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File:K Re L.pdf File:K Re L.pdf Visible light refers to all light sources that rely on wavelengths of light that are visible to the human eye. This generally encompasses 380 to about 750nm.

Image of wavelength of light

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Normal illumination

“Normal or reflected illumination provides a record of the appearance of the object as seen under standard viewing conditions. Generally, this means using relatively flat and uniform illumination, with minimal surface glare, although this can vary depending on the kind of object and the purpose of the documentation record. For Before Treatment photographs, this image will also serve to record the relative prominence of disfigurements as seen in a normal viewing situation. These disfigurements or conditions can then be recorded more definitively in subsequent images using non-standard (non-normal) illumination techniques.” (Kushel 2017, p. 113)

Raking illumination

Raking light is an illumination technique where a single light source is positioned almost parallel to the object. As a result of the light placement, the surface topography and texture of the subject plane are emphasized through the creation of shadows. This examination technique can amplify the appearance of indentations, undulations, inlays, textile weaves, plate marks,  paint cupping, relief objects, and tool marks amongst others. Kushel (2017, p.116). Raking light is used primarily when photographing 2D objects to capture the surface topography. This approach brings out surface variances including brush strokes, cracks, folds, lifting pigments, and undulations in the substructure. It is particularly useful in documenting the condition and treatment of drawings, paintings, prints, and watercolors. Raking light can also be used for documenting the surface of 3D objects, though it is more effective with low-relief object.

Schematic illustrating a the position of the light source and camera for raking light. The light source is placed low at to the left or top of the object as it is normally viewed. The camera is mounted to a copy stand and is facing straight down on the object.

Specular illumination

Overview

This technique involves positioning the camera and illumination source so that the camera captures a specular reflection off the surface of the object. Specular illumination is used to document surface topography and planarity,  variations  in  surface  sheen,  the  presence  of  coatings,  etc. With moderately reflective surfaces, such as paintings, specular illumination can sometimes be more informative  than  raking  illumination  in  rendering  surface anomalies,  since  these  will  be  visible  no  matter  their orientation. However, the depth or height of the surface texture  will  not  be  as  clearly  indicated  as  in  a  raking photograph.  There  are  two  basic  setups  for  specular illumination: axial and oblique. (AIC Guide 118)

Axial Specular

The light source and the viewer are on the same axis in this technique.  The  camera  film  or  sensor  plane  is  positioned parallel to the subject’s surface, and the lamps are placed adjacent to the camera. Kushel (2017, p.119)

Oblique Specular

The light source and the viewer are positioned on opposite sides of the subject and at the same angle from the plane of the subject in this technique. Although oblique specular is  by  far  the  most  commonly  used  specular  technique  in visual examination, it is the least commonly photographed. Standing opposite a window to examine the surface of an object lying on a table and kneeling down to examine the surface of a painting exhibited in a museum with overhead lighting are two common examples of the use of oblique specular illumination in everyday practice. Kushel (2017, p.119)

Transmitted illumination

Transmitted illumination

“With transmitted illuminations, the object is lit from the side opposite the viewing position. Light that is able to penetrate the object is recorded. Transmitted illumination is used to show variations in density, thickness, visual opacity, lacunae, ruptures, etc. Common applications include documenting paper structure, watermarks, and repairs, tears, and abrasions in prints and drawings; documenting characteristics of paint application or highlighting small losses, tears, and crack patterns in canvas paintings; and highlighting cracks, ruptures, or losses in freestanding objects. This last application is generally carried out by transilluminating the object with a localized source, such as a fiber optic light. For objects such as glass, for which transmitted light is part of the normal perception of the object, using transmitted light in combination with incident illumination can capture the “normal” appearance” Kushel (2017, p.121) from Section 6.2.4 of The AIC Guide.

Figure 1. Schematic illustrating a top view of a vertical backlit illumination and camera setup. The light sources are placed angled and equidistant behind an easel-mounted or otherwise supported diffuser. Extraneous light must be blocked using fabric or mat boards around the edges of the object once it is placed in front of the diffuser. The camera is also placed in front of the diffuser.

Best Practices

1. An ambient light source may be useful to light a reference target placed in the scene. Although the target illumination will be different from the object illumination, and thus may be minimally useful for quantitative calibration of the image, the inclusion of a repeatable target and target illumination strategy is a step toward a more standardized strategy for transmitted illumination setups, perhaps until transmitted targets become more common

Figure 1.a. Images showing theplacement of two light sources behind the easel-held frame.

Figure 1.b. Images showing the placement of two light sources behind the easel-held frame and diffusing material.

Figure 2. Image showing the camera positioned in front of the backlight setup after it has been shrouded in stray light-reducing black fabric.

Resources

1. A Harvard Art Museums blog post by Leonie Müller entitled Understanding Paper: Structures, Watermarks, and a Conservator’s Passion includes examples of images captured of the structure and watermarks of laid paper using transmitted light photographic techniques: https://harvardartmuseums.org/article/understanding-paper-structures-watermarks-and-a-conservator-s-passion
2. A blog post entitled Looking at Works of Art on Paper: An Overview of Examination and Imaging Techniques by Lindsey Tyne at The Morgan Library and Museum also describes the advantages of transmitted light imaging for visualization of the structure and features of laid paper: https://www.themorgan.org/blog/looking-works-art-paper-overview-examination-and-imaging-techniques
3. Kushel, Dan. 2017. “Transmitted Illumination.” In: Warda, J (ed.) The AIC Guide to Digital Photography and Conservation Documentation, 3rd ed. Washington, DC: American Institute for Conservation of Historic and Artist Works, 121-123. D. R. Wyble, “Spectral Imaging Method for Transmissive Media,” in Archiving 2021 Final Program and Proceedings, 2021, pp. 51–55.