Page 162 - The Indian Optician Digital Edition Jan-Feb 2020

LENS COATING by a certain amount, i.e. the lens 6. Edge Coating: The edges are
transmits more light in both the coated with the colour similar to
1. Scratch Resistant Coating (SRC): lightened and the darkened state. the frame to reduce the concentric
Also known as Hard Coated lens, rings visible to the observer. Other
such a coating develops more 4. Anti-Fog Coating: Useful for suitable alternatives are polishing
surface hardness so they reduce those who constantly change the edge of the lens, rolling the
lens reflection with higher light their environment's temperature, edge of the lens, AR coating of the
transmission. such lenses are coated with a lens or the use of high index lens
resin film that absorbs moisture to reduce edge thickness.
2. Colour Coating: An absorptive when the absorption reaches the
coating may be added through PHOTOCHROMATIC LENSES
the use of metallic oxide. For saturation point. The interfacial
such lenses, transmission curve activator (within the resin) For glass lenses, a reversible
is generally more even across changes water droplets into a thin darkening process occurs as a
the visible spectrum than the outer layer of water. result of silver halide crystal that is
internally tinted or dyed plastic. 5. Mirror Coating: It is applied by activated by UV and short visible
the vacuum process to the radiation (300-400 nm). Plastic
3. Anti-Reflecting Coating: A thin photochromatic lenses are made
layer/s is applied over the lens front surface of the lens, causing with a variety of ways as imbibition
surface to reduce unwanted the lens to have the same (surface is infused with the
reflection from the surface. It properties as a two-way mirror. photochromatic material), in mass
also increases the amount of It is applied either by metallised (photochromatic dyes mix into the
light passing through the lens. (absorbs and reflects as well) or liquid lens material, especially for
Furthermore, the coating also dielectric coating form (reflects glass lenses), multi matrix (1 mm
enables lightly-tinted lenses to a certain wavelength). There is a thickened layer is bonded over the
become more acceptable at night. reduction in the transmission of clear lens technique that is used by
On sunglasses, such coatings the lens simply due to the high Kodak Insta-shade), dip coating, front
reduce mirror-like reflection from percentage of light reflected. surface coating (lens is dipped in the
the black surface. Photochromatic These coatings are also used solution and then cured with a heat
lenses may also have such with the tint combination to process) and trans-bonding (this is
coatings to increase the maximum provide more protection against for polycarbonate and high index
and the minimum transmission intense sunlight. lenses where a surface treatment is

PHOTOCHROMIC
LENSES

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