Photometers produce electrical signals, which correspond to radiant flux energy, for a series of discrete wavelength intervals. They are used to measure wavelengths of light spectra and optical or atomic emissions. Photometry is the study of light from a radiant light source such as the sun, laser, or lamp. Radiation is separated into component wavebands in order to measure each individual band. These radiance measurements are expressed in different ways, depending on how radiation is collected and processed. Typically, wavelengths are measured in nanometers (nm). Performance criteria for photometers include spectral range and resolution. Spectral range is the range of wavelengths that a photometer can measure. Resolution is the smallest wavelength that a photometer can distinguish. Operating temperature and operating humidity are additional parameters to consider when selecting photometers.