Tints can be applied to most lens mediums including Glass, Plastic (CR-39), High Index, Polycarbonate, and Trivex. There are several methods and processes to tinting lenses. The lens material being used and the desired end result help to dictate the method and process used to tint the lenses. The most common used tinting method is the heat treated process using specially formulated chemical dyes. This process can be used on most lens materials excluding glass. Another method is the 'Constant Density' process, which is most commonly used with glass and Plano (non-prescription) lenses. Another process used on glass lenses is color fusion. See the lens mediums below for further detail on these processes. There are also several types of tints listed below, which include solid, gradient and multi-gradient See sample below.
Plastic (CR-39), High Index, Polycarbonate, and Trivex usually use a chemical heat treated process. Specially formulated chemical dyes are heated to around 200' Fahrenheit. Lenses are submerged into the dyes for a calculated time to achieve the desired shade. The darker the desired color the longer the lenses should remain submerged and so forth.
With the chemical dye process any color is possible, but one disadvantage is that most colored lenses treated with the chemical dye process will eventually fade after continuous exposure to the sun. In most cases, lenses can be re-tinted to an approximate match of the original color. There is always a risk to the actual lens and/or coatings on the lens when lenses are re-treated. See Polarized lenses for non-fading tints.
Polycarbonate and high index lenses require a backside hard coat. On these lenses the hard coat dictates how the tint takes. Some hard coats repel tints and/or coatings because of the anatomy of the chemical composition. These extreme hard coats will not tint or tint very little. There have been many technological advances with hard coats creating flexibility with tints and coatings without compromising the durability of the hard coat. However, the higher the index lens the denser the material, hence the harder the back side coat has to be. It is not uncommon for a High Index 1.71 and above to be non-tintable.
Most Plano (Non-Prescription) lenses and some glass lenses use a 'Constant Density' process. This process uses the actual lens material (Glass, Plastic , Polycarbonate) as the dyeing agent. So in essence the whole lens is consistently the desired color all the way through, not just the surface of the lens. These lenses will not fade with constant exposure to the sun. However, there are sometimes minor discrepancies with heavy prescriptions and this method. Prescriptions lenses have to have the backside of the lens surfaced. The surfacing process creates thin and thick points on the lens to accommodate the prescription desired. The thin and thick areas may create light and dark spots on the lenses depending on the prescription. This usually does not apply on mild prescriptions or Plano lenses.