Figure 4a. Amorphous selenium–based direct conversion DR systems. (a) Drawing illustrates a selenium drum–based system. A rotating selenium-dotted drum with a positive electrical surface charge is exposed to x-rays. Alteration of the charge pattern of the drum surface is proportional to the incident x-rays. The charge pattern is then converted into a digital image by an analog-to-digital (A/D) converter. (b) Drawing illustrates a selenium-based flat-panel detector system. Incident x-ray energy is directly converted into electrical charges within the fixed photo-conductor layer and read out by a linked TFT array beneath the detective layer.