Future Optometrists

  Dispensing Tips of Anti-reflection Coated Lens      The normal eye is easily adapted to different lighting conditions. Thus, the loss of light due to reflection is usually not serious enough to reduce visibility except where poor lighting condition already exist. It is the reflected light which enters the pupil of the eye along the same path as the light used in “seeing” an object that is of primary importance. Whenever light is incident on the boundary between two medium, some light is reflected and some is transmitted, and the transmitted light undergoes refraction into the second medium.      Anti-reflection coating are applied on the lens surface to efficiently manipulate the light transmission and reflection through it. It has been seen that the surface reflectance can be reduced by coating the lens with a film of some materials having a lower refractive index than that of the lens.       Although, there are now two reflections – one at the exposed surface of the film and anothe

  POLAROID LENSES      Under normal lighting conditions, light waves vibrate and travel in all directions – horizontally, vertically and also everywhere in between. When direct sunlight (which is non-polarized) or artificial light hit a smooth surface at a given angle, it reflects off the smooth surface and travels in the same direction, intensifying their effect on the eyes.       This phenomenon produces disturbing optical noise or glare. Glare results in eyestrain, loss of clear vision, poor colour saturation, loss of depth perception, causing discomfort.      The principle of polarized lenses can be best illustrated by observing Venetian blinds. The blinds block lights at certain angles, while allowing lights to transmit through it at certain angles. Polarization implies restricting the light waves to one direction. Polarized lenses block horizontally aligned light which causes glare, while allowing vertically aligned light to transmit.     APPLICATION OF POLARIZED LENSES      Pola

Story of Photochromic Lens      Photochromic lenses are lenses that visibly darken and fade indefinitely under bright and dull light respectively. They were developed by Dr WH Armistead and SD Stookey at Corning Incorporated in 1964.      A photochromic glass lens contains billions of microscopic crystals of Silver Halide. It is this metallic silver that absorbs the light. This reduces the amount of light passing through the lens and the lens turns darker. With the disappearance of UV rays or sunlight, the lens returns to its faded state by the reconvertion of deposited metallic silver into silver halide.       Since these crystals are within the lens material, the process of darkening and fading can be repeated. Factors affecting Photochromatism      Although, exposure to ultraviolet light is one of the conditions that influence photo chromic lens transmission most, several other factors also contribute to lighting and darkening phenomenon. They are somewhat temperature dependent.   

ABERRATIONS ASSOCIATED WITH SPECTACLE LENSES LENS ABERRATIONS     One of the first decisions to be made while dispensing a new prescription is the choice of lens form. When the eye views along optical axis of spectacle lens, the form of lens does not matter and image formed by lens is not afflicted with any defects or aberrations which might affect its sharpness or its shape.                   But in actual world, the eyes turn behind the lens to view through off-axis visual points and it is then the form assumes importance. Ideally, the off-axis performance of lens should be same as its performance at the optical centre. But it is never so.      The off-axis images are afflicted with various types of aberrations, which spoil the quality of image formed by the lens.     There are 6 major lens aberrations that work against obtaining a perfect image through the periphery of the lens. They are :      Chromatic Aberration      Spherical Aberration      Marginal Astigmatism      Coma      C