IR-Cut Filter
Definition
A filter placed in the camera optical path that blocks near-infrared radiation, allowing only visible-range light to reach the sensor; must be removed (or switched out electronically) for IR luminescence and IR transmission examination on the VSC.
- Function
- Blocks near-infrared radiation from the camera sensor
- Normal state
- In place for standard visible-light imaging
- Removal need
- Must be removed or switched for IR luminescence or transmission
- Common location
- VSC and similar document examination systems
Common questions
Why does an ordinary camera need this filter removed for document examination?+
Consumer camera sensors are inherently sensitive well into the near-infrared range, and the built-in cut filter exists to make everyday photos match human colour perception. Leaving it in place blocks exactly the wavelengths needed to reveal IR-responsive inks or obliterated text.
How does a VSC handle the IR-cut filter differently from a standard camera?+
A video spectral comparator uses a motorised or software-switchable filter wheel so the examiner can toggle between visible-light and IR-passing configurations on the same instrument during one examination, rather than swapping physical camera bodies.
Related terms
- ENFSI Document and Handwriting Working Group
- The European Network of Forensic Science Institutes working group responsible for harmonising best-practice guidelines for forensic document examination across European national laboratories.
- Foster+Freeman VSC8000
- The current-generation forensic document examination workstation produced by Foster+Freeman Ltd (UK), used by the FBI Laboratory, DSTL (UK), CFSL (India), AFP (Australia),...
- IR Luminescence
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- IR Transmission
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- Oblique-Light Illumination
- Illumination of a document surface at a very low angle (5-20 degrees) to create disproportionately long shadows from surface irregularities, revealing mechanical...
- Optical Brightening Agent (OBA)
- A fluorescent compound added to paper pulp or coatings that absorbs UV-A radiation and re-emits it as visible blue light, making paper...
- Transmitted-Light Examination
- Examination of a document with diffuse illumination from below, making paper thickness variation, watermarks, erasure thinning, and some obliterations visible as brightness...
- Video Spectral Comparator (VSC)
- A multispectral imaging instrument combining UV, visible, and near-infrared illumination with selectable emission filters and a calibrated digital camera; the primary bench...
- Watermark
- A pattern formed in paper during manufacture by localised fibre-density variation on the Fourdrinier wire; visible in transmitted light as a lighter...