Was introduced in 1983 and was the original high index lens material and It provided the ability to make thinner and lighter lenses.
It suffers from chromatic abberation [ colour fringes ] due to it's low abbe value but has great impact resistance, compared to newer technology, such as high index plastic 1.61 and 1.66 lenses.
It is often prescribed to peaple with active and dangerous working lives, ie: Emergency services.
Until the recent introduction of PPG's Trivex lens technology, it was first choice for rimless eyewear.