Knowledge about the lenses for fitting glasses

1.Resin sheets (the most representative one is CR-39)

1.1Characteristics of resin lenses:

The chemical name is allyl diethylene glycol carbonate, which is a hard and transparent substance. The refractive index is 1.499; Light transmittance: 92% Thermal stability: No deformation below 150 degrees Celsius; It has good water resistance and corrosion resistance (except strong acids), and is insoluble in common organic solvents.

1.2Advantages of resin lenses:

The specific gravity is 1.32, which is half that of glass and is light in weight. Impact-resistant, unbreakable and highly secure (in line with the standards of the US FDA); Comfortable to wear; It is convenient to process and has a wide range of applications (including those used in semi-frame and frameless spectacle frames). The product range is rich (single light, dual light, multifocal, cataract, color-changing, etc.); Its absorption capacity for ultraviolet rays is likely to be higher than that of glass lenses. It can be dyed in various colors; The thermal conductivity is low, and the situation of "water mist" caused by water vapor is better than that of glass lenses.

2.Special lenses

2.1Photochromic lenses

Silver halide particles are added to the raw materials of the lenses. Under the action of ultraviolet rays in sunlight, the silver halide decomposes into halogen ions and silver ions, thus changing color. The degree of discoloration varies according to the intensity of ultraviolet rays in sunlight. When the ultraviolet rays disappear, the lenses return to their original color.

2.2Polarized lens

--A lens that allows light to vibrate only in a certain direction to pass through is called a polarizing lens.
The purpose of using polarizing lenses is to block the dazzling light reflected on a certain plane.
--Precautions for use:
It has poor durability. When in prolonged contact with water, the surface film layer is prone to peeling off.
When installing the frame, if internal stress occurs, it will affect its polarization effect.

2.3Bifocal lenses

-Features:
There are two focal points on a pair of lenses, and a small lens with a certain degree of luminosity is superimposed on a regular lens. It is used alternately for presbyopia patients when looking at distant and near objects. The upper part is the luminosity when looking far (sometimes it is flat light), and the lower part is the luminosity when reading. The degree when looking at a distance is called upper light, and the degree when looking at a distance is called lower light. The difference between the upper and lower luminosity is called ADD (external luminosity). 

-Advantages:
Enable presbyopic patients to avoid changing glasses when looking at near or far objects.

2.4Progressive multifocal lenses

-Features:
Have multiple foci simultaneously on the same lens; The degree of the progressive band in the middle of the lens changes point by point from top to bottom.
-Advantages:
The same pair of lenses can be used to see far, medium and near distances. The lenses have no distinct boundaries and are not easily noticed by people. When looking at the distance from the vertical direction of the center, no jumping phenomenon is felt. Disadvantage:

2.5The characteristics of coated lenses

-Advantages:
Increase the transmittance and enhance the clarity. Aesthetically pleasing with no obvious reflection; Reduce the lens vortexes (vortexes are caused by the reflected light from the periphery of the lens being repeatedly reflected on both the front and back of the lens); Remove illusions (the inner surface of the lens receives incident light from behind and reflects it into the eye, which can easily cause visual fatigue); Increase the blocking of harmful light.

We suggest that： before getting glasses each time, one should learn about the wearer's work and study conditions. These conditions of his will determine what "tasks" the pair of glasses he is about to get fitted with will face for a considerable period of time in the future. Among them, the main dividing line is between outdoor workers and indoor workers, and indoor workers can be further subdivided into computer workers and others. Among them, the student group is a very special existence. Only by understanding the future working scenarios in which this pair of glasses will be fitted can it be adapted.