HOYA Training Center

Refractive index of lens is the value that illustrates the refraction of light in lens differently based on a kind of material. Refractive index of light in the air (Index in Air) is equal to one. The refractive index of light in any medium will always have value more than one when calculating from the following:

*Air (at STP) = 1.00029
  Water (at 25º C)= 1.333
  Glass= 1.5 to 1.8
  CR 39= 1.499
  Acrylic= 1.490
  Polyurethane= 1.560 to 1.609

Based on the above Figure, plastic lens has higher index value that causes less thickness in lens when comparing to the same value of vision. This helps wearers feel soft, comfortable and look more beautiful.

Abbe value is the value that illustrates color dispersion of lens in different materials. When light travels through prism of lens, different refraction occurs within each frequency current. From the Figure, it shows navy blue color ray (nf1) refracts from the horizontal line rather than the green color ray (ne) and the red color ray (nc1).

Higher Abbe value of lens can help wearers see clear, sharp image within the edge of lenses (from the sample Figure). Low Abbe value of lens causes unclear image like a dispersion of light.

For Abbe value, it is part of a component that is used to analyze the qualification of lens. However, if a wearer can adapt to the eyeglasses, different Abbe value will not bother his/her ability to see.

Single aspheric lens called Nulux is a kind of aspheric lens with smoother flatter peripheral front curvatures and less thick than the regular spheric lenses in general. With the indicated qualification, a visual field that is seen through lens is wider and becomes another alternative choice for people with higher vision value.

Double aspheric lens called Nulux EP is a kind of double aspheric with smooth flat curvatures on both sides in order to add beauty on eyeglasses. It is suitable for people who have higher vision value because the unclear image on the edge of lenses will be eliminated. Moreover, the unclear image will not occur to wearers when other people look into their eyes.

      What does anti-reflection mean?

Anti-reflective coating is the special surface that is coated on the lens surface in order to protect light reflection from the lens surface as this can bother seeing (from the Figure). When light transmitted through lens at most, efficiency of vision will be increased. Besides, it is easy to clean the lens surface and protect lens from any dusty scratch.

Nearsightedness is the symptom of having too long eyeball or too much ability to adjust the focus of retina and cornea, so the focus image received falls before reaching retina. When seeing an object located in a far distance, people cannot see clearly. Nearsightedness is the abnormality of eyesight that can be seen in common. Also, nearsightedness is mostly found among people who use computer for a long time, it tends to occur increasingly nowadays.

 

Far-sightedness is the symptom of too short eyeball or having less ability to adjust a focus of retina and cornea, so the focus image received falls behind retina. For people who have less far-sightedness due to younger age, they still see image normally. However, while their eyes catch images from both near and far distances, their sight will use more power to scrutinize and adjust the images received in order to be able to see normal images. Their eyes will be tired. This symptom somehow is difficult to find out when having eyesight checked up only.

 

Astigmatism occurs from the lack of symmetry in the curvature of the cornea or retina that is oblique in either horizontal or vertical direction. When having received a focus image, it falls into the wrong position from retina either horizontal or vertical direction. Therefore, when seeing the object, the image will be blurred or expanded. In some cases, the focus point will be seen in a line shape or oval shape. The effect of astigmatism can be also produced by misalignment of the lens. Astigmatism can occur with people who have either near-sightedness or far-sightedness.