II.2. Photography and Other Imaging Techniques for the Visualization of a Sculpture

Many bronze sculptures have reflective or glossy surfaces (fig. 293), and particularly in these cases, a uniform and diffuse light will help minimize specular reflection. Diffuse lighting means that light is directed at the subject from large-area sources, usually accomplished by use of reflection umbrellas or diffuser screens in front of the light sources or by using flat-panel LED arrays. Normal flash photography, which involves a single small light source near the camera, is usually undesirable. Normally, at least two light sources are recommended. For documentary photography, shadows should not be too “hard” and there should be more even light than for catalogue or commercial images, where dramatic lighting is often preferred.¹

Use lights that are designed for photography and have a high color rendering index (CRI) and always include a standard color and scale chart. For documentary photography, use only one type of illumination source at a time; if light sources of different color temperatures are used (such as window light in combination with photo lamps), the color rendition may be significantly different on different parts of the subject. In most circumstances, it is recommended to use a tripod with the camera set on “aperture priority.” Selection of a high f-stop will significantly improve sharpness and depth of field, but will require longer exposures, which in turn necessitates a tripod. In situations where a high f-stop is not adequate to capture the required depth of field, a series of images may be captured, with the focal point moving stepwise from the nearest to farthest point on the sculpture. So-called focus stacking software has become increasingly available in common image processing programs, allowing the image set to be blended into a single image where all areas appear in focus. A tripod is essential for focus stacking, and all individual images should be archived along with the stacked image.

Particularly with dark sculpture, the full dynamic range (the difference in luminance between the darkest and lightest areas) may not be captured in one exposure. In such cases multiple, “bracketed” images of different exposures should be captured. High dynamic range (HDR) algorithms can composite different exposures into a single image, but always retain the original files.

Open viewer Figure 293

For the documentation of fine surface topography such as tool marks and evidence of wear, raking-light photography may be useful. Here, instead of diffuse lighting, a single, strongly directional light source is positioned at a very low angle to the surface, typically around 10 degrees. This arrangement causes minor irregularities in surface topography to cast shadows, making them easier to perceive. Altering the direction of the light source can reveal different features on any given surface. Raking light can be useful for standard photography, macro photography, and even photomicrography (see II.2§2.1 below).

In many circumstances, close-up or macro photography will be necessary to resolve details of technical interest. Specialized macro lenses, which are designed to minimize distortion, are preferred for this purpose. The closer-up the image is, the smaller the depth of field will be, and thus the greater importance of using a high f-stop. In situations where it is desirable to hold the camera by hand, dedicated macro flash units, such as lens-mounted ring flash units, can provide sufficient diffuse or directional light for high speed and high f-stop macro imaging.

The quality of images produced will depend on both the resolution of the camera’s sensor (fig. 369) and the quality of the lenses used. For this reason, a professional-quality digital camera is recommended. As of the year 2023, a typical mirrorless camera may be equipped with a 8200 × 5500 pixel (45 megapixel) sensor, which will capture a one-meter-high statue with a vertical resolution of 82 pixels/cm, corresponding to a pixel size of 0.12 mm on the statue. Professional medium-format cameras with 100+ megapixel sensors can be of great benefit, but are cost prohibitive for many labs.

Open viewer Figure 369

The time required to photograph a bronze sculpture in a professional studio can vary dramatically depending on the nature of the object and the number of details required. The simple documentation of six low-gloss statuettes via four or five general views of each might take two and a half days: half a day for the setup, one day for the photographs, and one day for post-processing. On the other hand, the full documentation of a single large, dark, and glossy bronze with numerous details may take more than six days: half a day for the setup, two days for capturing images (which will require several lenses), and four days for post-processing.²

The imaging of surface features substantially smaller than one millimeter (polishing marks, details of tool marks, small inclusions, porosities, excrescences) may require higher magnification than macro photography, namely photomicrography. Traditional stereomicroscopes installed on articulated arms are very useful for this purpose, and these can be fitted with cameras for image capture.

High-quality digital microscopes (without eyepieces) offer additional capabilities and are increasingly widely used for technical examination of bronze sculpture. Most importantly, these microscopes usually incorporate automated focus stacking procedures that overcome the shallow depth of field inherent to photomicrography. Because the focus adjustments are precisely controlled by stepper motors, the microscope’s software can reconstruct a scaled 3D model of the field of view. Such topographic reconstructions can be useful for documenting, characterizing, and comparing fine features such as tool marks (figs. 273, 285, 286, see Case Study 4). With this type of 3D model, the sources of errors for precise calculations are numerous and measurements should therefore be considered with caution. Digital microscopes often have polarizers incorporated that can greatly reduce specular reflection, and some can be adapted for use with ultraviolet illumination.

If extremely fine, high-precision topographic measurements are required, a method called microtopography may be useful. Various techniques are available, with resolutions as small as 0.1 µm. This technique is expensive, slow, and can generally be used only on samples smaller than 20 cm across.³

There are also a wide range of low-cost handheld digital microscopes available with a variety of magnification ranges that can be tethered directly to a laptop computer. Some models are equipped with ultraviolet and/or polarized light sources. While the image quality is generally not as good as more expensive laboratory- and research-grade equipment, they may be adequate for routine examination.

