Interference filters are fabricated by a series of thin layers of various dielectric materials sandwiched by glass substrates. When multi-wavelength light passes through the filter interference occurs due to the differences in refractive index, resulting in a very high transmittance for certain wavelengths, while other wavelengths are either reflected or absorbed. For a given wavelength a very narrow bandwidth (3nm-10nm) can be obtained. Optical interference filters are angle-sensitive and should be used with normal or near normal incident light. If the incident angle exceeds 20° it is essential to specify s-polarization, p-polarization or random (the average of s and p performance) when ordering. Filters for the major laser wavelengths are available from stock. Those for other wavelengths and dimensions may be custom designed. These filters are extremely durable and withstand repeated cleaning.