title: A brief history of LED photopolymerization.
abstract: OBJECTIVE
  The majority of modern resin-based oral restorative biomaterials are cured via photopolymerization processes. A variety of light sources are available for this light curing of dental materials, such as composites or fissure sealants. Quartz-tungsten-halogen (QTH) light curing units (LCUs) have dominated light curing of dental materials for decades and are now almost entirely replaced by modern light emitting diode light curing units (LED LCUs). Exactly 50 years ago, visible LEDs were invented. Nevertheless, it was not before the 1990s that LEDs were seriously considered by scientists or manufactures of commercial LCUs as light sources to photopolymerize dental composites and other dental materials. The objective of this review paper is to give an overview of the scientific development and state-of-the-art of LED photopolymerization of oral biomaterials.
  
  
  METHODS
  The materials science of LED LCU devices and dental materials photopolymerized with LED LCU, as well as advantages and limits of LED photopolymerization of oral biomaterials, are discussed. This is mainly based on a review of the most frequently cited scientific papers in international peer reviewed journals. The developments of commercial LED LCUs as well as aspects of their clinical use are considered in this review.
  
  
  RESULTS
  The development of LED LCUs has progressed in steps and was made possible by (i) the invention of visible light emitting diodes 50 years ago; (ii) the introduction of high brightness blue light emitting GaN LEDs in 1994; and (iii) the creation of the first blue LED LCUs for the photopolymerization of oral biomaterials. The proof of concept of LED LCUs had to be demonstrated by the satisfactory performance of resin based restorative dental materials photopolymerized by these devices, before LED photopolymerization was generally accepted. Hallmarks of LED LCUs include a unique light emission spectrum, high curing efficiency, long life, low energy consumption and compact device form factor.
  
  
  CONCLUSIONS
  By understanding the physical principles of LEDs, the development of LED LCUs, their strengths and limitations and the specific benefits of LED photopolymerization will be better appreciated.
authors.affiliation: Faculty of Physics and Astronomy, Friedrich-Schiller-University Jena, Jena, Germany. k.Jandt@uni-jena.de
authors.forename: Klaus D
authors.initials: KD
authors.lastname: Jandt
authors.forename: Robin W
authors.initials: RW
authors.lastname: Mills
mesh_terms: D003188:Composite Resins / Q000266:history*; D055117:Curing Lights, Dental / Q000266:history*; D003764:Dental Materials / Q000266:history*; D049673:History, 20th Century; D049674:History, 21st Century; D055115:Light-Curing of Dental Adhesives / Q000266:history*; D058105:Polymerization
chemical_list: D003188:Composite Resins; D003764:Dental Materials