What is an Optical Longpass Filter?

An optical longpass filter is defined as an optical filter that passes all wavelengths from a given wavelength to the red of that wavelength. Emission filters are mostly longpass filters. Typically, long pass filters transmit long wavelengths and block the short wavelengths. They are colored glass filters.

The term used to denote the wavelength location of the longpass filter is ‘cut-on’. The wavelength at 50% of the absolute transmission is called the cut-on wavelength. Longpass filters that have a very sharp cut-on wavelength are more commonly referred to as edge filters. Longpass filters tend to have brighter backgrounds when compared to bandpass filters. This datasheet will look into the working, construction and applications of longpass filters.

Working Principle

Longpass filters transmit long wavelengths and block the shorter ones. The cut-on wavelength is the value at 50% of peak transmission, and all wavelengths above the cut-on are transmitted by the longpass filters.

Applications

Typical applications of longpass filters are given below:

• Fluorescence microscopy
• Cell biology microscopes
• Emission filters

Sources and Further Reading

• Optics - Filter Properties & manipulation of light in flow cytometry– Purdue University
• Basic Aspects of Light Filters – Molecular Expressions
• Filters for Microscopy – Pennington Biomedical Research Center