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 difference in refractive index, resulting a very high transmittance the differences 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, or p-polarization or random(the average of s and p polarization) when ordering. Filters for the major laser wavelengths are available from stock. Those for other wavelengths and dimensions may be custom designed. The interference filters are extremely durable and withstand repeated cleaning. Every filter comes in an anodized aluminum housing for protection against humidity and handling. The highly reflective side of the filter should generally face the source to minimize the thermal load on the absorptive glass and epoxies.