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Infrared (IR) Illumination

Industrial machine vision station with near-infrared 850 nm LED bar illuminator imaging dark plastic packaging with bandpass filter at the camera for ambient rejection

Near-Infrared 850/940 nm Imaging for Plastic Penetration and Ambient Rejection

  • 850 nm and 940 nm wavelengths compatible with standard silicon CMOS/CCD sensors at reduced quantum efficiency.
  • Penetrates many plastics and provides colour-invariant imaging across mixed-colour production lines.
  • Bandpass filter at the camera rejects visible ambient light — enables outdoor and uncontrolled lighting installations.
  • 850 nm faintly visible as red glow; 940 nm completely invisible for operator-distraction-free environments.
  • Visible-NIR lenses (400–1000 nm) recommended for high-performance NIR imaging to maintain focus across the band.
  • Backlit transmission mode through translucent containers and packaging delivered by the LED Backlight family.

Near-infrared LED illumination in the range of 850 to 940 nm is one of the most widely used spectral configurations in industrial machine vision. Its main advantages are penetration through many plastics, robust insensitivity to colour variations of the target, and effective rejection of ambient visible light through inexpensive bandpass filters at the camera. Near-infrared illumination is invisible to the human eye, which is also a practical advantage in environments where operator distraction from bright visible illumination must be avoided.

Working Principle of Near-Infrared LED Sources

Near-infrared LEDs are based on gallium arsenide and related semiconductor compositions that emit at wavelengths just above the visible range. The most common industrial wavelengths are 850 nm and 940 nm, chosen for the availability of high-power LED chips, the compatibility with standard silicon CMOS and CCD sensors, and the moderate atmospheric absorption that allows efficient transmission over short distances.

Standard silicon sensors retain useful quantum efficiency up to approximately 1000 nm, although the response declines significantly above 900 nm. At 850 nm, sensor sensitivity is typically 30 to 50 percent of the visible peak. At 940 nm, sensitivity drops to 10 to 20 percent, which requires longer exposure or higher illuminator intensity to compensate. Specialised NIR-enhanced sensors with thicker silicon or alternative pixel architectures extend the usable response into the 1000 nm range.

Optical Behavior of Materials Under NIR

Many materials that appear opaque or strongly coloured under visible light become semi-transparent or colour-neutral under near-infrared illumination. This is particularly true of polymers, where additives and dyes that absorb in the visible range often transmit in the near-infrared. As a result, NIR illumination provides invariant imaging across plastic products of different colours, simplifying inspection of mixed-colour production lines.

Typical Industrial Applications

Near-infrared LED illumination is widely used for inspection of dark or coloured plastic products on production lines where colour variations would otherwise complicate the algorithm; reading of codes and labels through transparent or translucent packaging that obscures visible-light imaging; inspection of contents inside semi-opaque containers such as filled bottles and blister packs; verification of seals and bonds on packaged products; quality control in environments with strong, uncontrolled ambient lighting, where NIR with bandpass filtering rejects visible interference; inspection in environments where visible LEDs would distract operators or interfere with other vision systems; and any application requiring penetration through plastics or strong ambient light rejection. NIR variants are available across the LED Ring Illuminators, LED Bar Illuminators and LED Panel Illuminators families.

Selection Criteria and Design Considerations

The choice between 850 nm and 940 nm depends primarily on sensor sensitivity and ambient light considerations. At 850 nm, the higher sensor response allows shorter exposure times and lower LED power, but a faint red glow may be visible from the LED chip itself, which can disturb operators. At 940 nm, the LED emission is completely invisible, but lower sensor response requires more LED power or longer exposure to achieve equivalent image quality.

The bandpass filter at the camera should be matched to the LED wavelength, with a passband typically 10 to 30 nm wide around the centre. Narrower filters provide better ambient light rejection at the cost of more critical alignment and slightly higher cost. The combination of a bandpass filter and monochromatic NIR LED illumination enables vision inspection in environments with very strong ambient lighting, including outdoor installations.

Lens Compatibility for NIR Illumination

Standard machine vision lenses corrected for the visible range often exhibit slight focus shift in the near-infrared, requiring lens refocusing when transitioning from visible to NIR imaging. Visible-NIR lenses optimised for the 400 to 1000 nm range are designed to maintain consistent focus across this band and are recommended for critical NIR applications.

Integration and Limitations

Near-infrared LED illuminators integrate identically to visible versions in all standard mechanical configurations, with the same diffusers, polarisers and beamsplitters. Power consumption and heat dissipation are comparable to visible LEDs at equivalent output flux. Non-standard NIR wavelengths or high-power NIR backlights are typically engineered within the Custom LED Illuminators portfolio, while transmitted-mode NIR inspection is delivered by the LED Backlight Illuminators family.

The principal limitation of NIR illumination is the loss of colour information, which makes it incompatible with applications requiring colour discrimination. NIR is also subject to slightly higher photon shot noise than visible illumination at equivalent image brightness, because the reduced sensor quantum efficiency requires higher photon flux to achieve the same digital signal. For applications where colour information is essential, white or RGB-switchable illumination should be used instead. For applications where invariance to target colour, ambient light rejection or plastic penetration is required, NIR is the most effective and cost-efficient choice.

RODER Vision Near-Infrared LED Illuminators

RODER Vision manufactures near-infrared LED illuminators at 850 nm and 940 nm across the full geometry portfolio for industrial vision applications requiring plastic penetration, colour invariance and ambient light rejection.

For high-speed NIR strobed inspection lines, the RODER catalogue includes dedicated LED drivers and electronic controllers compatible with industrial machine vision controllers and PLCs.