The True Colour of Stained Glass: Setting Standards for Photographic Imaging and Documentation using Digital Technologies

Nick Teed, MSc, ACR

Introduction

The recording of accurate information is the cornerstone of any conservation project on historic stained glass. As digital technology improves and expands, a wider and sometimes bewildering array of tools have become available to the conservator, and archival records created in the heritage sector are conforming to established guidelines, standards and disciplines. At the heart of these standards lies the assurance that digital image records represent the true nature and identity of the heritage asset. This paper will focus on how digital imaging technology can be harnessed to enable more accurate and authentic records of historic stained glass to be created. In particular, the paper will explore how it may be possible to record and scientifically quantify true colour accuracy in digital records of stained glass, along with the potential for technology and software to draw detail from the stained glass that is otherwise obscured or impossible to see without specialist equipment.  Examples of standardized digital imaging methodologies developed at the York Glaziers Trust for studio-based photographic recording will be discussed, alongside the use of the resulting images as a powerful tool within an in-house digital documentation system. As conservators frequently have the closest access to historic stained-glass panels, they are perfectly placed to be setting new standards of recording for the benefit of future research, publication, and public knowledge.

Digital Imaging of Cultural Heritage

The development of an in-house digital imaging methodology within the York Glaziers Trust has been a long and evolving process and plays a crucial part in the building of a comprehensive and appropriate documentation record.((Nick Teed, ‘Recording Our Fragile Heritage: The Development of Professional Stained Glass Photography and the Use of Digital Imaging in the Documentation of Stained Glass’, in Claudine Loisel and Isabelle Pallot-Frossard (eds.), Stained Glass: How to Take Care of a Fragile Heritage, 9th Forum for the Conservation and Technology of Historic Stained Glass, ICOMOS France, Corpus Vitrearum, LRMH, Paris, 2015, 96-101.)) As article 2.2 of the Corpus Vitrearum Guidelines, 2004 states: ‘Full documentation of the preliminary research and of all steps, methods, and materials of a conservation process is obligatory’.((Corpus Vitrearum, Guidelines for the Conservation and Restoration of Stained Glass, 2nd. Edition, Nuremberg, 2004:  http://www.cvma.ac.uk/conserv/guidelines.html  accessed 11/01/20)) This should include a full photographic record of all stained-glass panels in the care of a conservation studio, taken both before and after any conservation treatment. The ultimate aim in recording the stained-glass panels is to provide an authentic record of the historic material. Conservation studios dedicated to the care of stained glass are not generally affiliated with larger institutions or museums that offer access to a professional digitization department. For that reason the YGT chose to look further afield to see what could be learnt from practitioners in the field of cultural heritage digitization.

