A method for diagnosis and evaluation of tooth decay comprises: locating in an x-ray image the contour of the dento-enamel junction (DEJ); measuring optical density along contours substantially parallel to and on either side of the DEJ contour; and calculating at least one numerical decay value from the measured optical densities. A method for diagnosis and evaluation of periodontal disease comprises: measuring in an x-ray image a bone depth (BD) relative to the position of the cemento-enamel junctions (CEJs) of adjacent teeth; measuring bone density along a contour between the adjacent teeth; and calculating a numerical crestal density (CD) value from the measured bone density. Calibration standards may be employed for facilitating calculation of the numerical values. A dental digital x-ray imaging calibration method for at least partly correcting for variations of the optical densities of images acquired from the dental digital x-ray imaging system.