Original scientific paper
https://doi.org/10.17018/portal.2020.10
Cross-polarisation and Characterisation of Digital Still Camera (DSC) with Colour Test Targets
Ljubo Gamulin
; Croatian Conservation Institute, Section for Movable Heritage Documentation, Zagreb, Croatia
Abstract
This paper focuses on the use of cross-polarisation for documenting works of art with photographs, specifically for paintings made using the impasto technique at a geometry of light from 30°/0° to 45°/0°. In situations where it is necessary to emphasize the ridges of impasto paint layers of artistic two-dimensional objects, the technique of dominant lighting on the left or above in relation to the original is usually used. In addition to the lack of unevenness of the photographed surface, which is regulated by brightness uniformity, the geometry of light on the ridges of impasto paint layers can cause unwanted reflections. These can be removed by moving the light source, using a sharper lighting angle, or using a lens with a longer focal length, but this cannot be done if photographing in a confined space. Therefore, photographers use the technique of cross-polarisation, in which, due to dichroism (selective absorption of light), the reflection in the painting is completely removed. The main disadvantage of this technique is the ‘suffocation’ of the character of gold and silver and the effect of ‘beautification’, i.e. enhanced contrast and colour saturation in relation to the original.
In order to minimize the deviation of original colours from the obtained master file, usually present when using cross-polarized light, the digital still camera (DSC) characterisation techniques for photographing colour test targets should be used. Test photographs were taken under controlled conditions in a photographic studio using a DSC and a studio flash. The same work of art was photographed in three ways: without the use of polarizers and with factory characterization, with cross-polarisation and factory characterization, and with cross-polarisation and new characterisation. In addition to a visual comparison of the originals and master files, the color differences of the reference values of the CCPP test chart photographed with the artwork were also compared. The proposed method gave the least number of errors regarding colorimetric values, and met the criteria of the standard ISO/ TS 19264-1:2017 and other photographic guidelines for digitization of cultural heritage.
Keywords
photography; digitization; cross-polarisation; DSC characterisation; color difference
Hrčak ID:
250643
URI
Publication date:
20.12.2020.
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