John C. Crano

Photochromic compounds: Chemistry and application in ophthalmic lenses

The preparation of a commercially acceptable plastic photochromic ophthalmic lens requires the incorporation of a number of properties - a neutral activated color, reasonable activation and fade rates, good lifetime, etc.. In this paper we will concentrate on approaches to finding photochromic systems with relatively neutral colors and acceptable rates of activation and fade. While offering good photochromic properties with regard to kinetics and photostability, they suffer from the fact that the visible absorption bands of the activated, or open, forms have narrow band widths. Also, without difficult to achieve substitution patterns, the absorption bands appear within a relatively narrow range of wavelengths. Partial neutralization of the activated color of the naphthoxazines was achieved through the use of indolino spirobenzoxazines. The open forms within this family exhibited broad absorption bands but the photochromic activity of each of the members was moderate to low. Enhancement of the photochromic activity was possible with triplet sensitization but at the expense of reduced lifetime. True color neutralization was achieved by coupling pyridobenzoxazines with members of a completely different family of photochromic compounds, the naphtliopyrans. The indolino spironaphthoxazines and related compounds have been known for over 25 years.

Plastic photochromic eyewear: a status report

An estimated 10 million pairs of photochromic prescription lenses were dispensed in the United States in 1989, essentially all based on a silver halide system suspended in an inorganic glass. A significant trend within the ophthalmic industry has been the growth of light-weight plastic lenses. In the United States market, the percentage of prescription eyewear made of plastic is now greater than 70%. With this increasing market penetration of plastic lenses, the desire for an acceptable plastic photochromic lens has also increased. As with any commercial product, in order to achieve consumer acceptance there exist several technical requirements for a plastic photochromic lens. These include the light transmission and color of the lens in both the unactivated and activated states, the speeds of darkening and fading, and the fatigue resistance or lifetime of the photochromic system. These requirements will be defined along with approaches to achieving them. The properties of the commercially available plastic photochromic lenses will be compared with the defined requirements.