Presently, research continues on developing a simple, reliable, and cost-effective method and apparatus to produce and control the distinct red-green-blue point light sources necessary to electronically expose recorded images onto a conventional silver halide film. One approach that holds particular merit employs the physical phenomenon known as photoluminescence. Photoluminescence is the ability of certain solids, known as phosphors, to emit photons when irradiated by an external light source. Specifically, the use of "up-conversion" phosphors in conjunction with semiconductor laser diodes, typically GaAs diodes, hold the best promise for producing the necessary red-green-blue light for exposing silver halide film. Conceptually, "up-conversion" phosphors operate quite simply. A semiconductor laser diode can be used to irradiate infrared light onto the "up conversion" phosphor which, in turn, produces a visible and/or ultraviolet light therefrom. Practically, several issues must be resolved before "up conversion" phosphors can become a commercial reality.
Specifically, three problems impede the use of "up conversion" phosphors as the means to electronically expose images onto silver halide film. First, the host material for the "up conversion" phosphor must have low phonon energy, that is, the dopant atoms should not give off their energy to lattice vibrations of the host material when irradiated with photons. When the laser diode radiates photons onto the phosphor, the phosphor must use the photon energy to start the "up conversion" process, rather than to uselessly pump the energy into the lattice. Second, dopant atoms for the phosphor must have optically absorbing energy bands which correspond directly and integrally with the wave number associated with the light generated by the semiconductor laser diodes. This is so that the dopant atoms can absorb the radiated light from the semiconductor diode, rise to higher optically absorbing energy bands, and subsequently relax in energy and emit visible and/or ultraviolet light. In short, so that the phosphor can operate as an "up conversion" phosphor. Finally, the number of steps used by the "up conversion" phosphor must be carefully chosen so that the semiconductor laser diode can provide the necessary photon energy to initiate and continue the resulting "up conversion" process.