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Machine vision lighting is the practice of selecting an LED light source, its wavelength and its geometry, to maximize the contrast a camera needs to detect a feature reliably. The wavelength is matched to the material and the defect: UV (365–405 nm) to excite fluorescence and reveal surface flaws, visible (450–660 nm) for color and general contrast, NIR (780–940 nm) to see through surface coatings and cut glare, and SWIR (1050–1650 nm) to read moisture, fill level, and material composition that visible light can't. The geometry, bright field, dark field, dome, backlight, or coaxial, controls how that light strikes the part. Getting both right is what separates a robust inspection from a flaky one.
| Inspection goal | Wavelength | Geometry |
|---|---|---|
| Surface scratches, cracks, fluorescent markings | UV 365 nm / 405 nm | Dark field / low-angle |
| Color verification, print/label check | Visible (450–660 nm) | Bright field / dome |
| Glare reduction, see-through coatings | NIR (850–940 nm) | Diffuse / dome |
| Moisture, fill level, plastic ID, subsurface defects | SWIR (1050–1650 nm) | Backlight / diffuse |
| Edges, presence/absence, dimensional gauging | Any (high contrast) | Backlight (silhouette) |
What machine vision lighting does
A machine vision system is only as good as the image its camera captures. Lighting determines whether a defect appears as a strong signal or disappears into noise. The objective is contrast: making the feature of interest look as different as possible from everything around it, consistently, regardless of ambient light or part-to-part variation.
Two levers create that contrast, the spectral lever (which wavelength interacts with the material in a useful way) and the geometric lever (the angle and diffusion of the light relative to the part and camera). LEDs are the dominant source for both because they offer selectable narrow-band wavelengths, instant strobing, stable output for repeatable measurements, long life, and compact form factors that fit ring lights, bars, domes, and backlights.
Choosing the wavelength
Different wavelengths reveal different things because materials absorb, reflect, and fluoresce differently across the spectrum.
- UV (365–405 nm), excites fluorescence in inks, adhesives, and biological residues, and rakes across a surface to expose scratches and texture. Used for verifying UV-cure adhesive coverage, detecting contamination, and surface-crack inspection. See the UV LED Guide and the fluorescence excitation guide.
- Visible (450–660 nm), blue, green, and red for color discrimination and general contrast. Monochromatic visible light is sharper than white for edge and feature detection; a wavelength is often chosen to contrast against a part's own color (e.g. red light darkens a green feature).
- NIR (780–940 nm), penetrates some surface coatings, reduces specular glare, and is invisible, so it doesn't interfere with operators or ambient color. Common for reading through tinted films, suppressing surface print, and high-contrast silhouette work.
- SWIR (1050–1650 nm), reads what silicon-based vision cannot: water content (1450 nm), hydrocarbons and plastics (1650 nm), fill level through opaque containers, and subsurface defects. Requires an InGaAs camera. See the SWIR LED Lighting Guide.
Choosing the geometry
Geometry decides how the chosen wavelength interacts with the part's surface and reaches the camera.
| Technique | How it works | Best for |
|---|---|---|
| Bright field | Light reflects directly into the camera | Flat, matte surfaces; general illumination |
| Dark field (low-angle) | Light grazes the surface; only edges/texture scatter into the camera | Scratches, engravings, embossing, edge defects |
| Dome (diffuse) | Even, multi-angle light eliminates shadows and glare | Curved, shiny, or specular parts |
| Backlight | Light behind the part creates a silhouette | Edges, presence/absence, dimensional gauging, holes |
| Coaxial (on-axis) | Light injected along the camera axis via a beamsplitter | Flat specular surfaces (wafers, glass, mirrors) |
Strobing (pulsing the LED in sync with the camera) freezes motion on production lines and lets the LED run at higher peak intensity than continuous operation, improving signal without overheating.
Why LEDs for machine vision
- Spectral precision, narrow-band wavelengths selectable to the application, unlike broadband white sources.
- Strobe capability, microsecond pulsing synchronized to the camera freezes fast-moving parts.
- Stability and lifetime, constant output for repeatable measurements; 20,000–50,000 hour life cuts downtime versus fluorescent or halogen.
- Form factor, ring, bar, dome, backlight, and coaxial geometries in compact, integrable packages.
Tech-led supplies the LED emitters behind these systems across the full inspection spectrum, from UV through visible to IR/NIR and SWIR. For component selection, datasheets, and samples, contact Tech-led engineering.
Frequently asked questions
How do I choose the right wavelength for machine vision lighting?
Match the wavelength to the material and the feature. Use UV (365–405 nm) to excite fluorescence and reveal surface defects, visible light (450–660 nm) for color and general contrast, NIR (780–940 nm) to reduce glare and see through coatings, and SWIR (1050–1650 nm) to read moisture, fill level, or material composition. A useful rule: monochromatic light contrasting with the part's own color sharpens the feature.
What is the difference between bright field and dark field lighting?
Bright field directs light so it reflects straight into the camera, giving even illumination of flat matte surfaces. Dark field lights the part at a low grazing angle, so only edges, scratches, and texture scatter light back to the camera while flat areas stay dark, ideal for surface-defect and engraving inspection.
When should I use dome lighting?
Use a diffuse dome light for curved, shiny, or specular parts where direct lighting would create hotspots and shadows. The dome floods the part with even light from many angles, eliminating glare and giving uniform illumination for reliable inspection.
Why use infrared LEDs in machine vision?
NIR (780–940 nm) is invisible, so it doesn't interfere with operators or ambient color, it can penetrate some surface coatings and tinted films, and it suppresses specular glare. It's also useful for high-contrast silhouette and presence/absence checks.
What can SWIR lighting detect that visible light cannot?
SWIR (1050–1650 nm) reveals water content (around 1450 nm), plastics and hydrocarbons (around 1650 nm), fill level through opaque packaging, and subsurface defects, material properties that are invisible under visible or NIR light. SWIR imaging requires an InGaAs camera rather than a standard silicon sensor.
Why strobe machine vision lights?
Strobing pulses the LED in sync with the camera exposure to freeze motion on a moving line and to drive the LED at a higher peak intensity than continuous operation allows, increasing signal-to-noise without overheating the emitter.
Are LEDs better than fluorescent or halogen for machine vision?
Yes. LEDs offer selectable narrow-band wavelengths, microsecond strobing, stable repeatable output, 20,000–50,000 hour lifetimes, and compact geometries. Fluorescent and halogen sources are broadband, drift as they age, can't strobe cleanly, and have far shorter lives.
Related guides
- SWIR LED Lighting Guide, short-wave infrared for moisture, sorting, and material inspection
- Choosing an LED for Fluorescence Microscopy, wavelength-to-fluorophore matching
- Industrial UV LED Guide, UV for surface and fluorescence inspection
- LED Wavelength Guide: From UV to Infrared, the full spectrum map
Specifying lighting for an automated inspection system? Contact Tech-led engineering for LED wavelength and component recommendations across UV, visible, NIR, and SWIR.
