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 different types of filters, depending on the physical principle exploited during the manufacturing process: absorption filters (colour filters) and dielectric filters.
|Physical Principle||Price||Power Handling||Angle Tolerance||Thickness||Dysfunctions||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|
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 (Interference 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. These filters can be manufactured for use in the UV, visible and IR.
In order to reduce the number of coating layers – and thus the cost – a combination may be made by dielectric coating a colour (absorption type) glass substrate. This is common for some bandpass filters.
Interference filters have a complex coating structure consisting of several layers. They allow high transmission for a sharply limited wavelength range and also a high blocking of the remaining spectrum. The blocking of the light spectrum is described by the wavelength-dependent optical density OD. A transmission of 0.1% corresponds to an optical density of 4 and a transmission of 10% to an optical density of 1.
Different types of filters can be produced by the interference layers. The highest possible attenuation over a broadband wavelength range can be achieved by using neutral density filters. Bandpass filters characterize a highly transmissive region for single wavelengths or narrow wavelength ranges. Optical shortpass filters transmit light up to a certain wavelength and optical longpass filters transmit light only above a certain wavelength. Such filters are found, for example, as heat blocking filters, also known as hot mirrors, or correspondingly as cold blocking filters, also known as cold mirrors.
The filter types are represented below as symbolic transmission curves as a function of wavelength:
We offer filters cut to any shape. Black anodized aluminium rings may be provided for ease of mounting.
|Specifications||Absorption Filters||Dielectric Filters|
|Range of dimensions||1mm – 300mm||1mm – 300mm|
|Highest available surface figure||λ/4 (DIN: 3/0,5)||λ/4 (DIN: 3/0,5)|
|Highest available surface quality||40-20 S/D (DIN: 5/2×0,16)||40-20 S/D (DIN: 5/2×0,16)|
|Highest available tolerance of beam deviation||3” (arcsec)||3” (arcsec)|
|Blocking||—||OD3 (standard), OD5 (high blocking)|