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Principles and Applications of Different Types of Optical Filters

Optical filters are critical optical components whose core principle is to control specific wavelengths of light through selection effects (absorption, interference, diffraction, etc.), thereby achieving functions such as spectral selection and intensity control.

1.Core Working Principles of Filters
Different types of filters operate on distinct mechanisms, primarily categorized into three classes:

1. 1.1. Absorption Filters: Based on selective absorption of materials. They utilize the light-absorption characteristics of filter materials (e.g., glass doped with metal oxides, organic dye films) for specific wavelengths. Unwanted light is converted into heat and dissipated, allowing only the target wavelength to pass through.

2. 1.2. Interference Filters: Based on thin-film interference effects. Multiple layers of high-refractive-index (e.g., titanium dioxide) and low-refractive-index (e.g., silicon dioxide) films are alternately coated onto a glass substrate. They achieve precise filtering by utilizing constructive interference (to pass the target wavelength) and destructive interference (to suppress other wavelengths) resulting from reflection and transmission of light at the film interfaces.

3. 1.3. Diffraction Filters: Based on the principle of light separation by diffraction. Periodic grating structures (e.g., stripes, grids) are etched onto the substrate surface. They leverage the phenomenon of diffraction (where light of different wavelengths diffracts at different angles) to decompose composite light into monochromatic light, thereby selecting the target wavelength.

2.Typical Application Scenarios for Filters
Filter applications span numerous fields including optics, electronics, and medicine. Their core function is to solve practical problems through "light filtering":

1. 2.1.Imaging and Photography:

   • (1).Eliminating Stray Light: Polarizing filters (a type of interference filter) can filter reflected light from surfaces (e.g., water, glass glare), making subjects in photos clearer.

   • (2).Controlling Exposure: Neutral density filters (ND filters, absorption or interference type) uniformly attenuate light intensity across the spectrum, enabling long exposures in bright conditions.

   • (3).Color Correction: Using color filters to adjust image tones.

2. 2.2.Biological and Medical Detection:

   • (1).Fluorescence Microscopy: Utilizes a set of three filters (excitation filter, dichroic mirror/beam splitter, emission filter) to precisely separate excitation light from fluorescent signals, allowing observation of fluorescently labeled proteins, viruses within cells (e.g., in COVID-19 fluorescence tests).

   • (2).Spectral Analysis: Using near-infrared filters combined with spectrometers to analyze hemoglobin concentration in blood (via light absorption characteristics in the 700-900 nm band), enabling non-invasive blood glucose detection.

3. 2.3.Optical Communication and Laser Technology:

   • (1).Wavelength Division Multiplexing (WDM): In fiber optic communications, interference-type bandpass filters are used to combine optical signals of different wavelengths (e.g., 1310 nm, 1550 nm) into a single fiber for transmission, greatly increasing communication capacity.

   • (2).Laser Protection: Edge filters or notch filters are used in laser systems to allow the working wavelength (e.g., 532 nm green light) to pass while blocking high-energy pump light, protecting operators and equipment.

The fundamental principle of optical filters is the "precise control of light wavelength." Their applications solve the problem of "what light is needed and what light must be excluded" in various scenarios. From everyday photography to advanced aerospace technology, the role of filters permeates the entire process of light generation, transmission, and detection, making them an indispensable "optical switch" in modern optical systems.