Formally called progressive addition lenses (PAL), progressive lenses are the essential part of a pair of progressive reading glasses. These special reading glasses have won its fame because of incomparable features of these progressive lenses. In some cases, progressive lenses are also called progressive power lenses, graduated lenses, no-line bifocals and varifocal lenses. All of these variations indicate some features of progressive reading glasses. For instance, the names including progressive power lenses, varifocal lenses and graduated lenses suggest that these lenses have a gradient of increasing lens powers, helping the users correct other refractive errors. Such a gradient starts at a minimum at the top of the lens and reaches a maximum addition power at the bottom of the lens. Typically, the length of the power gradient on the lens surface usually stretches 15 to 22 mm. And the additional power is usually between 1.00 and 2.50 diopters. In addition, the name of no-line bifocals suggests that progressive lenses are superior to bifocal lenses. They will never exert visible lines on the wearer’s face. In this case, progressive reading glasses can also be named as no line bifocal reading glasses.
Evidence shows that the first commercially available progressive lenses were developed by Duke Elder in 1922. Sold by Gowlland of Montreal, these lenses were based on an arrangement of aspheric surfaces. Progressive lenses of a modern design could be seen only after the invention of Varilux lens, which was developed by Bernard Maitenaz and patented in 1953. It is understandable that those progressive reading glasses had relatively crude design and workmanship. Nowadays, progressive reading glasses are more mature and they have gained greater patient acceptance. In general, there are three vision zones in a progressive lens for near vision, distance vision and intermediate vision respectively. The near zone usually sits at the bottom; the distance section is usually situated at the top part; and the intermediate section is between the first two. Modern technologically advancements have developed several variations of such a basic design. For instance, progressive lenses can be customized for computer use. Moreover, they can be made of enlarged near or distance section. These customized progressive reading glasses are a great help for individuals with particular occupations.
Modern technologies used by manufacturers are still trying to minimize unwanted aberrations on progressive reading glasses. They improve mathematical modeling of surfaces and conduct extensive wearer trials. Furthermore, computer-controlled machines are used to cut the complex surfaces of a progressive lens.