Instrument that measures the spectrum of a light source. A spectrometer consists of an optical bench with a slit, a grating or diffraction grating, and a detector such as a photographic plate, photomultiplier tube, or charge-coupled device (CCD). The incident light is focused onto the slit by means of a collimating lens; the resulting beam of parallel rays passes through the slit and strikes the grating at an angle determined by the wavelength of interest. The grating disperses the light into its component wavelengths, which are then focused by another lens onto the detector. Depending on its design, a spectrometer may be used to measure either continuous spectra in which all wavelengths are present simultaneously (as in sunlight), or discrete spectral lines emitted by atoms and molecules (as in emission spectroscopy).
A variety of different types of spectrometers exist. The simplest kind is the absorption spectrometer, which consists of little more than a light source, collimating lens, diffraction grating, and detector. This type of instrument is used to study absorption lines in stellar and planetary atmospheres; because it requires only a single measurement for each wavelength region under study, it is well suited for surveying large areas of sky quickly. However, because it cannot resolve closely spaced spectral lines, it is not suitable for detailed studies requiring high spectral resolution.
The second type of spectrometer is the emission spectrometer. This instrument uses two gratings: one to dispersed incoming light from a continuum source (such as an incandescent lamp) into its component wavelengths; the other to disperse radiation emitted by atoms or molecules excited by this primary beam into their characteristic spectral lines. The resulting spectrum contains both continuum background radiation and discrete emission lines superimposed upon it; by subtracting out the former using appropriate software algorithms, astronomers can obtain “pure” emission line profiles for analysis. Emission spectroscopy is widely used in astronomy to study nebulae and other gaseous objects whose intrinsic brightness makes them difficult to observe with other techniques.
The third type of spectrometer is the reflection/refractionspectrometer. This instrument employs two separate optics systems: one for collecting reflected or refracted sunlight (or other electromagnetic radiation), and another for measuring its spectrum. Reflection/refraction instruments are generally much larger than absorption or emission Spectroscopers due to their need for two complete sets mirrors/lenses; they are also considerably more complex mechanically speaking since they must be able accurately align these two separate optics systems so that they share a common point focus somewhere within the instrument’s structure. As such instruments are relatively rare compared to their simpler cousins mentioned above