A bandpass filter is a filter that transmits a continuous range (band) of wavelengths but reflects or absorbs wavelengths above and below this range. Bandpass filters are specified by the full width at half maximum (FWHM) of their transmission peak: wideband (greater than 60 nanometers FWHM), medium band (20 to 60 nm FWHM), and narrowband (less than 20 nm FWHM).