UV LED Illumination for Fluorescence Inspection and Surface Defect Detection

Ultraviolet illumination makes two classes of defect visible that a white-light inspection station simply cannot see: sub-micron surface damage on smooth materials, and contamination or coatings that fluoresce. This page covers how UVA LED emitters are used in those inspections, how to pick between 365, 385 and 405 nm, and what to specify when you design the illuminator.

TeraPower LED is the authorized United States distributor of REVOX, Inc., a Japanese manufacturer of high-power industrial LED components and illumination systems. We supply UVA LED emitters as components to manufacturers, system integrators, universities, and research institutions across the United States, and we also supply REVOX finished UV light sources for teams that prefer a complete unit to a bare emitter.

Why UV reveals defects that visible light misses

Shorter wavelengths scatter more strongly from small surface features. REVOX describes its near-UV light source as being suited to "inspection of fine surface defects on materials such as silicon and metal, making use of the high scattering characteristics of ultraviolet light", and as a replacement for mercury and deuterium lamps in those applications. On a polished wafer or a machined metal face, a scratch or particle that returns almost no contrast under white light can produce a clearly visible scatter signal under 365 nm.

The second mechanism is fluorescence. Many organic materials absorb UVA and re-emit in the visible band. REVOX lists "fluorescence observation by ultraviolet excitation" as a primary use for the same light source. Because the excitation is invisible to the camera once you fit a UV-blocking filter, the fluorescing material appears as bright signal on a dark background, which is one of the highest-contrast imaging conditions available in machine vision.

Two inspection modes, two optical setups

UV scatter imaging Fluorescence imaging
What you detect Scratches, particles, pits and edge damage on smooth surfaces Residues, adhesives, coatings, oils, and markings that fluoresce
Camera sees Reflected UV, or visible light if the surface scatters broadly Visible emission only
Filtering Camera must be sensitive at the illumination wavelength; most silicon sensors have reduced but usable UVA response UV-blocking (longpass) filter on the lens is mandatory, so none of the excitation reaches the sensor
Geometry Dark-field or low-angle, so specular reflection misses the lens Flexible; signal comes from the material, not the reflection angle
Limiting factor Available irradiance and sensor UV response Quantum yield of the target material and stray visible light

In practice the fluorescence setup is more forgiving optically but more demanding on light-leak control: any ambient visible light that reaches the sensor lands directly on top of your signal. Enclose the station.

Choosing between 365, 385 and 405 nm

REVOX offers exactly these three wavelengths in its own near-UV light source, and the same three are available as high-power COB arrays in our catalog. For inspection work the trade-off is spectral separation versus available power.

365 nm is the default for fluorescence. It sits furthest from the visible band, so a longpass filter can reject the excitation cleanly while passing almost all of the emission, and it excites the widest range of fluorophores. It is also the hardest of the three to produce efficiently, which is why its typical radiant flux is the lowest of the three in the same package.

385 nm gives the highest radiant flux (18.6 W typical at 9 A). Choose it when the target fluoresces strongly and you need throughput, or for scatter imaging where the exact wavelength matters less than the photon count.

405 nm is at the visible boundary, which makes alignment and setup easier because you can see the beam. The cost is filtering: with emission in the blue-violet, separating excitation from signal takes a steeper filter and you lose part of the usable emission band.

If the material you are inspecting has a published excitation spectrum, match it. If it does not, request samples at two wavelengths and compare contrast on your actual parts; fluorescence yield varies far more between materials than between these three wavelengths.

Why LED replaced mercury and deuterium lamps

UV inspection stations were built around mercury and deuterium lamps for decades. REVOX states that its near-UV LED light source offers "a lifetime more than 20 times longer than ultraviolet lamps (30,000 hours)" and that it is "easier to switch on and off than a UV lamp, contributing to energy savings". That 30,000-hour figure is the expected life of the REVOX light source unit at 385 and 405 nm, not a guaranteed value and not a rating for a bare emitter, but the direction is the point: lamps age continuously from the moment they are struck, so a lamp-based station needs warm-up time, periodic recalibration, and scheduled replacement.

An LED station can be switched per part, strobed in sync with the camera, and dimmed under closed-loop control, which is why most new UV inspection designs start from an LED emitter rather than a lamp.

