Optical Longpass Filters block incoming shorter wavelength light by either absorption or reflection, while allowing longer wavelength light to transmit through the optic. Longpass Filters are also commonly referred to as High Pass, Sharp-Cut Edge Filters.
Longpass filters are characterized by their "cut-on" wavelength, which is the wavelength at the 50% point along the transmission slope.
Longpass Filters are often used to separate light sources such as fluorescence emission from a lower wavelength excitation source to prevent crosstalk. They can be used along with Shortpass Filters to create custom Bandpass Filters.