There is currently a growing international community of specialists dedicated to the digitization of cultural heritage, supported by such organisations as the Association of Historic and Fine Art Photographers (AHFAP). The Association hosts an annual conference in the UK and has an extensive website offering advice and links to resources.((Association for Historical and Fine Art Photography (AHFAP), webpage: https://ahfap.org.uk/  accessed 7/01/20.)) The biennial symposium hosted by the Rijksmuseum, Amsterdam entitled ‘2D & 3D Photography: Practice and Prophesies’, inaugurated in 2015, has quickly become a focal point in the gathering together of international experts whose aim is to encourage the sharing of knowledge and to promote the use of standardised practices.1 At present the most widely used digital imaging guidelines in the cultural heritage community are the U.S. National Archives and Records Administration’s ‘Federal Agencies Digital Imaging Guidelines’ (FADGI),((Thomas Rieger (ed.) Technical Guidelines for Digitizing Cultural Heritage Materials, Federal Agencies Digital Guidelines Initiative (FADGI), Library of Congress, Washington, 2016,  webpage: http://www.digitizationguidelines.gov/guidelines/FADGI%20Federal%20%20Agencies%20Digital%20Guidelines%20Initiative-2016%20Final_rev1.pdf accessed 7/01/20)) and the National Library of the Netherlands’ ‘Metamorfoze Guidelines’.((Hans Van Dormolen, Metamorfoze Preservation Imaging Guidelines, The Hague, 2012, webpage: https://www.metamorfoze.nl/sites/default/files/publicatie_documenten/Metamorfoze_Preservation_Imaging_Guidelines_1.0.pdf accessed 11/01/20)) The common themes of these guidelines, developed over many years, are the adherence to disciplined methodologies to obtain consistent image quality, even tonality and illumination, accuracy of scale, accuracy of colour, and optimum resolution. The Rijksmuseum, for example, has committed to digitizing the entire collection of the museum within the framework of the Metamorfoze Guidelines, and offers an innovative free online photoservice for the use of images in research.((Rijksmuseum Photoservice, webpage: https://www.rijksmuseum.nl/en/photoservice accessed 11/01/20)) Realising the growing importance of setting in-house standards for digital photographic recording, the YGT decided to begin a review of its own practices, reflecting on what could be improved upon and how, and also exploring the benefits of greater accuracy in the recording of the historic stained glass in its care. The key areas identified for improvement in studio-based photography were: Lighting for both transmitted and reflected light sources, with the objective of even illumination and colour balance; File-naming, archiving and in the development of an effective use of the possibilities of metadata; Colour accuracy. The majority of professional photographers working within the field of cultural heritage employ colour management systems, using technical targets to create colour profiles that control colour accuracy. In digital industries the concept of colour management was formalised in 1993 by the International Colour Consortium (ICC) anabling a cross-platform, vendor-neutral system of assigning calculated colour profiles.((Roy S. Berns, Principles of Colour Technology, Fourth Edition, Hoboken, 2019, 191)) It is easy to assume that when you invest in expensive digital equipment it will record information accurately.  However, most camera manufacturers look towards commercial markets for their custom. In most cases, this leads to a desire to record colour in a vibrant and aesthetically pleasing way rather than in a truly accurate rendering. The additional complication in photographing stained glass is the need to use transmitted light. The information found in the published guidelines focuses in general on digitizing subjects using reflected light, making our objective far from straightforward.

Developing A Methodology for the Digital Imaging of Stained Glass at the York Glaziers Trust

Lighting options for studio-based photography of stained glass commonly include natural lighting, fluorescent tube, LED and flash. Over many years the YGT has used all of these options, and all have advantages and disadvantages. Natural lighting, for example, is very effective at showing the character and quality of the glass, however it can be very difficult to eliminate the interference of buildings or objects behind the screen and changing weather conditions can lead to inconsistencies. Fluorescent tube lighting was once a commonplace solution, however the tubes can vary in their colour balance, can change colour or brightness over time, and require electronic ballast units to avoid flickering. There is also a need to consider the fading of light between the light tubes. LED lights provide an economical and efficient solution for the illumination of stained glass. Light screens can be made which evenly distribute the LED light source embedded at the perimeter of adapted acrylic sheets, however there can be a noticeable fading of light at the centre of these sheets if their width exceeds the capability of the light power source.

The YGT solution has been to adopt the use of studio flash lamps as a light source, placed behind an opal acrylic diffusing screen for transmitted light photography, and in front of the screen for reflected light images. A studio flash lighting system requires a sizeable soft box diffuser (in our own case 1800mm x 1200mm for transmitted light, and 1800mm x 300mm for reflected light) and therefore the studio space needs to be able to accommodate the equipment. Depending on the type of lighting, and power output chosen, flash lighting can be a significant investment, however it is then possible to guarantee that all lighting is consistent and will have a consistent optimal colour temperature of 5500 degrees Kelvin. This enables far greater accuracy in setting the white balance in the camera software correctly. Another key advantage is the ability to select very fast flash sync speeds. The YGT uses as standard an exposure of 1/500th of a second at aperture f.11 and ISO 100, allowing complete consistency of approach, optimal sharpness of the resulting image and a reduced chance that accidental camera movement will affect results.

