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1) What is the 1.56 Photochromic Flat-top lens.
Photochromic lenses are eyeglass lenses that are clear (or nearly clear) indoors and darken automatically when exposed to sunlight. Other terms sometimes used for photochromic lenses include "light-adaptive lenses" and "variable tint lenses."
Flat-top lenses were used to correct two different eye prescriptions. Bifocals were easy to spot - they had a line dividing the lens in two, with the upper half for distance vision, and the lower half for reading.
2) How the photochromic work?
The molecules responsible for causing photochromic lenses to darken are activated by the sun's ultraviolet radiation. Because UV rays penetrate clouds, photochromic lenses will darken on overcast days as well as sunny days.
Photochromic lenses typically will not darken inside a vehicle because the windshield glass blocks most UV rays. Recent advancements in technology allow some photochromic lenses to activate with both UV and visible light, providing some darkening behind the windshield.
3) Why choose the Flat-top?
Bifocals have two different lens powers that aid in seeing objects at various distances once the aging process reduces your ability to do so. These are usually prescribed to adults older than 40 years, although some children and younger adults may also need them for focusing problems, which strain the eyes while reading.
The coating used and the curved nature of lenses mean that up to 10% of light can be lost through reflections at the lens surfaces, these reflections can be distracting and appear unattractive.
By adding a coating it allows around 99% of light through the lens resulting in increased visual comfort and performance.