IF NORMAL DIGITIZATION has an alter ego, it is reflectance transformation imaging (RTI). Digitization generally treats all surface details as equal in the eyes of lighting. If shadows occur, they are removed, using techniques such as flat-field correction. This, in effect, turns a page into a flat surface. By contrast, RTI celebrates a page as a play of light. It revels in how each detail participates, capturing these dynamics for pigment, parchment, and ink. For example, as discussed in Chapter 1, recovering the yellow pigment orpiment is important because it emulates divine wisdom, its light pouring forth from the page. To make the effect of light more dynamic, artists layered pigments, generating textured swells of colour that increase the complexity of reflected light. For the later Book of Kells, Bernard Meehan has speculated that such layering had a further aim: through aging, cracking, and chipping, layered pigments would generate an evolving and enriched interplay of colour and light. For an illuminated manuscript such as the St. Chad Gospels, this interplay is an essential part of religious expression. Unfortunately, normal digitization fails to capture its dynamics.

By removing shadow, digitization not only diminishes the visual experience of a page, but it also conceals material details. As mentioned, multiple scholars critique interpretation that overlooks the contributions of materiality to meaning. In the example of the portrait of Mark, the shimmer of orpiment is echoed in how the artist portrays him, stepping onto the decorative border, the threshold of the page. It implies that Mark is stepping into the world, opening a path from the sacred and into the physical. To miss the material effect of orpiment is to miss its echoing of Mark's gesture, an emphasis that provides a cosmological orientation for entering the gospels. Such use of materiality is central to complex artistic expression, its triumph perhaps best expressed by the twelfth-century Welshman Giraldus Cambrensis. Upon seeing an early Irish gospelbook, he exclaims that it “must have been the result of work, not of men, but of angels.” Contemplating the rich history of such artistic expression, the twentieth-century philosopher Hans-Georg Gadamer concludes that aesthetic experience is the epistemic contribution of the humanities. By stifling the experience of materiality, normal digitization limits access to humanistic epistemic content.