1. A method for calibrating a CT system, comprising: acquiring an air scan at a specified peak voltage (kVp); acquiring a phantom scan at the kVp, wherein the phantom scan is acquired by scanning the respective phantom on a table; processing projections associated with the air scan and the phantom scan with a preliminary beam hardening correction function; reconstructing an image using the corresponding corrected projections; segmenting the image to remove non-phantom components; processing the segmented image to generate an image pair comprising a phantom image with artifacts and a phantom image without artifacts; projecting the image pair to generate a projection pair; deriving a respective deviation ratio for the projection pair; repeating the acquiring, processing projections, reconstructing segmenting, processing the segmented image, projecting, and deriving steps for a specified range of kVp, filters, and phantoms covering the attenuation range of the CT system; and deriving spectral calibration vectors based on the respective deviation ratios.