A white Light Source is a Light Source with a broad optical Bandwidth (usually 100 NM or more).

However, two quite different meanings of the term are in use:

• In some cases, a light source really emits visible white light. Such sources are required e.g. for lighting applications or for colorimetry. They may contain something like a bulb (e.g. a tungsten Halogen lamp), emitting a smooth and very broadband optical spectrum. Spectra of fluorescent lamps or other gas discharge lamps (e.g. xenon lamps) can also look white but are much more structured.
• In other cases, a broadband light source is meant, which does not necessarily emit in the visible spectral region. Such sources can be superluminescent sources, e.g. superluminescent diodes, and typically exhibit a high spatial coherence, while the temporal Coherence is low according to the large bandwidth. Typical applications of this kind of white light sources are white light interferometry, characterization of optical components, spectroscopy, etc.