Greg Bearman et al.: A Non-Contact Imaging Method to Measure and Map Water Content in Parchment

This presentation will report on a method to measure water content of parchment using an approach borrowed from biomedical imaging of skin. This is an imaging method, which can be done through glass and other transparent materials. It can map out water content for an entire document, detecting local high and low areas and edge effects. These water maps can be co-registered with other images, such as colour or spectral image cubes to guide the conservator.

The method relies on a novel approach to characterizing photon scattering, called Spatial Frequency Domain Imaging (SFDI), which has been previously shown to provide quantitative optical properties in living tissues. Photons can be either absorbed or scattered and it is very difficult to separate the effects. Isolating and determining the absorption component is critical, since this is what is used to calculate chromophore concentrations (e.g. water) via standard “Beer’s Law” absorption theory. For example, one can calculate tissue oxygenation since the absorption spectra of oxygenated and deoxygenated blood are well known, but first one needs to know how much of the measured photon interaction with tissue is absorption and not scattering.

To unravel scattering and absorption, our SFDI method uses spatial modulation of the illumination in combination with model-based image analysis. SFDI acquisition is fast (~1s), can cover a wide field of view (10s of cm) and can be constructed with consumer-grade projection and imaging electronics. The method has been demonstrated and validated for skin imaging in animals and people to detect and quantitate oxygenation, blood volume, and edema (water). Since parchment is simply skin, these results translate over. Water has an absorption feature at ~ 980 nm that we can use for absorption measurements.

The method can create dynamic video images to map the kinetics of water uptake and release by parchments. We can watch, in real time, water absorption and diffusion kinetics in the parchment.

Some examples from biomedical applications and some preliminary data on modern parchment will be shown.