Following a trial of the advantages of a soft box studio flash system, the YGT invested in two Broncolor lamps connected to a separate power pack. The power output of each lamp can be adjusted between zero and 10 points in 0.1 degree intervals without any change of colour balance. This allows us to maintain uniform camera settings but to adjust the flash power appropriately for lighter or darker stained glass panels. Photographing directly into Hasselblad Phocus © software we are able to apply a feature known as ‘scene calibration’ as an integral part of our working set-up. By initially photographing the blank photographic screen in transmitted light, the scene calibration function can then be applied to even out the light fall-off from the centre of the lamp to the edges of the frame, providing totally even illumination across the scene.  Using the second lamp positioned in front of and to the side of the screen for reflected light it is also possible to even out the light from left to right. These settings are then automatically applied to subsequent images. This feature proved to be particularly important in allowing the next stage of our development, the calibration of the colour of the stained glass.

In the experience of the YGT, a disciplined approach to the camera set up for stained glass photography (use of tripod and correct exposure, for example) usually achieves good results, but it was observed that the colour accuracy of the results did not always reflect what see. Certain stained glass colours prove to be consistently difficult to capture, particularly certain types of blue and purple, and it has been suggested that some densely saturated glass colours could fall out of the recordable spectral boundary or colour gamut of digital cameras.((Lindsay Macdonald (et al.), ‘Imaging of Stained Glass Windows’, in Lindsay Macdonald (ed.), Digital Heritage: Applying Digital Imaging to Cultural Heritage, Oxford, 2006, 411-44.)) In many widely available software applications it is possible selectively to adjust colours to improve them, but this approach can be arbitrary and subjective and is scientifically unquantifiable. It is important also to be aware that colour is perceived differently from one person to another and under different lighting conditions, times of day or in differing states of mood or alertness.((Michael Langford, Langford’s Advanced Photography, 8th edition, Oxford, 2011.)) It is very important in trying to compare the colours captured by a camera with those of a stained glass panel being viewed on a light-screen, that the two are lit and illuminated in the same way. The computer screen or monitor must be correctly colour-calibrated using appropriate software, and should also be set to the same level of measured luminance as the photographic screen. An independent monitor will allow for such precise adjustments to be made.

 

Fig. 1. York Minster, window SX, panel 4c, showing colour checker panels. Photo: York Glaziers Trust, reproduced with the permission of the Chapter of York

Fig. 1. York Minster, window SX, panel 4c, showing colour checker panels. Photo: York Glaziers Trust, reproduced with the permission of the Chapter of York

We were interested in why our cameras had difficulty recording certain colours accurately, and how we could make improvements. A breakthrough came in 2018 in discussion with photographer and Hasselblad technician Stuart Culley. Stuart, an expert in digital colour management, was immediately interested in the YGT’s objective and agreed to collaborate on a project to develop a quantifiable stained glass colour test panel. Choosing a range of 30 colours identified as commonly used in stained glass, but which are also difficult to photograph accurately, a panel was made using mouth-blown traditional glass and lead. Our hope was that if we could measure these colours accurately, we could then calibrate the camera to match them, and a colour profile could be created to consistently manage the colour in our photographic studio set-up. Our colour test panel is shown in Fig. 1 alongside two commercially produced colour checker cards during calibration trials.

The colour test panel was measured using an xrite R1 spectrophotometer. This produced quantifiable values for many of the glass patches, but certain colours could not be measured (the blues and the purples in particular). Other researchers have suggested that ‘in stained glass the colorant acts as a filter to exclude certain wavelengths of transmitted light’.((Phil Green, ‘Colour Management in Heritage’”, in Macdonald, Digital Heritage, 293-326.)) This would appear to be consistent with our own findings, that while certain glass colours are easily measurable with standard spectrophotometers, others are not due to the interaction of the metallic oxides used to colour the glass. If these metallic oxide components within the glasses diffract or absorb the light transmitted, this will prohibit accurate colour readings using equipment designed to measure with reflected light. This has also led us to query whether the metallic colourants of stained glass are a factor in digital camera inaccuracy.

