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 most common type of spectrometer, the USB 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.

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

Optical Spectrometers

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, 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 range of colours (the spectrum) produced by a light source will help decide how good it is for a given application. Examples of different light sources include fluorescent tubes, halogen bulbs, lasers, broadband white light sources or LED light sources. Indoor lighting generally requires a wide range of wavelengths but LEDs for colour displays should emit 'pure' light at a known wavelength. Meanwhile, the wavelength of a laser determines how far signals can travel when it is used for fiber optic communication.

Characterising light sources with an spectrometer is simple and easy, providing that your optical spectrometer operates in the desired range. The Ossila USB Spectrometer can measure the spectrum of light from the UV-A band to the near infrared.

(Anti)reflection Coating Efficiency Measurements

Researchers and device manufacturers often use thin film coatings to change the way light interacts with a materials surface. For example, a layer of metal can be used to create a mirror effect, or layers of transparent dielectric materials can increase or reduce the reflectivity of a surface.

Dielectric coatings are used to create antireflection coatings on eyeglasses, camera lenses, lasers, and microscopes. Spectrometers like the Ossila USB Spectrometer can be used to measure both the effectiveness of a coating and how this varies with wavelength.

Measuring Light Absorbaance

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 can reveal information about the nature of chemical bonds in the material. The amount of light absorbed by a sample can also give a quantitative measure of the concentration of a 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 different 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. You can use it to measure the absorption spectrum of the photovoltaic material and compare this 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, 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 amount 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).