Oblique Light
Definition
Side lighting at a low angle (5 to 15 degrees from the paper plane) that throws indentation shadows. The simplest indented-writing technique, used before any chemical or electrostatic step.
- Field
- Questioned document examination, indented writing
- Angle
- 5 to 15 degrees from the paper plane
- Sequence
- Used before chemical or electrostatic methods
- Effect
- Throws shadows from indentation depressions
Common questions
Why is oblique light tried before electrostatic detection apparatus (ESDA) on indented writing?+
Oblique light is nondestructive and requires no equipment beyond a raking light source, so it costs nothing to try first. If it reveals the indentation clearly, the document can be photographed without the risk ESDA carries of subjecting the paper to an electrostatic charge and toner application.
What kinds of indentations does oblique light fail to reveal?+
Very faint or old indentations where the paper fibres have relaxed back toward their original shape often do not throw a visible shadow under raking light, since the technique depends on surviving physical depression in the paper surface, not on any residual chemical trace of the writing.
Does the direction of the oblique light matter for reading indented writing?+
Yes, indentation shadows are strongest when the light strikes perpendicular to the direction of the strokes and weakest when parallel to them, so examiners typically rotate the light source or the document through multiple directions to capture the full text.
Related terms
- Charred Document
- Paper that has been burned to the carbonised stage but not fully reduced to ash; the cellulose has pyrolysed and the sheet...
- ESDA
- Electrostatic Detection Apparatus; detects latent indentations in paper by charging the surface and cascading toner particles, revealing tracing stylus pressure and writing...
- Glycerin Chamber
- Sealed humidification chamber held at 60 to 80 percent relative humidity using glycerin-water solutions to restore flexibility to charred or brittle paper...
- Indentation
- Latent pressure impression left on a lower sheet of paper when writing on an upper sheet; depth depends on pen pressure, paper...
- PVA Spray
- Polyvinyl-acetate solution sprayed in a fine mist to consolidate the surface of a charred sheet so it can be handled, mounted and...
- Secondary Writing
- Term for indented impressions: writing recovered from a sheet beneath the original, also called secondary or transferred writing.
- Steganography
- Concealing one file inside another in a way that hides the existence of the hidden file. LSB image manipulation, JPEG DCT coefficient...
- Sympathetic Ink
- Older name for invisible ink; the writing develops (becomes 'sympathetic') only after a heat, chemical or UV stimulus is applied.
- VSC
- Video Spectral Comparator (Foster + Freeman VSC-8000 series). Workhorse QD instrument combining UV, visible and IR sources with IR-sensitive cameras for ink,...