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HIGH INDEX

MADE BY : SWATI PANARAFROM : BHARTIMAIYA COLLEGE OF

OPTOMETRY2nd YEAR 3rd SEMESTER

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WHAT IS THE REFRACTIVE INDEX?

• The ratio of speed of monochromatic light in vacuum to speed of light in another medium is called refractive index.

OR The ratio of sin of angle of

incidence to sin of angle of refraction.

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REFRACTIVE INDEX =Sin i / sin r

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• In simple terms, index is a measurement of how much light is bent by the lens material. The higher the number, the more the light is bent by the lens.

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HI INDEX The term “hi index” refers to the lenses whose index of refraction is higher than 1.49 in plastics and more than 1.523 in

glass.

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• Normal index – 1.48 to 1.53.

• Mid index – 1.54 to 1.63.

• High index – 1.64 to 1.73.

• Very high index – 1.74 to above.

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• For example, a -7.00D lens ground on a 1.70 index material will be approximately 50 percent thinner than a -7.00D lens ground on a 1.50 index material if all parameters are equal.

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• Hi index glasses are made of lanthanum or titanium rather than barium as it gives similar optical properties and also weight less.

• Some plastic lenses are made from polyurethane.

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• There are several plastic materials now available in high index lenses, with index ratios between 1.53 and 1.71.

• High index glass lenses are also available,

with a index of 1.90.

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• Lenses of index = 1.5, 1.6, 1.7, 1.8 and 1.9 are used for single-vision, solid bifocal, and progressive prescription requirements; another range of high-index barium glasses is used for near-vision correction in multi focal lens segments.

Refractive index

Abbe value Specific gravity

1.52 58.9 2.54

1.60 42.0 2.59

1.60 42.5 2.58

1.60 42.2 2.73

1.601 42.2 2.67

1.604 41.8 2.67

1.70 51.0 3.38

1.702 40.2 2.99

1.706 39.3 3.20

1.800 35.4 3.60

1.805 26.4 3.37

1.806 33.3 3.47

1.830 32.0 3.59 11

• Available in 2 forms 1) Glass high-index 2) Plastic high-index• GHI are made up of titanium oxide, barium oxide or lead

oxide • PHI are made up of Thermosetting plastics• These lenses have higher than standard R.I.• Most cosmetically acceptable lenses• It enables the lens to bend light "faster". • Hence they are lighter and thinner than standard lenses.

Types Refractive Index

Abbe value Specific gravity

Glass High Index

1.600 36 1.45

Plastic High Index

1.597 37 1.34

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High index lens designs

• As technology has improved, so has the availability of high index lens designs.

• High index is most commonly available in multiple progressive and single vision designs.

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• Aspheric and atoric designs are also becoming more readily available to the eye care provider in high index materials.

• In aspheric and atoric designs, the lenses are gradually steepened along the periphery in minus designs and the periphery is gradually flattened in plus designs.

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• High index is also available with polarization or

photo chromic options.

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• When to recommend a high index lens

• A general guideline is to recommend high index materials for any patient with a prescription of +/-3.00 D or more.

• Anything less than this prescription and the weight and thinness benefits are usually minimal.

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• High index, especially the index of 1.67 and 1.70, is also an excellent choice for individuals who want the minimalist look of drilled rimless eyeglasses.

• There are some exceptions to fitting a high index lens, however. One instance is children or individuals who have severely decreased vision in one eye. In these cases, the safety and impact resistance of polycarbonate or Trivex is necessary to protect the patient’s vision.

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ADVANTAGE OF HI INDEX LENSES

• Power rings round the edges of the lens is reduced.• Reduce thickness.• Look flatter.• Thinner.• Lighter.• The benefit is greater peripheral vision as well as less

magnification or minification of the eye.

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DISADVANTAGE OF HIGH INDEX LENSES

• Low Abbe number, increase chromatic aberration.

• Poor light transmission and increased backside and inner surface reflection increasing so importance of anti-reflective coating.

• More expensive than normal lenses.• Greater distortion away from the optical

centre.

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MANUFACTURING COMPANY

• Schott• Hoya • Nikon • Corning • Rodenstock • Pilkington• Kodak

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•Thank you