The Effect of the Refractive Index of a Varnish on the Appearance of Oil Paintings

An optical model was derived that predicted the spectral reflectance of a varnished oil painting based on the illumination angle of incidence, refractive indices of the varnish resin and paint-layer surface, spectral transmittance of the varnish, and internal spectral reflectance of the paint layer. A computational analysis was performed to predict common visual phenomena caused by varnishing, such as the increase in color gamut and contrast. Of particular interest was the importance of varnish resin refractive index on color appearance. The analysis revealed that the refractive index of the varnish resin has a very small impact on changing the appearance of a varnished painting, about one CIEDE2000 color-difference unit under exaggerated conditions. The model was also used to analyze how the addition of first-surface reflectance, caused by a varnish resin (telegraphing' (replicating) the surface roughness of a paint layer, affects color appearance. In this case, large color changes occurred, about 20 CIEDE2000 color-difference units. Given that the ability of a varnish resin to level or replicate surface roughness is dependent on its molecular weight, it is clear that of the two physical parameters, refractive index and molecular weight, molecular weight is the dominant parameter in how a varnish resin affects the appearance of an oil painting.