Machine-vision illumination is selected by matching peak wavelength, beam geometry, and trigger mode to the workpiece reflectance and stand-off distance — wrong choices cause 70–90% of returned lighting installations in integrator field reports [S2].
The four mainstream families — area LED, ring LED, bar/dome diffuse, and structured laser line — cover roughly 90% of industrial machine vision system cells, while halogen-fiber, UV, and IR remain specialised [S2].
Wavelength and Colour Band Selection
Red 620–660 nm LED bars remain the default for general metallic and PCB inspection because silicon sensors peak near 550–600 nm and most line-scan cameras retain strong red responsivity [S2]. Blue 450–470 nm sources resolve sub-pixel features on bare silicon wafers and solder joints by exploiting shorter wavelength diffraction limits [S2]. Green 520–540 nm is widely specified for red-printed text and pharmaceutical blister-foil inspection where red-on-red contrast must be lifted [S2]. White 4000–6500 K broadband LEDs are used for colour verification on consumer packaging because RGB channels are captured simultaneously [S2].
Infrared 850 nm and 940 nm options exist for through-translucent-plastic inspection and covert illumination, while UV 365–395 nm sources trigger fluorescence in adhesives, conformal coatings, and currency paper [S2]. Polarised filters paired with the chosen band cut specular glare by roughly an order of magnitude on shiny metal and glass substrates [S2].
Geometry: Ring, Bar, Dome, Backlight, Coaxial
Ring lights at 0–45° incidence suit axial-mount cameras and small cylindrical parts, but produce hot-spot reflection on polished metal — a dome or coaxial source typically gives more uniform grey-field response on those surfaces [S2]. Bar lights at 15–30° give sharp shadow edges for edge-detection and crack inspection on machined components, with two crossed bars raising contrast on complex 3-D surfaces [S2].
Backlight panels — flat-field LED arrays behind the part — produce the highest-contrast silhouette for dimensional gauging on a vision measuring machine, with edge transition typically under 50 µm for a 100 mm field at f/8 aperture [S2]. Coaxial lights through a 50/50 beam-splitter cube illuminate specular targets (wafers, polished glass, LCD panels) with on-axis light, removing the shadow that ring or bar sources create [S2].
Strobe, Current, and Heat Trade-offs
Pulsed overdrive at 2–10× nominal current for 50–500 µs raises LED output roughly proportionally while keeping average junction temperature within rated limits, and is the standard method for freezing motion on conveyor speeds above 1 m/s [S2]. A vision controller trigger pulse width and latency must be matched to the vision light source driver’s enable input, with most industrial drivers accepting 3.3–24 V TTL and returning a sync output to the camera [S2].
Halogen-fiber units, although progressively replaced by LED, still win on continuous-wave spectral stability across 400–2000 nm and are used in line-scan web inspection where LED banding artefacts must be avoided at exposure times above 5 ms [S2]. LED lifetime at 25 °C ambient is typically specified at 50 000 h to L70, but degrades faster with continuous high-current operation — fan-cooled housings or pulsed modes are used to preserve the published figure [S2].
Compatibility with Camera and Lens Stack
CMOS monochrome cameras deliver roughly 2× responsivity at 660 nm compared with equivalent colour sensors, so red illumination is the most photon-efficient pairing for high-speed lines [S2]. Filter choice on the lens (bandpass matching the LED peak ±20 nm) cuts ambient interference from factory fluorescent and LED ceiling lighting by 60–80% in field measurements [S2].
Working distance and spot uniformity are linked: a 50 mm ring at 100 mm stand-off gives roughly 60% centre-to-edge uniformity, whereas a 100 mm dome at the same distance exceeds 90% across the same field of view [S2]. Telecentric lenses, used for metrology, must be paired with collimated or back-light sources to avoid perspective distortion of the projected edges [S2].
Comparison: LED Bar vs Ring vs Dome vs Laser Line
On four practical criteria — cost, contrast on shiny metal, motion-freezing ability, and setup tolerance — the four dominant vision light source geometries fall out as follows. LED bar lights score low on cost (entry roughly USD 80–200 for 100–200 mm length) and high on edge contrast but require precise angling within ±5° for repeatability [S2].
Ring lights cost USD 50–150, score medium on shiny-metal contrast unless polarised, and offer fast setup with tolerance of ±10° tilt [S2]. Dome lights cost USD 200–600, score high on shiny-metal contrast, and tolerate ±15° tilt but cannot strobe above roughly 2× nominal current without diffuser damage [S2]. Laser line projectors cost USD 400–1500, score medium on diffuse targets but very high on dark or low-contrast 3-D profile scans at 10–30 m stand-off [S2].
Who Should NOT Pick the Default LED Bar
Engineers specifying a 660 nm LED bar as the universal default hit three common failure modes: hot-spot reflection on mirror-polished stainless, red-washout on red-printed packaging, and motion blur on conveyor lines above 2 m/s without strobe overdrive [S2]. For any of these cases — specular metals, same-hue part/print, or fast motion — a polarised coaxial source, a complementary blue or green band, or a high-current strobe driver respectively should replace the bar [S2].
Halogen-fiber is also the wrong pick for high-throughput cells where 24/7 LED-on cycles are expected: bulb life of 2 000–5 000 h is roughly 10× shorter than LED at comparable output, and spectral output drifts visibly after 1 000 h [S2]. Likewise, fibre bundles are mechanically fragile and should be avoided on cells with vibration above 5 g RMS unless shock-mounted [S2].
Shortlist Logic and Selection Flow
Start from the part surface: specular metal, glass, or wafer → coaxial or polarised dome; matte metal or plastic → bar at 15–30° or ring; translucent film or fluid → backlight or IR transmission; printed colour artwork → white dome or RGB ring [S2]. Then match wavelength to the contrast polarity needed: red on red requires green or blue; dark on dark requires IR or UV; colour verification requires white broadband [S2].
Cross-checking geometry, wavelength, and strobe budget in that order yields a defensible shortlist for any integrator RFQ, and pairs naturally with the laser marker selection criteria workflow when a 3-D profile scan precedes marking. For cells sharing a controller with barcode reading, the industrial barcode scanner selection flow also needs to be reconciled to the same illumination budget. Two trackable signals for the next planning cycle: 365 nm UV-LED cost per mW falling below USD 0.10, and 450 nm blue overdrive strobe drivers rated at 24 V/2 A becoming the new default for solder-paste inspection.