Our next step was to collaborate with a printing specialist who offered to assist with the measurement using a Barbieri spectrophotometer, designed to measure translucent materials.  Initially these measurements have also proved to be inconclusive, however we are continuing to research a solution to enable a fully accurate record of the test panel to be made. We proceeded to establish a test using our lighting arrangement, an industry standard xrite colour checker SG colour target and our own test panel. Photographing a panel from window SIX from York Minster using the controlled even lighting, we were able to calibrate our Hasselblad H5D camera using the colour checker SG card, recognized by the Hasselblad Phocus software. An ICC (International Colour Consortium) colour profile was then built from the data within the calibrated RAW file using xrite profiling software.((International Colour Consortium, webpage: http://www.color.org/index.xalter accessed 7/01/20)) The advantage of assigning an ICC colour profile to your images is that it allows a colour standard to be set for your specific working situation that can lead to consistent results given the consistent lighting. This profile identification can then be read across multiple formats due to the nature of the cross compatibility of the software attribution.

The results from our initial tests have proved to be remarkably good. Whilst we have principally used a methodology common to those working in reflected light with manuscripts or paintings (such as for the Heironymus Bosch research project conducted in the Netherlands, completed in 2016)((Luuk Hoogstede, Ron Spronk et al., Technical Studies: Heironymus Bosch, Painter and Draughtsman, Brussels, 2016.)) we have made significant improvements to the accuracy of colour capture in terms of our visual assessments using calibrated monitors with matching luminance to the photographic screen. While this methodology has moved us forward in terms of colour accuracy, we have more work to do to be able to state quantifiably that we have captured the true colour of the glass. We do, however, believe that if we can work with specialists with the right colour measuring equipment, and finally make an accurate reading of our test panel, we will be able to make a new profile that takes us forward towards absolute accuracy and authenticity of recording. Once that can be achieved we will be able to build a record of colour from project to project, allowing direct comparisons to be made in the future, based on value readings from the digital image files.

Another aspect of the setting of new standards at the YGT has been in the use of metadata to embed within the images a wealth of information created as part of our conservation work. We are currently drawing up and refining our own methodologies for this data, influenced by the standards already set by the International Press Telecommunications Council (IPTC) and by Historic England.((International Press Telecommunications Council (IPTC) Webpage: https://iptc.org/standards/photo-metadata/photo-metadata/. Steve Cole and Paul Backhouse, Digital Image Capture and File Storage: Guidelines for Best Practice, Swindon, 2015.)) Our images are labeled with information about building and location, window location, panel number, before or after conservation status, and year. Within the metadata we can also create further information on authorship of the image, all necessary camera setting information, special notations on the project, copyright information and keywords to assist in future image searches. Our digital archive consists of the original camera RAW files and TIFF format files; currently recognized as the most stable file types for long-term archival storage.((Steve Cole, Photographing Historic Buildings, Swindon 2017, 42.)) TIFF files carry the maximum amount of information relative to detail and colour. Optimum archival files are 16 bitt TIFFS, although 8 bitt TIFF files are also acceptable. Our own digital files are stored on a dedicated server with three additional backup drives, stored at an off-site, fireproof location in constant operation.

Digital Imaging and Documentation

The digital images of stained glass that are made during the conservation process at the YGT feed directly into the package that comprises the entire suite of documentation information. This includes reports, dimensional information, template information, documentation notes and diagrams and post conservation reports and treatment records. The direct and close access to the historic glass creates a privileged opportunity to record information photographically, facilitating future research. Images are also made on a macro level to encompass corrosion aspects and phenomena, details of painting styles, condition of paint and the condition of lead-work. Using very high-resolution equipment and precision macro lenses we can even record on effectively a microscopic level, going deep into the body of the glass, which gives potential information about glass making methodologies (Fig. 2).