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Endoscopes (alternately called borescopes) are common tools for those studying the techniques of bronze sculpture. They are rigid or flexible tubes with a lens at one end and some optical or digital means to transmit an image through the lens to an eyepiece, camera, or digital sensor. In most cases, a light source is incorporated into the endoscope’s tip. They can be used to examine the interior of a hollow sculpture by inserting the boroscope into the base (or any aperture) (figs. 38, 370). Image quality tends to be low, and images generated by a boroscope tend to be difficult to interpret because there is no external frame of reference, so careful attention to image annotation and captioning is critical. Recording video with audio narrative can also be an effective method of documentation.

The cost of endoscopes can vary tremendously. Models produced for industry or the medical field that have articulated heads (whose direction can be controlled from the exterior) and software that enables making measurements may be priced in the tens of thousands of US dollars, while simple USB models are available for less than one hundred US dollars.

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Reflectance transformation imaging (RTI) can be of great utility in documenting and visualizing fine surface topography as well as color and gloss. In this method, both the object and the camera are fixed; only the point light source (typically a flash unit) moves. A large number of photographs are taken of the object (usually between twenty-four and one hundred) as the light source is systematically moved around the object (fig. 371). A reflective sphere is placed into the frame of each image and the position of the specular reflection of the light source on the surface of the sphere is used to calculate the direction of the light source. The software for compositing and viewing RTI images is open-source and free, making the technique readily accessible (fig. 241). Several mathematical image enhancement features are available that allow interactive relighting and enhancement of color and surface shape attributes.⁴

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Organic materials such as varnish, pigments, glue, resins, and a variety of restoration materials may fluoresce visible light when exposed to (invisible) ultraviolet light (UV). Photography of UV-induced visible fluorescence (often misleadingly called UV photography for short) may thus prove useful to visualize such materials on the surface of a sculpture (fig. 191). The method may be used in conjunction with macro photography and photomicrography.

Several types of UV light sources are available, including LEDs, fluorescent tubes, and arc lamps. Generally, lamps emitting at 365 nm with minimal visible light emission are preferred. Standard digital cameras can be used for UV fluorescence photography, but require special filters in front of the lens to block UV and infrared (IR) transmission to the camera’s sensor. For details see Pozeilov 2015.

Open viewer Figure 191

Repair patches, inlays, and other discontinuous areas of a bronze are often thermally isolated from their surrounding regions. When the surface is heated, for example by the sun or a tungsten lamp, the heat will be kept longer there, dissipating more slowly than on the surrounding large surfaces. IR thermography (IRT) typically uses a camera sensitive to long-wave IR light (about 9000–14,000 nm) to image heat transfer and buildup on an object’s surface. The thermograms thus obtained have been used on bronze sculpture to investigate and map defects, mechanical and cast-on repairs, as well as inlays.⁵ Thermography in the mid-IR (about 3000–5000 nm) has also been applied to cultural heritage.⁶ Imaging in the near-IR (about 700–10.1 µm) or short-wave IR (1100–2500 nm), commonly used on many other types of artwork, is generally of limited utility in the examination of bronze sculpture.

In photogrammetry, many photographs are taken of the subject from different angles and a 3D model is derived from a computational analysis of the individual images using dedicated software (fig. 373). Most photogrammetric work done in cultural heritage has been carried out with professional-grade digital cameras, though it is also possible to build very high resolution models using medium-format cameras that record images of 100 megapixels and higher. Applications for smartphones are also now also available at lower resolution.

Depending on the quality and resolution of the source images, photogrammetry can provide a quite high-fidelity model of a bronze sculpture with good color accuracy, particularly if the photography is done in a studio with controlled lighting. These images can be useful for both public and scholarly audiences, providing the opportunity for high-quality web-based interactive viewing. Photogrammetry can also be carried out using UV and IR photography.¹⁶ Photogrammetry is being used more and more for bronze sculpture, notably to design models (fig. 374) and mounts (fig. 375), or to test reassembly of fragmentary statues. Large-scale bronzes can be captured using scanners mounted to drones.¹⁷

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Structured light scanners shine a regular pattern of light (usually a series of parallel lines) onto an object and then record an image of the pattern with a camera that is offset from the projection source. Software then analyzes the apparent distortion of the light patterns by the three-dimensional surface and generates a computer model of the form. Most structured light scanners also record standard photographic images of the subject using onboard flash lighting, which alternates at high speed with structured light pulses.

The lenses and sensors used in structured light scanners are generally of lower quality than a professional digital camera, and lighting and color management are often a challenge.¹⁸ As a result, structured light scans often yield a final model that is less realistic and detailed in appearance than a photogrammetric model (figs. 376, 377), though the dimensional accuracy may be as good or superior. It may thus be used for precise measurements (fig. 378, see II.4§2.2.2).

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Most laser scanners used in cultural heritage employ a rapidly rotating mirror to deflect a visible or invisible laser beam in a pattern that sweeps across the subject. The scanner integrates a range finder that accurately measures the distance to each of the millions of points that the beam traces, building up a high-resolution 3D model of the form. Often multiple scans from different vantage points are combined to produce the final model (fig. 379). Most laser scanners integrate cameras to provide “texture,” or photographic detail, that can be overlaid onto the 3D model (figs. 380, 381). However, as with structured light scanning, the relatively low quality of the cameras used can result in models that are less realistic and detailed in appearance than a good photogrammetric scan.

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