Confocal Microscopes Used In Art Galleries
Aside from just their concerns about the appearance of the art display in their galleries, curators of art galleries are also very much concerned about the conservation of the artwork and the art works authenticity. Microscopes have played a role in these latter activities since the about the time of the 1930s. Various imaging techniques, including X-radiography, reflectography, macrophotography, UV-fluorescence and raking light source at a low angle to the surface imaging have all their advantages and share of disadvantages. Confocal microscopy is most often useful compared to the other methods for the purpose of examination of subsurface structure. However, the close working distance of just a few mm involved when using the confocal microscope makes it precarious to use on liable masterpieces. More recently, Haida Liang, Marta Cid, Radu Cucu, George Dobre, Adrian Podoleanu, Justin Pedro, and David Sauders have demonstrated the usefulness of the optical coherence tomography or OCT for the nondestructive examination of the artworks in galleries.
This method called Optical Coherence Tomography is said to be more commonly used to examine biological specimens. Liang and the other members used two different OCT systems, each operating at two different wavelengths, to examine specimens en-face. This means that the specimens are scanned in layers rather than in a series of cross-sections. They showed the OCT, compared to the confocal microscope, gives a higher dynamic range through the thickness of the paint. This is because the OCT takes advantage of the coherence properties of light and registers only correlated signals. Furthermore, the OCT method amplifies the weak signal from the object arm when examining the specimen, mixing it with the strong signal from the reference arm. This technique is believed to give approximately twice the penetration depth of confocal microscopy in samples that strongly scatter light. Example of such as layers are that of aged varnish and paints. Perhaps most importantly, the OCT method requires a working distance around 2 to 3 centimeters, thuskeeping the instrument safely away from the art specimen. The en-face OCT images could be acquired in a way that easily relates to what is seen with the naked eye, making navigation around a painting very intuitive. Information in the z axis showed the thickness of the layers of varnish, paints, and even the underdrawing or the sketch made prior to the application of paint, and show the type of drawing if it is solid or liquid based, and the layer on which the drawing was made.
The ability to acquire this type of information in a non-destructive way with the help of the OCT method as compared to the traditional confocal microscope has profound implications for art conservators and curators. For example, the layers of varnish provide objective data about the history of conservation efforts. The study of underdrawings is particularly useful for understanding painting techniques and for attributing works of art to specific artists. Liang and the other members convincingly demonstrated that OCT provides better microscopic images of the surface of the varnish and paint layers than any other system, not just that of the confocal microscope technique, that is currently employed in the examination of museum paintings. It also gives the best dynamic range and resolution of images of underdrawings than other techniques because this interferometric technique takes advantage of the coherence properties of light. OCT is particularly well suited for the examination of paintings because it provides non-invasive imaging across the surface of the specimen, and the modes of acquisition can be changed to give additional information. One would predict that the method of OCT will become a major player in the armamentarium of art conservators around the world. The art and science of conservation of artworks just got better, and the possibility of forging artworks just got harder with the help of this new technique.