A synchrotron light source is a source of electromagnetic radiation (EM) usually produced by a storage ring, for scientific and technical purposes. First observed in synchrotrons, synchrotron light is now produced by storage rings and other specialized particle accelerators, typically accelerating electrons. Once the high-energy electron beam has been generated, it is directed into auxiliary components such as bending magnets and insertion devices (undulators or wigglers) in storage rings and free electron lasers. These supply the strong magnetic fields perpendicular to the beam that are needed to stimulate the high energy electrons to emit photons.

The major applications of synchrotron light are in condensed matter physics, materials science, biology and medicine. A large fraction of experiments using synchrotron light involve probing the structure of matter from the sub-nanometer level of electronic structure to the micrometer and millimeter levels important in medical imaging. An example of a practical industrial application is the manufacturing of microstructures by the LIGA process.

Synchrotron is one of the most expensive kinds of light source known, but it is practically the only viable luminous source of wide-band radiation in far infrared wavelength range for some applications, such as far-infrared absorption spectrometry.

The primary figure of merit used to compare different sources of synchrotron radiation has been referred to as the "brightness", the "brilliance", and the "spectral brightness", with the latter term being recommended as the best choice by the Working Group on Synchrotron Nomenclature. Regardless of the name chosen, the term is a measure of the total flux of photons in a given six-dimensional phase space per unit bandwidth (BW).

The spectral brightness is given by

where ${\displaystyle {\dot {N}}_{\text{ph}}}$ is the number of photons per second in the beam, ${\displaystyle \sigma _{x}}$ and ${\displaystyle \sigma _{y}}$ are the root mean square values for the size of the beam in the axes perpendicular to the beam direction, ${\displaystyle \sigma _{x'}}$ and ${\displaystyle \sigma _{y'}}$ are the RMS values for the beam solid angle in the x and y dimensions, and ${\textstyle {\frac {d\omega }{\omega }}}$ is the relative bandwidth, or spread in beam frequency around the central frequency. The customary value for bandwidth is 0.1%.

Spectral brightness has units of time⁻¹⋅distance⁻²⋅angle⁻²⋅(% bandwidth)⁻¹.

Especially when artificially produced, synchrotron radiation is notable for its: