Ratio of velocity of light in vaccum to in optical medium

Normal index n _> 1.48 but <1.54

Mid index n _> 1.54 but < 1.64

High index n _> 1.64 but < 1.74

Very high index n _> 1.74

The abbe number is the reciprocal of the dispersive power of the materials

Indicates the amount of chromatic aberration present in lens

Lower Abbe value higher amount of chromatic aberration

As R.I. increase Abbe value decreases

low dispersion V _> 45

Medium dispersion V _> 39 but<45

High dispersion V <39

variation in surface power when material used is other than crown glass

CVF is simply the ratio of the refractive of crown glass to that of the chosen material , 0.523/(nd-1)

It compares the actual curves which are obtained on crown glass and the material in question for given curvature of the surface . Plastic materials are compared to CR 39 .

CVF enable a direct comparison of thickness to be obtained

For example 1.700Ds index material has CVF of 0.75, which informs us that the reduction in thickness will be about 25% if this material is substituted for crown glass

One of the most practical uses for the CVF is to convert the power of the lens that is to be made into is crown glass equivalent

This done, simply by multiplying the power of the lens by the CVF for the material .

For example, suppose we wish to dispense a -10.00D lens in 1.7 index material, the crown glass equivalent is 0.75*-10 or 0.750D

the use of 1.700 index material wuld result in lens that has power of -10.00D but, in all other respects, look like a -7.5D lens made in crown glass

Specific gravity

Density of high refractive index materials are seen to be greater than that of crown glass

for example, if the density of a material is quoted as 3.0, it means that the material is 20% heavier than crown glass.

Crown glass

Flint glass

Barium crown glass

Glass

High index refers to lens materials index of refraction

These lenses are advanced lenses that are thinner than conventional plastic or glass lenses

In simple terms high material bends or refracts light more efficiently than low index material and this efficiency translates to lenses that can be made thinner

Thinner

Lighter

Most of today's fashionable frames are made of plastic or metal with rims thinner than the lens itself.

In either case, the lens edges are highly visible, and thicker edges can detract from the appearance of the eyewear.

Also very popular are rimless mountings, where the lens edges are completely exposed.

For weak eyes prescription is higher and lenses must bend light more to provide clear vision which is possible with strong powered lenses which result in thick edges in case of minus lenses

So here high index lenses are preferred over regular lenses

Commonly, plastic lenses are made from a hard resin that is cast or molded in the wet state into lens blanks. Then these plastic blanks can be ground into specific shapes to fit any lens frame. The most common plastic lens material is called CR-39.

Available in glass or plastic

Available in range of lens types including photochromic, multifocals etc.

Thinner than conventional lens material

Cosmetically more appealing

Lighter sometimes

Greatest benefits of these lenses seen by myopic patients

Can be multicoated, tinted, UV treated etc..

Available in wide range of blank size finished as well as semi-finished

Not necessarily lighter even though thinner

Greater distortions away from optical center

Sometimes time consuming to produce

Less impact resistant

More costly than conventional lens materials

Tints often brighter and more vivid than conventional lens material

Adaptation time is longer for some patients

Myopia

Hypermetropia

1.6 index

1.7 index

1.8 index

1.9 index

The thickest lens has a refractive index of 1.5

1.60 index lens is approx. 20% thinner than a 1.5 index lens

1.67 index lens is approx. 40% thinner than a 1.5 index lens

a 1.70 index lens is approx. 50% thinner than a 1.5 index lens

a 1.80 index lens is approx. 60% thinner than a 1.5 index lens

a 1.90 index lens is approx. 75% thinner than a 1.5 index lens

People who prefer ultra thin lens option

Great appearance for higher scripts

Superior strength in today’s lightweight rimless fashion frames

Myopes

High powered patients

All lens materials block some light from passing through the lens, reflect light inside of the lens

This internal reflection causes thick lenses to have a "ring" or "coke bottle" look to them

A minor side effect of higher index lens materials is that they block slightly more light (10 to 12 percent).

However, when an anti-reflective coating is applied, cause internal reflection to decrease

High-index lenses transmit 99.5 percent of the light

For this and other reasons, anti-reflective coatings are a valuable addition when you order high-index lenses.

High-index lenses are heavy….

While the specific gravity of high index is more than CR-39 or polycarbonate, it does not necessarily mean the lens will be heavier in its finished form

However, we are comparing lenses with total refractive abilities, not just center and edge thickness parameters.

Because we need less material to get the same amount of refraction of light, it will be lighter

The following chart illustrates the various materials and lists their index of refraction, Abbe value and density.