Fig. 2. York Minster, window SXI, panel 5d. A 100% crop of a macro image, in this case at 50 megapixel resolution, showing microscopic air bubble structure within the glass. Photo: York Glaziers Trust, reproduced with the permission of the Chapter of York

Fig. 2. York Minster, window SXI, panel 5d. A 100% crop of a macro image, in this case at 50 megapixel resolution, showing microscopic air bubble structure within the glass. Photo: York Glaziers Trust, reproduced with the permission of the Chapter of York

The York Glaziers Trust is committed to the development of templates for the digitization of documentation. This ensures clarity of detail and information and allows each package of project documentation to be publication-ready. The digital images also now feed into the diagrammatic documentation, providing the backing to each template, effectively providing a double layer of accurate information. Each documentation symbol overlays precisely the photographic information which, when enlarged on a suitable monitor, becomes a very powerful research tool (Fig. 3).

Fig. 3. York Minster, window SXI, panel 4a, documentation diagram example. © York Glaziers Trust.

Fig. 3. York Minster, window SXI, panel 4a, documentation diagram example. © York Glaziers Trust.

Controlled studio lighting is particularly important for capturing the key record images required as part of any conservation project, but it is well worth considering other creative uses of light to illustrate interesting phenomena that can greatly enhance the information that feeds into the documentation record. Delicate back-painting, selective glass corrosion and inscribed graffiti are revealed particularly well when captured at a raking angle or with raking light sources. Critical close-up images, captured at extreme angles, can be made in focus from front to back using easily obtainable focus stacking software, which will automatically stitch several images focused at differing points. The use of multiple lighting positions can dramatically reveal details that are otherwise difficult to discern, such as the marks left by fired paint that has since corroded away or suffered from environmental deterioration. Software is available to allow the set-up of a system of reflectance transformation imaging (RTI),((Cultural Heritage Imaging, Webpage, http://culturalheritageimaging.org/Technologies/RTI/ accessed 7/01/20)) however ingenuity is required in the setting up of the light sources. Recently produced commercial products such as the Broncolor Scope D50((Broncolor, Webpage, https://broncolor.swiss/scope/ accessed 7/01/20)) bring an enhanced sophistication to the principles of RTI imaging with integrated software.  Experiments at the YGT have included the use of a camera with its infrared filter removed, thus allowing the full range of infrared light wavelengths to reach the camera sensor. This approach has often been used to reveal under-painting or writing in paintings or manuscripts but we have found it useful for revealing detail in very densely corroded glasses.((Duilio Bertani and Luca Consolandi, ‘High Resolution Imaging in the Near Infrared’, in Macdonald, Digital Heritage, 211-38.)) An infrared modified camera will also potentially pick up reddish brown pigments with great clarity where they may previously have been difficult to discern.

Whilst this paper has focused on studio based photographic recording, and the benefits that modern technologies can bring, it is worth noting how significant gains can be made when applying a similar disciplined approach to the photography of stained glass in situ. It is often the case that the conservator is awarded uncommon access, from high-level viewpoints or from purpose-built scaffolding. These opportunities during survey work should ideally be taken to add high resolution and high quality images to the conservation record (Fig. 4).

Fig. 4. York Minster, Great East Window, after conservation 2019, photographed from the scaffolding erected for the conservation of the Great Organ. Photo: York Glaziers Trust, reproduced with the permission of the Chapter of York.

Fig. 4. York Minster, Great East Window, after conservation 2019, photographed from the scaffolding erected for the conservation of the Great Organ. Photo: York Glaziers Trust, reproduced with the permission of the Chapter of York.

Acknowledgements

I would like to thank Stuart Culley for his skill and enthusiasm in assisting us to develop a colour management system, Sarah Brown for greatly supporting our endeavours to improve standards, and Anna Milsom who has provided photographic support and advice throughout this project.

Nick Teed, MSc, ACR worked at the York Glaziers Trust between 1999 and the summer of 2022, latterly as conservation manager. In this capacity he took the lead in developing standards of conservation photography.

  1. ‘2+3D Photography: Practice and Prophecies’ webpage: https://www.rijksmuseum.nl/en/2and3d accessed 7/01/20 []

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