This work presents a unified approach, from the applied engineering point of view, to radiometry, optical devices, sources, and receivers. It combines modern optics, electric circuits, and system analysis into a unified treatment. The text provides physical concepts together with numerous data for sources and systems, and offers basic analytical tools for a host of practical applications. Reference sources, such as a glossary with explanatory text for specialized optical terminology, are included. Also, there are illustrative examples and problems with solutions. Diverse areas are covered, such as medical thermography, fibre optical communications, and CCD cameras. In addition, topics such as D*, NEP, f number, RA product, BER, and shot noise are explained.