The study of wood diameter is needed for the understanding of wood supply management. This type of approach, which was launched in the 1990s, has improved considerably as radius of curvature measurement has gained in precision through the introduction of new equipment: stereomicroscope equipped with a camera, and image analysis software. This paper focuses on the first step of the wood diameter study: radius of curvature measurement. This measurement is unusable as it stands, but the data set can be integrated into models to provide better information on wood diameter. In addition to the circle tool technique based on tree-ring curvature, three new techniques are presented here, based on wood rays: Thales' theorem, trigonometry in a right-angled triangle, and trigonometry in an isosceles triangle. This study compares results on perfectly graduated targets and on wood samples for four radii of curvature measurement techniques, to assess their reliability and limits. Three parameters are examined: angle between two wood rays, distance between two wood rays, and radius of curvature.