What is a longpass filter?

A longpass filter is an optical filter that blocks incident light below a certain wavelength and allows light above this wavelength to pass through. This threshold wavelength is also called the cut-on wavelength. When choosing the right longpass filter, in addition to the cut-on wavelength, the actual blocking band must be taken into account. This is because the longpass filter is still permeable to the smallest parts of light. The transmission band of the filter also plays a role, the upper limit of which is dependent on the substrate used.

The wavelength range in which the light is blocked by the longpass filter is called the blocking band. The unit for the quality of blocking is usually given as the optical density (OD) and this is an essential feature of a filter’s quality. Long pass filters are available both as interference filters and as coloured glass filters, and are used, for example, in spectrometers and monochromators to suppress the higher orders of the diffraction grating. In this case they are also called order sorting filters.