The opacity of smoke-stack emissions was measured by three methods at thirteen different plants and the results compared. The three opacity measurement methods are trained observer, in-stack transmissometer, and laser radar (lidar). The instrumental methods, lidar and in-stack transmissometer, correlated better with each other than with the observer method and were also more precise than the observer method. Observer measurements were generally lower than instrumental measurements. Data show that for the range of opacity emission standards (betwen 10 and 40% opacity), the observer readings averaged 6 and 11% opacity less than the instrumental readings of lidar and transmissometer, respectively, and varied by as much as 25% opacity below to 8% opacity above the instrumental readings.