Theory
Function of Varnishes
In order to understand how the surface properties affect the visual appearance of paintings, consider the following diagrams that show the path of the light caused by a varnish. In the first diagram, the varnish is smooth and the light reflecting at the angle opposite the angle of incidence is considered to be specular reflection [2]. In figure 2, the varnish is no longer smooth and the light is no longer reflecting in the specular direction. Instead, it is scattered in many directions, thus making the light more diffuse, reducing the gloss and apparent chroma of colors within the painting.
Research on the various optical properties of various varnishes is particularly useful to individuals involved in conservation science. It is useful to have an idea of how a particular varnish will affect a painting before it is applied. A conservator who has an idea of the response of a particular varnish and knows the material of the painting will be able to safely apply an appropriate varnish that will restore its original appearance and minimize leaching due to application and later removal.
Spatial Analysis
Generally, the apparent sharpness of an image is a subjective quality. Over the years, objective measurements of sharpness have been created. For this experiment, the Cascade Modular Transfer (CMT) Acutance, proposed by Gendron[5], was used. This procedure follows the understanding that a system modulation transfer function (MTF) is simply the cascade or multiplication of the system component MTF's. In order to assign a usable metric to the varnish, the Spatial Frequency Response (SFR) of the system from the PIMA Image Analyzer was used[Pima Reference Here]. The SFR has been proven to be an analagous metric to the MTF[4]. The varnish SFR was then isolated and used in the CMT measurement.
Colorimetric Analysis
The effect of a varnish can be determined by using appropriate color difference equations. In order to obtain these data without performing direct contact measurements, it was decided that colorimetric data could be derived through image information. By optically obtaining these measurements, a better simulation of viewing conditions could be achieved, making the overall simulation more realistic.