A spectrometer is an instrument for measuring some property of light as a function of some portion of the electromagnetic spectrum. The measured variable is often the light intensity but could be the polarization state, for instance. The independent variable is often the wavelength of the light, usually expressed as some fraction of a meter, but it is sometimes expressed as some unit directly proportional to the photon energy, such as wavenumber or electron volts, which has a reciprocal relationship to wavelength. A spectrometer is used in spectroscopy. Spectrometer is a term that applied to instruments that operate over a very wide range of wavelengths, from gamma rays and x rays into the far infrared. In general any particular instrument will operate over a small portion of this total range because of the different techniques used to measure different portions of the spectrum. The radio electronics spectrum analyzer is a closely related device.
When a material is heated to incandescence it emits light that is characteristic of the atomic makeup of the material. In the original spectroscope design in the early 19th century, light entered a slit and a collimating lens transformed the light into a thin beam of parallel rays. The light was then passed through a prism that refracted the beam into a spectrum because different wavelengths were refracted different amounts because of dispersion. This image is then viewed through a tube with a scale that was transposed upon the spectral image, enabling its direct measurement. Particular light frequencies give rise to sharply defined bands on the scale which can be thought of as fingerprints. The element sodium has a very characteristic double yellow band known as the Sodium D-lines at 588.9950 and 589.5924 nanometers.
With the development of photographic film, the more accurate spectrograph was created. It was based on the same principle as the spectroscope, but it had a camera in place of the viewing tube. In recent years the electronic circuits built around the photomultiplier tube have replaced the camera, allowing real-time spectrographic analysis with far greater accuracy. Arrays of photosensors are also used in place of film in spectrographic systems. Such spectral analysis, or spectroscopy, has become an important scientific tool for analyzing the composition of unknown material and for studying astronomical phenomena and testing astronomical theories.