Visible light imaging

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Brief intro to visible light imaging ....

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Normal illumination[edit | edit source]

The following definition is from Section 6.2.1 of the "AIC Guide to Digital Photography and Conservation Documentation" (Warda et al. 2017, p. 113):

“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.”

Two-Dimensional Objects on a Copystand[edit | edit source]

Equipment[edit | edit source]

This section is in progress.

Lighting[edit | edit source]

The following workflow information is from Section 6.2.1 of the "AIC Guide to Digital Photography and Conservation Documentation" (Warda et al. 2017, p. 113):

“The light source can be simple incandescent fixtures (photofloods) or diffuse photographic light sources, such as fluorescent fixtures or HID sources (commonly installed in enclosures with diffusing screens). For relatively flat objects with pronounced texture (e.g., flat textiles), diffuse light sources, or a combination of diffuse and a single focused source, are generally best in order to avoid creating confusing sets of double shadows.

Lamps should be positioned approximately 25˚ from the surface plane of the subject; this will minimize surface glare. Positioning lights at 45˚, often mentioned in photographic manuals dealing with copy photography, is not recommended. This angle will nearly always result in unacceptable glare on any reflective surface unless other measures are taken (e.g., using polarized illumination or increasing the camera distance by using a longer focal length lens). In the studio, painting reference lines on the floor to indicate light placement increases efficiency and standardization.

To achieve uniform illumination across a subject’s surface, position lamps as far from the subject as possible. They should be aimed slightly beyond the far edges of a relatively flat object, not at its center. Uniformity of illumination can be measured with an incident light meter held at the center and all four corners of the subject.

Using barn doors on the lamps can further help even the illumination (especially at the edge of the object nearest the light) and help prevent light from striking the camera lens. If working on a copy stand, a lens hood or lens shade is essential. Always check the front of the camera lens to make sure that light from the lamps is not striking it. Standard lens shades often do not provide sufficient protection in these situations. Black paper taped to the shade can provide additional shading, if needed.”

Workflow[edit | edit source]
Best Practices[edit | edit source]
Case Studies[edit | edit source]
Creative Solutions[edit | edit source]
Resources[edit | edit source]

Two-Dimensional Objects on an Easel or Slantboard[edit | edit source]

Three-Dimensional Objects[edit | edit source]

Raking illumination[edit | edit source]

Specular illumination[edit | edit source]

Transmitted illumination[edit | edit source]

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.

