Light curing, or photopolymerization, depends on converting individual resin monomer units into connected polymer chains, thereby decreasing the viscosity of the resin material until it becomes solid. This process is initiated by molecules called photoinitiators, which generate free radicals when they absorb light of a specific wavelength. The free radicals interact with the resin monomers to change their chemical bonds so that they can link to each other, releasing more free radicals in the process to help the conversion continue throughout the material. The higher the degree of conversion, the more durable the resulting restoration. However, because the process also leads to a degree of material shrinkage, care must be taken to avoid stress at the tooth-composite interface, which can lead to secondary caries.
The degree of conversion is affected by several factors, such as the intensity of the light; the shade, composition, and depth of the restorative material; the distance and angle of the light to the composite; and the time of exposure. In addition, different photoinitiators respond to different wavelengths of light, making the need to choose the right light for the right material critical to success.