Multispectral Imaging and Thermofaxes

Duke University Libraries has successfully used multispectral imaging to make ancient manuscripts more readable, but the same technology can also be employed on modern collection materials.  Over the summer, our multispectral imaging working group teamed up with Rubenstein Library staff to investigate whether advanced imaging techniques could be used to make certain faded photocopies more legible. Duke’s archival collections are filled with copies of documents made using a wide range of printing techniques. Some print processes which were popular before the proliferation of “laser” printing (electrostatic prints) can become badly and irreversibly faded.  The Thermo-Fax was a specific type of copying process, introduced by 3M in 1950, but has become a common proprietary eponym for this whole category of collection materials.

Robert Hill Papers, Electrolytic print under normal illumination

When confronted with a badly faded photocopy, library staff have traditionally resorted to one preservation tactic: create a more stable reproduction of the document on high quality paper. While the original print will continue to fade over time, the “preservation photocopy” should remain just as legible. In some cases, the decision may be made to retain just the preservation copy and deaccession the original. If the document is completely illegible, however, a preservation photocopy won’t be effective and the original may not be retained at all; it’s difficult to determine the value of a letter that can’t be read. If alternative imaging techniques could be used to produce a better reproduction, it might dramatically change how we assess and work with faded documents.

Meghan Lyon, Manuscript Processing Archivist at Rubenstein Library Technical Services, helped to identify several examples of early photocopies from various collections. Our first trial was with a print, probably dating from the 1970s, from the Robert A. Hill Collection of the Marcus Garvey and Universal Negro Improvement Association Papers Project Records (see image above). This collection contains many reproductions of newspaper articles from microfilm. After multispectral imaging and software processing , we achieved these results:

Robert HIll Papers, PCA with False Color

The above image is more legible than under regular illumination, but really not the unequivocal win for which we had hoped. Research about machines that would print copies from microfilm suggested that this type of print, an electrolytic print process sometimes called “Photoconductography”, may not be the best candidate. The image is generated from metallic silver, rather than a dye. Therefore, this item likely became damaged from a water event, rather than fading. We went back to our friends in Technical Services for more examples.

Radio Haiti Collection during processing

The next candidate for imaging came from the Radio Haiti Archives. This collection contains more modern prints (from the 1990s), which are completely faded. Our test document (pictured below) almost looks like a blank sheet of paper. The only visible markings are from a ballpoint pen. Based upon descriptions of paper types and qualities of this object, we identified this as a form of direct thermal printing.

Radio Haiti, Direct thermal print under normal illumination

The paper used in this type of direct thermal printing is impregnated with a colorless dye precursor and coupler system. Thermoplastic material separates the two ingredients. When exposed to heat, the separator melts and the precursor and coupler react to form the colored print. Dyes are notoriously unstable over time, so it is not uncommon for thermal prints to completely disappear. Our document exhibits other interesting properties. For example, the faded text is visible when the page is held up to a light source. We often take transmitted light photos as part of conservation treatment documentation, so I imaged the object on top of a lightbox in a camera copy stand. This technique was pretty successful in delivering a readable image.

Radio Haiti, Transmitted light imaging

It is helpful to know that a low-tech method for reading or imaging this faded documents can be employed. It is easy to imagine library staff using a simple lightbox to identify, assess, or arrange a large collection of prints like this one.

Multispectral imaging of the letter yielded much better results. Interestingly, additional visual noise was introduced from fingerprints and hand marks on the page.

Radio Haiti, PCA with false color

Looking through the image stack, it appears the text is most legible under ultraviolet light (370nm) with a neutral filter in front of the camera.

Radio Haiti, 370nm

This result is actually quite promising when we think about scaling up an imaging project for an entire collection. Creating a full 18 image stack and processing those images takes a great deal of time; however, if we know that one frequency of light and filter combination is effective, we could easily set the imaging equipment to operate more like a normal copy stand.  In this way, we could bring the requisite imaging time and storage costs closer to those of our regular imaging projects.

Many modern print types have inherent vices that cause them to fade rapidly and large collections of them are common in library archives. Advanced imaging techniques, like multispectral imaging, potentially offer opportunities to identify, arrange, and preserve volatile or unreadable prints. Creating a full image stack may not be possible for every item in a large collection, but this investigation illustrates that full MSI may not be required to achieve the needed results.