Oblique Illumination
Definition
Lighting applied at a very low angle to the document surface (below 10 degrees from horizontal) to reveal topographic features: writing indentations, stamp embossing, paper-fibre disturbance at erasure sites, and surface texture.
- Light angle (documents)
- Below 10 degrees from horizontal
- Light angle (tool marks)
- 15 to 30 degrees from horizontal
- Primary use
- Reveals indentations, embossing, erasures, and surface texture in document and tool-mark examination
Common questions
What is oblique illumination and why use it on documents?+
Oblique illumination applies light at a very shallow angle (below 10 degrees from horizontal) to a document surface. The angled light rakes across the surface and reveals features invisible under normal lighting: indentations from pen pressure, embossed stamps, disturbed paper fibers at erasure sites, and surface texture details. It is a workhorse technique in questioned document examination.
How does oblique illumination differ in tool-mark microscopy?+
In tool-mark comparison work, a slightly steeper angle of 15 to 30 degrees from horizontal is used. This angle maximizes shadow contrast and makes fine striations on tool marks visible, helping examiners compare marks and link tools to surfaces.
What features can oblique illumination reveal?+
The raking light uncovers writing indentations (pressure marks from a pen or stylus), embossed impressions from stamps or seals, disturbances in paper fiber where material has been erased or scraped, and other surface texturing that indicates tampering or prior writing.
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.
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- ESDA (Electrostatic Detection Apparatus)
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- IR Reflectance
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- Likelihood Ratio (LR)
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Explained in these topics
- Lab Equipment, Evidence Handling and Chain of Custody in QDELighting applied at a very low angle to the document surface (below 10 degrees from horizontal) to reveal topographic features: writing indentations, stamp emb...
- Tool-Mark Comparison Microscopy and 3D ImagingA lighting angle of 15-30 degrees from horizontal used in tool-mark comparison microscopy to maximise shadow contrast and visibility of fine striations.