Photometry and radiometry study the energy transport of visible light (photometry) and the general laws of energy transport in optical systems (radiometry). Brief introduction is given to radiometric and photometric units. Propagation of energy from source to image as a functional of the parameters of an imaging system is described and expressions governing the illuminance of optical image are derived. A brief introduction to the sources and sensors of light is given, the major parameters of light sources and sensors are defined providing basic expressions for radiometric calculations in optical systems with different kinds of sources and sensors.