What is a spectrometer?

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- GoPhotonics

Aug 19, 2022

The spectrometer is an instrument that is used to measure and study the properties of light. It is also called a spectrograph or spectroscope. It is used to identify materials by studying the light emitted from or reflected from the materials. The spectrometer was invented by the German optical scientist Joseph Von Fraunhofer in 1814.  Those spectrometers used a prism and a telescope to investigate the properties of light reflecting from or emitting from an object. The light from the source or the material passes through a collimator, which has a vertical slit. The light passing through the collimator slit becomes parallel. This emitted parallel beam of light is directed to a prism that separates different frequencies (wavelengths) or resolves the spectrum, hence increasing the ability to see even the minor changes in the visible spectrum. This resolved spectrum of light is observed through a telescope where magnification increases the visibility furthermore.

 

When seen through the spectrometer, the spectrum of light from a light source contains absorption and emission lines in the spectrum which are identical to the specific transitions of the materials through which the light has passed or of the source material. This provides a method for determining unidentified materials by the study of these spectral lines. This process is known as Spectrometry.

The modern spectrometer uses diffraction grating as a monochromator instead of a prism which makes it less bulky and easy to use. The diffraction grating can be made to spread the colors over a bigger angle than a prism. Another advantage of using diffraction grating in the place of a prism is that the prism has a higher dispersion only in the UV region, but grating has a high and constant dispersion across the UV, visible, and IR spectrum.

The detector captures the light spectra and measures the intensity of light as a function of wavelength. These data are then digitized and plotted using the software. There is a wide variety of detectors that are used in spectrometers. Some of the commonly used detectors are the photomultiplier tube (PMT), photodiode, charge-coupled device (CCD), bolometer, and multi-channel analyzer (MCA).

Key parameters of a Spectrometer:

The integration time of a spectrometer is the time duration over which the spectrometer collects the photon, i.e. it is the amount of time available for the device to obtain the measurements during which there is no change in the level of the incoming signal (source).

Stray light in any system is the unwanted light present that may interfere with the intended performance of an optical system. They may originate from the source from which the light is originating or any other emitters present in the system or due to the temperature variations in the system. Stray light is mainly of two types: ghosts and flare or veiling glare. 

Ghosts occur due to unintended reflections between the imaging surfaces, or due to higher order diffractions from the gratings, or due to the secondary images formed by any bright scattering surface.

Flare or veiling glare results from the light incident on the system from any outside optical source or due to the presence of any bright sources present within the field of view or light scattering.

Sunlight is often considered a stray light during spectroscopic measurements and thus a dark room is always preferred for carrying out such measurements.

The spectral resolution of a spectrometer or an optical system, in general, is its ability to resolve the components of the electromagnetic spectrum. It is the maximum number of spectral lines or peaks that can be resolved by a spectrometer. For example, if a spectrometer with a wavelength range of 200 nm possessed a spectral resolution of 2 nm, then the system would be able to able to resolve a maximum of 100 individual wavelengths (peaks) across a spectrum.

The slit width is one of the main parameters that determine the resolution of a spectrometer and controls the amount of light that enters the spectrometer for processing. Larger slit width leads to an increase in the optical power available for the analysis, this will increase the wavelength difference between the components of the incident light, i.e. it will allow more wavelengths to pass through. This increase in the wavelength separation affects the resolution of the spectrometer. So, the widest possible slit width that satisfies the resolution condition should be chosen for better results.

Diffraction grating consists of a material that contains a periodic variation in its optical properties, usually a periodically varying refractive index is chosen, which is usually called grooves. It separates the wavelength component of the light falling on it by directing each wavelength at different angles. This change in the output angle with respect to different wavelengths is known as angular dispersion and it plays an important role in determining the wavelength resolution of a spectrometer. The groove density, i.e. the number of grooves per unit length controls the dispersion of light. A high groove density grating will spread the light over a larger area and thus increases the spectral resolution.

Types of optical spectrometers:

Spectrophotometer

They are types of spectrometers that use a monochromatic light that passes through a sample and a photodetector detects the output light. The changes in the output light compared to the source light allow the instrument to plot an output graph of the absorbed frequencies. 

Spectrofluorometer (Fluorescence/ photoluminescence spectrometer)

A spectrofluorometer is a type of spectrometer that measures the fluorescence emission from a sample. Here usually xenon lamps are used as the source of light as their high brightness is required to measure even the weak fluorescence emission. The sample is excited by the desired wavelength and the fluorescence emission is detected using a detector. 

The two most important spectral measurements in a spectrofluorometer are the excitation and the emission spectra. To measure the excitation spectrum, the emission monochromator is kept at a wavelength of strong fluorescence emission, the excitation monochromator is scanned across the particular wavelength region and the change in fluorescence intensity reveals the absorption characteristic of the sample. In the case of emission spectra, the wavelength of the excitation monochromator is set to a value where there is strong absorption by the sample. The emission monochromator is scanned across the particular wavelength region and the change in fluorescence reveals the fluorescence characteristics of the sample.

Raman spectrometers

Raman spectrometers are used to measure the Raman scattering of light from a sample. Here the light source used is laser light. The reason for this is that Raman Effect is a scattering effect so no light is absorbed by the sample. Thus we don’t require a broadband light source to match the absorption features of the sample. Also, the Raman Effect is a weak effect and so it requires a high power source for its detection. 

Here the laser light passes through the sample and is scattered due to Raman Effect. The scattered light is passed through a filter to filter out the weak Raman scattered lines. These lines are then analyzed to get a Raman spectrum.