Contact lenses in focus

What are contact lenses made of and how do they help those with impaired sight?

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Modern contact lenses are shaped, gelatine polymer discs that, when worn, correct a range of visual deficiencies. These include astigmatisms (non-uniform cornea or crystalline lens curvature), myopia (nearsightedness) and hyperopia (farsightedness). They achieve this by modifying image focusing on the wearer’s retina, which is typically out of alignment.

Contact lenses are made in a rapid yet complex process. Firstly, a user’s prescription data is analysed to determine the lens shape, size and optical power. This data is entered into a diamond-tipped digital lathe, capable of 6,000 revolutions per minute. The lathe is fed a flat polymer disc and sculpts out what will be the inner face of the lens – ie the part that touches the eye. The disc is then polished with an abrasive paste and measured for thickness.

The lens returns to the lathe for outer curvature sculpting, with the diamond tip removing nanoscale layers of polymer at a time. The outer surface is then coated with oil and paste, before its rough rims are polished.

The lens is now the correct shape and power for the user, but it needs to be hydrated. It is bathed in a balanced pH saline solution for 24 hours – a process that sees the polymer gelatine disc absorbing liquid and expanding.

Finally, the lens is tested for quality and accuracy with an optical topographer machine, which determines the spread of optical power across the surface, and a frontal focal meter, which measures the precision of the lens compared with the user’s prescription.

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