Choose the wavelength before you choose the light. A defect that disappears under white light can jump out under a single LED band, and a part that looks dull grey in the visible can fluoresce hard under UV-A. This page is the band map behind those decisions: where ultraviolet, visible and infrared light sit in nanometres, which colour sub-bands the visible range splits into, what a silicon camera actually detects, and how wavelength changes what your lens and your part do. The numbers are set out so you can carry each range straight into a lighting specification. For a task-driven walkthrough rather than a band map, start with the machine vision illumination types guide.
The bands from UV to mid-IR
Machine vision works in the slice of the electromagnetic spectrum from about 100 nm to 50 µm. Your eye covers only the middle strip: visible light runs from about 380 to 760 nm, and a typical observer responds from roughly 380 to 750 nm (Visible spectrum). Sensors see beyond both ends of that, which is the whole reason band selection matters.
| Band | Range | Machine-vision use |
|---|---|---|
| UV-C | 100-280 nm | Germicidal lamps; not used for imaging |
| UV-B | 280-315 nm | Very strong fluorescence but a serious eye and skin hazard; rare |
| UV-A | 315-400 nm | Fluorescence excitation of adhesives, sealants, coatings, grease, penetrants |
| Visible | 380-760 nm | Default band; white, red, green, blue and violet lights |
| Near IR | 780 nm-2.5 µm | Silicon sensors reach about 1100 nm; 730, 850 and 940 nm LED bands |
| Mid IR | 2.5-50 µm | Thermal imaging; beyond silicon sensors and normal machine-vision optics |
The visible band divides into six colour sub-bands:
| Colour | Range |
|---|---|
| Violet | 380-450 nm |
| Blue | 450-495 nm |
| Green | 495-570 nm |
| Yellow | 570-590 nm |
| Orange | 590-620 nm |
| Red | 620-700 nm |
The boundaries overlap by convention: the 380-400 nm strip counts as violet in colour tables and as UV-A in hazard tables, which is why a 405 nm UV LED still glows dim violet. Between 700 and 780 nm sits deep red, nearly invisible to the eye but strong on a silicon sensor, and the 730 nm lights in the backlight category sit exactly on that band. UV proper divides into UV-A (315-400 nm), UV-B (280-315 nm) and UV-C (100-280 nm) (Ultraviolet); the UV LEDs you will actually meet are 365 nm and 395-405 nm, both UV-A.
Wavelength, frequency and photon energy
Frequency f comes out in hertz when c = 3×10⁸ m/s and λ is entered in metres. Photon energy E comes out in electronvolts when λ is entered in nanometres.
Those numbers decide what a sensor can catch. Silicon, the material of almost every industrial camera sensor, absorbs photons of at least 1.12 eV and lets weaker ones pass straight through, which puts a hard edge at about 1107 nm.
A single-colour LED is not a laser line, either. Expect a spectral width of roughly 15-40 nm full width at half maximum, so an 850 nm rated light delivers useful energy from about 830 to 870 nm; match a bandpass filter to that width, not to the peak number alone. White light is a different case again: it is a phosphor-converted broadband source whose balance matters whenever a colour camera must judge print or colour (see white LED colour temperature).
What each band does to a part
A part's colour is its absorption spectrum in action. A red part reflects the 620-700 nm band and absorbs most of the rest, so red light makes it bright on a monochrome camera while blue or green light drives it dark. Choosing the wavelength that separates part from background is the highest-leverage lighting decision in the chain; the colour contrast guide works the pairing band by band.
UV-A does something different: it excites fluorescence. Many adhesives, sealants, coatings, greases and crack penetrants absorb 365 nm light and re-emit it at longer visible wavelengths, so with a filter that blocks the excitation the mark glows on a black field. The 395-405 nm LEDs excite many of the same materials more weakly, with less hazard and friendlier optics.
Short wavelengths scatter hard, because Rayleigh scatter scales with 1/λ⁴, so blue and violet light bring up dust, haze, micro-scratches and surface texture that red light slides over. Use that when texture is the defect; avoid it when you want a clean silhouette. Shiny parts add polarised glare, which a crossed polarizer pair tames at a cost in stops that the polarizer light loss article quantifies.
