This International Standard applies to the family of fibre optic filters. These components have all of the following general features: – they are passive for the reason that they contain no optoelectronic or other transducing elements which can process the optical signal launched into the input port; – they modify the spectral intensity distribution in order to select some wavelengths and inhibit others; – they are fixed, i.e. the modification of the spectral intensity distribution is fixed and can not be tuned; – they have input and output ports or a common port (having both functions of input and output) for the transmission of optical power; the ports are optical fibre or optical fibre connectors; – they differ according to their characteristics. They can be divided into the following categories: • short-wave pass (only wavelengths lower than or equal to a specified value are passed); • long-wave pass (only wavelengths greater than or equal to a specified value are passed); • band-pass (only an optical window is allowed); • notch (only an optical window is inhibited). It is also possible to have a combination of the above categories. This standard establishes uniform requirements for optical, mechanical and environmental properties.