There are many different types of lens materials on the market today.  Lenses correct vision by bending or changing the direction of light.  The amount of bending or change of direction depends on the shape of the lens and the density of the material from which the lens is made.  Virtually every material on the market is available to the patient at Kirkland Vision Center.  Below are brief descriptions of these products.  

CR-39 , usually referred to as plastic, is the most commonly used material.  It's optics approach that of crown glass but it is 50% lighter weight, making it more comfortable to wear.

CROWN GLASS was the first material used for making eyeglass lenses.  Because of its weight and potential for shattering on impact, it is rarely used today.  

HI-INDEX lenses are lenses with an index of refraction higher than CR-39 or crown glass.  The greater the density of the lens material, the more efficiently it bends light. The density rating of a material is referred to as its index of refraction.  The denser the material the higher its index of refraction.  Air, which has a negligible effect on light, has an index of refraction of 1.  Water, being denser than air, has an index of refraction of 1.33.  So the direction of light is changed as it hits the surface of water.  This is why a stick in water appears bent.  CR-39 has an index of refraction of 1.49.  Crown glass, with an index of refraction of 1.523, bends light slightly more efficiently than CR-39.  Beyond these basic materials, there is a variety of high index plastics and glass materials with an index of refraction as high as 1.9.  So the greater the index the more efficiently the lenses bend light.  Therefore, lenses can be thinner. 

There is a downside to high index materials.  As the index of refraction increases, the potential for optical aberrations also increase. These aberrations cause images to appear distorted or blurred as one looks through the periphery of a lens.  There is more inherent aberration in polycarbonate than any other lens material.  In addition, polycarbonate is softer than other materials, making it more scratchable.  On the other hand, polycarbonate is virtually shatterproof making it an excellent industrial safety material.  It is the responsibility of the optician to carefully match the patients needs with the most appropriate lens material.

SINGLE VISION is a lens with one point of focus.  Single vision lenses are prescribed for focusing at one distance.    

BI-FOCALS are lenses with two focal points.  Commonly prescribed for distance vision and reading vision.  They have a segment for which plus power is added for shortening the focal length.  With this additional segment, a very distinguished line is present.

TRI-FOCALS are lenses with three focal points.  Commonly used for distance, intermediate, and near vision.  Like bi-focals, they also have a segment which plus power is added for shortening the focal length.  However the middle or upper portion of the segment has half the power of the lower portion, allowing vision for intermediate use.  This gives two lines to have to view through.

PROGRESSIVES are lenses with an infinite number of focal points.  This allows vision from distance to near and everything in between.  The advantage of progressives is that there is no line separating distance vision from near vision.  Just a gradual increase of power to allow good vision at any given focal length.

POLARIZED lenses are lenses that filter glare.  From the moment of reflection, light becomes polarized and forms visual noise-blinding glare that interferes with the real image.  The only way to eliminate this glare is to place polarized lenses in its path.  By blocking glare, vision becomes more enhanced, colorful, and contrast is at its best. 

SCRATCH COAT is a transparent coat that is applied to the surface of CR-39.  This protects the lens material from scratching.  The coating can be scratched if not properly cleaned, however scratches are not as visible. 

UV-400 is a protective coating that can be applied to your lenses.  It filters harmful UV rays that can lead to damage to the eye.

ANTI-REFLECTIVE coatings improve transparency and minimize reflections on the surfaces of the lenses.  Lenses become almost invisible when treated with anti-reflective coats.  Night vision is sharper and in greater detail without reflections, which improves visibility while driving.

SOLID TINT is added primarily for reducing light transmission through a lens.  This can be applied to almost all lenses.  Whether it be sunglass lenses, computer lenses, or shooting lenses there is a certain color of tint that is suitable for your needs.