Filters are optical elements with specific spectral transmission characteristics. They are used to modify the properties of a light source or to block unwanted wavelengths of light.
Absorption filters (colour filters) function on the basis of choosing a glass recipe with chromophoric constituents which absorb a specific range of wavelengths. Typically these are long pass edge filters, but some bandpass types with limited functionality are available. Note that this filter type may fluoresce when used for blocking UV light.