What's the best longpass for low-light NIR imaging?

The best longpass filter for low-light near-infrared (NIR) imaging depends on your illumination source. For systems using 850 nm LEDs, a longpass filter with an ~800 nm cut-on is ideal, while for 940 nm LEDs, a longpass filter with a ~915–920 nm cut-on provides the best performance.

What is Low-Light NIR Imaging?

Near-infrared (NIR) light, which spans from approximately 700 nm to 1100 nm, is visible to standard silicon camera sensors but mostly invisible to the human eye. Most 'low-light NIR imaging' is actually active imaging. This means you illuminate a scene with your own NIR light source, such as 850 nm or 940 nm LEDs, and use a filter on the camera to see only that light. The result is a clean, clear image at night that is free from the interference of visible ambient light like headlights or city glow.

• Active NIR Imaging: This is the most common method. You flood the scene with IR light and use a filter to isolate that wavelength for the camera. This is used in most CCTV and driver monitoring systems.
• Passive NIR Imaging: This less common technique relies on ambient NIR light from sources like moonlight and skyglow. It typically requires long exposures or image intensifiers to produce a usable image. This is distinct from thermal imaging, which uses different sensors to detect mid- or long-wave infrared radiation.

Why Use a Filter for NIR Imaging?

A filter is essential for achieving high-quality images in active NIR systems by improving the signal-to-noise ratio (SNR). It works by blocking unwanted visible light from reaching the sensor, ensuring the camera primarily captures the reflection from your specific IR illuminator.

• Longpass (LP) Filters: These filters block visible light (typically 400–700 nm) while allowing all longer, NIR wavelengths to pass through. This is effective at suppressing ambient light pollution.
• Bandpass (BP) Filters: These are even more restrictive, passing only a very narrow band of light that is centered on your LED's wavelength. For nighttime applications, a bandpass filter often yields the highest possible SNR.

How to Choose the Right Longpass Filter

To select the optimal filter, consider the following factors in your system design.

• Match the Illuminator: Your filter's cut-on wavelength should be matched to your light source. Use an LP780–LP800 filter for 850 nm LEDs and an LP915–LP920 filter for 940 nm LEDs.
• Maximize Signal Throughput: Look for a filter with an average transmission of 95% or higher in your passband. Double-sided anti-reflection (AR) coatings are critical for maximizing signal.
• Block Unwanted Light: To eliminate stray visible light, the filter should provide high optical density (OD). Aim for OD ≥ 4, and for best results in low-light scenes, specify OD ≥ 6 in the visible spectrum.
• Consider Lens and Angle: Dielectric (interference) filters are angle-sensitive; the cut-on wavelength shifts to shorter wavelengths at higher angles of incidence (AOI). For systems with fast lenses (f/2.0 or lower) or a wide field of view, this can cause light leakage. In these cases, an absorptive glass filter is a better choice as its performance is insensitive to angle.

Recommended Longpass Filters

For 850 nm Systems

• Best for SNR: A dielectric LP800 filter with OD ≥ 6 in the visible range and ≥95% transmission from 820–1,100 nm. This is best for systems with an AOI of ±10° or less.
• For Fast Lenses/Wide AOI: An absorptive RG715 or Hoya R72 glass filter with a NIR AR coating. This minimizes angle shift and ghosting.

For 940 nm Systems

• Best for SNR: A dielectric LP915–LP920 filter with OD ≥ 6 in the visible range and ≥95% transmission from 930–1,050 nm.
• For Fast Lenses/Wide AOI: An absorptive RG850 glass filter with a NIR AR coating.

Simple Rules of Thumb

• Use a dielectric interference filter when your lens is not excessively fast and you need the steepest cut-on edge and maximum light transmission.
• Use an absorptive glass filter when your system has wide angles, a fast lens (f/≤2.0), or is sensitive to ghosting and flare.
• If ambient light is very strong (e.g., daytime or bright indoor lighting), a bandpass filter matched to your LED will likely outperform any longpass filter.