A short wave pass filter is an optical filtering device whose core function is to allow short-wavelength light (high-frequency light) to transmit completely while strongly reflecting or absorbing long-wavelength light (low-frequency light). It only permits light below a specific cut-off wavelength to pass through, making it a fundamental category in optical filtering technology (complementary to long wave pass filters [LWP] and band pass filters [BPF]).
II. Key Parameters (Core Indicators for Selection)
Parameter Name
Definition
Importance
Cut-off Wavelength (λc)
The wavelength at which transmittance reaches 50% (or a specified ratio), serving as the critical threshold distinguishing “short waves” from “long waves” (e.g., λc=500nm means light below 500nm is transmitted, while light above is blocked).
★★★★★ (Core basis for selection)
Transmittance (T)
Transmission efficiency of target short-wavelength light (ideal value >90%, high-end devices can reach over 95%).
★★★★★ (Affects optical signal intensity)
Cut-off Steepness
The width of the wavelength range from high transmittance (short waves) to low transmittance (long waves) (narrower range indicates higher filtering precision).
★★★★☆ (Essential for precision optical scenarios)
Blocking Degree (OD Value)
Ability to block long-wavelength light (OD value = -log10(transmittance), e.g., OD4 means long-wave transmittance <0.01%).
★★★★☆ (Avoids stray light interference)
Operating Waveband
Applicable spectral range (e.g., ultraviolet, visible, near-infrared).
★★★★☆ (Matches usage scenarios)
Size and Installation
Physical dimensions and mounting method of the filter (e.g., threaded, card slot).
★★★☆☆ (Compatible with equipment)
III. Working Principle
Based on optical thin-film interference and absorption characteristics:
Deposit multiple alternating layers of high-refractive-index/low-refractive-index thin films (e.g., TiO₂, SiO₂) on an optical substrate (e.g., glass, quartz);
Short-wavelength light undergoes constructive interference in the thin-film layers, with most energy transmitting through the substrate;
Long-wavelength light undergoes destructive interference or is absorbed by specific materials (e.g., colored glass), preventing transmission;
Precisely control the cut-off wavelength and filtering performance by adjusting the number of film layers, thickness, and materials.
IV. Common Types and Application Scenarios
1. Classification by Operating Waveband
2. Classification by Manufacturing Process
Thin-Film Interference Type: High transmittance and excellent cut-off steepness, suitable for precision optical equipment (e.g., scientific research instruments, high-end cameras);
Absorptive Type: Based on the absorption characteristics of colored glass, cost-effective with low angle dependence, suitable for general optical scenarios (e.g., security monitoring, teaching experiments).
V. Selection Notes
Match Cut-off Wavelength: Clearly define the “short waves to be transmitted” and “long waves to be blocked” based on requirements, and select a λc that accurately covers the critical value (e.g., to transmit blue light below 450nm and block green/red light above 500nm, a SWPF with λc=480nm is optional);
Consider Environmental Adaptability: For high-temperature and high-humidity environments, choose high-temperature-resistant and corrosion-resistant films (e.g., hard coating) to prevent film peeling;
Account for Incident Angle: The cut-off wavelength of thin-film interference SWPFs shifts blueward (toward shorter wavelengths) as the incident angle increases; calibration is required in large-angle incidence scenarios;
Balance Cost and Performance: Select thin-film type with high transmittance and high blocking degree for precision scenarios (e.g., scientific research, medical care); choose cost-effective absorptive type for general scenarios;
Adapt to Equipment Interface: Confirm that the filter size and mounting method are compatible with the equipment (e.g., camera lens thread specifications, optical platform card slot dimensions).
VI. Supplementary Information
Difference from Long Wave Pass Filter (LWP): SWPF transmits short waves and blocks long waves, while LWP does the opposite (transmits long waves and blocks short waves). The two can be combined to achieve narrower band filtering;
Common Brands: Foreign brands (e.g., Edmund Optics, Thorlabs) and domestic brands (e.g., Sunny Optical, China Electronics Technology Group Corporation No.44 Research Institute) are available, and can be selected based on budget and precision requirements;
Maintenance Tips: Avoid touching the coated surface with hands; clean with a lint-free cloth dipped in anhydrous ethanol and wipe gently; store in a dry, dark environment.
