What is a light source?

A light source enables all spectroscopy measurements. Here you can learn more about different kinds of light sources and their functionality.

Light is an electromagnetic radiation which in general ranges from the UV to the IR range

Light sources can be divided into monochromatic light sources and white light sources. 


Monochromatic light sources

The monochromatic light source is a radiation with a specific, single wavelength (frequency). Monochromatic light sources do not exist in nature, they must be manufactured artificially.      Monochromatic light beam is a product of an emission of electrons in solid state or atoms. First, the electron is excited with the adequate energy via electric field or light beam. Electron absorbs the energy and it transfers to the higher energy state. Excited electron, as a result of the recombination process, emit excess energy in the form of monochromatic light. 

The most well known monochromatic light sources are lasers, electroluminescence diodes (LED) and fluorescence lamps. 


White light sources

The white light source is a mixture of many different wavelengths (frequencies) in a specific range. However, it can be split into separate wavelengths by means of a prism or a diffraction grating. 

White light is usually a product of a thermal source, whereas illumination is a result of heating up a solid. Examples of thermal sources include: stars, bulbs, arc lamps and halogen lamps. 


Light source in UV- VIS spectrophotometry

The ideal light source should have a constant intensity over all wavelengths with low noise and long-term stability. In reality, unfortunately, such a source does not exist. There are two sources that are commonly used in UV-visible spectrophotometers.

1) Deuterium arc lamp: it yields a good intensity continuum in the UV region and in the visible region. Although modern deuterium arc lamps have low noise, which is often the limiting factor in overall instrument noise performance. Over time, the intensity of light from a deuterium arc lamp decreases steadily. 

2) Tungsten-halogen lamp: it has good intensity over part of the UV spectrum and over the entire visible range. This type of lamp has very low noise and low drift.

Most spectrophotometers use both types of lamps. In such instruments, either a source selector is used to switch between the lamps as appropriate, or the light from the two sources is mixed to yield a single broadband source.



1. https://www.rp-photonics.com/monochromatic_light.html

2. https://www.rp-photonics.com/white_light_sources.html

3. Fundamentals of modern UV-visible spectroscopy, Tony Owen, Agilent Technologies 2000