Surviving fragment of a wall painting from the Tomb of the Nasonii in Rome (left), now in the British Museum (1883,0505.4), and a visible-induced luminescence image of the fragment showing the presence of Egyptian blue as glowing white (middle). Photos: ©The Trustees of the British Museum. Shared under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) license. A seventeenth-century watercolor (right) executed by Pietro Santi Bartoli (1615-1700) shortly after the discovery of the tomb. The watercolor—which constitutes a prephotography example of “imaging”—appears to be more faithful to the original painting than the surviving fragment, which has undergone extensive repainting. Photo: Courtesy of the University of Glasgow Library Department of Special Collections.

One normally thinks about the imaging of cultural heritage in terms of photography—whether film or digital. But the word imaging derives from the Latin term imago, which can be translated as phantom, statue, or likeness. In this broad sense, imaging of cultural heritage occurred long before cameras.

Drawings or sculptural replicas, which captured the likeness of their original objects, are precursors of camera imaging. A drawing representing a building and a Roman sculpture duplicating an earlier Greek model are forms of imaging, in 2-D and 3-D, respectively. Other notable examples of imaging through the centuries include the work of the amanuenses who copied earlier texts in the Middle Ages; the seventeenth-century watercolors by Bartoli of the now badly deteriorated Tomb of the Nasonii in Rome; the early nineteenth-century engravings in the Description de l’Égypte, which recorded various aspects of contemporary and ancient Egypt; and later, the campaigns to record in easel paintings the murals at Ajanta. The importance of these images from the past can hardly be overestimated, as they are often the only surviving witnesses to lost originals.

In modern terms, imaging may be defined as the recording and representation of the spatial distribution of information over a surface (in 2-D or 3-D) and across time (video or time-lapse). An important difference between historic and modern imaging is that the latter attempts to reduce to a minimum human interpretation—inevitable in a manual reproduction of an object—by employing a reproducible scientific methodology. In the cultural sector, this recording and representation of information about objects, collections, buildings, sites, and intangible heritage, or any combination thereof—all of which we can refer to as assemblies—can take multiple forms and different scales and serve numerous functions. Examples of information capture through imaging include the reflection of visible light on a surface (as in conventional photography); the transmission of X-rays through an object (as in X-ray radiography); and the effects of acoustic vibration on detached plaster (as in laser speckle interferometry). The information recorded through imaging may also include material identification (such as pigments, fibers, and metals) and condition (for example, deterioration or damage due to the environment or human activity). All fields of heritage studies benefit from the use of imaging techniques—from archaeology to architecture and from science to conservation.

The efficacy of the overall methodology used to capture information is as important as the specific methods themselves. Within a sequence of investigations designed to efficiently answer a question about an assembly, where does imaging sit?

Following archival research on the physical, curatorial, and conservation history of an assembly, the next step is visual observation, with and without magnification. Based on this preliminary information, imaging techniques can offer further details. By providing information on a surface, they can permit meaningful comparisons within the assembly itself, for example by providing a distribution map of the presence of a pigment—as well as offer comparison with other assemblies, for instance by determining which objects on a shelf contain uranium glass. Moreover, they may inform more targeted investigations, which could be invasive or noninvasive, and could be image based or employ point analysis. This investigative process, iterative and incremental, maximizes the representative accuracy of the analysis and, in the case of invasive sampling, minimizes damage. In Cambodia, Light Detection and Ranging (LiDAR)—a surveying method measuring the distance to a target with laser imaging—revealed the unexpected extent of archaeological remains and the level of civilization; LiDAR enabled the identification of areas to excavate and strategies to protect such sites from deforestation, urbanization, and looting. A proper conservation management plan is difficult to design without an appropriate map of an archaeological area.

Imaging, rather than an end in itself, should be part of a methodology that selects the most suitable imaging techniques to answer initial questions and then combines those techniques with other appropriate investigative technologies.

Since its development in the nineteenth century, photography has been used in the cultural heritage fields to document and scrutinize. The wide-ranging commercialization of digital cameras in the 1990s prompted a step change in the development and application of imaging techniques. Since technological development, including that of digital cameras, is normally driven by military, medical, scientific, or consumer needs, heritage professionals have had to adapt others’ innovations to their specific requirements.

