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What Changes the X-ray Emission Spectrum
Amplitude shows quantity and position shows quality. mAs changes only the height; kVp, filtration, target Z and waveform also shift the curve.
Reading the graph
The x-ray emission spectrum is a graph of the beam leaving the tube. The x-axis is photon energy in keV, and the y-axis is the number of photons at each energy. The smooth hump is bremsstrahlung, and the sharp spikes are characteristic x-rays. Three features matter. The area under the curve is the beam quantity, the total number of photons. The position of the curve along the energy axis shows quality: the farther right, the higher the average energy. The right-hand endpoint is the maximum energy, which in keV equals the kVp. With a tungsten target, K-characteristic spikes appear at about 57 to 69 keV, but only when the kVp is above tungsten's K-shell binding energy of 69.5 keV.