Visual

Visible light is the most obvious way we can look at objects. Most surfaces reflect light, allowing us to interpret colour and texture. The charter was examined using light in different ways.

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Reflectance Transformation Imaging (RTI) uses photographs of an object with light from different angles to produce an interactive digitisation which allows detailed examination of the object’s surface features.

Reflectance Transformation Imaging

RTI anchor

Click and drag to change the position of the light source. Click the full screen button ( ) to view a larger version (press escape to exit).

Capturing the photographs to create the RTI digitisation.

To create an RTI digitisation, a camera is mounted on a tripod, facing directly down onto the object. Photographs of the object are taken with a light source placed at different angles, with the distance between the object and the light kept the same in each photograph. To produce the RTI for the Henry VI charter, we took 36 photographs. We set the light at 3 different angles: a low angle (‘raking light’) to accentuate shadows and produce high contrast, a high angle with relatively even lighting, and an angle between the two. 12 photographs at each angle were taken from different positions around the charter.

A shiny ball was placed next to the charter. Each photograph needs something in it that makes the direction of light obvious: the highlight on the ball serves this function. The computer programme that creates the RTI uses the highlights to process the photographs.

RTI has become popular in the heritage sector to display the three-dimensionality of an object, creating a more interactive and revealing digitisation which allows the viewer to understand an object’s physical appearance better than photographs – this dimensionality enhances detailed surfaces like relief sculptures and inscriptions (MacDonald, 2017, p. 239). The text on the seal can be easily read in the RTI, and the folds and creases of the parchment are much more perceptible than in a photograph.

Photography for documenting cultural heritage objects usually uses even lighting, rendering objects somewhat flat – this approach can conceal important aspects of an object, like the play of light on its surface; difficult to see features like ruled lines underneath text and dry-point (un-inked writing or illustration drawn with a stylus, leaving only a depression in the surface of the parchment) can also be revealed (Endres, 2019, pp. 33-34).

RTI can also be used to monitor an object’s deterioration because it captures surface detail so well: an object could be digitised this way multiple times over a long period, and the digitisations compared with statistical analysis which would detect changes over time (Manfredi et al., 2013). This would be useful for monitoring the effect of the storage environment on an object.