Photoinitiators are molecules that can be split into two or more parts by exposure to light. At least one of these parts is capable of reacting with both the monomers and binders to link them together. Photoinitiators are only sensitive to specific wavelengths of light. While there are initiators that work with visible light, most of the ones used in additive fabrication are sensitive to ultraviolet radiation. The spectrum of the light source used must overlap that for which the initiator is sensitive. Both lasers and other types of light sources such as arc lamps may be used. Photoinitiators comprise just a few percent of the photopolymer mixture.