By the late 19th and early 20th centuries, regulations were being passed that sought to control air pollution predominantly for smoke and odour control. Traditionally, regulators were concerned with the visual impact of the discharge from a stack or chimney. Therefore, emission limits were expressed in terms of colour or opacity.

Modern methods for opacity measurement still use the darkness of the stack gases to measure the amount of smoke or dust emitted within the exhaust gases.

Dynoptic opacity monitors use the light transmission technique to measure the visible opacity of exhaust gas in a duct, stack or flue, in which a high intensity light beam is projected across the stack. Any dust, smoke or particulate present will attenuate the transmitted light and the decrease in light intensity, due to absorption and scattering, is measured.

One advantage of opacity systems is that they are insensitive to interference from other flue gas components since the beam spectrum is within the photopic region (400-700 nm) and most other flue gas constituents do not absorb in this spectral region.

There are two types of opacity monitors; Single Pass and Double Pass.

Single pass opacity monitors have a Transmitter/Receiver arrangement with the light beam emitted from the Transmitter passing across the stack to a Receiver. Double pass opacity monitors have a folded beam Transceiver/Reflector arrangement with the light beam emitted from the Transceiver (TRX) passing across the duct to a Reflector, which then returns the light to the Transceiver.

Single pass opacity monitors are the simplest type of instrument and offer a very cost effective solution to meet monitoring requirements. One advantage to single pass systems is that they are suitable for applications with long path lengths, i.e. large stack diameters and Dynoptic single pass opacity monitors can be used for path lengths of up to 20m.