Figure 2.1 depicts the setup of a spectral CT x-ray attenuation measurement. The source S emits a flux of x-ray quanta at an object. The object consists of x-ray-attenuating materials that are described by their spectral attenuation coefficient μ( E , r ) at position r . The detector registers the quanta passing through the object. Two independent measurements with and without the object are performed. The resulting intensities I and I 0 yield the attenuation A = I = I 0 ∈ [0,1], which describes the relative decrease in intensity caused by x-ray attenuation in the object. In the following, we briefly review the spectral characteristics of the source, object, and detector.