Abstract: A Photohardenable composition consisting essentially of a free radical polymerizable compound, an electron donating coinitiator, and a benzospiropyran, wherein the benzospiropyran undergoes ring opening to form a merocyanine upon exposure to a actinic radiation or heat and exposure to a second 296 radiation causes the composition to harden.

Abstract: A polymerizable composition and a method are disclosed. The composition includes a photopolymerizable monomer and a photoinitiator system for the monomer. The system, in one embodiment, is one wherein a ray of activating radiation which enters a body of the composition through a surface thereof activates the photoinitiator system and causes polymerization at each of a succession of points, the first such point being immediately adjacent the surface and subsequent points being successively farther from the surface in the direction of travel of the ray of activating radiation, the distance from the surface to the point of the succession farthest therefrom varying as a function of at least one of(a) the intensity of the activating radiation and(b) the time during which the activating radiation enters the body through the surface.