IEC 61977:2020 is available as IEC 61977:2020 RLV which contains the International Standard and its Redline version, showing all changes of the technical content compared to the previous edition.

IEC 61977:2020 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 optical, mechanical and environmental properties. This fourth edition cancels and replaces the third edition published in 2015. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
- change of the title and the scope for the limitation to fibre optic fixed filters;
- addition of new terms and definitions reflecting new title;
- removal of terms and definitions duplicated in IEC TS 62627-09;
- harmonization of the vertical axis of Figures 1 to 5;
- restructuration of Clause 4 reflecting the latest technical and market situation.