This document 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 cannot 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); " gain flattening (compensating the spectral profile of the device). It is also possible to have a combination of the above categories. This document provides the generic information including terminology of IEC 61753-04x series documents. Published IEC 61753-04x series documents are listed in the Bibliography. This document establishes uniform requirements for the following: optical, mechanical and environmental properties.