FWHM or Full Width at Half Maximum is the width of the output spectrum of a light source measured at half of its maximum amplitude. It is measured between the point on the output spectrum curve where power has decayed to half of its peak value. FWHM is a quantitative method used to specify the spectral width of a light source.

FWHM is used to define the resolution of a light source. Ideally, no light source is monochromatic i.e. they do not produce a single wavelength of light and so the laser output is generally a range of wavelengths. The resolution of any device is its ability to distinguish between the closely spaced wavelength components present in the light source. The wavelength peaks which have an overlapping FWHM are considered to be indistinguishable and thus cannot be resolved.