Long pass Filters separate wavelength regions by reflecting the shorter wavelengths whilst transmitting longer wavelengths. Both portions of light can be utilised. The position of the filter is defined by the transition or ‘half power’ wavelength.
The width of the transmission and reflection zones can be specified independently as can the steepness of the transition edge between these regions. Different angles can be accommodated as can polarisation splitting. Very steep edges with low ripple and high %T utilise sophisticated designs with many layers. Such filters are available for the 300-8000nm region in a large variety of formats.