What is a white light source?
Thermal light sources with a sufficiently high temperature, e.g., halogen lamps (3200K), produce a continuous, broad spectrum that is perceived as white light. The spectral distribution can be approximately described as a blackbody. At lower temperatures (<2000K) the light appears reddish. At very high temperatures, such as in an arc lamp (6000K), the light is perceived as increasingly bluish and cold. The designations warm white, neutral white and cold white used in lighting technology actually contradict the true temperature of such (thermal) light sources of radiation.
If human color perception is in the foreground, white light sources are those that enable particularly natural color rendering (CRI, color rendering index). However, these light sources do not necessarily have a smooth, continuous spectrum. It is also possible – with limitations – to create the color impression using only a few spectral bands in the primary colors red, green and blue. Examples of this are light sources for video projectors that work with colored LEDs or filter wheels.
Today, phosphor-converted LEDs are used for general lighting purposes, in which blue primary light is converted into white light by special dyes. For some time now, medical, physiological, and ergonomic considerations have come into focus (see e.g., circadian rhythm): Artificial illumination should resemble as much as possible natural illumination to which humans have been adapted by evolution. Modern LED-based white light sources therefore have a broad, homogeneous spectrum that even goes beyond visual perception.
In spectroscopic applications, the observed spectral range extends beyond human perception into the UV and IR range. Here, a suitable white light source ideally covers the entire spectral range of the detector technology used. An example of this would be laser-pumped plasma lamps, which have a very broad and smooth spectrum from the deep UV to the mid-IR. As brilliant white light sources, these allow to fully exploit the spectral sensitivity of Si- or InGaAs-based detectors.