Editing Progressive lens (section)

==History==
The first patent for a PAL was British Patent 15,735, granted to Owen Aves with a 1907 priority date. This patent included the manufacturing process and design which was however never commercialized.  Unlike modern PALs, it consisted of a conical back surface and a cylindrical front with opposing axis in order to create a power progression.<ref name=Aves/>

While there were several intermediate steps (H. Newbold appears to have designed a similar lens to Aves around 1913), there is evidence<ref>Bennett A. (1973) Variable and Progressive power lenses. Manufacturing Optics Int. Mar, 137–141.</ref> to suggest that [[Duke Elder]] in 1922 developed the world's first commercially available PAL (Ultrifo) sold by "Gowlland of Montreal". This was based on an arrangement of aspherical surfaces.

The [[Carl Zeiss AG]] & [[Varilux]] lenses were the first PAL of modern design.  Bernard Maitenaz, patented Varilux in 1953, and the product was introduced in 1959 by [[Essilor|Société des Lunetiers]] (now Essilor). The first Varilux lenses' surface structure was however still close to a bifocal lens, with an upper, aberration-free half of the surface for far vision and a rather large "segment" for clear near vision.  The breakthrough in user adaptation and comfort, as well as peripheral and dynamic vision however occurred in 1972 with the introduction of Varilux 2, for which Maitenaz created a totally aspheric design and manufacturing process.<ref>{{Cite web |url=http://wernerkoeppen.com/ |title="Progressive Memories & Calculus" |access-date=2014-11-06 |archive-date=2014-11-06 |archive-url=https://web.archive.org/web/20141106084514/http://wernerkoeppen.com/ |url-status=live }}</ref>  [[Carl Zeiss AG]] developed freeform technology in 1983 with its own patented progressive series Gradal HS.<ref>{{cite web|url=https://www.zeiss.com/vision-care/en_us/about-zeiss/company-history/milestones-zeiss-spectacle-lenses.html|website=Zeiss.com|title=Milestones in the history of ZEISS eyeglass lenses|accessdate=24 September 2017|archiveurl=https://web.archive.org/web/20160415110006/https://www.zeiss.com/vision-care/en_us/about-zeiss/company-history/milestones-zeiss-spectacle-lenses.html|archivedate=15 April 2016}}</ref>

Early progressive lenses were relatively crude designs. Right and left were identical variable power lenses with distance and reading power centers in the upper and lower part of the lens, respectively. The glazing was made to accommodate eye position changes from distance viewing to reading. The point of reading is about 14&nbsp;mm below and 2&nbsp;mm to the nasal side in comparison to distance viewing. By tilting the reading power towards the nasal side in perfect symmetry, appropriate reading power was given to the wearer.

The symmetric design, however, was difficult to accept for patients, because the eyes in general work asymmetrically. When you look to your right, your right eye views distal (''i.e.'' looking through the lens near to the arm of the spectacles) while your left eye views nasal (''i.e.'' looking through the lens near to the bridge). Modern sophisticated progressive lenses are designed asymmetrically for greater patient acceptance and include special designs to cater to many separate types of wearer application: for example progressive addition lenses may be designed with distance to intermediate or intermediate to near prescriptions specifically for use as an occupational lens, or to offer enlarged near and intermediate view areas.

The typical progressive lens is produced from a so-called semi-finished lens. The semi-finished lens is molded with an asymmetrical power pattern on the front. On the back side a custom surfacing is made to adjust the power for each patient. This method is however problematic, especially for astigmatic prescriptions. The reason being that the semi-finished front pattern is designed for a spherical prescription. Freeform designs are tailored to each prescription and do not have this problem.<ref>{{cite journal|last=Meister|first=Darryl J.|title=Free-Form Surfacing Technology Makes Possible New Levels of Optical Sophistication for Spectacles|journal=Refractive Eyecare for Ophthalmologists|date=June 2005|volume=9|issue=6|pages=1–4}}</ref>

Since the 1980s, manufacturers have been able to minimize unwanted aberrations by:
* improvements in mathematical modeling of surfaces, allowing greater design control;
* extensive wearer trials; and
* improved lens manufacturing and measurement technology.

Today the complex surfaces of a progressive lens can be cut and polished on computer-controlled machines, allowing 'freeform surfacing', as opposed to the earlier casting process, thus explaining the difference in price. In short, the price is based on the technology used and the year the lens came to market.