The application of dual-energy principles allows accurate measurement of bone density using conventional radiologic equipment. Digital subtraction of two images generated with X-ray beams of different energies yields a bone signal proportional to the amount of bone in the beam path which is free from error due to uncertainty in the amount of overlying soft tissue. In this study the two images are generated simultaneously by selective filtration of the X-ray beam. Computer simulations were performed to optimize the X-ray spectra with respect to noise and separation of high- and low-beam energies. The effects of tube voltage, filter materials, filter thicknesses, soft tissue thickness and bone thickness on measurement uncertainty were examined. The authors' simulations indicate that, with appropriate filter selection, bone thickness may be measured with a coefficient of variation of less than 1%.