THE USE OF DIGITAL IMAGE PROCESSING TO CLARIFY THE RADIOGRAPHY OF UNDERPAINTING
James R. Druzik, David L. Glackin, Donald L. Lynn, & Raim Quiros
WHAT THIS ASPECT of the study generated was a series of images which taken alone yield some, but not a great deal of new information compared to what was present in the original material. There are three features in Figure 4 which are obscure in the original x-ray, particularly when displayed on a high resolution computer monitor. These features are the face of the seated woman, the left hand of the man in the left part of the painting, and what may be a person looking in through the window. After these features are pointed out, they can be found with difficulty in Figure 2, but they required the processed image to find them initially. The hand stands out better after the removal of the wood grain striations, because the striations were parallel to the fingers. The face stands out better after removal of the over-image.
Digital image processing does not create information which is not contained in the original images in some form. What is achieved, rather, is the isolation of discrete units of visual information with a common numerical “denominator.” Taken as a block of visual images and information, and aided by a knowledge of the processing steps, the conservator is led through the many levels of visual data step by step to clarify the relative contributions of both images and, perhaps, to give a little more insight into the structure and state of completion of the under-image.
It should be kept in mind that this was a very complex example, as several factors interfered to make this study more difficult than what might be expected with other Northern European paintings. In particular, the fact that a large amount of lead white occurs in the regions where we had hoped to gain the most insight into the lower painting created an unfortunate and unanticipated complication.
THE AUTHORS would like to acknowledge the patience, encouragement and continued support of William Liesher, Head of Conservation, and Dr. Pieter Meyers, Senior Research Scientist, Conservation Center, Los Angeles County Museum of Art; Glenn Garneau for assistance with the computer analysis and Jean Lorre for his many helpful technical discussions, Jet Propulsion Laboratory, Pasadena, California. Additional appreciation goes to Joe Fulton for his PDS microdensitometry at JPL, and Steve Cristen-Poucher for his many suggestions early in the design of this project. Lastly, a very special acknowledgement goes to John Gebhardt, Conservation Photographer, Los Angeles County Museum of Art, for his excellent images.
This work was supported under the Cal Tech President's Fund # PF-183. This paper presents one phase of research conducted at the Jet Propulsion Laboratory/California Institute of Technology, under NAS 7–100, Aeronautics and Space Administration.