ABSTRACT
This chapter describes the measurement of opacity, or optical density, and to the instruments used to make these measurements. The objective of opacity measurement is to observe visible emissions the same way as they are observed by the human eye. In order to reproduce the human eye, the spectral distribution of the light source in an opacity meter should also be photopic. Opacity can be related linearly to the Ringlemann numbers used by human observers and is also the parameter typically regulated by the Environmental Protection Agency to insure proper operation and maintenance of particulate control equipment. Opacity of smoke plumes is often related to the visual effects seen by a human observer. Human observation of smoke and dust was, of course, the initial stimulus to develop better techniques for control and monitoring. Several techniques have been developed to enable an observer to monitor smoke plume characteristics from a remote location.
