You use spectrometers for measuring wavelengths of light. They are analytical or scientific instruments. An emission (as of particles or radiation) is dispersed according to a property (such as mass or energy) of the emission, and then the amount of dispersion is measured. Spectrometer is a term you would use to describe instruments that measure a continuous variable of a phenomenon where the spectral components are somehow mixed. In visible light a spectrometer can separate white light and measure individual narrow bands of colour. This is called a spectrum. A mass spectrometer measures the spectrum of the masses of the atoms or molecules present in a gas. The first spectrometers were used to split light into an array of separate colours. Because spectrometers can determine chemical composition it rapidly drove their advancement. This continues to be one of their primary uses. Spectrometers are used to analyse the chemical composition of the stars and planets in astronomy!