A narrowband filter is an optical filter designed with a narrow bandwidth. It is a type of bandpass filter that can selectively transmit light in a very narrow specific wavelength range while blocking light of other wavelengths. Its core feature is that the bandwidth is very narrow, usually only a few nanometers (nm) to tens of nanometers. The purpose of the narrowband filter design is to extract optical signals of specific wavelengths in a complex spectral environment, and it is often used in application scenarios that require high wavelength selectivity.
Key parameters of narrowband filters
- Center wavelength (CWL): The wavelength with the highest transmittance of the filter.
- Bandwidth (FWHM): The wavelength range at half the peak transmittance, usually very narrow (such as 1nm, 10nm, etc.).
- Transmittance: The light transmission efficiency at the center wavelength, usually higher than 90%.
- Cut-off depth: The ability to suppress non-target wavelengths, usually expressed in optical density (OD). The higher the OD value, the stronger the suppression ability.
- Angle of incidence: The angle of incidence of light has an impact on the performance of the filter, and is usually designed to be 0 degrees.
Narrowband filters are a classification of bandpass filters, with a narrower light passband. Their design purpose is to achieve more accurate light band screening to achieve the extraction of a certain band. They are usually used in various high-end application environments, including laser purification, fluorescence detection, Raman spectroscopy, biometrics, laser ranging and other applications; not only that, they are also used in biomedicine, industrial testing, aerospace, and astronomical observation.
The design of narrowband filters is based on the principle of thin film interference. Its uniqueness lies in the film layer. The bandwidth of the passband is small. The higher the cutoff depth, the more difficult it is to design. Secondly, since narrowband filters are often used in various high-precision environments, they require extremely high transmittance standards, so it is also a great test for the transmission material and film design of narrowband filters.
Bandpass filters are generally divided into narrow bandpass filters (narrowband filters) and wide bandpass filters (wideband filters). In terms of the name, we (GIAI) often use bandpass filters (Chinese name) as a general term for wide bandpass filters, while narrowband filters have a separate name!
