This invention provides a mechanism for clearing debris and vapors from the region around the optical axis of a vision system that employs a directed airflow in the region. The airflow is guided by an air knife that surrounds a viewing gap placed in front of the camera optics. The air knife delivers airflow in a manner that takes advantage of the Coanda effect to generate an airflow that prevents infiltration of debris and contaminants into the optical path. Illustratively, the air knife defines a geometry that effectively multiplies the delivered airflow approximately fifty times (twenty-five times on each of two air-knife sides) that of the supplied compressed air. This provides an extremely strong air curtain along the scan direction that essentially blocks infiltration of environmental contamination to the optics of the camera.