Abstract: A method of increasing the efficiency of a multiphoton absorption process and apparatus. The method includes: providing a photoreactive composition; providing a source of sufficient light for simultaneous absorption of at least two photons; exposing the photoreactive composition to at least one transit of light from the light source; and directing at least a portion of the first transit of the light back into the photoreactive composition using at least one optical element, wherein a plurality of photons not absorbed in at least one transit are used to expose the photoreactive composition in a subsequent transit.