A hybrid measurement-simulation approach to determine the reflectance map of a historic tapestry

Illumination levels in historic buildings need to be monitored for conservation purposes, especially in daylit spaces which are invariably subject to large spatio-temporal variations in light levels. However, light meters record only at a single point, and numbers are limited. A recently demonstrated imagebased technique using digital cameras to measure indirectly the prevailing daylight illumination in spaces is extended. As a prerequisite, the technique requires a reflection map of the ‘target’ - here, a historic tapestry. Acquiring this without disturbance to the scene required a novel combination of in-situ measurement and lighting simulation. How this was achieved is described in the paper.