Vincent van Gogh (1853-1890), one of the founding fathers of modern painting, is best known for his vivid colors, his vibrant painting style, and his short but highly productive career. His productivity is even higher than generally realized, as many of his known paintings cover a previous composition. This is thought to be the case in one-third of his early period paintings. Van Gogh would often reuse the canvas of an abandoned painting and paint a new or modified composition on top. These hidden paintings offer a unique and intimate insight into the genesis of his works. Yet, current museum-based imaging tools are unable to properly visualize many of these hidden images. We present the first-time use of synchrotron radiation based X-ray fluorescence mapping, applied to visualize a woman's head hidden under the work Patch of Grass by Van Gogh. We recorded decimeter-scale, X-ray fluorescence intensity maps, reflecting the distribution of specific elements in the paint layers. In doing so we succeeded in visualizing the hidden face with unprecedented detail. In particular, the distribution of Hg and Sb in the red and light tones, respectively, enabled an approximate color reconstruction of the flesh tones. This reconstruction proved to be the missing link for the comparison of the hidden face with Van Gogh's known paintings. Our approach literally opens up new vistas in the nondestructive study of hidden paint layers, which applies to the oeuvre of Van Gogh in particular and to old master paintings in general.Vincent van Gogh is generally recognized as one of the founding fathers of modern painting. 1 In recent decades his work has undergone extensive art historical and technical study. One striking feature that emerged is Van Gogh's frequent reuse of paintings in order to recycle the canvas. 2,3 The artist would simply paint a new composition on top of an existing work. This is usually attributed to the artist's lifelong economic hardship and the rapid, energetic evolution of his artistic ideas. Visualizing such hidden paintings is of interest to both specialists in the field of Van Gogh and the public alike. Covered paintings in general provide an insight into the making of artworks and the underlying conceptual changes. In the case of Van Gogh, they also present a touchstone for comparison with preparatory drawings and the abundant literary record. The extensive correspondence with his brother Theo van Gogh, an art dealer based in Paris, is full of remarks by Vincent on his work.Nondestructive imaging of such hidden paint layers is usually realized by means of tube-based X-ray radiation transmission radiography (XRR). The absorption contrast in these images is mostly caused by the heavy metal components of pigments employed, such as lead in lead white or mercury in vermillion. Conventional XRR, however, has a number of important limitations. First of all, the observed X-ray absorbance is a summation of all element-specific absorbancies. This implies that the contribution to the overall image contrast due to (l...

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