Spectacle Lenses

Spectacle Lenses  
  • Spectacle Lens materials:

Materials for spectacle lenses must comply with the following optical characteristics:

  • Good optical homogeneity
  • High luminous transmission
  • Low scattered light creation
  • Chemical resistance
  • Temperature resistance
  • Good mechanical resilience
  1. Mineral / Glass material Lenses:
  • Glass lenses are the oldest material used for spectacle lenses but rarely used today as it is brittle and heavy.
  • 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.
  Advantages:
  • High transparency
  • High Abbe Value
  • Highly scratch resistant
  • Good Optical Quality
  • Resistant to chemical fumes and temperature fluctuation
  • Available in photochromatic & sunglass option
  Refractive Index:
  • Mineral 1.5
  • Mineral 1.60: 20% thinner than Mineral 1.5
  • Mineral 1.70: 30% thinner than Mineral 1.5
  • Mineral 1.80: 40% thinner than Mineral 1.5
  • Mineral 1.90: 50% thinner than Mineral 1.5
  Disadvantages:
  • Heavy
  • Easily breakable
 
  1. Organic lens material lenses:
Advantages:
  • High transparency
  • High Abbe Value
  • Popular Optical material
  • Good Optical Quality
  • Resistant to chemical fumes and temperature fluctuation
  • Available in sunglass option – photochromatic, tint and polarised
  Refractive Index:
  • Organic 1.50 (CR 39): 40% lighter than mineral lenses
  • Organic 1.6 :  20% thinner & lighter than organic 1.5
  • Organic 1.67 :  35% thinner & 30% lighter than organic 1.5
  • Organic 1.74 :  43% thinner & 32% lighter than organic 1.5
  Disadvantages:
  • Thicker than glass
  • Gets scratched when not coated
 
  1. Polycarbonate Material:
Advantages:
  • 23% thinner & 25% lighterthan Organic 1.50
  • 12 times moreshatter-proof than Organic 1.50
  • Has a very high impact resistance - ideal for protective eyewear and sports glasses
  • Has a very high mechanical resistance
  • Can be ground very thinly – very light
  • Good option for rimless and kids as it has high impact resistance
  Refractive Index:
  • Available in 1.6 index only
  Disadvantages:
  • Has a tendency to get stress induced cracks
  • Low scratch resistance – is systematically produced with a hard coating
  • Can be tinted to a limited degree
  • Polycarbonate has a lower abbe number leading to lower optical image properties
 
  1. Trivex Material:
Advantages:
  • 20% thinnerthan Organic 1.50
  • lightestlens material
  • Ideal for rimless frames
  • 12 times moreshatter-proof than Organic 1.50
  • Has almost the same refraction index as CR39 but can be ground more thinly
  • Has a very high impact resistance– ideal for protective eyewear and sports lenses
  • Is chemically resistantto solvents
  • Has very good optical characteristics
  • Is with 1.11 g/m3the lightest material for lenses
  • Is tough and hard-wearing– easy to drill, groove and cut
  • Provides an alternative to Organic 1.50 as a standard lens because it is lighter and tougher
  • Is systematically produced with a hard coating
  • Trivex has better optical image properties than polycarbonate.
  Refractive Index:
  • Available in 1.53 index only
 
  • Anti-reflection Coating:
  • To improve both the vision through the lenses and the whole appearance of the glasses, an anti-reflective coating can be applied to the lenses. AR coatings make the lenses clearer to look at and through.
  • They consist of several layers of metal oxides applied to the front or back lens surfaces. Because of the layering effect, AR coatings sometimes have a hint of green or purple colour, depending on the individual manufacturer's formula.  Each layer is scientifically calculated to block reflected light. The result is that you'll see a reduction in glare, annoying reflections and halos around lights while wearing the lenses. This is a great safety benefit when you're driving at night.
  • Anti-reflective coating reduces both internal and external reflections on the lenses themselves, creating a nicer cosmetic appearance. Internal reflections appear, as rings that make lenses look thick. External reflections mask your eyes from a clear, complete view when someone is looking at you. So with an anti-reflective coating, lenses appear thin or non-existent, and your eyes look more natural.
  • Anyone on TV or whose photo is taken often benefits tremendously from the better appearance of AR coated lenses, but really, all eyeglass wearers would benefit from an anti-reflective coating from a cosmetic point of view. If you have a strong prescription, you can use the AR coating in conjunction with high-index or thinner lenses to make your glasses look and feel as thin as possible.
 
