High index lenses
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High index lenses
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
Classification of abbe value
low dispersion V _> 45
Medium dispersion V _> 39 but<45
High dispersion V <39
curve Variation Factor CVF
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
Ratio of weight of substance to the weight of water
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.
Varieties of optical glass material
Calcium oxide 11.13%
Sodium oxide 14-16%
Percentage of potassium, borax, antimony and arsenic
45-65% of lead oxide
25-45% of silica
10% mixture of soda and potassium oxide
RI from 1.580 to 1.690
Abbe value from 30-40
Disadvantage of chromatic abberation
Barium crown glass
25% to 40% bariumm oxide
RI from 1.541 to 1.616
Abbe value from 55 to 59
Advantage of high index without increase in chromatic abberation
Extra dense flint
What are high index lenses?
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
High index material bends light to greater density which results in thinner lens because less curvature is needed to produce a given power
i.e. Curvature is inversely proportional to R.I.
Provides slimmer glasses and a wider choice of frames
The benefit is that if you have a strong prescription, you are no longer forced into unsightly "Coke-bottle" lenses
As these lenses are made of greater density and high index-- less material is required to provide the prescribed amount of vision correction and results in LIGHTER lens
But it is important to remember that thinner and denser does not really imply that the lens will be lighter in same proportion
Earlier H.I lenses were slightly heavier than CR-39 and crown glass due to barium or lead content which was then replaced by titanium oxide which resulted in lighter weight lenses
Thinner edges require lens less material, which reduces the overall weight of the lens
Lens made of H.I. Plastic are lighter than conventional plastics and vice versa in case of glass lenses
Different b/t high index lens and regular lens
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
How are they made?
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
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
In case of myopia or myopia combined with astigmatism the lens edges are thick as degree of myopia rises
The solution for more attractive lenses is to use high index lenses
The suggested index to use depends on degree of myopia
In cases of hypermetropia over 2-3 diopters of prescription have problem with central thickness
Which results in heavy lenses, magnification and distortion of your eyes
The vision also detoriates while looking through these glasses
Solution to this is use high index aspheric lenses
The asphericity makes lens flatter in order to achieve less thickness at the center
The result is thinner lens, light weight, less peripherial distortion and less magnification of eyes
The difference can be seen clearly and appears as……..
For slight and medium correction
Noticeable thickness reduction which brings general improvement in performance
For medium to strong correction
Thickness reduction brings immediate improvement in appearance and visual comfort
For very strong correction
This eliminates thickness problem and makes spectacle wear more acceptable
This is hi-tech lens
Thinnest mineral lens in the world
Higher Indices …will be Thinner
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
High powered patients
AR Coating: A Perfect Companion for High-Index Lenses
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.