An in-cylinder optical sensor has been developed to measure and control the exhaust smoke or soot emission from heavy-duty diesel engines. The sensor directly measures the radiant emission from incandescent soot particles during and after main combustion. Results show a strong correlation between both the measured duration and end of radiant emission, and the amount of soot emitted by the engine. Test results also demonstrated some potential benefits of in-cylinder control using the optical sensor. In one test, the optical signal was used to control high-load soot emission during changes in air intake pressure and temperature. In a second test, the optical signal was used to minimize the variability in exhaust soot levels caused by manufacturing variations in fuel-injector flow characteristics.