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

Abstract

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. Key Innovations• Hybrid lighting measurement and simulation approach to assess luminous environment in historic buildings. • Method to derive reflectance of complex patterned surfaces. • Characterisation of the albedo map of a historic tapestry. • Derivation of long-term illumination (i.e. light dose) from autonomous HDR capture. Practical ImplicationsQuantifying the amount of light exposure on artwork and fabrics in historic properties is essential for their conservation. The method presented here combines simulation and measurement, as well as laboratory and field work, to reduce errors and derive accurate albedo maps of complex patterned surfaces. Historic spaces that are open to public and that showcase fixed, light-sensitive features will benefit from the non-intrusive, non-destructive analysis of long-term light exposure, for which the albedo map is a necessary prerequisite.