Page 10 - NAME OF CONDITION: REFRACTIVE ERRORS

Plastic Frames
Most ‘plastic’ frames are actually all natural. The common constituent of this frame is

Cellulose acetate, which is derived from treated wood pulp. These are sometimes called
“shell” frames since the tortoise shell was initially used to make frames and later bone and
animal horns were also used. Though this practice no longer exists, the name has
remained.
All these shell frames are less allergenic, flexible and elastic – so they last longer. However,
low-grade plastic frames which are more thermosetting (become rigid with heating) tend
to become brittle and cannot be reused. Thermoplastic frames on the other hand become
pliable on heating and can be reused when one wants a change of lenses.

Rimless Frame
These frames are actually disjointed three parts of a frame – the two temples of the frame
and the nose bridge. These are then attached to the lenses and held in place by screws.
These are available in both metal and shell frames. The advantage here is that the lens

shape can be free form. However, these must be handled carefully as the lenses have no
protection.
Half-Rimless Frames

These frames appear to have the frame rim running only along one side of the frame, but if
at a closer look one can see a clear nylon wire running along a groove holding the lens in
place. These are quite fashionable and are more reliable than rimless frames.

Lenses
Once the frame is chosen the lenses need to be selected. Lenses are available in three basic
materials:

Glass Lenses
Traditionally lenses were made out of glass. These had very high clarity and were easy to
produce and fit into frames. Alternatives for lens materials were sought because glass was
heavy and broke easily. This was a cause for concern where wearers had an active lifestyle.
Plastic Lenses
Lenses made of plastic are lightweight and do not break easily. However, as plastic is softer
than glass, it tends to scratch easily. But today with the developments of high quality

scratch resistant coatings plastic is fast replacing glass in the market. Plastic lenses weight
only 40% as much as glass and are especially suitable for an active lifestyle or a high
refractive error. It is especially safe for children, as it does not shatter like glass lenses.
Polycarbonate Lenses

These are called “bulletproof” lenses and for good reason too. They are impact resistant,
that makes them long lasting and lightweight. They have a very high refractive index, which
means that these lenses are very thin even for very high powers. Polycarbonates are
usually used for sports eyewear. But their lightweight, longevity combined with a good
scratch resistant coating make them one of the best choices.

10