This equation assumes monochromatic x-rays. For a polychromatic x-ray spectrum, μ can be considered as the linear attenuation coefficient of the mean beam energy. At a given tube potential, the incident and detected x-ray intensities are both proportional to the tube current and exposure time (gantry rotation time). Automatic exposure control systems, however, modulate the tube current only and keep the rotation time constant. Thus, tube current, instead of tube current-time product, will be used in the following analysis.