An introduction to the evolution of progressive lens designs and probably more then you ever wanted to know about progressive lens optics.
The first progressive lens was made in 1959. The earliest progressives created about 4 Diopters of unwarranted peripheral distortion (called astigmatism) for every 1 Diopter of reading power. There was no difference between the design for the right versus the left eye. The second generation of progressives created 2 Diopters of unwarranted distortion for every 1 Diopter of reading power. The later generations of progressives create 1 D of unwarranted distortion for every 1 D of reading power. They are partially aspheric and they are asymmetrically designed so the right eye is designed differently from the left eye. They feature variable inset of the near vision depending on the power of the reading segment. The designs are different for each of the twelve reading powers. These lenses represent 65% of the progressives in use today.
The next generation of progressives uses global design management to optimize the features of the right and left eyes together. This creates equal powers in each eye when looking down and to the right or left. They are fully aspheric. This creates wider fields of vision across the entire lens surface and reduces peripheral distortions. “Swim” is reduced and adaptation is increased. They provide better visual clarity than prior progressives.
Newer technology allowed for the development of standard freeform progressives using wavefront technology to correct higher order aberrations like coma. All standard freeform lenses are made from digital diamond lathe cut molds with the lens design on the front surface. This lens also orients all unwarranted astigmatism (toricity) in the vertical meridian. They create the most accurate and sharpest vision with the widest field of view in distance, intermediate and near field of vision and the least amount of unwarranted toricity which reduces adaptation problems with standard molded freeform progressive lenses.
Digital Freeform back surface lenses are the most advanced lens design to date. The progressive design and Rx are digitally surfaced onto the backside of a single vision blank. This allows for complete customization of the Rx as worn in the particular frame you choose (i.e. the “as worn” Rx will vary slightly depending on the eyeglass frame due to A and B measurements vertex distance, face form etc.) There are few limitations on lens options because any lens that is manufactured can be turned in a digital freeform back surface progressive! These lenses can be compared to a tailor made suit where a standard PAL is like wearing a suit “off the rack.”
1.) Short corridors
A very important development with modern progressives is the “short corridor” progressives that satisfy the cosmetic needs for small frames where one cannot attain the 19 mm segment fitting height required for many progressives. It is always unacceptable to use a progressive lens with insufficient segment height. It does not work to increase the reading power – it just destroys the mathematical model or design system.
2.) Progressives for improved walking vision
Other specialty progressives offer distance vision below the reading segment to allow better vision for walking on stairs.
3.) Progressives for high base curves necessary for wrap around frames
These progressive lenses are designed to reduce annoying peripheral aberrations induced with high base curves required for wrap around frames.
4.) Progressives for optimum computer vision
The computer progressive is underutilized. It will optimize computer vision for presbyopes, pre presbyopes and some non presbyopes. The idea is to place the computer Rx at the center of the lens in front of the pupil. A “dynamic power” is calculated so that the plus power decreases when you look to the top of the lens to give you optimum vision out to 13 ½ feet to 7 feet depending on the reading power and the bottom of the lens will give you good reading vision.
5.) Fixed design digital freeform progressives
These progressive lenses offer the largest possible reading area with a “fixed” intermediate area.
6.) Variable design digital freeform progressives
These progressive lenses offer more intermediate vision.