IR Transmission
Definition
Examination of a document with infrared radiation transmitted from below; exploits the IR-transparency of titanium dioxide in correction fluid to image underlying text through an otherwise opaque obliteration layer.
- Method
- IR light transmitted through the document from below
- Exploits
- IR-transparency of titanium dioxide in correction fluid
- Reveals
- Text hidden under an obliteration layer
- Instrument
- Typically performed on a VSC
Common questions
Why does correction fluid fail to hide text under IR transmission when it hides it visually?+
Titanium dioxide, the white pigment in most correction fluids, scatters visible light effectively enough to look opaque to the eye, but it is largely transparent at near-infrared wavelengths, so IR passing through the page reaches the underlying ink and the camera behind it.
What obliteration methods defeat IR transmission examination?+
Opaque coverings that are not titanium-dioxide based, such as certain black markers or genuinely opaque tapes, or physical erasure and re-writing rather than covering, do not have the same IR-transparency gap and will not reveal underlying text this way.
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-Cut Filter
- A filter placed in the camera optical path that blocks near-infrared radiation, allowing only visible-range light to reach the sensor; must be...
- IR Luminescence
- Emission of near-infrared radiation by ink dye molecules when excited by visible light; different ink formulations luminesce at different intensities, allowing differentiation...
- 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...