Ultraviolet Excitation for Fluorescence Inspection of Adhesives, Coatings and Security Marks
- 365 nm (UV-A) and 395 nm wavelengths excite fluorescence in adhesives, security inks, coatings and biological contaminants.
- Visible-band camera with UV-blocking filter captures the fluorescent re-emission as bright features on dark background.
- Best fit for adhesive verification, security mark inspection, UV-curable coatings, contamination detection.
- 365 nm produces stronger fluorescence; 395 nm offers higher LED efficiency and longer lifetime.
- Operator safety enclosures required to prevent prolonged direct UV exposure of eyes and skin.
- UV-grade quartz optics required only for direct UV imaging; standard glass lenses sufficient for visible fluorescence capture.
Ultraviolet LED illumination opens up a class of inspection applications that are simply impossible under visible light: detection of fluorescent markers, inspection of UV-sensitive coatings, verification of security features and identification of organic contaminants. By emitting at wavelengths below 400 nm, UV LEDs excite fluorescent emission in many materials, which can then be captured by a visible-band camera as bright features against a dark background.
Working Principle of UV LED Sources
Ultraviolet LEDs are based on gallium nitride and related wide-bandgap semiconductors that emit at wavelengths in the near-UV range. The two most common industrial wavelengths are 365 nm (long-wave UV, also called UV-A) and 395 nm (very near-UV, on the boundary with visible violet). Shorter UV wavelengths (mid-UV at 280-315 nm and deep-UV at 200-280 nm) are available with specialised LEDs at significantly higher cost and are reserved for specific applications such as disinfection and certain spectroscopic measurements.
UV illumination at 365 nm and 395 nm primarily generates fluorescence, the phenomenon by which certain materials absorb UV photons and re-emit them at longer (visible) wavelengths. The re-emitted light typically falls in the blue, green or yellow range and can be captured by a standard visible-band camera. A UV-blocking filter at the camera lens removes any residual UV from the image and leaves only the fluorescent signal.
Fluorescent Materials in Industrial Inspection
Many industrial materials exhibit fluorescence under UV excitation. Common examples include cyanoacrylate and epoxy adhesives, which fluoresce intensely and allow detection of bonding lines and excess adhesive; security inks and fibres incorporated into banknotes and tax stamps, which produce distinctive fluorescent patterns; surface coatings such as primers, paints and protective films that include fluorescent tracers for quality control; biological contaminants such as oils and proteins that fluoresce naturally; and various plastics, papers and textiles whose fluorescent additives can be exploited for sorting and identification.
Typical Industrial Applications
UV LED illumination is essential for inspection of adhesive application on assembled products, where presence, position and quantity of adhesive must be verified; quality control of security marks on banknotes, identity documents, pharmaceutical packaging and high-value branded products; verification of UV-curable coatings on glass, plastic and metal substrates; detection of surface contamination by oils, biological materials and process fluids; inspection of phosphor-based displays and signage; quality control of fluorescent textiles and sorted recycling streams; and any application where the feature of interest can be made visible through fluorescence excitation. UV-specific assemblies are engineered within the Custom LED Illuminators portfolio, with compact directional sources also available from the LED Spot Illuminators family.
Selection Criteria and Design Considerations
The wavelength is the first selection parameter. 395 nm UV is more efficient (higher LED wall-plug efficiency, longer lifetime, lower cost) but excites fluorescence less strongly than 365 nm. 365 nm UV produces more intense fluorescence and is the standard choice for security mark inspection and high-sensitivity applications, at the cost of slightly higher cost and shorter LED lifetime. Mid-UV and deep-UV are reserved for specialised applications.
The fluorescence response of the target must be characterised before designing the inspection system. Not all materials fluoresce, and the wavelength and intensity of the fluorescent emission depend on the specific material composition. Spectrofluorimetric measurement or empirical evaluation under candidate UV wavelengths is recommended at the design stage.
Safety and Filter Requirements
UV-A LED illumination at 365 nm and above is considered relatively safe for short-term exposure, but prolonged direct exposure of the eyes or skin must be avoided. Industrial UV illuminators include enclosures and warning labels to prevent operator exposure. Below 320 nm, additional safety measures are required including UV-rated protective eyewear and dedicated enclosures.
The camera must include a UV-blocking filter to prevent residual UV from saturating the visible image. Standard machine vision cameras often include such a filter as part of the IR-cut filter assembly, but UV-specific filters provide more effective UV rejection and are recommended for high-sensitivity fluorescence inspection.
Integration and Limitations
UV LED illuminators require careful selection of optical components. Standard glass lenses absorb significantly in the UV range and may not transmit enough light to excite fluorescence efficiently. UV-grade quartz or fused silica lenses are required for direct UV imaging, but for fluorescence inspection where only the visible re-emission is captured, standard glass lenses can be used on the camera side as long as the UV illumination reaches the target through a UV-transparent path.
The principal limitation of UV illumination is its dependence on the fluorescent response of the target. Materials that do not fluoresce produce no signal and cannot be inspected with this technique. The intensity of fluorescent emission is also typically much lower than the intensity of direct reflection under visible light, which requires longer exposure times or more sensitive cameras. UV illumination is best deployed as a complementary technique on inspection stations that combine multiple illumination modes, rather than as a single-source solution for all inspection tasks.
RODER Vision UV LED Illuminators
RODER Vision engineers application-specific UV LED illuminators at 365 nm and 395 nm for industrial vision inspection of fluorescent features, with directional spot sources and panel geometries available on request.
- Compact directional UV sources for targeted fluorescence excitation — LED Spot Illuminators
- Application-specific UV ring, bar, panel and area assemblies — Custom LED Illuminators
For pulsed UV inspection lines and synchronised fluorescence acquisition, the RODER catalogue includes dedicated LED drivers and electronic controllers compatible with industrial machine vision controllers and PLCs.