Essilor Orma 1.5 index lenses

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 possibilites.

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.