Exploring the Optical Properties of Picture Varnishes Using Imaging Techniques

An experimental paradigm was developed in which a varnished painting was considered as ani111aging system such as a camera: the unvarnished painting was equivalent to a scene, the varnish was equivalent to an optical system, and the observer was equivalent to the sensor of the camera. Methodologies used to evaluate image quality were employed to evaluate the quality changes that occur when a varnish is applied. An unvarnished painting was simulated by a photographic transparency in physical contact with triple-thickness window glass sandblasted on one side. Varnish resins applied to the sandblasted surface simulated a varnished paint surface. Back-illuminated images transmitted through the varnished glass were digitized using a near-colorimetricdigital camera. Differences in physical. properties between varnish resins would result in changes in image quality. These were quantified by spatial analyses (using a modulation transferfunction), colorimetricanalyses, and visualization using a reproduction of an Old Master painting. The results confirmed the utility of this approach to quantifying the effects of varnishing on the appearance if paintings.