Plastics
Photochromic plastic lenses
Technically speaking, photochromic plastic is an entirely different world from photochromic glass. Not only are most of the components used to achieve specific color, refraction, constringence and darkening/fading speed characteristics different, but the photochromic mechanism that takes place in a plastic photochromic lens is also very specific.
The photochromic dye is an organic molecule which undergoes a reversible chemical reaction when exposed to ultraviolet A (UVA): the opening or closing of its structure, respectively causing the fading or darkening of the lens (see our animation).
Most of the time, a photochromic plastic lens will incorporate a specific mix of several dyes, all of them photochromic.
Two technologies: in-mass and coating
The in-mass photochromic technology disperses photochromic dyes throughout the lens material (liquid monomer as a medium + other technical components) with a specific manufacturing process ensuring that they are distributed evenly. As ultraviolet (UV) exposure may, in the long run, degrade some of these molecules (those closer to the convex curve), others as yet inactivated replace them, providing a longer-lasting, more consistent color over the life of the prescription.
The coating technology offers both high performance and flexibility. Photochromic coating means applying a photochromic layer (a few microns thick) onto various kinds of substrates such as 1.60 and 1.67 high-index lenses or polycarbonate.
Because they are no longer bound by the refraction or constringence limitations of the resin as with the “in-mass technology”, and in order to benefit from improved darkening and speed of reaction properties, our customers increasingly use photochromic coating for their high-end lens designs, including “free-form backside” progressive lenses.
Corning Ophthalmic offers a complete product range of patented in-mass and coating photochromic plastic solutions, fully compatible with most anti-reflective (AR) and hard-coating (HC) treatments.