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. There are two principle types of optical filter: absorption filters and dielectric filters. 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. Dielectric filters absorb very little light. The functionality is based on reflection of the unwanted spectral regions based on optical interference within the layer structure of the filter coating. In order to reduce the number of coating layers - and thus the cost - a combination may be made by dielectric coating a colour glass substrate. This is common for some bandpass filters. The following table gives an overview of the main criteria for deciding between these two filter types. Heating due to absorption: may break at high power High in Small volumes. Low to mid in high volumes Withstand high optical power due to low absorption Artifex Engineering e.K. Dortmunder Str. 16-18 26723 Emden, Germany May fluoresce when blocking UV light Wide angle tolerance Level of blocking depends on thickness. Typ. 2-3mm Functional spectrum blue shifts with angle Managing Director: Dr. Steven Wright Registry Number: HRA 200036 RoHS Conform Many types "No" but with "legal exeption" status Low in transmission region, high in blocking region Tel: +49-(0)4921-58908-0 eMail: [email protected] Web: http://www.afx-eng.de/