Transmitted-Light Examination
Definition
Examination of a document with diffuse illumination from below, making paper thickness variation, watermarks, erasure thinning, and some obliterations visible as brightness variations in the transmitted image.
- Illumination
- Diffuse light from below the document
- Reveals
- Thickness variation, watermarks, erasure thinning, obliterations
- Output
- Brightness variation in the transmitted image
- Category
- One of the oblique/transmitted/UV/IR/VSC detection methods
Common questions
What distinguishes transmitted-light examination from transmitted-light photography?+
The examination is the visual inspection step performed at the light source or viewer, while the photography step records that same effect permanently on film or digitally so it can be documented, compared, and presented as evidence.
Can transmitted-light examination reveal an obliteration hidden by an overwritten mark?+
Sometimes. If the obliterating ink and the underlying ink transmit light differently, or if the paper itself was disturbed underneath, the difference can show up as a brightness variation even when the surface looks uniform.
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...
- IR Transmission
- Examination of a document with infrared radiation transmitted from below; exploits the IR-transparency of titanium dioxide in correction fluid to image underlying...
- 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...
- 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...