Near IR does the opposite: it goes through. Thin plastic films, silicone and many inks and dyes turn transparent around 850-940 nm, so printed codes vanish and substrate defects appear — the trick behind NIR lighting for printed packaging and much internal feature inspection. Carbon black is the standing exception: it absorbs across the IR and kills transmission in black-filled plastics.
What the camera actually detects
Silicon sensors respond from about 400 nm out to roughly 1100 nm, usually peaking in the red and near IR; the SWIR, NIR and visible comparison maps which sensor family covers which band. Two consequences matter daily.
First, colour sensors are poorer infrared detectors than their sheets suggest: the Bayer colour filter array works behind an IR-cut filter that blocks most energy beyond roughly 650-700 nm. For 730-940 nm work use a monochrome sensor or a colour model whose IR-cut filter can be removed; the IR-cut filter options article sets out the choices. Second, the short end is soft: at 405 nm the sensor still catches photons (3.06 eV), but quantum efficiency drops and sensor window and lens glass absorb increasingly below about 380 nm, so plan on longer exposures than the same scene in red.
For transmission checks the working tool is a long-wavelength backlight. The 850 nm infrared backlight with a 136.4 x 136.4 mm lit area runs at 24 V, and its constant-on design needs no controller (product page).
- Size
- 100-200 mm
- Type
- Standard
- Voltage
- 24 V
- Wavelength
- 850 nm
Wavelength, diffraction and focus
The lens joins the decision, because diffraction blur grows with wavelength:
d is the Airy disk diameter, λ the wavelength and N the f-number, in matching units (micrometres are convenient).
For sub-10 µm edge gauging, shorter wavelengths hold the edge sharper, and they compound with geometry — a collimated backlight with violet or blue light is the sharp-edge recipe. If the band must be infrared for transmission or ambient rejection, run the widest aperture your depth of field allows. Focus also shifts: visible-corrected achromats move their focal plane in UV-A and deep infrared, so refocus when you change bands rather than trimming an existing setting.
For gauging where the band is still open, a 120 x 120 mm collimated backlight at 24 V keeps that edge transition steep whichever wavelength you settle on.
- Size
- 100-200 mm
- Type
- Collimated
- Voltage
- 24 V
Choosing the band: a decision path
- Write down what the defect does differently from the good part: absorbs, reflects, fluoresces, transmits or scatters. The band that maximises that difference wins.
- Bright part on a dark field: light with the colour the part reflects. Dark defect on a bright field: light with the colour the defect absorbs. Quick colour wheel: blue light darkens orange, red and yellow surfaces; red light darkens blue and green ones.
- Set geometry first and band second — the surface or silhouette decision sorts out the angle, then wavelength tunes the contrast.
- Marks under print or inside plastic: 850 nm or 940 nm transmission through a backlight.
- Fluorescing marks: 365 nm or 405 nm excitation, with a visible bandpass on the camera.
- Bright, uncontrolled ambient: match a narrow bandpass filter to the LED's peak and width, and strobe to freeze motion.
- Two jobs on one camera: sequence two wavelengths in the same cycle rather than compromising on one — the multi-wavelength sequencing article covers the wiring and timing.
Prove the shortlist on real parts before ordering. An easy first probe is red: the 150 x 150 mm red flat backlight runs at 24 V on a monochrome camera, and ten minutes swapping it against a white light tells you whether band choice will move your contrast (product page).
- Size
- 100-200 mm
- Type
- Standard
- Voltage
- 24 V
Driving UV and infrared lights
UV LEDs turn drive current into usable UV photons inefficiently, so UV heads need more current for the same inspection signal; infrared arrays are efficient but often run at high power to cover large fields. Size the controller for the strobed current, not the constant-on current: pulse overdrive of several times the continuous rating is normal in machine vision, and the duty-cycle arithmetic that keeps the LEDs alive is the same for every colour — the strobe duty cycle guide works it through.
For strobed infrared or UV, pick a constant-current controller with headroom: this two-channel high power constant current unit delivers up to 240 W per channel, 320 W total, with analog control (product page).
