Transmitted Illumination
Definition
Lighting passed through the document from the back face to reveal paper-body features invisible from the surface: watermarks, security threads, and paper-thickness variation at erasure sites.
- Direction
- Light passed through the document from behind
- Reveals
- Watermarks, security threads, paper-thickness variation
- Detects
- Thinning at erasure sites
- Field
- Questioned document lab equipment
Common questions
How does transmitted illumination differ from oblique lighting in document examination?+
Oblique light rakes across the surface to reveal indentations and texture, while transmitted light passes through the paper body itself, so the two techniques expose different classes of feature and are typically used together.
Why does an erasure site show up under transmitted light?+
Scraping or chemical removal of writing thins the paper fibres at that spot, and thinner paper transmits more light, so the erased area appears as a brighter patch against the surrounding sheet.
Related terms
- Acid-Free Packaging
- Paper folders or envelopes manufactured without acidic buffers, used for long-term storage of questioned documents to prevent acid-catalysed paper and ink degradation.
- Chain of Custody
- The documented chronological record of who collected, handled, transferred, and examined a piece of evidence. For digital evidence, chain of custody includes...
- ESDA (Electrostatic Detection Apparatus)
- A non-destructive technique for detecting indented impressions on paper caused by writing pressure. A physical ink signature on paper produces detectable ESDA...
- 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 Reflectance
- Imaging under near-infrared illumination (715-1000 nm) to differentiate inks by their IR absorption: carbon-based inks remain dark, many synthetic dye inks become...
- Mylar (Polyester) Sleeve
- A chemically inert, optically clear polyester film sleeve used to protect questioned documents during examination and archival storage without introducing chemical contaminants...
- Oblique Illumination
- Lighting applied at a very low angle to the document surface (below 10 degrees from horizontal) to reveal topographic features: writing indentations,...
- UV Fluorescence Examination
- Examination under 365 nm or 254 nm ultraviolet light to reveal optical brightener fluorescence in paper, ink luminescence, and areas of altered...
- 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...