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
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
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
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
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 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.
