3 types of materials are used for the manufacturing of spectacle lenses.

1) Glass
2) Plastic
3) Polycarbonate

Crown glass is the oldest material with refractive index of 1.523

Benefits:

Relatively difficult to scratch

Doesn't discolor on aging

Disadvantages:

Heavy in weight

Less impact resistance (Gets easily broken)

Hi Index lenses

If the patient has a very high powered correction, then lenses will look thicker at edge in case of minus lenses and at centre in case of plus lenses.

To reduce thickness, titanium oxide is added to glass material and this type of lens is known as a Hi Index lens. They are available in 3 different refractive indices in glass material

1.7 Index 30% thinner than regular glass lens (1.523), also lighter than regular glass.
(Comparison between 1.7 & 1.523 for -10.00DS)

1.8 Index 40% thinner than regular glass & 10% thinner than 1.7 Hi Index glass.
(Comparison between 1.7 & 1.523 for -10.00DS)

1.9 Index 50% thinner than regular glass, 20% thinner than 1.7 Hi Index glass & 10% thinner than 1.8 Hi Index glass.
(Comparison between 1.7 & 1.523 for -10.00DS)


CR-39 is the most commonly used plastic material with refractive index of 1.498

Benefits:

Light weight than glass, 40% light weight than regular glass

Better impact resistance than glass

Doesn't fog up as easily as glass lens.

Can easily be tinted to any colour

Disadvantages:

Gets scratched easily [but this can be reduced by Scratch Resistant Coating (SRC)]

Hi Index Plastic lenses
1.56 Index 15% thinner than normal plastic (1.498) .
1.61 Index 20% thinner & lighter than normal plastic
(Comparison between 1.61 & 1.498 for -8.00DS)

1.67 Index 40% thinner than regular glass & 10% thinner than 1.7 Hi Index glass.
(Comparison between 1.67 & 1.498 for -8.00DS)

1.74 Index 45% thinner than normal plastic; 10% thinner than 1.67; 55% lighter than 1.7 Glass Hi Index



Classified as thermoplastic with refractive index of 1.586, is extremely soft & flexible material.

Benefits:

100% U.V. protection

20% thinner than traditional plastic lens.

38% flatter than normal plastic lens because of aspheric design

43% lightweight than normal plastic lens.

Highest impact resistance (Unbreakable)

Disadvantages:

Scratches easily (but this can be reduced by SRC)

There are three type of Lens.

1) Single Vision Lens
2) Bifocal
3) Progressive Addition Lens (Multifocal Lens)

As the name suggests it has only one focal length.

A lens with the same power (Sphere & Cylinder) throughout the whole lens.

They are prescribed for either distance vision or for reading purpose.

As the name suggests it has two focal lengths.

Lens having two areas of viewing, each of its own power.

Usually the upper portion of the lens is for distance vision, the lower for near vision. Upper portion is called "Major Lens" & lower "Segment"

Benefits:

No need to keep two spectacles, each for distance & near

Disadvantages:

Dividing line seen, so cosmetically looks poor.

Intermediate targets are blurred.

Ring scotoma around the segment

Most commonly used bifocals in India:

Bifocal Style	Benefits	Disadvantages	Availability
Executive	widest near portion	- cosmetically unappealing - heavier in weight	Glass
Round Segment (Kryptok)	- cosmetically better than any other bifocal - cheapest of all bifocals	- most downward eye movement required by patient to read - high image jump while shifting focus from distance to near	Glass & Plastic
D' or Flat Top	- larger useable reading area - less downward eye movement required by patient to read - less amount of image jump experienced while shifting focus from distance to near	costliest of all bifocals	Glass, Plastic & Polycarbonate

A lens having optics that varies in power gradually from the distance to near zone.





Advantages:

No dividing line is seen.

No image jump experienced

No ring scotoma around the segment

Continuous vision at all distances

Disadvantages:

Peripheral distortion.

Less width of near field as compared to most bifocals.

3 type of Coating are used for spectacle lenses.

1) Transition (Photochromatic) lenses
2) Anti Reflection Coating (ARC)
3) Polarizing lenses
4) Mirror (Mercury) Coating

The Transition (Photochromatic) lens changes its transmission when exposed to light. The lens darkens automatically on exposure to Ultraviolet (U.V.) light & lightens on withdrawal of it.

Benefits:

Photochromatic lens gives 100% U.V. protection

Although not a complete substitute for the regular tinted sunlens, it does offer flexibility to use the same lens for both outdoor & indoor purposes.

To an extent Photochromatics do reduce glare.

Availability :

All Photochromatics are available in two shades, Grey & Brown.

Available in Glass, Plastic & Polycarbonate. Most commonly used Plastic & polycarbonate photochromatic is Transition. Transition IV & Transition V are currently used widely. Transition V has got distinct advantage of getting dark faster than Transition IV.

Heavy in weight All single vision lenses can be made as photochromatics. While all glass bifocals can be made in photochromatic, only flat top (D') bifocal is produced in plastic photochromatic. Polycarbonate bifocal transition is not currently available. All kind of PALs can be made as photochromatics.

All photochromatic lenses can be coated with Anti Reflection Coating (ARC) to improve visibility.

An ARC is a thin, clear layer or layers applied to the surface of a lens. Initially ARC lenses were available as a single layer coating of Magnesium Fluoride. This posed certain limitations of the effectiveness of the coating. Moderns ARCs are multilayer coatings that reflect little light & look more attractive.

Benefits:

Reduces unwanted reflections from the lens surface thereby improving the cosmetic appearance of spectacles.

Increases the amount of light that actually pass through the lens to the eye.

It reduces the disturbing multiple image reflections caused by oncoming headlights & streetlights, thereby improving night vision.

It also helps make high minus lenses more acceptable cosmetically, since it decreases the visibility of the "rings" around the edges of the lens.

When light falls on water, snow, highways & metallic surfaces, it gets reflected back in to the eyes and causes severe glare. Polarizing filter is used to reduce this glare. When a polarizing filter is place in the path of any reflected light, it allows light to travel in only one direction and absorbs light rays in all other directions.