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SEEING THE WOOD FROM THE TREES - AN EXAMPLE OF OPTIMISED DIGITAL PHOTOGRAMMETRIC 
DEFORMATION DETECTION. 
S.Robson, A.Brewer*, M.A.R.Cooper, T.A..Clarke, J.Chen, H.B.Setan.& T.Short. 
Centre for Digital Image Measurement and Analysis *Hamilton Kerr Institute, University of Cambridge 
School of Engineering, City University, London Whittlesford, Cambridgeshire. 
Tel: +44 171 477 8000 x 3628 77^ 
Fax: +44 171 477 8570 
e-mail stuart@city.ac.uk 
KEY WORDS: Digital, Deformation detection, Panel painting conservation. 
ABSTRACT 
Many historically important paintings have been produced on wood panels. Unfortunately variations in environmental 
conditions can have a detrimental effect on the condition of such paintings. This paper describes a multi-image digital 
photogrammetric approach and some of the subsequent data analysis undertaken to evaluate the deformations 
occurring in 74 retro-targeted mock panel paintings over a three month period of controlled environmental variation. 
Target image locations and correspondences were computed automatically to sub-pixel accuracy. Bundle adjustment 
procedures based on scale constraints were then applied to produce three dimensional target coordinates with their 
associated full covariance matrix for each panel at each measurement epoch. A suite of rigorous deformation analysis 
software was used to determine the changes in shape occurring in each panel. Finally the deformed surface information 
was transferred to a CAD environment for visualisation by the art conservator. Visualisation included the use of NURB 
surfaces and advanced rendering techniques. 
1. INTRODUCTION 
Advances over the last decade in digital camera sensor technology and computer hardware mean that automated 
inspection methods based on the measurement of digital images are feasible. The geometric fundamentals of CCD 
sensors are now sufficiently understood to enable photogrammetric algorithms, traditionally applied to photochemical 
based imagery, to be used reliably in a modified form with imaging systems based on silicon sensors. This paper 
describes an example of the use of such a system and some of the techniques developed for measurement of 
deformation in the field of fine art conservation. 
Many historically important paintings have been made on wood panels. For their preservation in storage, transit, or 
exhibition, many such paintings require additional reinforcement that still allows for wood movement. Unfortunately 
variations in temperature and humidity interact with paintings and reinforcements and can lead to deformations, which 
have a detrimental effect on a panel painting's condition. The use of techniques such as "thinning", whereby the wood 
constituting the painting support has been reduced in thickness and a supporting lattice or cradle added, has often 
intensified such effects. To investigate such deformations, a controlled environment study that would subject panel 
painting mock-ups to cycled humidity and the restraining effects of a series of overall reinforcements, whose designs 
were drawn from traditional and current practices, was planned. Approximately twenty different types of reinforcement 
were constructed and applied to each of four wood types used traditionally as panel painting supports. Digital 
photogrammetric analysis was selected as a technique which could provide data appropriate for the evaluation of the 
behaviour of each of the experimental panel painting types. 
2. IMAGING SITUATION. 
The measurement requirements of this project were dictated by the needs of the art conservator who required an 
objective measure of panel surface shape and size. A sequence of 25 panel surface shape measurements was required 
over a three month period on each of 74 different mock panel paintings. Measurements of the surface of each panel had 
to be of sufficient precision (aprox. 100jum) to not only detect change in shape and size, but also to provide data suitable 
for further research into the mechanical properties of the different wood panel construction types. 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995