A COMS is an instrument that continuously measures opacity, which is a measure of the amount of light attenuated by particulate matter in effluent emissions. The percentage of visible light attenuated is defined as the opacity of the emissions. Transparent stack emissions that do not attenuate light will have a transmittance of 100 percent or an opacity of zero percent. Opaque stack emissions that attenuate all of the visible light will have a transmittance of zero percent or an opacity of 100 percent. Opacity often is used as an indicator of the degree of particulate matter emissions.