940nm Filter Selection Guide: Configuration Analysis for Typical Applications
I. Security Surveillance Night Vision Systems
Application Scenario:In covert surveillance scenarios (e.g., banks, sensitive facilities), 940nm infrared LEDs serve as invisible light sources (no red glow phenomenon), working with filters to achieve clear night imaging. The system must accurately capture infrared signals amid natural light interference while preventing visible light from contaminating the image.Filter Configuration Requirements:
1. Central Wavelength & Bandwidth:
Strictly match the 940nm light source with a narrow-band pass design (bandwidth ±10nm) to ensure only the target wavelength passes through, suppressing interference from adjacent bands (e.g., 850nm light source leakage).
2. Transmittance & Cut-off Depth:
- Passband transmittance should be ≥80% to compensate for the lower luminous efficiency of 940nm LEDs (requiring nearly twice the light output to achieve the same brightness as 850nm for photosensitive sensors).
- Stopband cut-off depth must reach OD5 (transmittance <0.001% in the visible spectrum, 400-700nm) to effectively isolate ambient light (e.g., moonlight, streetlights) and improve the signal-to-noise ratio.
3. Substrate & Coating Technology:
- Use fused silica or B270 glass substrates to balance mechanical strength and infrared transmittance.
- Adopt multi-layer hard coating technology to enhance scratch resistance and weatherability, suitable for complex outdoor environments.
Selection Logic & Problem Solving:
- Narrow-band Design: The ±10nm bandwidth precisely filters 940nm infrared light, preventing ambient visible light from entering the sensor and solving issues like white noise or excessive grain in night vision images.
- High Cut-off Depth: OD5-level stopband suppression (full-band isolation from 300-1100nm) ensures pure infrared imaging under strong natural light interference (e.g., during dusk transitions), avoiding the risk of red glow exposure that traditional 850nm filters may cause.
- Material Optimization: Hard coating technology minimizes film layer detachment during long-term use, ensuring stable performance in high-temperature and high-humidity environments and reducing maintenance costs.
II. Biometric and Health Monitoring Devices
Application Scenario:
In the heart rate monitoring module of wearable devices (e.g., smartwatches), 940nm infrared light penetrates the skin surface to calculate heart rate by detecting changes in blood absorption of infrared light. The system must maintain high-precision measurements in dynamic scenarios with varying light conditions and movements.
Filter Configuration Requirements:
1. Wavelength Compatibility:
- For pure 940nm applications, use a single-band pass design with a relaxed bandwidth of ±20nm to accommodate wavelength fluctuations of different light sources (e.g., LED central wavelength tolerance ±3nm).
2. Transmittance & Anti-Reflection:
- Passband transmittance should exceed 90% to enhance light signal strength and compensate for infrared light attenuation in skin tissue (penetration depth ~1-2mm).
- Double-sided anti-reflective (AR) coatings reduce surface reflectivity to <0.2%, minimizing optical path loss and improving signal quality.
3. Size & Integration:
- Employ ultra-thin substrates (e.g., 0.5mm float glass) to fit compact sensor modules.
- Edge blackening treatment (e.g., laser cutting + black ink coating) eliminates stray light interference.
Selection Logic & Problem Solving:
- Broad-band Design: The ±20nm bandwidth covers the wavelength drift range of 940nm light sources (actual emission may vary between 930-950nm), preventing signal loss due to light source individual differences and resolving heart rate data instability during motion.
- High Transmittance & Anti-Reflection: AR coatings and high-transmittance films ensure sufficient light intensity even in dynamic environments (e.g., sweat covering the sensor during running), avoiding measurement errors caused by signal attenuation.
- Miniaturized Design: Ultra-thin substrates and edge blackening enable lightweight device integration (e.g., watch thickness <10mm) while eliminating internal reflective light interference with photodiodes, enhancing data stability.
III. Cross-Scenario Selection Principles
1. Light Source Matching Priority:
Select filters based on the actual spectral characteristics of 940nm LEDs or lasers (e.g., full width at half maximum, central wavelength deviation). Avoid excessive bandwidth narrowing that could cause signal loss (e.g., filter bandwidth should be ≥25nm if the LED's FWHM is 20nm).
2. Environmental Adaptability:
- For outdoor devices, choose UV-resistant substrates (e.g., fused silica) to prevent film layer aging from long-term light exposure.
- In high-temperature scenarios (e.g., industrial inspection), verify the filter's thermal stability (e.g., operating temperature range: -40°C to +80°C).
3. Cost-Performance Balance:
Narrow-band filters (e.g., ±10nm) are higher in cost but suitable for high-precision applications; broad-band filters (e.g., ±40nm) offer better cost-effectiveness for applications with lower wavelength accuracy requirements (e.g., smart home infrared sensing).
By following these configurations, 940nm filters enable efficient signal filtering in security, health monitoring, and other fields, addressing core challenges like environmental interference and device miniaturization while balancing long-term reliability and cost control.
