Full-Width at Half Maximum (FWHM)
A source which is said to be one color, or monochromatic, will always contain some spread of wavelengths, even if this is relatively small. Often this is defined as the full-width at half maximum, FWHM, Δλ, on a plot of output power against wavelength. Sometimes it is expressed in terms of frequency.
Related Terms: Emission Spectra, Wavelength