Machine vision light wavelengths: pick the LED band for your part

A wavelength is the one lighting choice that changes image contrast without moving a single part. The same camera, lens and geometry can make a feature glow or vanish just by swapping a 455 nm head for an infrared one. This guide maps the bands machine vision lights actually cover, from 365 nm to 1550 nm, and gives the rule for each: UV near 365 nm for fluorescence and fine surface detail, visible 455-660 nm for color contrast, near-infrared for ambient rejection and seeing through inks, and 1200-1550 nm SWIR for moisture and subsurface features. You will leave able to name a band for your feature, order the right emitter color, and confirm it with one test image.

The band map: what every wavelength range does

Machine vision lights are specified by peak wavelength in nanometers. White is the default because it works on everything, but each band below is a standard option across the spot light, bar light and special light categories, and the light finder filters the range by wavelength.

White light is the compromise: it returns signal from feature and background at once. A narrow band returns signal from one and starves the other, and that asymmetry is where extreme contrast comes from. The skill is matching the band to the feature's spectral response, not to how the part looks to your eye.

UV, 365-405 nm: fluorescence and surface detail

UV photons carry enough energy to make many adhesives, coatings, lubricants and optical brighteners fluoresce. The surface absorbs the UV and re-emits visible light, so the mark reads bright against a near-black background; a visible-blocking filter on the lens strips the last reflected violet. This is the standard approach for glue-bead checks, sealant coverage and contamination.

The band also works without fluorescence. At 365 nm, fine scratches and residue that disappear under white light scatter UV strongly enough to image directly, which is why reflected-UV rigs appear on glass, polished metal and coated films. 365 and 395 nm are the most common UV peaks in machine vision, with violet as the high-output bridge between UV and blue. For filter choices read the fluorescence lighting guide; to decide which interaction your part offers, compare the reflected-UV and UV-fluorescence routes.

For a compact violet source, the LS-SPT-0068 packs 405 nm emitters into a 73.7 x 9.4 mm body with a PNP trigger input.

Visible, 455-660 nm: the color wheel decides

A surface reflects its own color and absorbs its opposite. Light a red feature in red and it goes bright; light it in green and it goes dark. That single rule, walked around the color wheel, generates most visible-band decisions on a monochrome camera — the color wheel guide pairs the colors feature by feature.

Blue near 455 nm darkens yellows and browns: rust, adhesive squeeze-out, handling marks on bright metal. Green near 530 nm sits close to a mono sensor's response peak, so it gives the shortest exposures while turning red marks black. Red at 620-660 nm brightens red features and keeps the cell comfortable for the people next to it. The monochromatic lighting guide shows why one narrow peak beats white whenever the feature has a spectral signature.

Two spot formats cover the blue and cyan bands:

The LS-SPT-0024 pairs a 455 nm emitter with a 32° lens and the LS-SPT-0037 pairs 505 nm cyan with a 21° lens; both run from 24 V. When red light has to run the length of a long part, the LS-BAR-1168 is a 423 x 40 mm red diffused bar light.

Infrared, 850 nm and the longer peak: ambient rejection and seeing through ink

Infrared buys two things visible light cannot. It is invisible, fully so at the longer near-IR peak, so inspection runs beside operators without flashing the cell. And near-infrared passes through many inks, paints and thin plastics that stop visible light, so under-print codes and marks beneath a label layer image directly.

The two peaks trade signal against stealth. As silicon's response rolls off toward its 1100 nm cutoff, a typical mono sensor returns roughly twice the signal at 850 nm as at 940 nm, and about five to ten times less at that longer peak than at green through identical optics. Recover the difference with longer exposure, more gain or overdrive strobing. In exchange, the longer peak produces no glow at all, while 850 nm shows a faint deep-red dot in a dark room. Both peaks run across the spot and bar ranges, and both pair with strobed ambient rejection: a microsecond pulse under a controller freezes motion and drowns shop lighting at once.

730 nm deep red is the bridge band: still visible enough for alignment, dim enough not to distract, and easier on the sensor than 850 nm. Two checks before committing to IR. Color cameras carry an IR-cut filter that blocks both peaks, so IR work needs a mono camera. And mains-powered shop lighting flickers at line frequency; if black bars appear in some frames, the fix is strobe timing, not exposure.