Specify radiant flux in watts, not lumens

UVA is outside the photopic response curve, so lumens and lux are meaningless for these devices. Our UVA emitters are specified in watts of radiant flux at a stated drive current. What determines whether your inspection works is the irradiance in W/cm² at the work surface, which depends on your optics and working distance as well as the emitter, so budget the optical chain rather than the device rating alone.

Drive the emitters with a constant-current source. Forward voltage spreads from 3.4 to 4.2 V on the 365 nm device and shifts with junction temperature; a constant-voltage supply will not hold radiant flux steady, and in fluorescence work an unstable source shows up directly as measurement drift.

Thermal design and the glass lens

The UV COB arrays dissipate 93 to 101 W. Die-to-case thermal resistance is 0.31 °C/W, which is excellent, but the case-to-ambient path is entirely your design. UV LEDs lose output faster than visible LEDs when run hot, so size the heat sink for the worst-case duty cycle and use the built-in 10 kΩ NTC thermistor to monitor junction conditions in service.

The SMD device uses a glass lens rather than a silicone or epoxy encapsulant. Organic encapsulants yellow under sustained 365 nm exposure, which shows up as a slow loss of output and a shifting baseline, exactly the failure mode you do not want in a quantitative fluorescence station. Glass does not.

UV safety

These emitters produce hazardous levels of UVA. REVOX classifies its near-UV light source as Risk Group 3. Enclose the beam path, fit interlocks on access panels, and specify UV-blocking eyewear for anyone who may be exposed to an open beam during setup or maintenance. At 405 nm the beam is partly visible and at 365 nm it is nearly invisible, so do not rely on the operator seeing that the source is on; use an indicator.

UVA emitters we supply

Peak wavelength Model number Package Radiant flux (Typ.) Spectral half-width
365 nm 365L-120D-12C-2827COB-02 12-chip COB 15.3 W @ 9 A 13 nm
385 nm 385L-120D-12C-2827COB-01 12-chip COB 18.6 W @ 9 A 12 nm
405 nm 405L-120D-12C-2827COB-01 12-chip COB 17.4 W @ 9 A 15 nm
367 nm 365L-50D-1C-6868SMD-01 Single-chip SMD, glass lens, 50° 5.5 W @ 3500 mA 12 nm

Full specifications, including forward voltage, current ratings, package dimensions and thermal data, are on each product page and on the UV LED emitters collection.

Finished UV light sources. The emitters above are components, but REVOX also builds complete UV illumination systems. The SLG-150V-UV is a near-ultraviolet fiber optic light source offered at 365, 385 and 405 nm for fine defect inspection and fluorescence observation - see fiber optic illuminators. For line-scan inspection, the SPX-TA200 LED line light can be ordered with UV LEDs at the same three wavelengths; the rest of the series is on the LED line lights page. All illumination systems are quoted individually.

Wavelengths outside this catalog

REVOX assembles its COB and SMD packages in-house in Japan, with die bonding, wire bonding and phosphor coating all performed on its own equipment. Owning the die bonding process is what allows the arrays to be packed this tightly: chips are placed with a chip-to-chip gap of 60 µm, which keeps the emitting area compact enough to couple efficiently into lenses and fiber optics. Because the whole assembly line is in-house, UVA wavelengths between and beyond these three, different chip counts, array layouts, substrates and package dimensions can be produced to order. If your application needs a specific excitation wavelength to match a fluorophore, tell us the target and we will quote it.

Standard products cover 365 to 1,650 nm. Wavelengths outside that range, and other chip counts, array layouts or package sizes, are made to order. Tell us the wavelength and quantity you need and we will confirm feasibility with REVOX.

What to include in a request for quote

  1. Target wavelength and acceptable tolerance. State whether you need a specific peak or a band.
  2. Required irradiance at the work surface, and the working distance. This determines whether a COB array or an SMD with secondary optics is the right starting point.
  3. Duty cycle and drive mode. Continuous, strobed with the camera, or pulsed; this drives the thermal design and the choice of driver.
  4. Annual quantity and target date. Pricing and lead time both depend on volume.

Use the Request a Quote form on any product page, submit the Custom Order Form for non-standard requirements, or email sales@terapowerled.com.

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