In a normal human eye, power increases as the object moves close to the eye. The increase in power occurs due to the accommodation of the eye lens. Accommodation is an automatic adjustment of the focal length of the eye lens. In bifocal and trifocal lens, even though the clarity of vision is achieved, the power increases in discrete steps from far distance to near distance and also the process of accommodation is absent. But progressive lenses are designed in such a way that, accommodation can be adjusted to any distance within the sight of a human being. Such a lens was first designed by Bernard Maitenaz in 1959 and the lens was called Essel's Varilux. After this model, many multidesign lenses were designed. These include Sola Percepta, Kodak and AO progressive lenses and Essilor's adaptor lenses.
What are Progressive Lenses?
These are categorized according to the distance of coverage, namely far, intermediate and near. In these lenses, the near addition (the change in power from far distance to near distance) is resembled by a 12 mm long channel that is present in the lens. If near addition is large and the size of the pupil is large, the person should not use such lenses and also avoid working continuously in places where large field of vision is required.
Few things which must be kept in mind while choosing the frame are discussed below.
- Ideally, the fitting height of the frame should 24 mm.
- The pantoscopic angle (tilt) of the frame should be 7° to 9°.
- The frame should have short vertex distance.
- Hard Lenses: In this type of lens, the rate of change of power and the level of astigmatism is high.
- Soft Lenses: The level of astigmatism is low in soft lens and the distance of the lens is narrow as compared to hard lens.
- Premium Lens: These lenses are digitally grounded and they provide a clear and distortion free vision. These lenses are referred to as 'free-form design' lenses.
- Standard Lens: In these lenses, wide area is present on the surface of the lens, which is suitable for reading purposes.
- Computer Lens: These lenses provide the coverage range from 16 inches to 6 feet. This lens is also known as office lens, as it provides near variable focus which is particularly designed for official use. It is ideal for people who work using computers, for long and continuous hours.
- Wide Corridor Lens: The intermediate range of these lenses are broad and so it can be used in large frames.
- Short Corridor Lens: The short corridor lenses are capable of fitting into a small frame. The distance between the intermediate part and near part of the lens is small.
- Polarized Lens: This type of lens is designed to protect the eyes from intense rays of the sun. Polarized lenses are very comfortable and basically act as a shield for our eyes.
- Transition Lens: These lenses are clear indoors and they become dark when exposed to sunlight.
- High-index Lens: The index range is from 1.67 to 1.74 and the lenses are thin and lightweight.
- Mid-index Lens: The index range is from 1.56 to 1.60 and it is thinner than other eyeglass lenses.
- Trivex Lens: They are high quality optical lenses with impact resistance.
Progressive lenses are made from glasses, plastics, polycarbonates, etc. The readers can vary the power of focus by tilting their heads and by viewing through a particular area of the lens. These lenses are designed without any demarcation lines, which give a younger look to those who wear it. However, the cost of these lenses is higher than bifocal and other single vision spectacle lenses.