Orma is Essilor's version of CR39 which was for many years, the world's most popular
Orma is a lightweight, tough plastic lens with 1.5 index that is
much safer than glass. It weighs far less than the equivalent
glass lens and it very easy to tint.
Hard coatings are recommended to increase scratch resistance and anti-reflection
coatings can be applied.
For ophthalmic optics, CR39 features a number of properties which account for its
success : a refractive index of 1.498 (close to that of Crown glass), a density
of 1.32 (almost half that of glass) and a Abbe value of 58-59 (thus a very low chromatism),
high impact resistance, excellent transparency, and multiple tinting and coating
Its major drawback is its limited resistance to abrasion relative to crown glass,
however, this can be overcome by the application of a Crizal Forte Scotchgard coating.