  1. Workflow
    • On a light box (flat objects such a paper, paintings, small 3D objects)
  1. Set-up the camera.
  2. Turn on the light box and custom white balance or input the color temperature of the lights. “To provide the best color rendition, the box should contain tubes with a color-rendering index above 90.” Kushel (2017, p.121) from Section 6.2.4 of The AIC Guide.
  3. Place the object on the light box.
  4. Place the color target and label at the edge of the object. Raise the target to be on the same level as the surface of the object. The target can be illuminated with a different light source. Fiber optic spot lights work well for this.
  5. Set-up ambient light source as needed.
  6. Frame the shot.
  7. Block all excess light from the light box with an opaque, dark material such as velvet. “If the light box surface extends beyond the edges of the subject, the image may be degraded by edge flare, causing an overall loss of contrast. An exposed light box surface should be masked. A very thin edge of the exposed surface may be desired in some cases (e.g. when losses along the subject’s edge need to be documented.” Kushel (2017, p.121) from Section 6.2.4 of The AIC Guide.
  8. Take the image using Aperture Priority or Manual.
  9. Adjust the exposure in Camera Raw using the histogram, ensuring there is no clipping.
  • On an easel or slant board (larger flat objects or objects that can not lay flat)
  1. Set-up the camera.
  2. White balance the light source using a custom white balance or input the color temperature of the lights.
  3. Secure the subject between two supports that don't obscure the back. This can be two easels, variable height horses or another mounting system. Smaller paintings and paper objects can be placed on a clear slant board.
  4. Place a light source behind the object, ensuring it evenly illuminates the area. “Illumination can be from a spot or fiber optic light or another studio source. Even larger subjects, such as paintings, can be supported between two easels and illuminated by a freestanding studio light, preferably one with barn doors or focusing capabilities, or both, to minimize spill.” Kushel (2017, p.122) from Section 6.2.4 of The AIC Guide.
  5. If possible, place the reference target and label at the edge of the object. It can be illuminated with a different light source.
  6. Set-up ambient light source as needed.
  7. Frame the shot closely around the subject to reduce light leaks around the edges.
  8. Take the image using Aperture Priority or Manual.
  9. Adjust the exposure in Camera Raw using the histogram, ensuring there is no clipping.
    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.
    • Interior 3D objects illumination (ex.basketry, porcelain, translucent 3D objects)
    1. Set-up the object and camera as you would for normal illumination.
    2. Place a light source outside of the field of view with the light aimed into the object. Fiber optic works well for this.
    3. Set-up ambient light source as needed.
    4. White balance the light source using a custom white balance or input the color temperature of the lights.
    5. Take the image using Aperture Priority or Manual.
    6. Adjust the exposure in Camera Raw using the histogram, ensuring there is no clipping.
    7. Equipment (specific to each workflow)
      • On a light box (flat objects)
        • Light table
        • Opaque cloth or other material to block light leaks
        • Camera mount or arm
        • When using a reflectance-based reference target: incident light source for visualizing the target
          1. Alternatively, or additionally, a transmissive target can be used to aid in setting exposure and white balance (e.g. the Library of Congress utilizes a Stouffer T2115 21 step densitometry target placed along the edge of the object for this purpose)
      • On an easel or slant board
        • Easel(s), variable height horses, clear slant board or other mounting system
        • Light to place behind the object
        • Camera mount, tripod
        • Optional diffusers for creating more even illumination
        • Incident light source for reference target
      • Interior 3D objects illumination
        • Fiber Optic or other light source for interior illumination
        • Camera mount, tripod
    8. Lighting
      • Light box
      • Fiber Optic light source
      • Studio lights with barn doors and/or focusing capabilities to minimize light spillage
      • Ambient light:  “It is frequently helpful to use transmitted light in conjunction with controlled incident ambient illumination. It is important that the color temperature of both sources be very similar. If a light box is used, similar fluorescent tubes can be placed in a nearby ceiling or desk fan. If a freestanding lamp is used, a second lamp can be pointed at the ceiling or at an adjacent, neutrally colored wall or surface. The appropriate intensity of these ambient illuminations relative to the transillumination can be judged by eye. Use of controlled ambient illumination can be used to solve many problems. Cracks, tears, and losses will be better rendered because flare from the intense light passing through them will be diminished, and localized losses or crack systems in otherwise opaque subjects will be rendered in context.” Kushel (2017, p.122) from Section 6.2.4 of The AIC Guide.
    9. Best practices
      • 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.
    10. Case studies
      • Paper/photo
        • Background
          1. A strategy for performing transmitted visible light imaging of film positives and negatives has been described by Wyble 2021. The setup is described below, and a schematic and images of a similar setup approximating that described in Wyble 2021 are provided.
        • Setup
          1. Two light sources are positioned behind an easel-mounted, diffusely transmitting material; in this case, the frame and diffuser removed from a GTI light booth originally designed for backlit examination of transmissive media.
          2. Frames made of heavy-duty black paper or cardstock are fashioned to the approximate size of the imaged targets and film samples in order to reduce light scatter to the camera.
          3. Clips can be used to secure the frames and film samples to the diffuser.
          4. The light sources, easel, and diffuser frame are also draped with black fabric to further reduce stray light.
          5. The camera is positioned in front of the draped backlight setup such that the diffusing area is centered and maximized in the field of view of the image.
        • Schematic and example setup images
          Figure 1.a. Images showing the placement 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.
          • Painting
            • Images of example setups utilizing 1) a translucent-backed table easel and 2) variable height horses can be found on page 123 of the AIC Guide.
          • Object
            • Images of a Native American basket lit from both the interior and exterior can be found on page 122 of the AIC Guide. This image provides an excellent example of transmitted illumination to visualize losses in the basket weave supplemented by ambient illumination to light both the color reference target and the external surface of the basket.
          1. Budget set up/ creative solutions
            • A window can be used as a light source on a bright day. A subject can be supported by an easel on either side, ensuring nothing is blocking the back of the object and placed in front of the window. Close all other window shades and light sources. Block any surrounding light from the window using curtains or fabric as needed.
          2. Resources
            • 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
            • 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
            • 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.