A bronze Tibetan Sakyamuni Buddha, dating from the fourteenth to fifteenth century. The X-ray radiograph image (left) does not reveal what the neutron transmission image (right) does—the presence of organic materials within the sculpture. Images: ©Paul Scherrer Institut, Switzerland, Neutron Imaging and Activation Group.

In the past three decades, digital cameras have become easily available to the public. Their popularity has grown exponentially as they have provided increasingly refined digital tools with improved spatial resolution and quality. In addition to visible radiation, digital camera sensors can measure infrared and ultraviolet radiation, making them appropriate tools for the analysis of heritage materials. Moreover, the widespread development of other sensor compounds for infrared radiation (indium gallium arsenide and lead selenide, among others) allows capture of valuable information in other spectral ranges at reasonable cost. In recent years, thermal imaging has become available as a feature for mobile phones, and radar imaging is following soon. This versatility has changed cultural heritage research by providing accessible tools that can address a variety of questions for conservators, curators, and scientists.

The most intuitive and common imaging techniques capture the interaction between light and the matter of which an assembly is composed. However, virtually any nondestructive investigative technique can be considered imaging if it involves more than point analysis and records spatial distribution in more than one dimension. This expands the definition of imaging to encompass a vast range of methods.

In recent years, imaging has evolved beyond basic photography and includes scientific methodologies like chemical and physical characterization to detect, for instance, the presence of organic or inorganic compounds. This type of imaging is normally referred to as chemical imaging. For example, X-ray fluorescence spectroscopy (XRF), which provides information on the presence of elements in the periodic table, is normally performed on a single point. However, when attached to an automated arm that scans the surface under investigation, XRF is transformed into an imaging technique and creates a map of the surface’s elemental composition. Advanced sensors are composed of pixel arrays, shortening acquisition time. Recent innovations include techniques such as Fourier transform infrared imaging, which allows capture of spatial molecular information, in situ and even remotely. It can measure gaseous pollutants in the environment or the chemical composition of a surface. This level of spatial information obtainable in a single scan was unthinkable a few years ago. Similarly, computed tomography, a 3-D technique developed by the medical and material science industries, creates complex volumetric information by mathematically analyzing a large array of 2-D information. The same quality and clarity of information cannot be easily retrieved through conventional radiography.

The complexity, scope, scale, and variety of imaging techniques is enormous and is constantly redefined as technologies evolve. Imaging techniques can be applied at a macroscopic or microscopic level, from satellite imaging, remote sensing, and unmanned aerial vehicle imaging, which investigates large areas of the surface of the earth, all the way down to scanning and transmission electron microscopy, which scrutinizes matter at a molecular and even atomic level. The depth of penetration is another important parameter. In general terms, imaging techniques using ultraviolet radiation provide information on the surface of an assembly, while infrared, X-ray, gamma-ray, and neutron imaging can be highly penetrative and deliver information on otherwise inaccessible underlying layers.

When imaging techniques are applied directly to an assembly, as in a visible image recording color or a thermograph of a building that captures features not visible to the naked eye, they may be defined broadly as noninvasive and, often, noncontact. However, even if a sample is not removed from an assembly, investigations may cause damage. For example, ultraviolet-induced luminescence exposes an assembly to ultraviolet radiation, which may contribute to the fading of sensitive materials like organic colorants and some inorganic pigments. The same imaging techniques can also be applied to samples and are therefore classified as invasive, as in the examination of thin sections of stone, plant and animal fibers, and cross sections of paint samples. It is therefore the nature of the application that qualifies a specific imaging technique as invasive or noninvasive. In general, however, imaging techniques are nondestructive, as the sample can, at least in principle, be reused for other investigations.

Cambodia's Angkor Wat. The image produced by LiDAR scanning (right) reveals the extent and the hidden features of the archaeological site. Images: Courtesy of PT Map Tiga Internasional (PTMI) – Indonesian Geomatics Company; President Director, Francisco Goncalves.

The specific purposes of imaging for the study and interpretation of the cultural and physical history of assemblies, and for their conservation, vary but can be consolidated into three main categories: documentation, investigation, and visualization/communication. These three categories are closely intertwined, and information from one category can also provide information for the others.

Documentation
The importance of documentation has long been recognized by heritage communities, and imaging is considered one of the most efficient means of creating a record for the future. Imaging can therefore provide a terminus post quem for characterization of change. This type of investigation seeks to understand the making of the object, the materials of which it is composed, and the technologies used in its production, history, and use, as well as interventions it may have experienced. It also attempts to record and understand condition. Generally, the study of assemblies faces different challenges depending on the nature of the heritage.

