Infrared examination is usually described as a single capability, as though a document is either under IR or it is not. In practice the near infrared covers a broad range, and where in that range you look changes what becomes visible.
Why the band matters
Inks do not switch off at one particular wavelength. They have a response curve across the infrared, and two inks that behave identically at one point in the range can diverge sharply at another.
That is the whole basis of the technique. If you only look at one fixed wavelength, you see the differences that happen to appear there and miss the ones that would have appeared elsewhere. A substitution that is invisible at a shorter band can be obvious a few hundred nanometres further out.
The practical regions
Around 800 to 900 nanometres. The closest to visible light and the most commonly used starting point. Many dye based inks begin to lose density here while carbon based inks and laser engraving remain dark. This band is often where an added entry first separates from an original.
Around 900 to 1000 nanometres. Further separation. Inks that still held density in the previous band frequently drop out here, so text that appeared consistent can suddenly show two populations.
Above 1000 nanometres. Useful for substrate and security thread behaviour, and for cases where the closer bands showed nothing. Sensor sensitivity falls away as you move out, which is why equipment specifications matter more at this end.
The exact behaviour depends on the specific inks involved, so these regions are a guide to where to look rather than a rule about what you will find.
Fixed wavelength versus multiple bands
A fixed wavelength source is simpler, cheaper and adequate for a defined task. If your workflow is checking one document type against a known behaviour, and you have established that the band you use separates what you need to separate, a single source works.
The limitation appears with unfamiliar documents and unfamiliar inks. Then a negative result at one wavelength tells you very little, because you do not know whether there was nothing to find or whether you were looking in the wrong part of the range.
For second-line work on varied material, the ability to view at more than one band is what turns IR from a confirmation tool into an investigative one. The general treatment is in infrared examination, and the reason IR sees what UV cannot is set out in what infrared reveals that UV cannot.
Laser engraving
Modern polycarbonate documents personalise by laser engraving rather than printing, and that process leaves carbonised material inside the substrate. Carbon absorbs broadly across the near infrared, so engraved personalisation stays dark across the range while a printed imitation of it often does not.
This makes IR a useful discriminator on exactly the documents that are hardest to assess visually, because a good print reproduction of an engraved portrait can look convincing under white light.
What limits results in practice
Sensor response. The camera has to be sensitive at the wavelength you are illuminating with. A source further into the infrared than the sensor can register produces a dark image and no information.
Illumination geometry. Reflections from a laminate or a hologram can swamp the detail underneath. Angling the source, or using diffuse rather than direct light, often does more for the result than changing wavelength.
Contamination by visible light. Ambient light with an infrared component reduces contrast. Working with the source as the dominant illumination gives a cleaner separation.
Absence of a reference. An IR image of a genuine specimen of the same document, taken under the same conditions, is what makes an anomaly recognisable. See building reference material.
Testing before you commit
The behaviour of a particular ink at a particular wavelength is difficult to predict from specifications alone, which makes a practical trial more informative than a datasheet comparison.
A useful trial is small. Take three or four documents representative of your caseload, including at least one with a known alteration if you have one, and view them at each wavelength the equipment offers. What you are looking for is whether the separation you need appears, and at which band. That answers the specification question directly rather than by inference.
If a supplier can arrange this, it is worth more than any amount of reading. If not, establishing which bands your existing sensor is genuinely sensitive to is the next best thing, because a source outside that range contributes nothing regardless of its output.
Frequently asked questions
Is one wavelength enough for first-line checks?
First-line checks do not usually need IR at all. Where IR is used at the front line, a single band suited to the documents seen most often is a reasonable compromise.
Does more infrared power give better images?
Only up to a point. Beyond adequate illumination, extra power adds heat and reflection problems rather than detail. Geometry matters more than intensity.
Can a modified consumer camera be used?
Cameras with the infrared blocking filter removed do register near infrared, and they can demonstrate the principle. For casework the concerns are repeatable geometry, known spectral response and a record you can defend, which is what purpose built equipment provides. See IR cameras for ID document authentication.
Will IR show every alteration?
No. Two inks with similar infrared behaviour will not separate, and carbon based inks are a common case. A negative result narrows the possibilities rather than closing the question.
Where does anti-Stokes fit?
Anti-Stokes features are excited by infrared and emit visible light, so they need an IR source but a normal visible light detector. That is a different arrangement from IR imaging. See anti-Stokes luminescence.
What to do next
If you are specifying IR capability, start from the documents and inks you actually examine, then check that the source and the sensor overlap where you need them to. Browse the full document authentication equipment range, or compare the IR camera options. For help matching wavelength and sensor to your casework, request a quote.
