SPECTACLE LENS MATERIALS
Glass Lenses
Glass lenses are the oldest material used for spectacle lenses but rarely used today. They have excellent optics properties and are scratch resistant but unfortunately they are approximately 33 percent heavier than traditional plastic lenses (CR39) and the least safe for impact resistance. The most common reason we will still use glass lenses is because of "photogrey" lenses which are sun sensitive and darken outside. However, there is a newer lens that changes colors (Transitions) that is available in plastic & hi-index lenses so glass lenses are used even less now.
Plastic Lenses (CR39)
Traditional plastic lenses (called CR 39 for the batch number of the original mixture created) are much lighter than glass lenses and safer in case of breakage. The index of CR39 is 1.498 which is a measure of it's density. They can be tinted almost any color and darkness. They are more easily scratched than glass lenses although the current lenses are much more resistant than the original ones. There is a scratch resistant coating that can be applied which greatly reduces scratches.
(In our office scratch resistance coatings come with a one year warranty against scratching and if they do scratch we will replace them free of charge one time).
Hi-Index Lenses
"Light, Thin and ohhhh so nice ..."
These are newer plastic materials that are denser (higher index) than traditional CR39 above. The end result is that the lenses can be made much thinner and lighter. Hi-Index lenses can still be tinted and they absorb the harmful UV light. The most commonly used high index lenses are from 1.58- 1.60. There are even newer materials go over 1.70 but the index is just one factor to consider. The higher the index, however there are stability issues to consider. Transitions lenses can be used in the high index materials so we can now offer lighter, thinner lenses that can change color depending on the outside UV.
(In our office Hi-Index lenses come automatically with a scratch resistance coating that has a one year warranty and if they do scratch we will replace them one time free of charge).
Polycarbonate Lenses
"Safety Lenses, Great for Children"The polycarbonate lens is one particular high index lens that was originally designed as a safety lens. It is about the most impact resistant lens material available and should be used for safety glasses. It is typically a less expensive option for those that desire a higher index on a budget. Personally, we prefer the optics of the newer materials over the polycarbonate lenses but highly recommend it for children's lenses and safety lenses.
(In our office Polycarbonate lenses come automatically with a scratch resistance coating that has a one year warranty and if they do scratch we will replace them one time free of charge).
Aspheric Lenses
The aspheric lens design that can be used in most lens materials. The design of the lens creates a flatter, thinner and less distorted lens than traditional designs. In general it achieves this by changing the radius of curvature the further from the center of the lens. Aspheric lenses are an excellent choice for plus powered lenses (where the center is thicker- as shown below). Aspherics can be also combined with high index materials and it creates an excellent design for cosmetic and optical purposes.
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Dr. Daniel L. Mason, Optometrist
(619)656-1081