Photopolymerizable compositions and elements for refractive index imaging

An element containing a refractive-index image formed by exposure to actinic radiation, said element comprising a substantially solid layer that contains said image and is formed by photopolymerizing a composition consisting essentially of, by weight:

• (a) 25 to 75% of a solvent soluble, thermoplastic polymeric binder;
• (b) 5 to 60% of a liquid ethylenically unsaturated monomer, said monomer having a boiling point above 100.degree. C. and being capable of addition polymerization;
• (c) 0.1 to 10% of a photoinitiator system that activates polymerization of said unsaturated monomer upon exposure to actinic radiation;
• wherein said unsaturated monomer is an acrylate or methacrylate that contains a substituent from the group consisting of (1) an aromatic compound selected from the group consisting of (i) unsubstituted phenyl, (ii) substituted phenyl, (iii) substituted or unsubstituted naphthyl and (iv) substituted or unsubstituted heterocyclic aromatic having up to three rings;
• (2) chlorine, (3) bromine, and mixtures thereof, and said thermoplastic binder component is substantially free of said substituents, the composition having a refractive index modulation of at least approximately 0.005 as determined with 632.8 nm radiation from a transmission grating having a spatial frequency of about 1000 lines per millimeter, which transmission grating is prepared holographically from a layer of said composition.