Cross polarisation in photographyWritten by Paul Bourke
The following is a brief introduction to cross polarisation techniques used in various photographic processes. Examples will be presented from ultra-high resolution scanning of paintings consisting of a semi glass oil paint. Other applications include photogrammetric capture of shiny and/or metal objects, or simply to achieve richer colours for an aesthetic improvement.
The basic principle is based upon the realisation that shiny surfaces (metals, specular materials) preserve polarisation of the light they reflect. Whereas surface that behave as diffuse reflectors will randomly depolarise incident polarised light. Therefore if one linearly (not circular) polarises an incident light source, and places a linear polariser on the lens of a camera oriented at 90 degrees to the incident polarisation, then any specular reflections will be greatly attenuated while diffuse perfections will not. Noting that one does lose at least 1.3 stops of light intensity of the diffuse reflections but this is not normally a problem in practice since one can use a bright light source.
Three examples are shown below taken from the digitisation of an oil based painting. The camera is the ixH PhaseOne, the lighting is an LED area light aligned to a 45 degree angle to the cameras optical axis. The alignment of the polarising filter on the camera is performed by photographing a metallic spoon while in live view mode and rotating the polariser until the highlight is suppressed.
In the above two example the specular highlights are largely on the leading curve of the paint strokes between the camera and the light source. In this third example the blue sky paint was more reflective and the specularity is much more pronounced even on the flat surfaces.