Science Behind the Lens

  • Talk about optical center - induced prism
  • Talk about EX3 - substrate matched, hardness value
  • Spherical, vs. aspherical vs. digitally surface
  • Trivex
  • Index of refraction
  • Barrel effect
  • Chromatic aberration (abbe value) aka the “no-poly-promise”
    • Human eyes can not detect the chromatic aberration if the Abbe number is above 40. Poly is 30
    •  low level Abbe values make for very difficult night vision
    • Trivex is 45
    • CR-39 is 58
    • Polycarbonate: This material was developed during the “space race” of the late 1960’s. It was used for helmets and visors for astronauts. It was soon adapted for use in safety eyewear applications due to its superior impact resistance. By 1978 the first single vision ophthalmic lenses were being produced. The following years brought improvements to resin purity, manufacturing, and lab processing.
    • Trivex: Trivex was originally developed for the military, as visual armor. PPG Industries adapted the technology for the optical industry. Trivex is a urethane-based pre-polymer. PPG named the material Trivex because of its three main properties which are good optics, light weight, and strength.
    • Index of Refraction Index of refraction is a number that compares the speed of light in a material to the speed of light in air. The higher the index number, the more a material will slow down and bend light. So less curvature is needed to create a given lens power. Poly has an index of refraction of 1.586. The index of Trivex is 1.530. Steeper curves would be needed to make an Rx in Trivex than in polycarbonate. The higher the lens power, the more difference there will be in curves and thickness.
    • Abbe Value The Abbe number, or V-value, is a measurement of the dispersive properties of a material. The lower the Abbe number the more a material will disperse light into its component colors. This causes chromatic aberration which may be seen as color fringes or blur toward the edges of the lens. Low Abbe value is seldom a concern when the corrective power of the lens is less than seven diopters. Also the point on the lens at which the chromatic aberration is significant enough to create a visual problem is the point through which the wearer does not normally look. High index materials tend to have lower Abbe numbers. If low Abbe value was truly an issue, more people would be wearing glass, with its Abbe of 58.5. The Abbe of poly is 30, Trivex is 43-45
    • Percent Transmittance As light travels through a lens, a certain percentage of that light is lost through absorption and reflection at each air-tosurface interface. The amount of original light available to the eyeglass wearer by the time it exits the lens will vary depending on the quality of the lens material and type and amount of anti-reflection coatings applied to the lens surface. This is an important factor that directly affects the actual brightness of an observed image. The term used to describe this percentage of light that is not lost is transmittance, and for most quality optical lenses this figure will usually be above 90 percent. Trivex lens material has one of the highest transmission levels of all commonly utilized lens materials at 91.4%. This means that patients will enjoy sharp, clear and crisp vision through lenses made from Trivex material.
    • Refractive Index Trivex lens material has a 1.53 index of refraction. This mid-index value enables lenses made from Trivex material to be thinner, lighter and more comfortable. For example, when compared to lenses made with CR-39 monomer (with an index of 1.50) of the same power and diameter, lenses made from Trivex material are up to 50% thinner and 50% lighter resulting in improved comfort for the wearer. Due to the strength of the Trivex material, it can also be surfaced to a 1.0mm thickness thereby reducing the weight and thinness of the lenses even further.