A perusal of the list of Intangible Cultural Heritage inscribed by UNESCO confirms that video recording and photography are pivotal in the documentation of this heritage, which includes performing arts, knowledge, and skills. The faithful capture of this type of heritage is fraught with difficulties, as all of it evolves through time.

While the museum environment is commonly controlled, making it generally easier to carry out imaging in the museum context, the documentation of museum collections nevertheless presents its own challenges. Collections are often highly heterogeneous and may comprise large numbers of objects. Some objects in museum collections may be extremely susceptible to damage and deterioration, making them difficult to document. For example, the texts and hidden contents of very fragile papyri and religiously sensitive objects remained inaccessible to researchers until the recent application of penetrative techniques like X-ray contrast imaging and neutron transmission facilitated the visualization of their contents. Imaging techniques can provide useful and ingenious solutions to a whole range of problems by analyzing and documenting multiple objects at the same time, in their location, and by offering clues for condition and risk assessments.

	A now destroyed arch from Palmyra, Syria, re-created in 2016 in London's Trafalgar Square using 3-D imaging and 3-D printing technologies. Photo: © Amanda Lewis, iStock Editorial, Getty Images.

The documentation of buildings and sites also has its challenges. Built heritage, by its nature heterogeneous, covers vast surfaces, may be difficult to access, and presents a variety of conservation conditions. Imaging techniques can offer suitable solutions, as many methods have evolved both to become highly portable and to provide quick results. Examples include photogrammetry, ground penetrating radar, 3-D laser scanning, LiDAR, and multispectral imaging in the ultraviolet, visible, and infrared ranges, which, when coupled with flashtubes, allows capture of spatial information in otherwise adverse conditions.

Investigation
The importance of imaging techniques for comprehending the cultural significance of heritage extends to all relevant disciplines—including conservation. Understanding the making of and the history of assemblies helps establish significance and provides the foundation for conservation interventions.

In some instances, the ability to visualize hidden information has crucial implications. Conservators sometimes face pressure to clean and remove superficial material from paintings to reveal hidden features, as with blackening and fire-related damage. Whereas the superficial material may not be original and therefore could in principle be removed, practical considerations may make its removal unsafe for the stability of the painting. And cleaning is of course labor intensive and costly. The ability to visualize what may lie underneath a darkened layer could facilitate a satisfactory compromise between the needs of documentation and study and those of safe conservation.

A fundamental role of conservation is understanding original and added materials; the latter may be valued, as in the case of historic interventions, or unwanted, as with past conservation treatments. Understanding the spatial distribution of materials informs conservation strategies and minimizes damage. Imaging can also help identify rates of change, diagnose condition, and investigate decay mechanisms. Imaging technologies have also proven essential in monitoring and assessing the efficacy of conservation interventions. The important contribution made by imaging techniques is the ability to extend information from a single point, to another point, to a surface, and through the passage of time, thereby enabling a careful interpretation of phenomena.

Visualization and Communication
An important aspect of imaging is visualizing the results of documentation and investigation. Images are an efficient means of communicating complex ideas to heritage professionals, funders, and the general public. Several institutions have begun systematic digitization campaigns, which, along with their documentary value, enable stakeholders to access information and advance cultural heritage study.

	A papyri scroll discovered at Herculaneum (PHerc.Paris. 4, in the collection of the Institut de France). X-ray phase-contrast tomography (XPCT) revealed Greek alphabet letters inside the papyri without unrolling it.

An important difference between digital and analog imaging is that the digital signal converts information into numerical values, which can be processed to improve visualization and, in some cases, make visible what is otherwise invisible to the naked eye. Developments in the computing abilities of modern processors have enabled complex data processing and presentation of results. The information is all contained in the images, but the extraction of the information of interest is the challenge.

	The letters, as revealed by XPCT, can be seen above on lines 1 and 2; on line 3 are infrared images of the same letters from an unrolled papyrus (PHerc. 1471 in the National Library in Naples), which was used as a reference for the writing style of scroll PHerc.Paris. 4. Photo/Images: Reprinted by permission from Macmillan Publishers Ltd: Nature Communications 6, Article number: 5895 (2015).

