Patent No. U.S. 6,268,055
Photochromic polyacid-cured epoxy coatings for optical use have been patented by PPG Industries Ohio Inc. The coatings have a specified hardness and darken in color on irradiation with UV light. The coatings are obtained from a composition containing epoxy resin, a polyacid curing agent, a photochromic compound and a basic catalyst. A non-tintable hardcoat may be used as the primer. Coated substrates are claimed in this patent.