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A Guide of Filter Coating Types and Selections

I. Core Coating Types and Functions
The functional diversity of filters primarily stems from combinations of different coating layers in their design:

1. 1.Anti-Reflection (AR) Coating:

   • Function: Primary goal is to reduce reflection loss at optical surfaces for specific wavelengths or bands, significantly enhancing transmission. It is the most widely used base coating.

   • Principle: Utilizes interference cancellation from single or multilayer films to make reflected light waves cancel each other out.

   • Application: Found on surfaces of almost all high-quality optical components (lenses, prisms, windows, sensor protectors). Single-layer AR coatings (e.g., MgF₂) are common for central visible wavelengths, while broadband AR coatings (e.g., multilayer broadband AR) cover wider spectra (e.g., entire visible or NIR). Key specs include minimum reflectivity, operating bandwidth, incidence angle tolerance, and environmental durability.

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2. 2.Cut-Off Coating:

   • Function: Forms the core functional layer for spectral "cutting":

     • Shortpass: Transmits light shorter than a specific cutoff wavelength, reflecting/absorbing longer wavelengths.

     • Longpass: Transmits light longer than a specific cutoff wavelength, reflecting/absorbing shorter wavelengths.

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3. 3.Bandpass Coating: Allows a specific wavelength range (passband) to transmit while strongly reflecting/absorbing light on both sides.

4. 4.Notch Coating: Strongly reflects/absorbs a very narrow/wide specific wavelength range (notch wavelength) while transmitting others.

   • Principle: Uses meticulously designed multilayer stacks of alternating high/low refractive index materials (e.g., TiO₂/SiO₂, Ta₂O₅/SiO₂, Nb₂O₅/SiO₂) to create high reflectivity (cut-off) in specific regions via interference.

   • Application: Absolute core of various functional filters (fluorescence filters, laser protection glasses, color wheels, spectrometer components, biosensing, machine vision). Specs are stringent: center wavelength, cutoff wavelength, passband width, average passband transmission, cutoff depth, cutoff width, passband ripple, edge steepness, angular sensitivity, temperature stability, laser damage threshold (LDT).

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5. 5.Beamsplitter Coating:

   • Function: Splits incident light by wavelength or energy ratio into two or more beams.

   • Types:

     • Dichroic: Reflects one band, transmits another (a special cut-off application).

     • Neutral: Reflects and transmits incident light in a fixed ratio (e.g., 50:50, 70:30) over a broad spectrum, maintaining spectral neutrality.

   • Application: Beamsplitters/combiners in microscopes, projectors, laser systems, optical instruments. Key specs: splitting ratio accuracy, spectral flatness, polarization dependence, angular dependence, aperture uniformity, LDT.

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6. 6.High-Reflective (HR) Coating:

   • Function: Achieves very high reflectivity (>99.9%) at specific wavelengths or broad bands.

   • Principle: Uses multilayer periodic structures (DBR) of high-index-contrast materials (e.g., TiO₂/SiO₂) or metal films (Al, Ag, Au).

   • Application: Laser cavity mirrors, interferometer mirrors, reflective filters. Key specs: peak reflectivity, bandwidth, absorption/scatter loss, LDT, stress control.

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7. 7.Metal Coating:

   • Function: Utilizes inherent broad-spectrum absorption/reflection of metals (Al for UV-Vis, Ag for Vis-NIR, Au for IR, Cr for absorption).

   • Application: Base reflective layers (often with protective layers), neutral density filters, beam blocks, apertures. Key specs: reflection/absorption curve, stability (oxidation resistance), surface finish, adhesion.

II. Coating Structure & Materials

• 1.Structure: Single-layer (simple), multilayer (most common, alternating materials), graded-index (refractive index variation for broader bandwidth/angle performance).

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• 2.Materials: Dielectrics (SiO₂, MgF₂, TiO₂, Ta₂O₅, Nb₂O₅, ZnS, ZnSe – low loss) and metals (Al, Ag, Au, Cr – broad reflection/absorption). Selection based on refractive index, transparency range, hardness, stability, stress, cost.

III. Design Theory & Manufacturing

• 1.Design Theory: Based on thin-film interference (Maxwell's equations). Methods: optical admittance matching, vector method, computer optimization (Needle, genetic algorithm) with software (TFCalc, Essential Macleod).

• 2.Manufacturing:

  • Physical Vapor Deposition (PVD):

    • Thermal Evaporation: Traditional, lower cost, for medium/small batches.

    • Magnetron Sputtering: Mainstream high-end process – dense layers, good adhesion, uniformity, repeatability.

    • Ion Beam Sputtering: Highest precision, best quality (low scatter/absorption, high LDT), expensive.

  • Chemical Vapor Deposition (CVD): Less common in optics.

  • Process Control: Thickness monitoring (optical, quartz crystal), deposition rate, vacuum/gas control, temperature, cleanliness.

IV. Key Application Standards
Performance must meet stringent practical requirements:

1. 1.Spectral Performance: Center/cutoff wavelength accuracy, bandwidth, transmission (average/peak), cutoff depth (OD value, e.g., OD4 = transmission <0.01%), edge steepness, passband ripple, background suppression.

2. 2.Optical Uniformity: Consistency across aperture.

3. 3.Angular Properties: Spectral shift with angle (blue shift, bandwidth change).

4. 4.Polarization Properties: Response difference to S/P polarization (critical at oblique incidence).

5. 5.Surface Quality: Surface finish (scratch-dig, e.g., 60-40), flatness (e.g., λ/4), wavefront distortion.

6. 6.Environmental Stability: Temperature shift (ppm/°C), humidity resistance (85°C/85% RH), mechanical durability (adhesion, abrasion), chemical resistance, LDT (for laser apps, depends on wavelength/pulse width).

7. 7.Substrate & Size: Glass type (e.g., fused silica, sapphire), thickness, dimensions, beveling, clear aperture.

V. Applications & Coating Selection
Coating technology permeates all optical fields:

• 1.Imaging & Photography: Lens AR, UV/IR cut filters, ND filters, color filters, polarizers.

• 2.Display: Color filters in LCD/OLED, dichroic mirrors/color wheels in projectors.

• 3.Biomedical/Life Sciences: Fluorescence microscopy (excitation/emission filters, dichroics), flow cytometers, analyzers.

• 4.Laser Technology: Cavity mirrors, line separation filters, laser safety glasses (high OD), harmonic generation crystal coatings. LDT is critical.

• 5.Spectroscopy: Grating/prism AR coatings; bandpass filters for wavelength selection; ND filters for intensity control.

• 6.Machine Vision/Industrial Inspection: Color/polarization filters for contrast enhancement/feature recognition.

• 7.Astronomy: Narrowband filters for light pollution suppression/specific emission lines; solar filters (high safety).

• 8.Communications/Sensing: DWDM filters in fiber optics; filtering in fiber sensors.

• 9.Security/Defense: IR AR coatings for thermal imaging; IR filters in missile seekers; laser protection filters (high OD/LDT).