1. A method for determining a correction for beam hardening in CT images in a CT imaging apparatus, said method comprising: obtaining air scans at a plurality of kVp's; determining detection efficiencies for detector elements of the CT imaging apparatus; estimating projection values through a combination of at least two different materials of different thicknesses; determining a transfer function that translates the estimated projection values into ideal projection values for at least two of said different materials; and storing the transfer function as the beam hardening calibration function for images of the CT imaging apparatus.