Original bifocals were designed with the most convex lenses (for close viewing) in the lower half of the frame and the least convex lenses on the upper. Up until the beginning of the 20th century two separate lenses were cut in half and combined together in the rim of the frame. The mounting of two half lenses into a single frame led to a number of early complications and rendered such spectacles quite fragile. A method for fusing the sections of the lenses together was developed by Louis de Wecker at the end of the 19th century and patented by Dr. John L. Borsch, Jr. in 1908. Today most bifocals are created by molding a reading segment into a primary lens and are available with the reading segments in a variety of shapes and sizes.The most popular is the D-segment, 28 mm wide. While the D-segment bifocal offers superior optics, an increasing number of people opt for progressive bifocal lenses.
Bifocals can cause headaches and even dizziness in some users. Acclimation to the small field of view offered by the reading segment of bifocals can take some time, as the user learns to move either the head or the reading material rather than the eyes are generally placed directly in front of users and can lead to muscle fatigue due to the unusual straight and constant movement of the head. This trouble is mitigated by the use of trifocal lenses or by the use of monofocal lenses for computer users. In an interesting legal case reported in the UK in 1969, the plaintiff’s ability to use bifocals was impaired by accident.
Research continues in an attempt to eliminate the limited field of vision in current bifocals. New materials and technologies may provide a method which can selectively adjust the optical power of a lens. Researchers have constructed such a lens using a layer sandwiched between two glass substrates.
Bifocals in the animal world
The aquatic larval stage of the has, in its principal eyes, two and two distinct focal planes that are substantially separated (in the manner of bifocals) to switch their vision from up-close to distance, for easy and efficient capture of their prey, mostly larvae. This is the first ever recorded use of bifocal technology in the world.
The circular button consists of two segments with different refractive indices. The top part of the button – known as its crown – has the same refractive index as the base lens, i. e. as the distance portion, while the lower part – the subsequent near portion – has a higher refractive index.
At the surfaces of the dividing edge which is later visible, the two segments are smoothed and then permanently fused in a furnace. The back surface of the button is ground and polished. The function of the button crown is to protect the dividing edge of the near portion when the button is being milled and to complete the distance portion above the near portion.
Crizal Forte UV gives your eyes complete protection against damaging UV rays.
Taking UV protection to a new level .
We all know UV can damage our skin. But what about our eyes? UV can seriously damage your vision.Normal UV protection helps shield your eyes from rays that pass through the front of your glasses. Crizal Forte goes further, protecting your eyes from UV light reflected off the back surface of your lenses too.
CRIZAL® FORTE UV FEATURES
Crizal Forte UV give your eyes the best available protection, on a clear lens, against damaging UV light. Essilor’s exclusive anti-reflective technology is applied to both sides of the lens so that it also blocks reflected UV rays, which can account for up to 50% of exposure.
Most clear prescription lenses claiming UV protection only reduce the transmission of UV through the front of the lens.Crizal UV Lenses are prescription lenses that protect your eyes from UV by reducing the amount of UV radiation reaching them from the front, through the lens, and also from the sides and behind, by minimising reflected UV rays.
DID YOU KNOW… UP TO 50% OF UV RADIATION PENETRATING THE EYES IS REFLECTED OFF THE BACK SURFACE OF THE PRESCRIPTION GLASSES.
CRIZAL® UV BENEFITS
Taking UV protection to a new level
We all know UV can damage our skin. But what about our eyes? UV can seriously damage your vision. Normal UV protection helps shield your eyes from rays that pass through the front of your glasses. Crizal UV goes further, protecting your eyes from UV light reflected off the back surface of your lenses too.
Crizal UV lens are available in thin and light options. Ask your optician for details.
CRIZAL® UV FEATURES
Crizal UV give your eyes the best available protection, on a clear lens, against damaging UV light. Essilor’s exclusive anti-reflective technology is applied to both sides of the lens so that it also blocks reflected UV rays, which can account for up to 50% of exposure. These lenses comes with E-SPF* 10 rating. Know more about E-SPF.
Crizal UV lenses comes with Titus hard coating making it a more durable against daily wear and tear.
CRIZAL® PREVENCIA BENEFITS
Maximises the benefits of light, while reducing the risks.
UV light from the sun can cause cataracts, while blue light can cause AMD. By choosing Crizal Prevencia, you are protecting your eyes on a daily basis and maintaining your long term eye health. At the same time, it lets in the good light that regulates sleep and mood, and boosts brain activity.
CRIZAL® PREVENCIA FEATURES
Our advanced technology provides the most effective UV protection possible, and filters out harmful blue-violet light. Look out for the Essilor Sun Protection Factor (E-SPF*) to be sure your eyes are protected. This anti-reflection lens gives you the best vision in all light conditions, and is resistant to smudges, scratches, dust and water.
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