Multispectral Imaging
Definition
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 in wildfire investigation to map char-depth gradients, residual heat signatures, and fire-spread direction from aerial survey data.
- Wavelength range
- Ultraviolet through visible light to mid-infrared (and near-infrared for some applications)
- Key principle
- Different materials absorb light differently at different wavelengths, allowing software to optimize contrast and reveal hidden content
- Common applications
- Burnt document recovery, water-damaged document restoration, wildfire investigation via drone mapping
Common questions
What is multispectral imaging used for in document examination?+
Multispectral imaging captures photographs of documents across multiple wavelength bands, from ultraviolet through visible light to infrared. This technique helps reveal hidden or damaged text by separating ink from stains or burn damage. By analyzing how different materials absorb light at different wavelengths, examiners can recover text that would otherwise be invisible to the naked eye.
How does multispectral imaging help recover burnt or water-stained documents?+
For burnt documents, the technique captures images across UV, visible, and mid-infrared wavelengths to find the best contrast between ink and charred background. For water-stained documents, it separates ink chromophores from stain chromophores. For example, haemoglobin in biological stains absorbs light at 415 nm, allowing software to isolate and enhance text hidden beneath the stain.
What other forensic applications use multispectral imaging?+
Multispectral cameras mounted on drones can map fire scenes from the air. They detect char-depth gradients, residual heat signatures, and fire-spread direction across near-infrared and short-wave infrared wavelengths. This allows investigators to reconstruct how a fire moved across a scene without physically accessing every burned area.
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...
- Cal Fire (California Department of Forestry and Fire Protection)
- California's state fire agency, which has integrated drone aerial survey and multispectral imaging into major wildfire investigations since 2018. Operational use in...
- Charring
- The stage of heat damage at approximately 200 to 400 degrees Celsius at which paper cellulose converts to amorphous carbon; the sheet...
- Cockling
- The wavy planar distortion of a paper sheet caused by differential shrinkage between areas that dried at different rates, typically visible as...
- 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 Air Drying
- Drying of a wet document flat on acid-free blotter at controlled temperature (18-20°C) and relative humidity (45-55%), separated from adjacent pages, with...
- Controlled Humidification
- Exposure of a charred document to 85 to 90 per cent relative humidity inside a sealed chamber, temporarily restoring pliability to the...
- ENFSI EWG AI Working Group
- The European Network of Forensic Science Institutes Explosives Working Group's artificial intelligence working group, active from 2021. Produced a 2023 position paper...
- Freeze-Drying (Vacuum Lyophilisation)
- A drying technique in which water is first frozen and then sublimed under reduced pressure (below 611 Pa), bypassing the liquid phase....
- 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...
- Handheld Raman
- A portable spectrometer using a near-infrared laser (typically 785 nm or 1064 nm) to measure inelastic light scattering from molecular bonds. Generates...
- HPMC (Hydroxypropyl Methylcellulose)
- A water-reversible cellulose ether polymer used in dilute aqueous solution as an ink fixative; applied by atomiser to wet document surfaces to...
Explained in these topics
- Charred and Burnt Document Recovery and DeciphermentCapture of a series of document images across multiple wavelength bands from UV through visible to mid-infrared, producing an image cube that allows software-b...
- Emerging Methods: Handheld Raman, LIBS, CT Imaging, MLPhotography or remote sensing in multiple wavelength bands beyond the visible spectrum, including near-infrared and short-wave infrared. Drone-mounted multispe...
- Water-Damaged, Wet and Stained Document RestorationCapture of document images across a range of wavelength bands from UV through visible to near-infrared, used in water-damage contexts to separate ink chromopho...