3 Light Sources
The light used for spectroscopy experiments comes from devices that convert electrical energy into light energy (and heat, since they are never 100% efficient). Light sources differ in the wavelength(s) of light emitted and other functional properties. Most light sources operate on the basis of either black-body radiation or emission from atoms, molecules, or other materials.
Black-Body Light Sources
Tungsten Halogen Lamp. This lamp comprise a tungsten filament in a glass or quartz bulb filled with inert gas and a small amount of a halogen (e.g. bromine, iodine). The filament is heated by an electric current to a temperature hot enough for a broad black-body-like spectrum of visible and infrared light to be emitted between ca. 400–3500 nm. Ultraviolet (UV) emission is not significant. The added halogen extends the lifetime of the tungsten filament.
Globar. This black-body-like light source is a silicon carbide rod that is heated by an electric current to emit a broad spectrum of infrared (IR) light between ca. 1–40 µm. UV light emission is negligible and visible emission is weak.
Arc Discharge Lamps
Xenon Lamp. This lamp comprise a quartz tube with electrodes at both ends and filled with high-pressure xenon gas. An electrical arc excites the xenon gas to induce light emission. Although this emission is quantized in principle, collisions between xenon atoms broaden their energy levels, resulting in a broad spectrum of UV and visible light between ca. 200–1100 nm.
Mercury Lamp. This lamp are designed and operate similar to a xenon lamp, but also include a small amount of mercury. The light output is a broad-spectrum background with multiple intense lines (UV and visible) from superimposed mercury emission. The useable range is ca. 250–600 nm.
Metal-Halide Lamp. This lamp have bromine or iodine added with the mercury, which extends the operational lifetime, slightly broadens the mercury emission lines, and increases the relative intensity of the broad-spectrum background light. The useable range is ca. 300–700 nm.
Deuterium Lamp. This lamp comprises a quartz tube with two electrodes and filled with deuterium. An electrical arc induces deuterium emission to yield a broad spectrum of UV light from ca. 200–400 nm.
Light-Emitting Diodes
LEDs produce light by passing an electric current through a p-n semiconductor junction. At this junction, electrons fall from a higher energy state to a lower energy state, emitting light in the process. The light from an LED is emitted over a much smaller range of wavelengths compared to lamps, appearing as a particular colour of light rather than white light, but is far from a single wavelength like laser output. LEDs are smaller, produce less heat, have much longer lifetimes than lamps, and are available at UV, visible, and IR wavelengths.
Lasers
The acronym LASER stands for light amplification by stimulated emission of radiation. Energy is pumped into a lasing material to excite it and induce spontaneous emission. The lasing material is located in a mirrored cavity, with one mirror intentionally having a small amount of transmission. Emitted photons bounce back and forth in the cavity and stimulate the emission of additional photons, which have the same energy and travel in phase and in the same direction as the stimulating photons. A coherent beam of light of a single wavelength (i.e. monochromatic) is formed by the photons that escape from the cavity. Depending on the technology, lasers can be very large or very small, and emit continuously or emit in pulses. Compared to other light sources, lasers provide the highest intensity light and are available with UV, visible, and IR wavelengths. While simple lasers have a fixed wavelength, lasers systems can be designed to have tuneable output.
Connections
The light sources used in instruments are primarily selected to match method requirements for intensity and wavelength, and secondarily selected based on size, cost, and operating lifetime.
- Tungsten, deuterium, and xenon lamps are used for UV-visible spectrophotometry (Ch. 7)
- Xenon lamps are used for fluorescence spectroscopy and mercury lamps and metal-halide lamps are used for fluorescence microscopy (Ch. 9)
- Xenon lamps are used in some modern atomic absorbance spectrometers (Ch. 11)
- Globars are used in infrared spectrometers (Ch. 13)
- Lasers are used for fluorescence spectroscopy and imaging (Ch. 9) and Raman spectroscopy (Ch. 15)
The choice of light source can impact signal-to-noise ratios, sensitivity, selectivity, and other figures of merit for an analysis.
Post-Reading Questions
- Classify all the light sources in this chapter into three categories: monochromatic, broad spectrum, narrow spectrum.
- Classify all the light sources in this chapter into four categories: UV emission, visible emission, UV-visible emission, and infrared emission. Some light sources may be listed in more than one category.
Topic Learning Objectives
The chapter is a primer for the following learning objectives, which will be covered in lecture and/or with additional assigned reading:
- Explain the operating principles of selected light sources
- Draw diagrams that illustrate the design and operating principles of selected light sources
- Relate the operating principle of a light source with the nature of its light output
- Match light sources with types of spectroscopy
- Summarize advantages and disadvantages for a particular light source when matched with a specific spectral region or type of spectroscopy
- Be prepared to select light sources with other instrument components to design a system capable of a desired type of measurement