Transmitted imaging
Imaging > Imaging Techniques > Visible light imaging > Transmitted imaging
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Transmitted imaging (also known as backlighting, trans-illumination or trans-irradiation) is a method that records how radiation passes through an object, revealing internal structures, material densities, and hidden details. It is commonly used to visualize and document characteristics that are not visible under reflected light; for example, it can reveal paper watermarks, hidden repairs in textiles,[1] or paint losses in a canvas painting. The technique exploits the principle of light attenuation, where materials absorb and scatter light to varying degrees, allowing for the visualization of internal density differences.
Both visible light and infrared radiation can be used as the main radiation source. The two methods are complementary to each other, and to other imaging techniques.[2][3] For these applications, a key limitation is the method's effectiveness on thick objects like lined paintings and backed works on paper.
Digitization of both positive and negative film and photographs, as well as stained glass, uses the same photographic setup.
Working principle[edit | edit source]
Transmitted imaging involves positioning the radiation source behind the object to be photographed.
Equipment[edit | edit source]
The minimal equipment for this experimental setup includes:
- A camera,
- A tripod or a column with a camera mount,
- A prime lens,
- An IR & UV-cut filter to mount on the lens or in front of the sensor,
- Light sources emitting visible light and/or infrared radiation,
- Light stands or other light mounts,
- Filters suitable for infrared imaging.
Additional recommended tools are:
- Light modifiers like softboxes, barn doors, black cardboard,
- A lens hood or snoot to block stray light,
- Mounting grips, arms, and clamps may be necessary to hold the black cardboard in place,
- Two easels and extension bars if a painting is to be photographed,
- An inert transparent support, like a polycarbonate or acrylic sheet,
- A working station to control the session in tethering mode.
Technical limitations[edit | edit source]
The thickness of a non-transmissive object plays a major role in the application of the method; wooden panels, paintings with a thick paint layer, and works on paper mounted on a backing board may not let enough radiation pass through the support to get a useful record of the internal structure. In that case, other imaging techniques with a more penetrating power, such as X-ray radiography, would be more appropriate.
Edge flare may occur whenever stray light bleeds from behind the object; the effect can be limited by installing a shield around the edges of the object.
General recommendations[edit | edit source]
Shooting an image of the radiation source directly, at an appropriate exposure, allows for flat-fielding and white balance corrections.
Adequate exposure is assessed visually by the amount and quality of the features to be recorded, rather than by measuring a reference target.
Paintings can be secured by placing them on a transparent support or by using two easels at each end.
When using heat-emitting lamps, a safety distance from the object should be considered.
It is often useful to record the object also under even illumination conditions to facilitate the interpretation of the images.
Further readings[edit | edit source]
Frey, Franziska S, Jeffrey Warda, Dawn Heller, Dan Kushel, Timothy Vitale, Gawain Weaver, and American Institute For Conservation Of Historic And Artistic Works. Digital Photographic Documentation Task Force. The AIC Guide to Digital Photography and Conservation Documentation. 3rd ed. Washington, DC: American Institute for Conservation of Historic and Artistic Works, 2017.
An all-round publication that comprises recommendations for numerous imaging methods used in conservation practices.
Müller, Leonie. “Understanding Paper: Structures, Watermarks, and a Conservator’s Passion” Harvard Art Museum, May 7, 2021.
A Harvard Art Museums blog post includes examples of images captured of the structure and watermarks of laid paper using transmitted light photographic techniques.
Thaw Conservation Center. “Looking at Works of Art on Paper: An Overview of Examination and Imaging Techniques.” The Morgan Library & Museum, May 1, 2020.
The blog post describes the advantages of transmitted light imaging for visualization of the structure and features of laid paper.
References[edit | edit source]
- ↑ Rauca, Adrian, Luminița Ghervase, Antonia Berdie, and Matei Agachi. “Unveiling the Secrets of an Artwork through Non-Invasive Investigations—Case Study of a 19th-Century Female Portrait.” Minerals 13, no. 9 (September 11, 2023): 1193–93.
- ↑ Cucci, Costanza, Marcello Picollo, and Muriel Vervat. “Trans-Illumination and Trans-Irradiation with Digital Cameras: Potentials and Limits of Two Imaging Techniques Used for the Diagnostic Investigation of Paintings.” Journal of Cultural Heritage 13, no. 1 (January 2012): 83–88.
- ↑ Herrero-Cortell, Miquel, Paola Artoni, and Marta Raïch. “Transmitted Light Imaging in VIS and IR, in the Study of Paintings: A Brief Report on the Behavior of the Main Historical Pigments.” Color Culture and Science Journal 12, no. 02 (July 1, 2020): 79–88.
