The ultraviolet spectrum may be investigated by these means to wavelengths of less than 60 mm (0.0000024 in); infrared spectra may be investigated by special means to regions beyond 0.01 cm (0.004 in).

Spectrophotometer
The spectrophotometer is widely used for measuring the intensity of a particular spectrum in comparison to the intensity of light from a standard source. The concentration of the substance that emits or absorbs the spectrum can be determined from this comparison. The spectrophotometers are also useful for studying spectra in the no visible areas because their detecting elements are bolometers or photoelectric cells. The former are particularly applicable to infrared spectrum analysis, and the latter to ultraviolet spectrum analysis.

Diffraction Grating
The second type of spectroscope in common use is the diffraction-grating spectroscope, first used in the early 1800s by the German physicist Joseph von Fraunhofer. This instrument consists of a metal or glass mirror surface on which a large number of parallel lines are ruled by a diamond, and light is dispersed by means of a diffraction grating rather than a prism. A good grating has a very high dispersive power, thus permitting the display of much greater detail in spectra. The lines of the diffraction grating may be inscribed on a concave mirror rather than on a transparent piece of glass, so that the grating also serves to focus the light and renders lenses unnecessary. In such a spectroscope, the light need not pass through any transparent substance, and these instruments have been used through the entire ultraviolet region into the region commonly considered X rays. Gratings may be adapted to spectrographs and spectrophotometers in the same manner as prisms.

Spectrum Analysis
Light is emitted and absorbed in minute units or corpuscles called photons or quanta. The energy e of a single photon is directly proportional to the frequency u, and therefore inversely proportional to the wavelength l. This is expressed by the simple formula, where h, the proportionality factor, is Planck's constant and c is the speed of light. The particular colours, or wavelengths (and thus energies), of the light quanta emitted or absorbed by an atom or a molecule depend in a rather complicated way on its structure and on the possible periodic motions of its constituent particles, because this structure and these periodic motions determine the total energy, potential plus kinetic, of the atom or molecule.