What is a Spectrometer? Types and Uses
A spectrometer is a device that measures a continuous, non-discrete physical characteristic by separating it into a spectrum of its constituent components. The study of this data is known as spectroscopy.
Different types of spectrometer measure different characteristics. The oldest and most common type of spectrometer, the optical spectrometer, measures the properties of light over a defined range of the electromagnetic spectrum. The spectral range measured varies from device to device depending on the design of the spectrometer and its intended use, but most operate around the visible part of the spectrum. Wide range optical spectrometers may also extend into the near-infrared and UV regions.
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Optical spectrometers are the most prevalent type of spectral device. Optical spectrometers can be used to study how light interacts with, or is emitted by, a sample. This is known as optical spectroscopy (or spectrometry).
The name 'optical spectrometer' is a broad term, as there are a number of different types of optical spectrometer. These are usually defined by the range of the electromagnetic spectrum that they cover, but can also be distinguished by their optical design, by their intended application, or by specific features that they offer.
Optical spectrometers have a wide range of applications across physics, chemistry, and biology. You can use them to measure the transmission, reflection, scattering, or absorption of light on a sample as well as electroluminescence or photoluminescence from an emitter. Each of these measurements can reveal a large amount of information about the material or structure in question, whether that be a thin film on a substrate, a 2D material, a chemical or electrochemical solution, a living cell or other biological material, or a distant star.
Characterisation of LEDs, Lasers, and More
The wavelength spectrum of the emitted light determines the suitability of an artificial light source (such as fluorescent tubes, halogen bulbs, lasers, and broadband white light or UV LEDs) for a given application. Indoor lighting generally requires a wide range of wavelengths and LEDs for colour displays should emit chromatically 'pure' light at a known wavelength. The wavelength of a laser, meanwhile, determines how far signals can travel when it is used for fiber optic communication.
Characterising light sources is a simple process, providing that your optical spectrometer operates in the desired range. The Ossila Optical Spectrometer can measure the spectrum of light from the UV-A band to the near infrared.
(Anti)reflection Coating Efficiency Measurements
Thin film coatings are often applied to materials to modify how light interacts with the surface. These coatings generally take the form of a layer of metal, to create a mirror, or of layers of transparent dielectric materials which rely on interference effects to increase or reduce the reflectivity of a surface.
Dielectric coatings are used for antireflection coatings on eyeglasses, camera lenses, lasers, and microscopes. Spectrometers like the Ossila Optical Spectrometer can be used to quantify both the effectiveness of a coating and how this varies with wavelength.
Investigations into Absorbing Materials
The light that is absorbed by a material can offer insight into its atomic structure. For example, peaks seen in the optical absorption spectrum of a molecule correspond to the energy separation between electron orbitals. This can reveal information about the nature of the chemical bonds in the material. The amount of light absorbed by a sample can also give a quantitative measure of the concentration of absorbing molecules within the sample through the Beer-Lambert law.
Fluorescence measurements are complimentary to absorption (and generally more far sensitive) and can provide information about the vibrational states of a molecule. They can also be used for detecting dopants/contaminants, identifying materials, and observing changes in a chemical environment. Highly fluorescent molecules are widely used in biological studies to 'tag' particular cells or proteins, making them more visible.
Efficient photovoltaic devices require matching the absorption spectrum of the device to the emission spectrum of the source (usually the sun). The use of a spectrometer here is two-fold. Firstly, it allows the direct measurement of the source spectrum. The solar 'standard' spectrum for solar cells is well documented, however, it varies with location, time of day, season and weather. A spectrometer allows a real-world measurement of the solar radiation spectrum. You can use it to measure the absorption spectrum of the photovoltaic material and compare this to the incident radiation.
Mass spectrometers produce spectra which show intensity as a function of mass-to-charge ratio. This is done by ionising the sample to be studied, supplying the ions with kinetic energy, and directing them through a magnetic field. The ions are then deflected by the field. Those with a lower mass-to-charge ratio are deflected more, and those with a higher mass-to-charge ratio are deflected less.
By varying the magnetic field in the mass spectrometer, ions of different mass-to-charge ratio will be deflected into the detector, producing a charge proportional to the number of ions. This spectrum can then be used to determine the relative abundance of isotopes of an atom, to identify chemical compounds, or to deduce the structure of a certain molecule.
Nuclear Magnetic Resonance Spectrometers
Nuclear magnetic resonance spectrometers are primarily used in organic chemistry and biochemistry to obtain information on the structure and composition of molecules. In the presence of an external magnetic field, some nuclei act like magnets. If a broad range of radiowave frequencies are directed onto the sample, the nuclei will resonate at different frequencies.
The resonant frequency depends on both the environment that the nuclei are in (e.g. how many and which atoms they are close to), and the magnitude of the applied magnetic field. The intensity of the NMR signal is proportional to the number of nuclei within each resonant frequency. This is plotted against the chemical shift which gives the resonant frequency with respect to a known reference, and has units of parts per million (ppm).
- W.G. Fastie, "Ebert Spectrometer Reflections", Physics Today 44(1), 37 (1991)
- W. Neumann, "Fundamentals of Dispersive Optical Spectroscopy Systems", SPIE (2014)
- G. Wünsch, A. Wennemer, J.W. McLaren, "On the design and performance of the Czerny-Turner monochromator in ICP-AES", Spectrochimica Acta Part B: Atomic Spectroscopy, 46(11), 1517-1531 (1991)
- A. Thevenon, et al., "Aberration-Corrected Plane Gratings", Proceedings of SPIE, 0815, 136-145 (1987)
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