Matthew M. Virends in his excellent book, Gouldian Finches: Everything About Purchase, Hosing, Care, Nutrition Breeding, and Diseases (pictured) recommends that finches kept indoors have the benefit of full spectrum light. He states: “A light with the whole color spectrum (especially the ultraviolet part) can have a significant influence on the birds’ biological functions, including the production of vitamins in the body and the fixing of calcium in the bone tissues." Id. p.24.  

Gouldian Finches: Everything About Purchase, Hosing, Care, Nutrition Breeding, and Diseases by Matthew M. Virends