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Cut-off filter coatings are designed to achieve high transmittance within a specific wavelength band while exhibiting a sudden transition to high reflection (suppression) for light outside this band. Under vertical incidence conditions, filters that suppress the short-wave region and transmit the long-wave region are termed long-wave pass (LWP) filters; conversely, those that suppress the long-wave region and transmit the short-wave region are called short-wave pass (SWP) filters.
A cut-off filter refers to an optical component that can eliminate all long-wave or short-wave light from composite light, retaining only the desired wavelength range. Generally divided into two types: short-wave cut-off filters and long-wave cut-off filters, their principle primarily relies on the interference effects of optical coatings and the transmission/reflection characteristics of specific wavelengths on the filter. Cut-off filters typically employ multilayer coating designs. These layers have specific refractive indices and thicknesses. When light passes through the filter, different wavelengths interact through interference between the layers—some wavelengths are reflected while others are transmitted. This mechanism allows maximum reduction or complete elimination of light transmission at specific wavelengths
Transmission and Reflection Properties
Cut-off filters exhibit high transmission within a specific wavelength range, while rapidly transitioning to high reflectivity for wavelengths outside this range. Specifically:
Long-Pass Filters: Suppress shorter wavelengths and allow longer wavelengths to pass through.
Short-Pass Filters: Suppress longer wavelengths and allow shorter wavelengths to pass through.
Cut-Off Wavelength
The cut-off wavelength marks the point where the transmission curve begins to drop (or rise), indicating the boundary between transmission and reflection.
Passband Transmittance
This refers to the spectral width and average transmittance of the high-transmission band, reflecting the filter's light transmission performance in a specific wavelength region.
Stopband Optical Density (OD)
This indicates the spectral width and average OD value of the low-transmission band. A higher OD value means lower transmittance in the stopband/cut-off region, resulting in better filtering performance.
Structural Forms
Cut-off filters typically have two structural forms:
Bandpass Filters: Formed by overlapping long-pass and short-pass filter coatings, creating a specific bandpass region.
Fabry-Pérot Interferometer-Based Filters: These can achieve narrower passbands but often require additional filters to broaden the stopband and deepen the cut-off region.
A cut-off filter is an optical filter designed to block light within a specific wavelength range while allowing only the desired light to pass. Widely used in optical instruments, lasers, and lighting systems, cut-off filters serve the following key applications:
Optical Instruments: In microscopes, telescopes, spectrometers, and other optical devices, cut-off filters help eliminate stray light, enhancing imaging quality.
Lasers: In laser systems, cut-off filters prevent interference from non-laser light, improving output power and stability.
Lighting Systems: In illumination setups, cut-off filters remove ultraviolet (UV) and infrared (IR) radiation, protecting objects from potential light-induced damage.
Selecting the appropriate cut-off filter requires consideration of the following factors:
Cut-off Wavelength: Choose the suitable cut-off wavelength based on specific application requirements.
Transmittance: Select the optimal transmittance level according to desired optical performance and budget constraints.
Coating Material: Choose the appropriate thin-film material based on optical performance needs and environmental conditions.
Size and Shape: Select the right dimensions and shape to match the application requirements.
In summary, a cut-off filter is an optical filter widely used in fields such as optical instruments, lasers, and lighting systems. Its working principle relies on a specific optical structure that enables the filtering of light within a designated wavelength range. When selecting a suitable cut-off filter, factors such as cut-off wavelength, transmittance, coating material, size, and shape must be carefully considered.
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