Commonly used 6 Coating of spectacle lenses
1. ARC Coating
2. Hard coating
3. Hydrophobic coating
4. Anti-Fog coating
5. Toughning coat
6. Surface tint coat
There are various kinds of surface treatments that are applied on either side of the spectacle lenses.
In addition to anti-reflection coating which has already been discussed separately, many other types of surface treatments are done to enhance optical and / or physical properties of the lens.
Optical properties can be enhanced by tinting, UV coating and mirror coating and the physical enhancements can be achieved by hard coating, hydrophobic coating, anti mist coating, anti fog coating and toughening.
Type of ophthalmic lenses
HARD COATING
Resin lenses are basically from two families of plastics—thermosetting materials and thermoplastic materials.
A thermosetting material is created in a mould by chemical reaction, the cross linking of molecules is very strong to form a rigid structure, and if they are subsequently heated will not soften.
CR39 and most other high index resin lenses used for spectacle lenses are thermosetting materials. Higher index materials are usually softer and must be hard coated.
Thermoplastic materials can be reheated and reformed. They consist of long chains of molecules that do not cross link with each other.
Most common thermoplastic material is polycarbonate. All known thermoplastic materials are very soft and they must be hard coated. Hard coating is a procedure by which a thin layer of material usually 0.5 micron to 10 micron thick is applied to the lens surface.
The most common material used is lacquer which is applied either by dipping or by spinning procedure. Vacuum deposited hard coats are less common. It may be tintable or non tintable. Non-tintable hard coating offers more scratch resistance than the tintable.
It is not possible to see the layer of hard coating except of the edges of bifocal segments and there is no optical effect of hard coating if the refractive index of the lacquer used is same as that of the base lens material.
If the refractive index differs, there is a change in surface reflection.
HYDROPHOBIC COATING
Anti-reflection coating needs to be kept clean. The reason is that a very small amount of oil or grease on top of coating can destroy its effect.
The result is loss of the purpose for which anti reflection is being applied. Since the effect of this does not occur evenly across the lens surface , this could be very annoying to the patient.
In fact, they are not so dirtier, the wearer is just more aware of it. This problem can be eased if a hydrophobic coating is applied to the lens surface.
The basic principle is to create a high surface wetting angle, which allows the water or oil to run off rather than wetting and then drying on the surface.
It can be applied simply by dipping process. Baking after dipping enhances the life. It can also be applied in the vacuum chamber as the final stage of the multi-layer anti-reflection coating process.
ANTI-FOG COATING
The potential for eye injuries in sports and battlefield has increased over the years. Fogging of spectacle lenses can occur from a combination of body heat and environmental factors which can result in blur vision from sweat beads streaming down the lenses.
A film of anti-fog coating can be applied on the back surface of the lens which is compatible with hard coating to protect the lens from being fogged.
A good anti-fogging treatment has not only initial anti-fogging properties but also good anti-fogging retainability.
At times anti-fogging lens cleaning solutions have been used in the form of sprays, the effect of the same has yet to be studied.
ANTI-MIST COATING
The same reason can be attributed to anti-mist coating on the lens surface.
The property to prevent the formation of mist behind the lens is an important feature.
TOUGHENING GLASS LENSES
Spectacle glass lenses can be strengthened by toughening process. The finished lens is heated to its softening point and is followed by rapid cooling, either by a stream of cold compressed air directed at the surfaces or by submerging the lens in oil.
This results in greater compressive strength in the glass since outer region of the lens material cools more rapidly than the interior, which remains hotter and more fluid for some time.
When the interior finally cools, it contracts and exerts tension on the rigid outer surface to produce an envelop that encloses the interior under great tensile stress.
Toughened glasses are far more durable and more resistant to scratches because of increased surface hardness. When they break, they shatter into relatively harmless cubes of glasses.
Glasses can also be toughened by a chemical process, in which a compressive envelop, is produced by ion exchange as a result of hot dip of the finished lens into a salt bath.
Chemically toughened lenses are stronger than air- quenched toughened lenses.
SURFACE TINTING OF LENSES
Tints are simply the methods of absorbing lights so that transmission is reduced. If all wavelengths of light are equally absorbed, then a neutral grey is produced.
If the absorption is different for different wavelengths, then the tint has a particular colour. Tinting resin lenses are relatively simple procedure.
The lenses are placed in a suitable bath of hot colour dye for an appropriate length of time. Excess darkness can be reduced and light colour can be made darker.
Full colour and graduated colours are all possible.
Glasses can also be coated by vacuum process which has a unique advantage of uniform tint throughout the lens surface which is otherwise not possible in glass lenses.
OPHTHALMIC LENS MATERIAL