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 physical principles which may be exploited to manufacture optical filters: absorption and interference.
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 physical principles of filters.
|Price||Power Handling||Angle Tolerance||Thickness||Dysfunction||Reflection||RoHS Conform|
|Absorption||Low||Heating due to absorption:may break at high power||Wide angle tolerance||Level of blocking depends on thickness. Typ. 2-3mm||May fluorescence when blocking UV light||Low||Many types "No" but with "legal" exeption status|
|Dielectric||High in small volumes. Low to mid in high volumes||Withstand high optical power due to low absorption||Functional spectrum blue shifts with angle||May be very thin eg 0.5mm||None||Low in transmission region. High in blocking region||Yes|
In addition, there are four basic functional types of filters. These are represented below as symbolic transmission curves as a function of wavelength.