  • Tints:
  • While some people think colouredcontact lenses are merely cosmetic functions, the right tint can enhance vision and comfort at specific tasks, from in the office in front of a computer screen, to outdoor activities such as golf or skiing. Tints come in solids, gradients or double gradients, and can be a fun unique touch to your lenses.
 
  • Pink or Rose:
  • This tint has been known to help reduce eyestrain in brightly-lit offices. This tint is often recommended for computer wearers.
 
  • Yellow:
  • Often known as blue blockers, yellow tints are a favorite of skiers and skeet shooters because they provide excellent depth perception and low light contrast.
  • Yellow lenses are not recommended when accurate color perception is necessary.
 
  • Brown:
  • Similar to the yellow tint, brown tints provide excellent contrast because they filter some blue light, but not as much as the yellow tint.
  • Brown lenses are excellent general purpose lenses.
 
  • Green:
  • The human eye is most sensitive to green light. Because of this, green tints give us the highest contrast and greatest visual acuity of any tint.
  • Green tints can enhance contrast in low-light conditions.
 
  • Gray:
  • The most popular tint, a gray tint provides good protection from glare and does not overly distort colors.
  • Gray tinted lenses are excellent for general purpose and for driving.
 
  • Purple:
  • A good choice for hunters, purple tints allows for natural color perception while offering some shade from the sun.
 
  • Blue:
  • Blue tints are not recommended for outdoor use because it can increase glare. It can be used as a fashion tint if used in lighter shades.
  • If blue coloured contact lenses are desired for outdoor wear, be sure to consider the option of a brown or gray polarized lens with a blue flash mirror coat
 
  • 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
  • Aspherics can be also combined with high index materials and it creates an excellent design for cosmetic and optical purposes.
 
  • Polarized Lenses:
  • Polarized lenses are used for prescription sunglasses.
  • Polarised filters are made of a chemical film applied to a transparent plastic or glass surface.
  • The chemical compound used will be composed of molecules that naturally align in parallel relation to one another.
  • When applied uniformly to the lens, the molecules create a microscopic filter that absorbs any light matching their alignment.
  • Most of the glare that causes you to wear sunglasses comes from reflection off of horizontal surfaces, such as water or a highway.
  • When light strikes a surface, the reflected light waves are polarized to match the angle of that surface. So, a highly reflective horizontal surface, such as a lake or the sea, will produce a lot of horizontally polarized light.
  • The polarized lenses in sunglasses are fixed at an angle that only allows vertically polarized light to enter the eye hence reducing the glare.
 
  • Photo chromatic Lenses:
  • Photo chromic lenses are eyeglass lenses that darken automatically when exposed to sunlight, then fade back when you return indoors.
  • In most cases, photo chromic lenses are clear (or nearly clear) indoors and darken to a medium sun tint outdoors. But there are exceptions.
  • Photo chromic lenses are available in nearly all lens materials and designs, high-index lenses, bifocals and progressive lenses.
  • An added benefit of photochromic lenses is that they shield your eyes from 100 percent of the sun's harmful UVA and UVB rays.
  • Because a person's lifetime exposure to sunlight and UV radiation has been associated with cataracts later in life, it's a good idea to consider photochromic lenses for children's eyewear as well as for eyeglasses for adults.

Leave a Reply

Your email address will not be published. Required fields are marked *