1. A brief history of white light sources

The oldest source of white light, and the first to be used scientifically by man, is the Sun. The first incandescent lamps appeared in the 19th century, and this principle was widely used as a light source until the invention of the laser in the 1960s by Maiman . The temporal and spatial coherence and power density of laser sources, which are far superior to thermal sources, soon put them at the top of the bill.

However, the qualities of lasers mean that they can only emit a single wavelength, which is limiting for certain applications. Scientists, and in particular Alfano...