A light source for a photo-reactive curing apparatus is provided, which includes a plurality of light source elements or modules, such as, UV or visible LEDs or LED arrays, arranged to provide a beam profile comprising irradiation zones separated by a dark zone. Photo-polymerization occurs during periods of irradiation and dark polymerization occurs during dark intervals between irradiation. The relative positioning or spacing of light source elements or modules is set to provide an exposure profile with a dark interval which matches the required dark reaction interval for optimal curing efficiency. In modular or adjustable light sources, the spacing is adjustable dependent on process parameters. For processes such as inkjet printing, the beam profile may be better matched to the ink chemistry, so as to control the polymerization reaction to meet a required process speed for single pass or multiple pass applications.