Special imaging techniques

 
Stylus incisions, piercing and pouncing marks in the surface of artworks and manuscripts reveal evidence of preparatory stages in their production or as a method for copying designs. These markings are often so narrow and shallow that they are impossible to capture in a conventional photograph, but photometric stereo is very effective in revealing them. Photometric stereo recording has been used to record hundreds of annotations in stylus from the margins of medieval manuscripts.

 
This technique can be extremely effective at revealing watermarks. This is especially useful in cases where transmitted light or radiography cannot be used. The structure of the paper can also be assessed. Renders made using this technique are capable of revealing the impression from the printing block or plate into the paper and in some cases, reveal regions of reworking.

 
Copper printing plates from the 17th and 18th centuries which have not undergone restoration are likely to have become discoloured thorough decay. Using this imaging technique, it is possible to reveal the original design with more clarity than conventional photography.

 
Photometric stereo imaging has several advantages over raked light photography and is typically more effective at revealing brush stokes and pigment layers in paintings, especially in dark, homogeneous regions. Photometric stereo recordings can in some cases, be used to examine the substrate on which the artwork has been made.

 
The Bodleian Library’s photometric stereo imaging system records originals at a minimum of a million pixels per square inch. The extremely high-resolution, 3D output can be used for the assessment of a number of conservation related features. These include the examination of historic repairs, insect damage and pigment loss. The height, width and depth of recorded elements can in many cases be measured. Recordings can be shared remotely with experts in other institutions.

 
Unlike many laser-based forms of 3D acquisition, photometric stereo imaging can be effective at recording the surface of originals through glass. Provided that the glass is clean and free from scratches, the surface of glass-mounted papyrus fragments and artworks which cannot be removed from their frames can be recorded.

 
Surface data captured using photometric stereo imaging can be 3D printed or milled in order to create a physical facsimile of the recorded original. Files which store the 3D data may need to be converted to a compatible format specific to the 3D printer.

 
Photometric stereo is not an appropriate recording technique for the recording of originals which have a height variation of more than 25mm or have undercuts. In these cases, photogrammetry is a preferable technique for accurately recording their surface.

 
In cases where text has been erased through rubbing the surface of the paper or parchment, it is unlikely that any depressions made by the tip of the writing tool can be recovered. The rubbed regions will appear rougher than the surrounding areas when looking at the surface render. If any traces of ink remain, it may be possible to enhance them either through the editing of a high-quality photographic image, by recording the original using multispectral imaging or in some cases by using transmitted light imaging.

 
Photometric stereo is unlikely to be successful in recovering text which has been redacted.  In many cases, the impression of the nib into the paper or parchment is likely to be extremely shallow and the ink used to make the redactions may have infilled these tiny grooves. However, if the text was written using a modern pen rather than a quill pen, or if the writer applied a good deal of force through the writing tool, it may be possible to reveal the text in a surface render. If the redactions were made using an ink of a different composition, it may be possible to use multispectral imaging to recover the hidden text. In some cases, transmitted light imaging may also be used.

 
Though photometric stereo imaging can be used very effectively to record metallic originals such as discoloured copper printing plates or plaquettes, if the surface of the original is highly polished, it may be difficult or impossible to record using photometric stereo.

 
The Bodleian Libraries Imaging Services department specialise in the 3D capture of book and manuscript bindings. Hundreds of our 3D models have been published and can be explored in our ARCHiOx gallery.

 
Originals which weigh less than 4kg and are smaller than a shoe box can be recorded in our photographic studio. Though the capture of the original is reasonably quick, it typically takes several hours to process the images into a 3D model and associated textures. Photogrammetry recordings of small originals held by the Bodleian include letter seals and portrait busts.

 
For originals, which cannot come to the studio, recordings can sometimes be made in-situ. These may include metal or stone statues or large museum pieces. A preliminary assessment will be required in order to plan for the recording. The success of the recording will depend on the complexity of the object’s shape, the available space surrounding the object, and the ambient light falling on it.

 
A successful recording is only possible if the shape and volume of the original remains unchanged during photography. Paperback books and manuscript with limp bindings are likely to be challenging or impossible to record successfully; and likewise, objects with flexible, hanging or draped elements. In some cases, it may be possible to produce a 3D model from these kinds of originals but with the underside remaining unrecorded.

 
Originals with jewels or glass components cannot be recorded using this technique. Highly polished metal surfaces may be challenging or impossible to record.

 
Along with radiography, transmitted light is a commonly used technique for photographing watermarks, chain lines and laid lines in paper. The effectiveness is dependant on the change in density of the paper at the region where the watermark was made.  

 
In some cases, preparatory underdrawings can be revealed using transmitted light. The success is dependant on the composition of the substrate and the density of the ink and pigments used on both sides of the original.

 
In cases where new material has been added to an original either for the purpose of alteration or for support, these features may be revealed with additional clarity through transmitted light imaging.

 
In cases where redactions were made using an ink of a different composition, it may be possible to use multispectral imaging to recover the hidden text. In some cases, transmitted light imaging may also be used though success will depend on the density of the ink used for the redactions.

 
Photometric stereo imaging is likely to yield better results.

 
Photometric stereo imaging is likely to yield better results.

 
Due to their length and aspect ratio, scrolls often need to be captured by photographing multiple image tiles. The photographer will ensure that an adequate overlap between tiles is recorded. If required, these image tiles can be stitched and blended together in order to create a single, high-resolution image.

 
Originals larger than A0 (841 x 1189mm) may need to be captured in sections. If required, these sections may be stitched together to form a single, high-resolution image.