OpenOptix ABO Study Guide - Laramy-K Optical

UV Coating
An ultra violet coating is added to CR-39 lenses to increase the absorption of harmful UV
rays. This type of coating can easily be done in-house. It is applied by dipping the lens
into a solution for a period of time so that the lens can absorb the UV blocking chemical.
The majority of new lens materials have UV filtering properties inherent in the material.
Mirror Coating
Mirror coatings are highly reflective and are used to reduce the light transmission through
a lens. They are produced in a vacuum process. The chemicals used vary greatly
depending on the color and density of the mirror desired. The coating generally will have
a combination of various metal oxides that when combined will result in the specified
color. Mirror coatings are available as a solid, gradient, double, and triple gradients.
Multifocal Lenses
A multifocal lens can be thought of as two lenses on one. The larger lens is sometimes
referred to as the carrier, while the smaller lens is typically called the segment. The
power of the segment is always a plus and is also commonly referred to as the reading
add. The add power is used to correct presbyopia, compensating for the lack of
accommodation, and allowing the wearer to clearly view objects at near.
The first multifocal lens was created by Benjamin Franklin out of necessity. Mr. Franklin
had a hard time viewing distances and when he had his distance spectacles on he had a
hard time seeing up close. So, around the 1750s Benjamin Franklin had two pairs of
spectacles one for distance and one for near cut in half and placed in one frame together
to create what was referred to as “double spectacles”. Although there is no evidence,
many credit Samuel Pierce as the optician who made the first multifocal lenses for
Benjamin Franklin. The term multifocal in it’s earliest sense was a bifocal however the
term can be used for any lens with multiple focal lengths from trifocals and double D
segments to blended and round segments. There are cemented, fused, and one piece
construction methods for creating multifocal lenses. Cemented multifocals are worked as
two separate lenses with the multifocal section being worked to the equal but opposite
curve of the surface it is being glued to. Cemented lenses used to be constructed with
balsam as the adhesive. However, balsam is susceptible to heat and shock. It also tends to
discolor over time. Although not as common, newer methods include using epoxies to
bond the lenses. One of these cements is Summer Optical’s lens bond. Fused
construction is made in glass where sections of higher index glass are fused using heat
into a countersunk corresponding curve in the lens blank. The addition of a fused
multifocal is created by the following relationship:
This document is licensed under the Creative Commons Attribution 3.0 License. 7/30/2009
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