Most current IR detectors that have been considered in the previous chapters are used in the direct detector mode. In direct detection, the output electrical signal is linearly proportional to the signal power. But in terms of the amplitude of the electric eld carried by radiation, photodetectors are quadratic. As such, no information regarding the phase of the signal is retained in the electrical output. When the radiation eld at the detector is only that of the signal, we talk of direct detection (Figure 4.1a), by far the most usual case in applications.