3 times thicker than normal glass lenses.

Photochromic lenses:

Photochromic lenses can turn dark when they meet sunshine, ultraviolation and bright light indoor. So they are suitable for people who need work for long hours in open air.

Polycarbonate lenses:

Polycarbonate lenses are the lightest and the most impact-resistant lenses than any other materials. The extra layer of protection makes polycarbonate lenses very suitable for kids. They are used for skiing glasses and goggles too.

Advantages:
1) Light – half of the weight of the normal glass.
2) Safe – The impact-resistance is 60 times more than glass lenses. It can be bullet proof is the thickness is over 2.5cm. Extremely suitable for rimless frames because it is unbreakable.

Disadvantages:
1) Difficult to process. The surface is very easy to be scratched.It gets soft when the temperature is above 100 degree.

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Date：2008-3-26　【Return】