Chapter 4: LENS OPTIONS

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Chapter 4: LENS OPTIONS
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2013-11-04 17:29:04
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  1. 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.
  2. Specific gravity
    describes the density of a lens material by comparing its density to the density of water.
  3. Abbe Value
    the higher the index of a lens material, the higher the chromatic aberration, and the lower the Abbe value.
  4. 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
  5. Transmittance
    The transmittance of a lens material describes the amount of light (usually specified for a given waveband) that will pass through that material.
  6. Glass
    • Pros:
    •    Superior optics
    •    Stable material
    •    Scratch resistant
    • Cons:
    •    Does not accept tint
    •    Not impact resistant
    •    Heavy
  7. 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
  8. 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
  9. 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)
  10. 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)
  11. 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
  12. Photochromic
    lens with a characteristic of changing state from clear to sunglass dark when exposed to light
  13. 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.
  14. PGX/PBX
    Photo Grey Extra and Photo Brown Extra are the original glass versions of a photochromic lens.
  15. 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
  16. Slab Offs
    or bicentric grinding, is a method of correcting vertical imbalance for patients with anisometropia.

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