A simple device for accurately measuring real-time air flows along a wall, and relating to a measuring device including an optical sensor configured to measure an illuminance distribution on a free surface of the optical sensor. A mobile member movably is mounted on the light sensor by partially covering its free surface, the mobile member being adapted to be freely directed by a local air flow pattern, thus, modulating the illuminance distribution on the free surface so as a current illuminance distribution measurement by the optical sensor indicates the current direction of the mobile member representing the current air flow pattern.