Long-pass filters are also known as edge filters. An edge filter is an optical component which blocks all the light in a defined spectral range and lets it through at high transparency in an adjacent range. If the transmission range has a longer wavelength than the blocked range, the term used is “long-wave pass filter” or “cut-on”, whilst the reverse situation is referred to as a “short-wave pass filter” or “cut-off”.
Blocking can be generated by means of either absorption, as in the case of coloured glass products, for example, or by reflection. If reflection is deliberately used in the blocking range, the industry also refers to dichroitic beam splitters. Suitable interference layer systems are used for blocking by means of reflection. Filters with extremely steep edges and high blocking properties in the blocking range are generally produced with the aid of ion-beam sputtering. This allows the edge position of the filter to be maintained very accurately and with a high degree of reproducibility. The IBS process helps guarantee the edge steepness and high level of optical density often desired.