1. MADE BY : SWATI PANARA
FROM : BHARTIMAIYA COLLEGE OF
OPTOMETRY
2nd YEAR 3rd SEMESTER
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2. 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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4. • 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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5. 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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6. • 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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7. • 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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8. • 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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9. • 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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10. • 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.
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12. • 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.
14. 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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15. • 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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16. • High index is also available with polarization or
photo chromic options.
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17. • 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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18. • 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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19. 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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22. 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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