Narrowband Coloured Emission for Selective Contrast on Pigmented Targets
- Narrowband emission (20–40 nm FWHM) in red, green, blue or amber wavelengths matched to the target colour reflectance.
- Colour-wheel pairing rule: maximum contrast achieved by illuminating with the complementary colour of the target pigment.
- Best fit for coloured printed codes, PCBs, pharmaceutical tablets, security marks and colour-coded assemblies.
- Bandpass filter at the camera matched to LED wavelength delivers superior ambient light rejection.
- Custom wavelengths (730 nm deep red, 590 nm amber) available for specialised contrast requirements.
- Requires monochrome camera — colour-discrimination applications need white or RGB-switchable configurations instead.
Monochromatic LED illumination uses narrowband emission in a single colour (red, green, blue, amber or other) to generate selective contrast on coloured targets. By exploiting the spectral reflectance and absorption properties of pigments, inks, coatings and natural materials, monochromatic light can produce contrast on coloured features that is impossible to obtain with broadband white illumination. The principle is straightforward: a coloured surface appears bright under illumination matching its reflectance peak and dark under illumination at its absorption peak.
Working Principle of Monochromatic LED Sources
A monochromatic LED emits over a relatively narrow spectral band, typically 20 to 40 nm wide at half maximum, centred on the nominal wavelength of the chip. Red LEDs are available at 620, 660 and 730 nm; green LEDs at 525, 530 and 565 nm; blue LEDs at 450 and 470 nm; amber LEDs at 590 nm. Each wavelength produces a specific colour interaction with the inspected target, governed by the spectral reflectance curve of the surface material.
When a monochromatic illuminator is used with a monochrome camera, the resulting image encodes the reflectance of the target at the specific illumination wavelength. Features whose reflectance differs at that wavelength produce contrast in the image, while features whose reflectance is identical at that wavelength become invisible. The same target imaged under different monochromatic wavelengths produces different images, each highlighting different features.
Spectral Reflectance and Contrast Optimisation
A practical rule of thumb is that the wavelength of maximum contrast between two coloured features is approximately the wavelength of maximum absorption of one of the two pigments. For example, a green printed mark on a white substrate is best imaged under red illumination, where the green pigment absorbs strongly and produces a dark mark on a bright background. A red mark on the same substrate is best imaged under green or blue illumination, where the red pigment absorbs strongly. The wavelength of optimum contrast is identified through the colour wheel: opposite colours on the wheel produce maximum contrast.
Typical Industrial Applications
Monochromatic LED illumination is the standard choice for reading of coloured printed marks, codes and labels where contrast must be maximised against a coloured background; inspection of multi-layer printed circuit boards where copper traces, solder mask and silkscreen must be selectively enhanced; quality control of coloured pharmaceutical tablets and capsules; verification of dye-based security marks; inspection of agricultural products where ripeness, defects or contamination produce colour-dependent contrast; quality control of textile dyeing for colour uniformity; and any application where the inspection target contains coloured features whose contrast can be enhanced by appropriate wavelength selection. Monochromatic variants are available across the LED Ring Illuminators, LED Bar Illuminators and LED Panel Illuminators families.
Selection Criteria and Design Considerations
The wavelength is the primary selection parameter, chosen to maximise contrast between the feature of interest and the background. Spectral reflectance measurements of the target materials, performed with a calibrated spectrophotometer, provide the most reliable basis for wavelength selection. In the absence of such measurements, an empirical evaluation through imaging under different monochromatic wavelengths is the recommended practical approach.
The bandwidth of the LED emission affects the precision of the contrast selection. Narrower bandwidth (10 to 20 nm) provides more selective contrast but lower flux output. Wider bandwidth (30 to 40 nm) provides higher flux at the cost of slightly reduced selectivity. For most industrial applications the standard LED bandwidth is adequate.
Combination with Bandpass Filters
Monochromatic LED illumination is frequently paired with matching bandpass filters at the camera lens, which transmit only the LED wavelength and reject all other light. This combination provides effective rejection of ambient lighting from overhead fluorescents, sunlight or neighbouring machinery, enabling reliable inspection in environments where ambient light control would otherwise be required.
Integration and Limitations
Monochromatic LED illuminators integrate identically to white versions in all standard mechanical and optical configurations. The principal practical advantage is the ability to combine the illumination with bandpass filtering at the camera for ambient light rejection, a capability that is unavailable with white sources. Non-standard wavelengths or asymmetric spectral configurations are typically engineered within the Custom LED Illuminators portfolio.
The principal limitation of monochromatic illumination is the loss of colour information at the camera. Monochrome cameras must be used to take advantage of monochromatic illumination, which means that any application requiring colour discrimination of the target must use white illumination instead, or must use multispectral or RGB-switchable configurations that combine multiple monochromatic wavelengths in sequence. Monochromatic illumination is also slightly less universal than white in that the wavelength must be selected specifically for each application; a station designed for green-on-white inspection cannot be reused for red-on-white without changing the illuminator.
RODER Vision Monochromatic LED Illuminators
RODER Vision manufactures LED illuminators with monochromatic emission across red, green, blue and amber wavelengths, available throughout the geometry portfolio for selective contrast enhancement in industrial vision inspection.
- Monochromatic ring geometries in red, green, blue and amber — LED Ring Illuminators
- Linear monochromatic configurations for elongated fields — LED Bar Illuminators
- Monochromatic panel geometries for large inspection areas — LED Panel Illuminators
- Compact monochromatic spot sources for targeted contrast — LED Spot Illuminators
- Application-specific wavelengths and asymmetric spectral configurations — Custom LED Illuminators
For monochromatic strobed inspection on high-speed lines, the RODER catalogue includes dedicated LED drivers and electronic controllers compatible with industrial machine vision controllers and PLCs.