Imaging also plays a vital role in the virtual reconstruction of deteriorated or lost assemblies. Efforts have been undertaken over the years to attempt to visualize, in 2-D and 3-D, the appearance of ancient landscapes, cities, buildings, and objects. For example, a physical reproduction of a monument closed to the public makes it available to all, resulting in less pressure to open it to tourism, which benefits preservation. This happened at Lascaux in southwestern France, where prehistoric paintings faced severe biodeterioration, forcing authorities to close the cave to the public and provide visitors instead with replicas of portions of the cave.

Digital imaging is a subset of digital heritage, which itself was the subject of a 2003 UNESCO charter. Whether born digital or converted into digital format, digital images are not only used for the conservation and study of nondigital objects but are themselves objects of conservation policies. The development of digital data standards is a field of research and development in its own right. Such standards will ensure data interoperability and long-term usability in a world struggling with increasing amounts of data and ever-changing technologies.

Perhaps more than many other investigative point-analysis techniques, imaging has seen an early democratization of many of its tools (often designed to be used in situ) for the analysis of large and immovable objects, for the identification of representative sampling areas, and for recording actions that cannot be repeated or reversed, such as an archaeological excavation or the act of cleaning a wall painting. Imaging is an invaluable tool for the study and communication of world history. Recent intentional destruction of cultural heritage has emphasized the importance of images, which, captured by professionals and tourists, allow for the virtual and sometimes physical reconstruction of lost heritage. Experienced and shared by millions, the world’s heritage lives on in those images. Tragically, the method chosen to advertise destruction of cultural heritage was also imaging, through truculent and disturbing videos.

With the advent of Internet-based media, information is produced, published, and shared as never before. In what some have called our current “post-truth” era, it is important to ensure robust quality control of the data captured and distributed, online or not. This is the case, for example, with the color reconstructions of lost polychromy of sculpture and architecture—a popular online subject—which often do injustice to the artists’ skills and trivialize the efforts of those seeking to study the subject seriously. A benefit of visual digital reconstructions over physical reconstructions is that they are more easily updated and improved when more robust scientific evidence is found. Moreover, it is likely that physical replicas will soon need conservation themselves; therefore, a good balance between research and development in the field of replication and research focused on conservation is crucial in a world of scarce resources. Consider that these new replicas could be the Harmodius and Aristogeiton of our time—lost now, but replicated in antiquity in different media. The engagement and participation of the public in heritage discourse make heritage more relevant and, when managed conscientiously, will help sustain it. For example, digital information is more easily shared and grouped, which helps to create ad hoc databases of information. In addition, accessible open source or free software allows the creation and sharing of 3-D models of built heritage and of museum objects. With public collaboration and a sensible allocation of limited resources, it will be possible to increase awareness of the importance of documentation and appropriate conservation strategies and to fund research for heritage understanding and preservation.

	Images at increasing magnification of a fragment from an Attic red-figure kylix (attributed to the Triptolemos Painter, Greek, 490–470 BCE, JPGM 90.AE.35.42). A sample of black gloss (A) was removed from the edge of the fragment and sectioned using focused ion beam milling (B) to produce a cross section (C), from which transmission electron microscopy can provide images of individual crystal grains (D), and atomic resolution electron microscopy can show the arrangement of individual atoms in the crystal lattice (E). Fragment photo: J. Paul Getty Museum. Microscopy images: GCI and The Aerospace Corporation.

With increasing public interest in imaging techniques and their capacity to visually communicate to experts and the public alike, it is possible that the demand for more sophisticated and accessible imaging techniques will continue to inspire technological progress, which, in turn, will expand the heritage professional’s toolbox (including the currently less accessible chemical and physical imaging). Imaging techniques are uniquely suited to promote dialogue between fields of enquiry, including archaeology, history, art history, science, and conservation. Ultimately, those charged with the responsibility for heritage preservation need to be prepared to adapt existing tools—and design new ones—to address the complexity of heritage protection. Through the use of imaging tools in heritage documentation, investigation, visualization, and communication, it is possible to better assess connections among assemblies, understand and define their significance more effectively, and, eventually, design conservation interventions, assess their efficiency, and monitor their long-term effects.

Like pixels of an image, heritage professionals and the public can diligently and conscientiously contribute to defining a clearer and brighter picture for the future of our past.

Giovanni Verri is a reader at the Courtauld Institute of Art in London.