Narrowband filter is a special bandpass filter with a narrow passband range. It can accurately control the transmission of light of a specific wavelength and block light of other wavelengths. Usually, the half-width (i.e. bandwidth) of the passband of a narrowband filter is between a few nanometers and tens of nanometers. Its implementation principle mainly relies on the interference, absorption and reflection effects of light. The purpose of precise wavelength selection is achieved by coating a specific optical film (such as an interference film or an absorption film) on the substrate.
Classification of narrowband filters
Classification by manufacturing process
Interference narrowband filter
The interference filter uses multiple layers of optical films with different refractive indices and is manufactured by accurately designing the thickness and refractive index of the film. This filter uses the interference effect of light to achieve selective transmission of light of a specific wavelength.
Advantages: extremely narrow bandwidth, high transmittance, and strong stability.
Disadvantages: complex manufacturing process and high cost.
Absorption narrowband filter
By adding specific absorbents to the filter substrate to absorb light of a specific wavelength, the filtering effect is achieved.
Advantages: simple manufacturing process and low cost.
Disadvantages: Wide bandwidth, transmittance and stability are not as good as interference filters.
Combined narrowband filters
Combining multiple filters to combine the advantages of different filters to obtain narrower passbands or more complex spectral characteristics.
Advantages: High flexibility, can meet a variety of complex needs.
Classification by spectral range
Ultraviolet narrowband filter
Used to select a narrow wavelength range in the ultraviolet band, commonly used in applications such as fluorescence analysis and ultraviolet imaging.
Visible narrowband filter
Mainly used to filter specific wavelengths in the visible light band, also known as color filter. It can be distinguished by color and used in photography, optical detection and biometrics.
Infrared narrowband filter
Used to accurately select specific wavelengths in the infrared light band. Widely used in infrared imaging, laser ranging, environmental monitoring and industrial automation.
Key parameters of narrowband filters
Center wavelength (CWL)
Refers to the wavelength at the center of the filter passband, usually matching the operating wavelength of the device.
Bandwidth (FWHM)
Indicates the distance between two wavelengths corresponding to half of the peak transmittance in the passband range, also known as half-maximum width.
Peak transmittance (Tmax)
The highest light transmittance in the passband range, reflecting the transmission performance of the filter.
Cut-off range
Refers to the wavelength range outside the passband, requiring extremely low light transmittance.
Cut-off depth (OD value)
Describes the transmittance of light in the cut-off range, usually expressed in optical density, with the formula OD=−log10(T)\text{OD} = -\log_{10}(T). For example, when the transmittance is 1%, the OD is 2.
Application fields of narrowband filters
Communication field
Narrowband filters are used in wavelength division multiplexing (WDM) technology in optical fiber communication, especially in dense wavelength division multiplexing (DWDM) systems, to help accurately separate optical signals of different wavelengths, thereby improving communication capacity and transmission efficiency. In addition, in wireless communication, it can be used as a filter to suppress interference signals and improve communication quality.
Optical detection field
Narrowband filters are used to select specific wavelengths of light in spectral analysis to improve the accuracy and sensitivity of analysis. For example, in atomic absorption spectrometers, they are used to detect the element content in samples; in optical sensors, they improve the accuracy of sensors by filtering background light.
Medical field
Narrowband filters are widely used in medical equipment, such as:
In the treatment of skin diseases, narrowband UV filters of specific wavelengths are used to treat diseases such as psoriasis.
In ophthalmic diagnosis, they are used to observe fundus vascular lesions.
In biomedical research, narrowband filters are used in fluorescence microscopes to select excitation light and emission light to improve imaging quality and the accuracy of experimental results.
Industrial Field
In the field of industrial automation, narrow-band filters are used in machine vision systems to improve image contrast and clarity, thereby performing efficient product quality inspection and classification. In addition, in laser processing, by selecting a laser with a specific wavelength, processing accuracy and efficiency can be improved.
Summary
Narrowband filters are indispensable components in the field of optics. Their high selectivity and precise wavelength control capabilities make them play an important role in communications, spectral analysis, medical and industrial fields. Narrowband filters can be divided into many types according to different manufacturing processes and application scopes, and each type provides solutions for specific optical needs. With the continuous advancement of optical technology, the performance and application scope of narrowband filters will be further expanded, bringing greater technological breakthroughs to multiple fields.
