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Emission Spectrum and Wavelength

Industrial machine vision station illustrating LED emission spectra: UV, visible white and monochromatic colours, NIR and SWIR bands for vision inspection

Selecting the Right Wavelength and Spectral Configuration for Industrial Vision

  • Wavelength selection generates contrast by exploiting the spectral reflectance and absorption of every material in the scene.
  • White light serves colour-critical inspection; monochromatic maximises contrast on coloured features by colour-wheel pairing.
  • Near-infrared (850/940 nm) penetrates plastics and rejects ambient light through bandpass filtering at the camera.
  • Ultraviolet (365/395 nm) excites fluorescence in adhesives, security marks and surface contaminants.
  • SWIR (900–1700 nm) transmits through silicon and dark plastics; requires InGaAs sensors.
  • Multispectral and RGB-switchable configurations enable runtime wavelength adaptation and material discrimination.

The spectral content of a machine vision illuminator interacts with the spectral reflectance and absorption of every material in the scene, generating contrast that would be invisible under broadband illumination. Selecting the right wavelength, narrowband or broadband, monochromatic or multispectral, is therefore one of the most powerful tools available to the vision engineer. Wavelength selection also affects depth of field, chromatic aberration of the lens, immunity to ambient light, and the response of the camera sensor across the visible, near-infrared and ultraviolet ranges.

Wavelength as a Contrast Engineering Tool

Materials reflect and absorb light differently at different wavelengths. A pigment that appears green to the human eye absorbs strongly in the red and blue and reflects in the green; the same pigment under red illumination appears dark, while under green illumination appears bright. This wavelength-dependent reflectance allows the vision engineer to generate contrast between similar-coloured features that would be indistinguishable under white light.

The principle extends well beyond the visible. Many plastics that appear opaque in the visible spectrum become transparent under near-infrared illumination, allowing inspection of internal features. Many adhesives that appear colourless under visible light fluoresce intensely under ultraviolet excitation, allowing detection of bonding and sealing defects. Wavelength selection effectively expands the dimensional space in which a vision system operates, providing degrees of freedom that no other illumination parameter can match.

Spectral Configurations Covered in This Section

White Light

White LED illumination provides broadband visible emission suitable for colour inspection, general imaging and applications requiring natural colour rendition of the inspected object. White LEDs are available in different colour temperatures (cool, neutral, warm) to match the target application. Standard white emission is available across the full LED Ring Illuminators, LED Bar Illuminators and LED Panel Illuminators portfolios.

Monochromatic Colored Light

Monochromatic LED illumination in red, green, blue or amber generates selective contrast by exploiting the spectral reflectance of coloured targets. A monochromatic illuminator at the right wavelength can produce contrast on coloured features that is impossible to obtain with white light.

Infrared (IR) Illumination

Near-infrared LED illumination at 850 nm and 940 nm penetrates many plastics, reduces influence from ambient visible light through narrowband filtering, and provides invariant imaging across coloured targets that would appear different under visible light.

Ultraviolet (UV) Illumination

Ultraviolet LED illumination at 365 nm and 395 nm excites fluorescence in adhesives, security marks, surface coatings and contaminants, enabling inspection of features invisible to the human eye under standard lighting. UV-specific geometries are typically engineered within the Custom LED Illuminators portfolio.

Short-Wave Infrared (SWIR)

Short-wave infrared illumination between roughly 900 nm and 1700 nm enables inspection through materials that are opaque to visible light, including silicon wafers, dark plastics and packaged products with non-transparent outer layers.

Multispectral and Hyperspectral Light

Multispectral illuminators combine multiple discrete wavelengths in a single fixture, enabling sequential or simultaneous imaging at different bands for material discrimination, defect characterisation and chemical-physical analysis.

RGB Switchable Illumination

RGB switchable illuminators integrate independently driven red, green and blue LEDs in the same fixture, allowing the inspection wavelength to be selected dynamically at runtime to optimise contrast on different targets.

Sensor Sensitivity and Wavelength Selection

The choice of wavelength must always consider the spectral sensitivity of the camera sensor. CMOS and CCD sensors typically peak around 500 to 600 nm in the visible range and decline rapidly above 900 nm. The use of near-infrared illumination above 900 nm therefore requires careful consideration of sensor responsivity and exposure time. Specialised sensors (silicon-enhanced NIR, InGaAs for SWIR, UV-enhanced for sub-400 nm) extend the usable spectral range at higher cost.

The lens must also be considered. Standard machine vision lenses are corrected for the visible spectrum and may exhibit significant chromatic aberration outside this range. Visible-NIR lenses optimised for the 400-1000 nm range are required for high-performance NIR applications. UV-grade quartz or fused silica lenses are required for sub-400 nm illumination, where standard glass lenses become absorbing.

Combining Wavelength with Other Parameters

Wavelength interacts with all the other illumination parameters: geometry, diffusion, angle of incidence and operating mode. Monochromatic illumination is more easily collimated than white because there is no chromatic dispersion in the collimating optics. Narrowband illumination through bandpass filters at the camera provides effective ambient light rejection, which is essential for installations near windows or under variable factory lighting. Pulsed monochromatic illumination at high power density delivers the photon flux required for high-speed inspection.

The dedicated pages in this section examine each spectral configuration in detail, including material interaction, sensor compatibility, filter requirements and typical industrial use cases that exploit the unique characteristics of each wavelength range.

RODER Vision LED Spectral Illuminators

RODER Vision manufactures LED illuminators across the full industrial spectral range, with standard white, monochromatic and near-infrared variants available throughout the geometry portfolio, and application-specific UV, SWIR and multispectral configurations engineered on demand.

For multispectral and pulsed wavelength-switched inspection, the RODER catalogue includes dedicated LED drivers and electronic controllers with independent channel control compatible with industrial machine vision controllers and PLCs.