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What is a Spectrometer? Types and Uses


Ossila Spectrometer
The Ossila Optical Spectrometer is a compact USB Vis-NIR Spectrometer

A spectrometer is a device that measures a continuous, non-discrete physical characteristic by first separating it into a spectrum of its constituent components.

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.

Optical spectroscopy (or spectrometry) is the use of an optical spectrometer to study how light interacts with, or is emitted by, a sample. Optical spectrometers can measure the transmission, reflection, scattering, or absorption of light on a sample and the luminescence 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. Optical spectrometers therefore have a wide range of applications across physics, chemistry, and biology.

Other types of spectrometer includes mass spectrometers and nuclear magnetic resonance (NMR) spectrometers, but unless otherwise stated, 'spectrometer' is generally used to refer to optical devices (and likewise 'spectroscopy' generally refers to optical spectroscopy).

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Optical Spectrometers


Optical spectrometers are the most prevalent type of spectral device, and are often referred to as just 'spectrometers'. The name 'optical spectrometer' itself 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.

UV-Vis-NIR spectrometers have numerous applications across a number of different fields.

Characterisation of LEDs, lasers, and more

The wavelength spectrum emitted by artificial light sources such as fluorescent tubes, halogen bulbs, LEDs and lasers determines their suitability for a given application. Indoor lighting generally requires a wide range of wavelengths, LEDs for colour displays should emit chromatically 'pure' light at a known wavelength, and the wavelength of a particular laser determines how far signals can travel when it is used for fiber optic communication. The Ossila 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 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, which 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 detection

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.

Photovoltaics

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. Secondly, the absorption spectrum of the photovoltaic material can be measured and compared to the incident radiation.

Mass Spectrometers


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, and 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 with 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).

  

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References


  1. W.G. Fastie, "Ebert Spectrometer Reflections", Physics Today 44(1), 37 (1991)
  2. W. Neumann, "Fundamentals of Dispersive Optical Spectroscopy Systems", SPIE (2014)
  3. 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)
  4. A. Thevenon, et al., "Aberration-Corrected Plane Gratings", Proceedings of SPIE, 0815, 136-145 (1987)

Contributing Authors


  • David Coles
  • Kirsty McGhee
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