Becoming a NIR-Sensitive Aerial Archaeologist
Ghent University - Department of Archaeology Blandijnberg 2 B-9000 Ghent (Belgium)
[email protected] Ph.D. fellowship of the Research Foundation - Flanders (FWO)
PRINCIPLES
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FUTURE PLANS
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380 nm
Infrared
Near InfraRed (NIR) radiation 700 nm
1400 nm
51% NIR
27% NIR
The images one the left show the same scene, shot simultaneously by a normal and the NIR-converted Nikon D50. Only the latter indicates archaeological features. photograph by F. Vermeulen
8% visible 13% visible
healthy green vegetation
Reflectance (%)
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0
stressed green vegetation
For the third image, the pixels produced with the blue-sensitive photodiodes were divided by the pixels resulting from the redsensitive diodes. This operation makes several linear features very noticeable.
Wavelength (nm)
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Digital NIR-photography Colour InfraRed (CIR) photography Colour photography
THEORY & PRINCIPLES 1
4 PROCESSING & RESULTS
AIMS 2 Archaeological features often can be discovered from the air by photographing stress-induced discolorations in vegetation, known as (positive or negative) cropmarks.
3 EQUIPMENT A normal digital camera contains a silicon image sensor covered with a Colour Filter Array (CFA) and a hot-mirror/NIR-block filter. This combination creates images with visible light only. NIR-radiation
hot-mirror
photograph by F. Vermeulen
hot-mirror + CFA + digital sensor
Due to the significant difference in NIR-reflectance between stressed and healthy green vegetation (see 1 ), NIR-photography should enhance these stress patterns, making them more pronounced (or visible at all).
1 Normalized response
Negative cropmark showing a Roman rural temple
0 All pictures and illustrations by the author unless otherwise indicated. (1) Adapted version of Moh, J. e.a., Characterization of the Nikon D70 Digital Camera, scien.stanford.edu/class/psych221/projects/05/joanmoh/index.html (2) Adapted version of HOYA Corporation USA Optics Division, Infrared Transmitting Filters (R72), www.hoyaoptics.com/color_filter/ir_transmitting.htm
An aerial NIR-photo straight from the camera is shown on the right. The second image is the result after histogram stretching and local contrast enhancement.
(1)
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Replacing the hot-mirror with an NIR-filter permits the acquisition of NIR-photographs, because silicon is very NIR-sensitive.
visible radiation
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Visible radiation
Visible
Photographing NIR is performed by capturing RAW imagery, while a preset White Balance (WB), manual exposure, an aperture of f/8.0 and ISO 200 are chosen.
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Building a rig to capture simultaneously NIR and colour images, enabling false CIR-imaging and calculation of Vegetation Indices (VIs).
CF A
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Determination of the colour filter spectral response curves, which are essential in choosing additional filters and image algorithms.
dio wa
ar mm Ga
Sun emits electromagnetic radiation
Geert Verhoeven
0 400 500 600 700 800 900 Wavelength (nm) (2)
The D-SLR used is a NIR-converted Nikon D50 combined with a Nikkor 20 mm f/3.5 AI-S or AF-S DX Nikkor 17-55 mm f/2.8G IF-ED, two lenses that do not produce hot spots. As no filter is needed on the lens, the viewfinder will not be blocked, allowing to compose and focus as usual.
