Publisher Summary This chapter discusses the absorption and reflection techniques. Measurement of the absorption of infrared radiation enables the quantity of many gases in a complex gas mixture to be measured in an industrial environment. Sometimes this is done without restricting the infrared frequencies used (dispersive). Sometimes only a narrow frequency band is used (nondispersive). The chapter also explains absorptions in the visible and ultraviolet. There are two instruments. First, is the Barringer remote sensing correlation spectrometer designed for the quantitative measurement of gases, such as nitrogen oxides or sulfur dioxide, in an optical path between the instrument and a suitable source of visible and ultraviolet radiant energy. The sensor is designed for maximum versatility in the remote measurement of gas clouds in the atmosphere, using the day sky or ground reflected solar illumination as the light source. Second, which covers absorption in the visible in liquids, is the Brinkmann Probe Colorimeter. This instrument is a standard colorimeter consisting of a tungsten light source, the output from which passes through one of a series of interchangeable filters covering the wavelength range 420-880 nm, then through a light pipe at the end of which is a probe cell.