Photochromic Lenses
Photochromic Lens Technology

Technical Similarities
Photochromic Technology Performance/Technical Similarities Let�s look at the performance characteristics between photochromic lenses made with imbibed and in-mass technologies. First, the similarities.

All photochromic lenses share certain traits. For example, in the darkened state, they all block potentially harmful UVB. They are all somewhat temperature dependent too. Temperature dependence means that the photochromic performance of the lenses is different, given different temperatures. Generally, photochromic lenses won�t get as dark on a very hot day as they will on a cool day. Likewise, the lenses fade more rapidly in a warm environment than a cool one.

Flourescent light, typical of many office environments, is richer in UV and shortwavelength light than incandescent lighting. As a result, photochromic lenses may not clear fully in flourescent illumination. At night, most photochromic lenses are clear, allowing for safe driving after dark. However, photochromic lenses used for medical conditions and photochromic lenses that are tinted for maximum sun protection may not be suitable for night driving.

How about darkening behind the windshield of a car? Although some plastic in-mass technology lenses claim partial activation behind the windshield of a car, neither technology provides true sunglass performance behind the windshield. Drivers need to be reminded of this to provide realistic expectations.

Imbibed and in-mass plastic photochromic lenses also are said to darken uniformly across the entire lens surface, regardless of lens power. However, the depth to which each lens darkens is different. With imbibed lenses, the photochromic action takes place in a thin layer �imbibed� on the front surface of the lens, and only darken to a minimal depth. Corning�s revolutionary technology allows their in-mass plastic lenses to darken uniformly to a 1.5mm depth across the exposed surface of the lens. As a result, lenses in strong plus or minus powers can be surfaced thinner than the leading imbibed technology lens in the center with no uneven darkening, sometimes referred to as �bulls eye� or �raccooning.� Uncut plus lenses with a knife edge of less than 1.5 mm may appear slightly lighter on the edge, but this is eliminated when the lenses are edged and mounted in a frame.

Photochromic Technology Performance

Technical Similarities

Blocks potentially harmful UVB rays Temperature dependent Darkens uniformly across lens surface Activated by UV wavelengths Return to clear state at night