In physics, a spectrometer is an apparatus to measure a spectrum. Generally, a spectrum is a graph that shows intensity as a function of wavelength, of frequency, of energy, of momentum, or of mass.
A spectrometer is an tool commonly used by astronomers which splits the light collected by a telescope into its colors. This allows astronomers see the details in the light from space. Astronomers know how to get a lot of special information about a space object by studying its light. By using spectrometers, we can find out the temperature of an object in space, learn which direction it is traveling, find out how fast it is going, figure out its weight and even find out what it is made of. Spectrometers help us learn all of this from light!
Over the past 20 years, miniature fiber optic spectrometers have evolved from a novelty to the spectrometer of choice for many modern spectroscopists. People are realizing the advanced utility and flexibility provided by their small size and compatibility with a plethora of sampling accessories.
The basic function of a spectrometer is to take in light, break it into its spectral components, digitize the signal as a function of wavelength, and read it out and display it through a computer. The first step in this process is to direct light through a fiber optic cable into the spectrometer through a narrow aperture known as an entrance slit. The slit vignettes the light as it enters the spectrometer. In most spectrometers, the divergent light is then collimated by a concave mirror and directed onto a grating. The grating then disperses the spectral components of the light at slightly varying angles, which is then focused by a second concave mirror and imaged onto the detector. Alternatively, a concave holographic grating can be used to perform all three of these functions simultaneously. This alternative has various advantages and disadvantages, which will be discussed in more detail later on.
Once the light is imaged onto the detector the photons are then converted into electrons which are digitized and read out through a USB (or serial port) to a computer. The software then interpolates the signal based on the number of pixels in the detector and the linear dispersion of the diffraction grating to create a calibration that enables the data to be plotted as a function of wavelength over the given spectral range. This data can then be used and manipulated for countless spectroscopic applications, some of which will be discussed here later on.
In the following sections we will explain the inner-workings of a spectrometer and how all of the components work together to achieve a desired outcome, so that no matter what your application is, you’ll know what to look for. We’ll first discuss each component individually so that you have a full understanding of their function in the workings of a spectrometer, then we’ll discuss the variety of configurations that are possible with those components, and why each of them has a different function. We’ll even touch on some of the accessories used to make your application as successful as it can possibly be.
EM radiation, including light, is a spectrum of different wavelengths. Spectroscopy is the detailed analysis of a light signal by wavelength. Ordinary color images break up light into 3 channels (red, green, and blue), but spectroscopy is generally concerned with breaking up light into a higher number of bands (e.g. 10, 100, or more), and a spectrometer is the instrument that does just that.
The basic principle of spectrometry is simple, various methods (the most ordinary being the use of a prism) can be used to cause the different wavelengths of light to follow different paths, which can be used in combination with a monochromatic imaging sensor to record the spectrum. Alternately, multiple images of the same scene can be recorded while using different narrow band filters (either separate filters or a device which can be adjusted to pass through different wavelengths such as a fabry-perot filter).
Spectrometry has multiple uses:
Ions of different elements have different emission spectra due to the differences in electron energy levels. This makes it possible to determine the elemental composition of objects that are significantly ionized such as stars (which are composed of high temperature plasma). Additionally, at lower temperatures molecules have characteristic absorption and emission spectra which can be used to determine the composition of lower temperature objects such as planets and asteroids.
The large scale structure of a light spectrum will be dominated by the characteristics of the black body spectrum, making it possible to determine an object's temperature.
As mentioned above the composition of an object will result in a very characteristic spectrum. However, this spectrum will be shifted a certain amount one way or another depending on whether the object is moving away or towards us, due to doppler shifting. This makes it possible to measure the relative velocity of an object along the line of sight. By studying changes in an object's motion we can infer certain information about the object such as whether or not it is orbited by another otherwise unseen object. To date this is one of the most prolific methods for detecting extrasolar planets.
Since spectroscopy splits up a light signal into many tiny buckets it's very helpful to have as much light to work with as possible, which is why most of the largest telescopes in the world (such as the Keck or VLT telescopes) spend a lot of their time collecting spectra and have very sophisticated spectrometers.
The invention of CCDs and other electronic imagers has been a gigantic boon to spectrometry, since such devices have very high quantum efficiency (meaning the vast majority of photons from the source light are converted into usable signals) and can have fairly flat spectral response curves. Most importantly, they are already finely divided into different bins spatially and they contain a huge number of individual detectors (pixels).
One of the most interesting advances in modern spectroscopy is the increasing predominance of "imaging spectrometers" in interplanetary spacecraft and observatories. Instead of merely collecting multiple color channel data for each pixel in an image these instruments collect entire spectra for every pixel. This dramatically increases the amount of data collected and the speed of data collection by a spacecraft many fold, making it possible to extract a lot more information from a single view of a planet, moon, rock or what-have-you than was possible before. A few examples of imaging spectrometers would be the Mars Reconnaissance Orbiter's CRISM, the JWST's NIRSpec, and Dawn's VIR instrument.