Opacity sensors and instruments measure the amount of light transmitted through a sample. Opacity sensors and instruments use two types of technology: transmissometry, where light is absorbed and scattered as it passes through a mixture of gas and particles, and compares the intensity of the emitted light with the received light, and the scattered light principle where a light beam emits infrared light that is scattered by particles in the gas. This method is used to detect very low concentrations.
These instruments work on a number of measuring principles. With extinction or extinction coefficient technology, the attenuation of light in its passage through an aerosol is due to (light) scattering and absorption. Percent opacity devices use a measure, usually in percent, of light extinction, defined as one minus the transmittance of an aerosol cloud. They are used mainly to assess the obscuration by smoke plumes. Percent transmission is the ratio of the detected irradiance to the source irradiance for a beam passing through an attenuating medium. The relationship between transmission and opacity follows: % Transmission (light through the smoke) + % Opacity (light blocked by the smoke) = 100%. Optical density sensors report the logarithm (base 10) of the reciprocal of the transmittance. Dust density sensors report the concentration of dust in the sample volume.
Detector range, in terms of distance between transmitter and receiver, is one of the most important specifications for opacity sensors and instruments. Accuracy is expressed as a percentage of the full-scale output. Response time, the time between the sensor’s detection and output signal, is another important specification.
Some popular features for opacity sensors and instruments are field calibration, built-in calibration, self-diagnostics, and heaters for outdoor or other cold temperature applications.
Electrical output types can be analog current or voltage, as well as a frequency. Output can also be a switch or an alarm. Displays for opacity sensors and instruments can be analog, digital or video terminals. Control and programming the devices can be done through an analog or digital front panel or by using a computer interface, typically serial or parallel.