UV and IR Machine Vision Lighting: When Visible Light Cannot See the Feature

Most lighting problems are solved within the visible band by changing geometry. Some are not: the feature and the background have the same visible reflectance, and no amount of repositioning will separate them. Two bands outside the visible range solve a specific subset of those cases — UV by making something fluoresce, near-IR by seeing through something or by suppressing colour.

Both are narrower tools than a white ring light, and both fail in ways visible light does not. All values below are published POMEAS figures for the UV and IR series. This is a selection method, not a compatibility claim, and no third-party specification is quoted.

What UV actually does in an inspection image

UV does not make a feature brighter — it makes a different feature visible. A 365 nm or 395 nm source excites fluorescent compounds in adhesives, inks and some polymers, and those compounds emit visible light that the camera records. The contrast comes from the emission, not from reflection.

The practical consequence: if the material has no fluorescent component, UV does nothing. POMEAS states this directly — if the adhesive contains no fluorescent component, the result has to be confirmed by sample testing. UV is not a general-purpose "more contrast" setting.

Published POMEAS UV series values:

Choosing between 365 nm and 395 nm

There is no rule that picks the wavelength from the material name. Both excite fluorescence, and different adhesives, inks and substrates respond differently to each. POMEAS recommends imaging the actual sample at both wavelengths and choosing the one with higher fluorescence brightness, less background interference and more stable contrast.

Two further points from the published guidance: the choice is per material, so a wavelength that works on one adhesive does not transfer to another; and the sample test has to be done with the intended camera and filter, because a filter that blocks the excitation wavelength changes the result.

A UV build also needs two items that visible-light builds do not: a lens filter matched to the emission band, and an enclosure assessed for the installation, because the visible component of the source will otherwise dominate the image.

What near-IR does, and the trap in it

Near-IR (850 nm / 940 nm) is used for two things: penetrating materials that block visible light, and removing colour variation from the image so that shape and structure are what remain.

Published POMEAS IR series values:

Choosing between the two: 850 nm generally gives higher camera response and a brighter image, and is the starting point where signal strength matters. 940 nm has weaker visible red glow, which matters where visible light is a nuisance — for example where operators work at the station, or where a visible indicator would be confusing.

The trap: a camera with an IR-cut filter will not see it. Many colour cameras carry a filter that blocks near-IR to preserve colour fidelity, and POMEAS notes that this can reduce near-IR image brightness. Confirm the sensor's response at 850 nm or 940 nm before specifying the light. A monochrome camera without an IR-cut filter is the usual pairing.

A second trap is judgement by eye. The human eye is far less sensitive to 850 nm and 940 nm than a sensor is, so a source that looks dim to an operator may be delivering plenty of signal — brightness has to be judged from the camera image and grey values, not from looking at the emitter.

Voltage and structure: the same caveat as the rest of the range

Ring, bar, area and coaxial structures in both series run at 24 V; point sources in both series run at 5 V. POMEAS states explicitly that the label and datasheet must be checked before connecting power. This matters because the UV and IR series share mechanical families with the visible-light series — a UV ring and a white ring look alike and do not share a drive voltage with the point-source versions.

Both series are recommended for use with a constant-current light controller. Where multiple bands are combined on one station — for example IR transmission and white reflection on the same part — each band needs its own channel, and the channel current budget has to be checked against the controller's per-channel limit.

Three configurations

A — UV adhesive bead inspection. 365 nm or 395 nm ring or coaxial source, 24 V, 1.7–6.2 W, with a lens filter matched to the emission. Sample-test both wavelengths on the actual adhesive; if the adhesive has no fluorescent component, this path fails and the problem needs a different contrast mechanism.

B — Seeing through a package or a coloured film. 850 nm area or bar source, 24 V, with a monochrome camera confirmed to have no IR-cut filter. Start at 850 nm for signal; move to 940 nm if visible glow is a problem.

C — Removing colour variation from a print inspection. 850 nm or 940 nm illumination with a monochrome camera: colour differences that dominate under white light largely disappear, leaving structure and registration to be judged.

What UV and IR do not do

They do not replace geometry. A specular surface still needs coaxial or dome illumination; UV and IR change what is visible, not how the light is arranged. They do not measure temperature. They do not make a low-resolution lens resolve more. And they do not remove the need for a sample test — material response to both bands varies with formulation, thickness, colour and surface treatment, and cannot be judged from appearance.

FAQ

When should I use UV instead of white light?

When the feature contains a fluorescent component — adhesive, ink, anti-counterfeit marking — and the visible contrast is insufficient. Without a fluorescent component, UV does nothing.

365 nm or 395 nm?

Test the actual sample at both. Different adhesives, inks and substrates respond differently; choose the one with higher fluorescence and less background.

850 nm or 940 nm?

850 nm generally gives higher camera response and a brighter image. 940 nm has weaker visible red glow, which matters where visible light is a nuisance.

Will my existing camera work with IR?

Only if the sensor responds at that wavelength and there is no IR-cut filter. Many colour cameras block near-IR; a monochrome camera is the usual pairing.

Why does the IR light look dim?

The eye is much less sensitive to 850 nm and 940 nm than a sensor is. Judge brightness from the camera image and grey values.

Are all UV and IR models 24 V?

No. Ring, bar, area and coaxial are 24 V; point sources are 5 V. Check the label and datasheet before connecting power.

Next Step

Send the material, the feature to be detected, the camera model and whether it carries an IR-cut filter, and POMEAS will arrange sample imaging at 365 nm and 395 nm or 850 nm and 940 nm and recommend the wavelength and structure.