A longpass filter is a device that selectively transmits light of longer wavelengths and blocks or reflects light of shorter wavelengths.
A longpass filter can be implemented as a glass plane or plastic device that is either dyed in the bulk or has interference coatings.
A longpass filter is characterized by its cut-on wavelength, which is the wavelength at which the transmission reaches 50% of the maximum value.
A longpass filter can be used for various applications, such as isolating spectral regions, enhancing contrast, blocking unwanted light, fluorescence microscopy, and multispectral imaging.
Comparison of different type of filters
Key Specifications of Longpass Filter
Transmission: the percentage of light that passes through the filter at a given wavelength or range of wavelengths.
Blocking: the amount of light that is rejected by the filter at a given wavelength or range of wavelengths, usually expressed as optical density (OD).
Cut-on slope: the steepness of the transition from blocking to transmission at the cut-on wavelength.
Full width at half maximum (FWHM): the width of the transmission band measured between the wavelengths where the transmission is half of the maximum value.
Center wavelength (CWL): the average of the wavelengths where the transmission is half of the maximum value.
Bandwidth: the difference between the wavelengths where the transmission is half of the maximum value.
OD and FWHM Explanation
OD and FWHM are two important parameters that affect the performance of a longpass filter.
OD describes how much light is attenuated by the filter at a certain wavelength or range of wavelengths. A higher OD value means lower transmission and higher blocking. For example, an OD of 2 means that only 1% of the light is transmitted and 99% is blocked. An OD of 4 means that only 0.01% of the light is transmitted and 99.99% is blocked.
FWHM describes how wide or narrow the transmission band of the filter is. A smaller FWHM means a narrower band and a sharper cut-on slope. A larger FWHM means a wider band and a smoother cut-on slope. For example, a FWHM of 10 nm means that the filter transmits light within 10 nm around the CWL. A FWHM of 100 nm means that the filter transmits light within 100 nm around the CWL.
The choice of OD and FWHM depends on the application and the desired trade-off between transmission, blocking, and resolution. A higher OD and a smaller FWHM can provide better blocking and resolution, but may also reduce the transmission and increase the cost. A lower OD and a larger FWHM can provide higher transmission and lower cost, but may also compromise the blocking and resolution.
Example of Standard and Performance Longpass Filter
Standard longpass filter: This filter has an OD of 2, a FWHM of 100 nm, a CWL of 600 nm, and a cut-on slope of about 10%. It can provide moderate transmission (>85%) in the pass band (550-1100 nm), moderate blocking (OD > 2) in the stop band (200-540 nm), and smooth transition at the cut-on wavelength. This filter is suitable for applications that do not require very high contrast, resolution, or blocking, such as general photography or color separation.