- Output
- Constant current
- Channels
- 2 channels
- Control
- Analog
- Feature
- Constant current
Keep the drive DC-regulated rather than mains-derived, so the light cannot beat with the mains cycle; if images flicker under plant lighting, the flicker diagnosis article separates ambient flicker from drive ripple. Whatever you choose from the light controller category, run a stability check before deployment, because LED output and peak wavelength both drift with junction temperature — the lighting stability test gives the procedure. If the light mounts away from the controller, the lighting accessories range carries the extension and drag-chain cabling.
Where the usual advice fails
White light is still right for colour work. If the camera must judge print colour, wire colour or food freshness, take the broadband source and control its colour temperature rather than chasing monochrome contrast.
A 405 nm light is not a 365 nm light with a different number. At 405 nm you sit at the violet edge of the visible with partial fluorescence, and weak fluorescers may barely respond; test both on your marks before specifying UV.
Sunlight is a broadband infrared lamp. Outdoors or under skylights, an 850 nm light on its own drowns; narrowband filtering plus strobing buys the contrast, not the wavelength alone — the same logic the ambient light rejection acceptance test turns into a pass criterion.
Judge every band on the sensor, not the eye. Deep red 730 nm looks dim to someone standing at the cell yet images strongly; the reverse trap is a white light that looks bright to the eye but delivers little under strobe.
Checks before you order
- The data sheet prints dominant wavelength and spectral width: match any bandpass filter to both, not just the peak.
- Monochrome sensor for 730 nm and above: colour arrays lose most of those photons behind the IR-cut filter.
- Lens transmission at the band, plus a refocus plan for UV-A: visible-corrected optics shift focus there.
- Light field larger than the FOV at working distance: oversize by roughly 10-20% for mounting tolerance.
- Controller rated for the strobed current and duty cycle, with headroom for overdrive.
- Two-band comparison images on sample parts, archived with the project, so the band decision rests on evidence.
Backlights in the range filter by wavelength — 405, 730, 850 and 940 nm as discrete options, plus white, red, green, blue, infrared and UV colour families — and the light finder narrows by size, type and voltage. Build the chain in the light configurator, browse the controller finder for the drive electronics, or start from the defect with machine vision lighting selection by inspection task. The rest of the lighting guides sit on the lighting topic page.
Questions
What wavelengths can a silicon machine-vision camera detect?
Most industrial sensors are silicon and respond from about 400 nm to roughly 1100 nm. The hard edge is photon energy: silicon's bandgap is 1.12 eV, so photons longer than about 1107 nm pass through unabsorbed. Below 400 nm, quantum efficiency falls and the sensor window plus lens glass absorb, so UV-A work takes longer exposures. Past 1100 nm you need InGaAs or another SWIR technology, not a silicon camera.
Should I pick an 850 nm or 940 nm infrared light?
Pick 850 nm when signal is the limit: silicon sensors respond more strongly there, so exposures stay short and contrast high. Pick 940 nm when the light must be fully invisible, because 850 nm shows a faint dark-red glow to an operator, or when the water-absorption band near 940 nm helps, as in moisture or cure checks. Expect longer exposures at 940 nm because sensor response has fallen steeply by that wavelength.
Can a colour camera see near-infrared light?
Only weakly. Bayer colour sensors sit behind an IR-cut filter that blocks most energy beyond roughly 650-700 nm, so a 730-940 nm light gives a dim, colour-shifted image at best. For near-IR work use a monochrome camera, or a colour model whose IR-cut filter can be removed or swapped, and confirm with a test exposure at your working distance before committing the band to the inspection.
Which UV wavelength gives the strongest fluorescence?
365 nm is the classic excitation band and drives most fluorescent sealants, adhesives, coatings and penetrants hardest. The 395-405 nm LEDs excite many of the same marks more weakly but with less eye hazard and friendlier optics, which is often enough signal. Whichever you pick, fit a bandpass filter on the lens that passes the visible fluorescence and blocks the excitation wavelength, so the mark reads as a bright object on a black field.
Why does lux tell me nothing about an infrared light?
Lux is weighted to the human eye's response, which peaks at 555 nm and falls effectively to zero inside the near IR, so a strong 850 nm floodlight can measure almost zero lux while saturating a monochrome sensor. Compare lights in camera units instead: same lens, same exposure, same part, then read the mean pixel value from a fixed region of interest. That comparison is repeatable and tracks what your algorithm actually sees.