Publisher Summary This chapter focuses on optoelectronics. The term “optoelectronics” covers the study of all electrically driven light-emitting devices, all devices that produce electrical effects when exposed to light, and all methods of conducting light through fixed pathways. A wide variety of units for measurement of light and illumination is found in the manufacturers' specifications for light sources and photocells. The chapter reviews only the relevant units of the Systeme International (S.I.). Incandescent lamps are still the predominant source of light where a steady illumination is required, either to indicate to a user that some particular circuit is operating, or to provide a beam of light across an intervening space on to a photosensitive device. To obtain a long life from an incandescent lamp, two points should be carefully considered. First, the lamp should be operated at a voltage lower than that recommended by the manufacturer for normal use. Second, the initial current surge when the lamp is switched on should be limited.