NIR Reflectance Imaging
Definition
Photography of a document surface using illumination and camera detection in the near-infrared band (700 to 1100 nm), which differentiates carbon-based ink residues from carbon char background through differences in infrared reflectance that are invisible to the naked eye.
- Wavelength band
- 700 to 1100 nm
- Target
- Carbon-based ink residue on charred documents
- Mechanism
- Differential infrared reflectance versus carbon char background
Common questions
Why does NIR reflectance specifically work on charred rather than merely faded documents?+
Both the paper char and many carbon-based inks appear visually black after burning, but they reflect infrared light at different intensities, so the technique exploits a physical property difference that only becomes useful once the visible contrast between ink and paper has been lost to charring.
What kinds of ink does this technique fail to differentiate?+
Inks that are not carbon-based, such as many dye-based or iron-gall inks, do not necessarily show the same infrared reflectance contrast against char, so the examiner must confirm the ink type responds to NIR before relying on this method for that document.
How fragile are charred documents during this imaging process?+
Charred paper is extremely brittle and imaging typically requires careful mounting or handling under controlled humidity before photography, since physical manipulation risks fragmenting the document before any content can be recovered.
Related terms
- Ashing
- The terminal stage of heat damage, above approximately 400 degrees Celsius, in which carbon oxidises to carbon dioxide and only inorganic mineral...
- Charring
- The stage of heat damage at approximately 200 to 400 degrees Celsius at which paper cellulose converts to amorphous carbon; the sheet...
- Consolidant
- A dilute polymer solution (Paraloid B-72, Klucel G, Cellugel) misted over a fragile char surface to stabilise the carbon matrix before handling...
- Controlled Humidification
- Exposure of a charred document to 85 to 90 per cent relative humidity inside a sealed chamber, temporarily restoring pliability to the...
- Gelatine Tissue Lift
- A technique in which a fine-gauge gelatine tissue sheet, slightly humidified, is applied to a charred surface to lift a fragile carbon...
- Multispectral Imaging
- Photography or remote sensing in multiple wavelength bands beyond the visible spectrum, including near-infrared and short-wave infrared. Drone-mounted multispectral cameras are used...
- Oblique-Light Photography
- Photography of a document surface using illumination at a low angle (10 to 15 degrees from the paper plane), which casts shadows...
- Principal Component Analysis (PCA)
- A multivariate statistical method used in fire debris research to reduce chromatographic data matrices to principal components that capture major variance. Used...
- Pyrolysis
- The thermal decomposition of cellulose in paper at approximately 150 to 300 degrees Celsius, disrupting glycosidic bonds and releasing water and carbon...