SWIR, 1200-1550 nm: moisture, silicon and subsurface features

Above roughly 1100 nm silicon sensors go blind: the photon no longer carries the 1.12 eV a silicon pixel needs to free a charge packet, so SWIR imaging runs on InGaAs detectors. The payoff is contrast no other band offers. Water absorbs strongly near 1450 nm, so moisture, wet coatings and bruised produce read dark. Silicon wafers turn transparent above about 1200 nm, which opens through-wafer and backside inspection.

Decide whether you need the band with the SWIR, NIR and visible comparison; the near-infrared selection guide for water and silicon works both cases in detail; and the SWIR lens primer covers the optics. LED heads in the shop categories top out at the longest near-IR peak. Past silicon's edge the emitter, sensor and lens all become SWIR-specific parts, so plan that chain as one purchase.

Worked example: photon energy from 365 nm across the bands

The inputs are the shop's own filter values: 365, 405, 455, 850 and 940 nm. The rule is photon energy: a photon's energy in electron-volts equals 1240 divided by its wavelength in nanometers, and a material only responds to light when the photon carries enough energy for that interaction.

The arithmetic, one line per band:

• 365 nm: 1240 / 365 = 3.40 eV
• 405 nm: 1240 / 405 = 3.06 eV
• 455 nm: 1240 / 455 = 2.73 eV
• 850 nm: 1240 / 850 = 1.46 eV
• 940 nm: 1240 / 940 = 1.32 eV

A 365 nm photon carries 3.40 eV, about 2.6 times the 1.32 eV of the longest photon listed. Fluorescent brighteners absorb inside that 3.0-3.4 eV window, which is why glue beads that ignore visible and IR light glow under UV peaks. The same arithmetic sets the camera side: silicon's 1.12 eV band gap puts its cutoff at 1107 nm, so no exposure setting pulls a silicon camera past it. The decisions: if a mark stays dark under violet, step down to 365 nm before abandoning fluorescence, because the extra 0.34 eV opens more absorption bands; and treat SWIR as a detector decision, never an exposure decision.

Choose the band in four steps

1. Write down the feature and the background: material, color and finish, and name the interaction you need — reflectance, fluorescence or transmission.
2. Take the band from the table above: reflectance to the visible color wheel, fluorescence to the UV band, through-ink and ambient rejection to near-infrared, moisture and subsurface to SWIR.
3. Shortlist two candidate heads in the light finder and shoot both on real parts at the same exposure.
4. If ambient light leaks into the frames, add a controller and strobe; the lighting controller guide lists what strobing buys, and the lighting step of the configurator pairs the head with one.

One camera position, several bands: a sequencing head such as the LS-SPC-0086 drives 8 zones from a 150 mm package through one controller, so each exposure gets its own wavelength.

When the usual answer is wrong

• A color camera under colored light. A narrow band illuminates only the pixels under one channel of the sensor's color filter array, so you pay color-camera money for mono-camera signal. Use mono for wavelength work.
• White as the safe default. White cannot see a fluorescent mark and cannot separate two colors. It is the right call only when the part has no spectral signature at all.
• More light instead of the right light. Saturating the sensor flattens contrast; when the histogram clips, shorten the pulse before raising the current.
• Fighting the geometry with wavelength. When no band separates feature from background, the setup is a geometry problem — the light geometry selection guide sorts lighting angle and arrangement by surface type.

Before you order: a six-point check

• Feature and background written down, with the interaction named: reflect, fluoresce or transmit.
• A primary band and one alternate picked from the table, both testable at the same exposure.
• Mono camera confirmed for any UV or near-IR work.
• Sensor window specified for UV transmission on any UV head.
• Controller channels sized for the sequence if several bands share one position.
• Shielding planned for UV heads, and a no-direct-view rule for UV and IR alike.

Send the shortlist, or a bench image that failed, through expert support: every order includes free expert support, and a quote with the right emitter color attached starts with one message. The band-by-band guides continue on the lighting topic hub.