Instruments for viewing spectra
A spectroscope is an instrument used to observe the atomic spectrum of a given material. Because atoms can absorb or emit radiation only at certain specific wavelengths defined by electron transitions, the spectrum of each type of atom is directly related to its structure. There are two classifications of atomic spectra: absorption and emission.
An absorption spectrum is produced when light passes through a cool gas. From quantum mechanics we know that the energy of light is directly proportional to its wavelength. For a given type of atom, a photon of light at some specific wavelength can transfer its energy to an electron, moving that electron into a higher energy level. The atom is then in an "excited state." The electron absorbs the energy of the photon during this process. Thus, a white light spectrum will show a dark line where light of that energy/wavelength has been absorbed as it passed through the gas. This is called an absorption spectrum.
The energy transfer is reversible. Consider the excited state photon in the example above. When that electron relaxes into its normal state, a photon of the same wavelength of light will be emitted. If a gas is heated, rather than bombarded with light, the electrons can be pushed into an excited state and emit photons in much the same way. A spectrum of this emission will show bright lines at specific wavelengths. This is known as an emission spectrum.
Light entering a spectroscope is carrying spectral information. The information is decoded by splitting light into its spectral components. In its simplest form, a spectroscope is a viewing instrument consisting of a slit, a collimator, a dispersing element, and a focusing objective (see Figure 1). Light passes through the slit and enters the collimator. A collimator is a special type of lens that "straightens out" light coming in at various angles so that all of the light is travelling the same direction. The wavefront is converted into a planar wavefront; if you wish to think of light as rays, all the light rays are made to travel in parallel.
Next, light enters the dispersing element. A dispersing element spreads light of multiple wavelengths into discrete colors. A prism is an example of a dispersing element. White light entering the prism is separated out into the colors of the spectrum. Another type of dispersing element is a diffraction grating. A diffraction grating redirects light at a slightly different angle depending on the wavelength of the light. Diffraction gratings can be either reflection gratings or transmission gratings. A grating is made of a series of fine, closely spaced lines. Light incident on the grating is reflected at an angle that varies as wavelength. Thus, white light will be divided into the spectral colors, and each color will appear at a discretely spaced position. A transmission grating works similar to a reflection grating, except that light travels through it and is refracted or bent at different angles depending on wavelength. The focusing objective is just a lens system, such as that on a telescope, that magnifies the spectrum and focuses it for viewing by eye.
A spectroscope gives useful information, but it is only temporary. To capture spectroscopic data permanently, the spectrograph was developed. A spectrograph operates on the same principles as a spectroscope, but it contains some means to permanently capture an image of the spectrum. Early spectrographs contained photographic cameras that captured the images on film. Modern spectrographs contain sophisticated charge coupled device (CCD) cameras that convert an optical signal into an electrical signal; they capture the image and transfer it to video or computer for further analysis.
A spectroscopic instrument in great demand today is the spectrometer. A spectrometer can provide information about the amount of radiation that a source emits at a certain wavelength. It is similar to the spectroscope described above, except that it has the additional capability to determine the quantity of light detected at a given wavelength.
There are three basic types of spectrometers: monochromators, scanning monochromators, and polychromators. A monochromator selects only one wavelength from the source light, whereas a scanning monochromator is a motorized monochromator that scans an entire wavelength region. A polychromator selects multiple wavelengths from the source.
A spectrophotometer is an instrument for recording absorption spectra. It contains a radiant light source, a sample holder, a dispersive element, and a detector. A sample can be put into the holder in front of the source, and the resulting light is dispersed and captured by a photographic camera, a CCD array, or some other detector.
An important class of spectrometer is called an imaging spectrometer. These are remote sensing instruments capable of acquiring images of Earth's surface from an aircraft or from a satellite in orbit. Quantitative data about the radiant intensity or reflectivity of the scene can be calculated, yielding important diagnostic information about that region. For example, a number of important rock-forming minerals have absorption features in the infrared spectral region. When sunlight hits these rocks and is reflected back, characteristic wavelengths of the light are absorbed for each type of rock. An imaging spectrometer takes a picture of a small region of rocks, splits the light from the image into different wavelengths, and measures how much reflected light is detected at each wavelength. By determining which quantities and wavelengths of light are absorbed by the region being imaged, scientists can determine the composition of the rocks. With similar techniques, imaging spectrometers can be used to map vegetation, track acid rain damage in forests, and track pollutants and effluent in coastal waters.
Another class of spectrometer highly useful to the laser industry is the spectrum analyzer. Although lasers are nominally monochromatic sources, there are actually slight variations in the wavelengths of light emitted. Spectrum analyzers provide detailed information about the wavelength and quality of the laser output, critical information for many scientific applications.
Parker, Sybil, ed. The Spectroscopy Sourcebook. New York: McGraw-Hill, 1987.
Spex, Jobin Yvon. Guide for Spectroscopy. Edison, NJ: Instruments S.A.