The visual appearance of a painting is the result of variations in pigments, layers and layer thicknesses. An important role can be played by ground colour. The ground is the first preparation of the support before the actual painting. Medieval painters worked on white grounds. Coloured grounds appeared during the late 15th century in Italy before spreading to the northern part of Europe around 1550. Painters like Rembrandt or Rubens exploited dark or light coloured grounds to create spectacular, realistic effects. But how can we understand how they do this? To answer this question and add some quantifiable analysis, one needs to resolve the spectral reflectivity profile of a painting in three spatial dimensions. In addition, as the main intent is to relate the optical effects with colours, one needs to operate using visible light. The answer to these challenges is visible-light Optical Coherence Tomography (vis-OCT). In this study, we investigate the use of vis-OCT to measure the reflectivity profile of multi-layered paint samples in three dimensions. Using mock-up paint samples we prepared with two thin paint layers on a black and a white paper support, we measured the reflectivity profiles of the separate colours and of the superposed colours over either the black or white (back)ground. This paper first describes the signal processing involved in rebuilding the spectral reflectivity in three dimensions. Secondly, we will present the results obtained from the mock-up samples and draw conclusions on the possibility to discriminate colours in a three-dimensional context. All reflectivity measurements will be compared to a classical, commercially available hyperspectral imaging setup. This study is executed in the context of the NWO Down to the Ground Project. The results of the OCT measurements will be used by technical art historians and conservators for investigations into coloured grounds.
In this study, we compare the hyperspectral imaging capabilities of a custom-built visible light OCT with those of a commercial grade hyperspectral camera. Using the Short-Time-Fourier-Transform algorithm on the OCT signal, we estimate the variation in the surface spectral response from two paint pigments. Our study aims at comparing the spectral measurements obtained from calibration samples and to estimate an optimal working point for OCT-based hyperspectral processing.
In this study we investigate the possibility of spectra stitching in the context of Spectral Domain – Optical Coherence Tomography (SD-OCT). The aim is to reach a high axial resolution while keeping sampling issues to a low level (slow decay in depth) but still operating with the fastest camera line rate available. The paper focuses mainly on simulations of spectrometer signals and the stitching procedure. It briefly introduces the experimental system. The findings of this study are relevant to most of the SD-OCT systems and could also be transferred to Swept Source OCT (SS-OCT) where they will help to increase the axial resolution capabilities.
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