An optical receiver converts an optical signal, transmitted through an optical fibre cable into an electrical signal suitable for a receiving device installed at the other end of the communication system. One application of an optical receiver is the conversion of an optical signal into a digital form. This chapter describes the fundamental principles of semiconductors. The most common type of optical detector is the semiconductor photodiode, which produces current in response to incident light. The fundamental principle behind the photodetection process is optical absorption. The quantum efficiency of a photodetector is a measure of how effectively the detector converts light into electrical current. A photodetector usually detects the energy of some photons better than others. Detection sensitivity is a function of the photon’s energy being detected. The chapter presents two experimental cases: measuring light power using two types of photodetectors, and measuring the power output and calculating the efficiency of a solar cell with/without a filter and/or a lens.