Filters

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.

 

PricePower HandlingAngle ToleranceThicknessDysfunctionReflectionRoHS Conform
AbsorptionLowHeating due to absorption:may break at high powerWide angle toleranceLevel of blocking depends on thickness. Typ. 2-3mmMay fluorescence when blocking UV lightLowMany types "No" but with "legal" exeption status
DielectricHigh in small volumes. Low to mid in high volumesWithstand high optical power due to low absorptionFunctional spectrum blue shifts with angleMay be very thin eg 0.5mmNoneLow in transmission region. High in blocking regionYes
 

 

In addition, there are four basic functional types of filters. These are represented below as symbolic transmission curves as a function of wavelength.

 

Functional Types:

 

bandpass longpass shortpass neutral density
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