 A technical definition of an atomic line filter is as an "ultra-narrow-band, large-acceptance-angle, isotropic optical filter". "Ultra-narrow-band" defines the thin range of frequencies that an ALF may accept; an ALF generally has a passband on the order of 0.001 nanometer. That atomic line filters also have wide acceptance angles (near 180 °) is another important characteristic of the devices; conventional dielectric filters based on the spacing of reflective or refractive layers change their effective spacing when light enters at an angle.
