Best Bandpass Filter Bandwidth for 850 nm NIR LED Imaging

Choosing the right near-infrared (NIR) bandpass filter for 850 nm LED imaging directly impacts your image clarity, signal-to-noise ratio, and how well you reject ambient light. Here is an easy-to-follow guide for engineers, integrators, and buyers about selecting the best filter bandwidth for this task.

Most 850 nm LEDs emit light with a full-width-half-maximum (FWHM) between 26 and 50 nm. A bandpass filter with about 40–50 nm FWHM, centered at 850 nm, passes the bulk of your LED's energy but still blocks a significant amount of unwanted background and ambient light. This is a great starting point for most setups.

• Controlled Indoor Scenes (little ambient IR):
  Use a wide bandpass (e.g., 820–910 nm, FWHM ≈ 160 nm) to maximize light transmission and tolerance to LED or filter angle shifts. Broad filters are ideal for labs, factories, or any setup with minimal sunlight.
• Mixed Light Environments or Moderate Ambient IR:
  Go medium (40–50 nm FWHM). This matches typical 850 nm LEDs well and rejects more ambient spill than broader filters. Perfect for spaces with windows, skylights, or multiple artificial IR sources.
• Harsh Ambient IR (sunlight), Multiple IR Sources, or Tight Spectral Needs:
  Choose a narrow band (10–25 nm FWHM). These filters offer the best rejection of stray IR or overlapping sources, but may be more expensive and require strict mounting (keep filter angle as close to 0° as possible to prevent cutting off your LED's emission).

Why Not Always Use a Narrow Filter?
850 nm LEDs are not single-wavelength; many have FWHM of 26–50 nm. If your filter is too narrow, you risk blocking a chunk of your LED's signal. Also, LEDs shift with temperature (about 0.2 nm/°C), and interference filters blue-shift with angle, so mid-width filters give more tolerance.

Quick Pick: If you are unsure, choose a 40 nm FWHM filter at 850 nm and keep it close to normal incidence (perpendicular to the light). Then adjust smaller for more blocking or wider for brighter images, as needed.

• Security, CCTV & Machine Vision: 850 nm gives brighter images and longer range for silicon sensors versus 940 nm, with a faint red glow from the illuminator.
• Biometrics and Tracking: Common in iris, face, or eye tracking for access control or analytics, riding near CMOS detector sensitivity peaks.
• Automotive In-Cabin Sensing: Used in driver-monitoring and interior imaging, thanks to high sensor responsivity and true night vision.
• Industrial Inspection: 850 nm NIR penetrates films and inks, exposing hidden features for automated inspection.
• General Night-Vision: Powers many common IR illuminators and torches for off-the-shelf compatibility.

• Keep filter angle (AOI) as close to 0° as possible; interference filters blue-shift at higher angles.
• Leave thermal margin: avoid ultra-narrow filters if LED or setup can change temperature a lot.
• If you need total 'invisible' IR, consider 940 nm LEDs/filters—but expect shorter range and dimmer images versus 850 nm.

• Around 40–50 nm FWHM at 850 nm is the 'Goldilocks' choice for most NIR LED imaging.
• Go narrower only when you must block strong backgrounds or need spectral isolation; go wider only if you are certain ambient IR will not cause problems.
• If you know your LED's FWHM and your scene lighting, you can fine-tune the exact bandwidth for best results.