The photo below is of an Inuit man taken in 1933 most likely in the North West Territories of Canada. For thousands of years the people living in the polar regions of the northern hemisphere fashioned protective sun eyewear to guard against the bright sun particularly during the spring as the sun began to get higher in the sky. They were often made of whale bone or antler, and in more recent times from wood as it became more readily available. The inside of the goggle was often blackened with soot to cut down on any reflections off the back of the eyewear, a simple but probably effective anti reflctive coating or perhaps closer to being a tint! They were held in place by a strap of some natural material such as the sinue from a caribou. The thin slots in them would also have given some assistance to any individuals with a refractive error as the pin hole effect would have been significant as long as the slits were cut finely enough. What a fine example this is of the inventiveness of the Inuit people and their ability to survive in such a harsh climate.
You may have heard of the term ‘grind lenses’ but probably don’t really know what it really means. The same may apply to the term ‘stock lenses’. Anyway, stock lenses are finished on both the front and back surface, come in various powers including correction for astigmatism and are delivered as a round blank of perhaps 65 or 70mm in diameter. The lens is then shaped by a machine so that it can be fitted into a frame. The picture below is of two pairs of glasses with the one above made in stock lenses and the one below in grind. Look at the thickness difference!
Grind lenses begin as a ‘semi finished’ blank, whereby the front surface only is finished and the back surface is finished by a machine called a generator. Because grind lenses are specially surfaced, they can be made to the correct thickness for the dimensions of the frame and a thinner, more comfortable pair of glasses can result. Pictured below is a semi-finished lens blank as is straight out of the packet.
Here is our stock lens cupboard. Shown are a range of plus powers only in various corrections for astigmatism.
Below is a stock lens before being cut.
Sometimes however, whether the lens is grind or stock makes no difference. It all depends on the prescription, frame size, distance between the pupils, amount and angle of astigmatism. Only a well trained and experienced optical mechanic or dispenser can interpret all that information and really know which way to go. Doing the wrong thing can result in ugly, heavy glasses or unnecessary expense.
Benjamin Franklin is reputedly the inventor of the first bifocal lenses. He had his optician split his distance and near lenses in half and join them together down the middle. This allowed excellent distance and near vision with one pair of glasses. Of course, times have changed and the manufacture of multifocal lenses has become much more sophisticated. There is still a place however for the original bifocal design. In this case our client is an airline captain and his visual needs are quite demanding. Many pilots do quite well with standard progressive lenses, however my client feels uncomfortable with the slight peripheral blur experienced with them.
Below is a photo of the pair that I made for him. They have his full distance in the top, gauges distance in the middle and the near is for the fine detail of filling out paperwork. All set at the correct heights for easy usage.
There are many instances when patients can be assisted with Franklin split bifocals/trifocals, in particular eye muscle balance issues and problems that arise when the lens power is different in the right and left eye. Really, the use for these lenses is only limited by our imagination.
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