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

2. 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

3. 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

4. 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, including 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 withcataracts later in life, it’s a good idea to consider photochromic lenses for children’s eyewear as well as for eyeglasses for adults.