Proceedings e report - Firenze University Press

WOOD SCIENCE FOR CONSERVATION OF CULTURAL HERITAGE
The hardware needed for DICT is simply a digital camera suited for photogrammetry and having a
reasonably high resolution and a homogenous illumination (Fig.4 left). Until recently resolutions
exceeding 10 Megapixel could only be achieved by mechanical scanning. For example, the results
presented below were obtained using a Rollei SX6006 having a CCD line detector with 5000 pixels
(size: 15 µm x 15 µm). The detector was scanned mechanically in 5048 steps over the image plane
resulting in a resolution of more than 25 Megapixel. Even today the so-called micro-scan principle is
frequently used to increase the resolution of matrix detectors. In this type of camera, the detector can
be shifted 1/2 pixel (or some other fraction of a pixel) in both directions of the detector plane. In such
a way four images with different detector positions can be taken and combined into one highresolution
image. A detector having 2048 x 2048 pixels makes thus images having an effective
resolution of 4096 x 4096 pixels.
Lamp
Object
PC
Camera
Lamp
Screen with
correlation fringes
Fig. 4: Set-up for DICT measurement on beams of half-timbered houses (left) and a typical result (right)
By acquiring a reference image before and a final image after applying a load to an object (e. g., a
climatic change) the displacement can be calculated by dedicated algorithms. These compare a given
pixel in the reference image with every pixel in the final image in the following way: A subset of n x n
pixels centred around each of the pixels to be compared is defined. The cross correlation function
between these two subsets is calculated. The pixel in the final image giving the largest correlation
coefficient is identified with the pixel in the reference image so that the displacement can be calculated
(Fig. 4 right). The procedure is repeated for every pixel in the reference image. This is obviously a
time-consuming procedure. For practical reasons it is often necessary to limit the number of final
pixels to be compared with a given reference pixel. This is normally done by defining a search region
of size N x N around the reference pixel with n < N ≤ 100.
Fig. 5: Displacements of a beam of a half-timbered house in x- and y-direction along a line parallel to the beam
(left) and comparison of DICT and strain gauge data (right)
The displacements obtained in such a way are often displayed by vectors as shown in the right part of
Fig.4 for the example of a beam of a half-timbered house. The image shows also a conventional strain
gauge which is mounted in order to compare their data with DICT measurements. The displacements
along a line parallel to beam are plotted in the left part of Fig.5 separately for the x- and the y-
171
mm
DICT displacement
Strain gauge displacement
Date