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Safety Lenses 179

tension in the lens mass. This produces a compression tension coat, often
referred to as the compression envelop. This improves the impact resistance.
After the lenses have been manufactured, they are fitted into the frame or
housing.
Advantages
1. Heat toughening is comparatively quick and cheap process.
2. It does not need skilled labour.
3. The equipment needed is inexpensive and requires little bench space.

Disadvantages
1. Heat toughened lens will always be thicker than an untoughened lens
of equivalent power.
2. Prescription lenses over + 5.00 D are not ideal for toughening, as the
bulk of glasses require prolonged heating which can cause warping,
degrading the optical qualities. Lenses over – 5.00 D have poor impact
resistance due to the relatively thin centre substance.
3. Impact resistance is markedly reduced by scratches and other surface
abrasions. Hence, they should be periodically inspected and replaced
when distinct scratches are present.
4. The heat toughening process has an adverse effect on the photo
chromatic lenses. The lens does not lighten to its original transmission.

Chemically Toughened Glasses
In this process the lenses are first preheated and then lowered into a
potassium nitrate solution at 470°C for 16 hours. The compression coat is
produced by exchanging the larger potassium ions present in the solution
for the smaller sodium ions present in the glass. As this treatment occurs
on the surface of the glass only, it produces a very thin but very tough
compression coat. For photo chromatic lens toughening, the solution
normally used is 40% potassium nitrate and 60% sodium nitrate at a reduced
temperature of 400°C. This process does not generally affect the photo
chromatic action of the lenses.
Advantages
1. Although, chemically toughened lenses are thinner than heat toughened
lenses, they have been shown to possess a greater impact resistance.
2. The toughening process takes the same time for all types of lenses.
3. The temperature required for chemical toughening is lower than that
used for the heat treatment of lenses; therefore, warping is not a problem.

Disadvantages
1. Chemical toughening is an expensive process, as it requires equipments
that must withstand the chemicals and the temperatures involved.