Abstract. More than 2 people out of 10 suffer from allergies, which can take various forms, from eczema to anaphylactic reactions with possible lethal consequences. Diagnosis is achieved through so-called prick-tests or IDR (intradermo-reaction): the injection of a small quantity of substances suspected to cause the allergic manifestation induces an erythema, the size of which is a useful indicator for the diagnosis. The manual surface measurement is timeconsuming and inaccurate. This article presents a method for the semi-automatic measurement of the erythema from a photograph of the skin, taken in such conditions that lighting problems are minimized. The method is based on region growing and takes advantage of the most significant color spaces; the Lab space appears to be the best suited. It was tested on nearly 100 images, taken by various operators, on patients with various skin pigmentations; it gave promising results and proved to be robust.

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