The notch filter, in contrast to a narrow band filter, intercepts light in a particular wavelength range by transmitting light at most wavelengths.

Notch filters are useful in applications where it is necessary to filter out the laser light. For example, in drug bioassays, in order to obtain a good signal-to-noise ratio in Raman spectroscopy experiments, it is necessary to suppress the pumping laser, which can be achieved by placing a notch filter on the detection channel. Notch optical filters are also often used in laser-based fluorescence instruments and in biomedical laser systems.