The context of strong local sensitivity to dust emissions from an integrated steel plant like Sollac Dunkerque justifies the monitoring of the emissions of abnormally coloured smokes from this plant. In a first step, the watch was done "visually" by screening and counting the puff emissions through a set of seven cameras and magnetoscopes. The development of a new device making automatic picture analysis allowed to render the inspection automatic. The new system detects and counts the incidents and sends an alarm to the process operator. This way for automatic detection can be extended, after some tests, to other uses in the environmental field.