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Refractive Index
a lens material indicates how much the material will refract or bend light as it enters the material from air, by comparing the speed of light in a given material to the speed of light in air.
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Specific gravity
describes the density of a lens material by comparing its density to the density of water.
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Abbe Value
the higher the index of a lens material, the higher the chromatic aberration, and the lower the Abbe value.
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Reflectance
- The reflectance of the material describes the percentage of incident light reflected from a highly polished surface of that material and is calculated from the refractive index of a material. When light hits a lens surface in air normally, the percentage of light reflected at each surface is given by:
- R = (n - 1)2/(n + 1)2 * 100%
- Thus a material of refractive index 1.5, has a reflectance of
- (0.5/2.5)2*100 = 4% per surface
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Transmittance
The transmittance of a lens material describes the amount of light (usually specified for a given waveband) that will pass through that material.
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Glass
- Pros:
- Superior optics
- Stable material
- Scratch resistant
- Cons:
- Does not accept tint
- Not impact resistant
- Heavy
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CR-39
- Pros:
- Lighter than glass
- Readily tint-able
- Less likely to fog
- Cons:
- Susceptible to scratching (correctable by coating)
- Lower index of refraction makes it less suitable for higher powered prescriptions
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Polycarbonate
- Pros:
- Thinner and lighter than glass and plastic
- Highly impact resistant (used for safety glasses)
- Inherent UV protection
- Cons:
- Poor optical quality
- Susceptible to scratching (correctable by coating)
- Susceptible to stress fractures in drill mounts
- Does not readily accept tint
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Hi-Index
- Pros:
- Thinner and lighter than glass and plastic
- Better optical quality than polycarbonate
- Cons:
- Susceptible to scratching (correctable by coating)
- Susceptible to backside and inner-surface reflections (correctable with AR)
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Trivex
- Pros:
- Impact resistance of polycarbonate
- Better optical quality than polycarbonate
- Tint-able
- Lightest material on the market
- Inherent UV protection
- High tensile strength (ideal for drill mounts)
- Cons:
- Susceptible to scratching (correctable by coating)
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Anti-reflective
- Reduces glare by 99%
- Eye fatigue, Itchy eyes
- Reduces glare: Driving, Computer (Phone/tablet), Artificial light reduces glare
- Makes lenses appear thinner
- UV, scratch
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Photochromic
lens with a characteristic of changing state from clear to sunglass dark when exposed to light
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Transitions
- Transition lenses were created by PPG Industries
- Transitions lenses use a technology called imbibing to place the photochromic dye photosol a few microns below the front surface of the lens.
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PGX/PBX
Photo Grey Extra and Photo Brown Extra are the original glass versions of a photochromic lens.
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Multifocals types
- • Flat Top – 25mm, 28mm, 35mm, 45mm, 7x28mm, 8x35mm, Double D 28mm, Double D 35mm, Quadrifocal 28mm
- • Curved Top – 28mm
- • Round – Achromat, Kryptok, Ultex, 22mm, 25mm, Double Round
- • Panoptik
- • Ribbon
- • Executive – Bifocal, Trifocal, ED Trifocal
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Slab Offs
or bicentric grinding, is a method of correcting vertical imbalance for patients with anisometropia.
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