The Next Generation of Optical Coherence Tomography (OCT) for Art Conservation - in situ non-invasive imaging of subsurface microstructure of objects

对艺术作品进行科学检查对于保护、保存和了解材料变化至关重要。理想情况下，需要使用非侵入性检查方法。光学相干断层扫描（OCT）是一种非侵入性、非接触式成像技术，专为眼睛和其他生物组织的体内成像而设计。OCT是一种快速扫描迈克尔逊干涉仪，能够对地下微观结构进行三维成像。2004年，首席研究员领导了一项开创性的合作，将OCT应用于绘画。同年，另外两个小组也报告了对玉器、陶瓷和绘画的OCT检查。除了对油漆层和清漆层进行无创地层学检查外，由于其高动态范围和深度选择能力，OCT也被证明是揭示油漆层下预备底纹的最敏感技术。OCT已被用于动态监测不同清漆的润湿和干燥，使用溶剂去除清漆，清漆层的实时激光消融以及由于环境变化而导致的帆布变形的跟踪。OCT已在古代玻璃、珐琅、陶瓷、玉石、彩陶和羊皮纸的检测中得到应用。我们目前的研究表明，OCT有潜力成为博物馆中常规的非侵入性工具，允许在没有其他方法获得地下信息的完整物体的任何地方进行横断面成像。OCT可以超越定性成像，走向光学特性的定量测量，提供老化过程的信息，并协助材料识别。虽然目前的oct在这一领域已经显示出潜力，但它们针对生物医学应用进行了优化。一些主要的限制是：(i)与传统的油漆截面显微检查相比，深度分辨率较低；（ii）通过高散射涂料探测深度有限。需要小于~4微米的深度分辨率来分辨最薄的清漆和油漆层，并且需要更长的操作波长来增加穿透深度。OCT的深度分辨率与源带宽成正比。近年来的OCT研究已朝着开发新型宽带信号源的方向发展。用于生物医学应用的OCT系统通常限制在800nm和1300nm之间的波长，以便在吸水和组织散射之间取得最佳折衷。然而，艺术保护的要求却大不相同；我们最近对历史艺术家颜料透明度的调查显示，三分之一的油画颜料在波长为2000nm时的透明度比在波长为900nm时的透明度提高了50倍。很少有超过1300纳米的OCT系统建成。商用OCT在任何波长下的最佳分辨率都在6微米左右。我们建议使用新型超荧光光纤源探索更高分辨率和2000nm波长的OCT系统，推动OCT的界限，以匹配目前使用的样品横截面显微检查所提供的信息内容。本项目旨在探索新一代艺术OCT可以帮助解决的保护和艺术史上的新问题，并推动非生物材料近红外OCT成像的界限。它将通过增加深度分辨率和穿透度来显著提高OCT的能力，以减少采样的需要，并使无法采样的完整物体上的地下微观结构能够成像，鼓励更频繁和彻底地检查整个物体，以早期预警恶化，提高艺术史研究的可见性和分辨率。更好地为保护策略提供信息，并在保护成本方面创造长期节省，从而牢固地确立OCT作为遗产领域非侵入性成像工具的地位。

Scientific examination of works of art is essential for conservation, preservation and understanding of material change. Ideally non-invasive methods of examination need to be used. Optical Coherence Tomography (OCT) is a non-invasive, non-contact imaging technique designed for in vivo imaging of the eye and other biological tissues. OCT is a fast scanning Michelson interferometer capable of 3D imaging of subsurface microstructure. In 2004, the principal investigator led a collaboration pioneering the application of OCT to paintings. In the same year, two other groups also reported OCT examination of jade, ceramics and paintings. Apart from the non-invasive examination of the stratigraphy of paint and varnish layers, OCT has also been shown to be the most sensitive technique for revealing preparatory underdrawings beneath paint layers owing to its high dynamic range and depth selection capabilities. OCT has been used for dynamic monitoring of the wetting and drying of different varnishes, varnish removal using solvents, real time laser ablation of varnish layers and tracking of canvas deformation due to environmental changes. OCT has found application in the examination of ancient glass, enamel, ceramics, jade, faience and parchment. Our current research has shown that OCT has the potential to become a routine non-invasive tool in museums allowing cross-section imaging anywhere on an intact object where there are no other methods of obtaining subsurface information. OCT can go beyond qualitative imaging toward quantitative measurement of optical properties giving information on ageing processes and assisting material identification.While current OCTs have shown potential in this field, they are optimised for biomedical applications. Some major limitations are: (i) lower depth resolution compared to conventional microscopic examination of paint cross-sections; (ii) limited probing depth through highly scattering paint. A depth resolution of less than ~4 microns is needed to resolve the thinnest varnish and paint layers, and the operation wavelength needs to be longer to increase the penetration depth. The depth resolution of OCT is proportional to the source bandwidth. OCT research in recent years has moved towards development of novel wideband sources. OCT systems for biomedical applications are generally restricted to wavelengths between 800nm and 1300nm for the best compromise between water absorption and tissue scattering. However, the requirements of art conservation are very different; our recent survey of the transparency of historical artists' pigments has shown that a third of pigments in oil have >5 times improvement in transparency at wavelnegth of 2000nm compared with 900nm. Few OCT systems have been built beyond 1300 nm. The best resolution commercial OCT at any wavelength is around 6 microns. We propose to explore OCT systems at higher resolutions and at 2000nm wavelength using novel superfluorescent fibre sources, pushing the boundaries of OCT to match the information content given by the microscopic examination of sample cross-sections currently employed. This project intends to explore new problems in conservation and art history that the next generation OCT for art can help to solve and push the boundaries in near infrared OCT imaging for non-biological material. It will significantly improve the capabilities of OCT through increasing the depth resolution and penetration in order to reduce the need for sampling and enable the subsurface microstructure to be imaged on intact objects where sampling is not possible, encourage more frequent and thorough examination of the whole object for early warning of deterioration, improve the visibility and resolution of underdrawing for art historical research, better inform conservation strategy and create long term savings in the cost of conservation, and hence firmly establish OCT as a tool for non-invasive imaging in the heritage field.