Patent ID: 7442119

Claim:
A ventilation device comprising: an exhaust hood comprising a drive unit for producing an air flow; a transmitter device affixed to a first side of the exhaust hood generating a laser light beam directed to a second side of the exhaust hood, wherein said transmitter device comprises a first lens through which said laser light beam passes through resulting in an expanded laser beam; a receiver device affixed to said second side, receiving said expanded laser light beam and generating an electrical signal in response, said receiver device comprising an optical sensor, wherein: said expanded light beam forms a light spot comprising a plurality of maxima and minima light spots of varied sizes, at least one maxima light spot detected by said optical sensor, wherein said light spot is at least 5 millimetres wider than said optical sensor, and said plurality of maxima and minima light spots moving in response to changing ambient conditions of said air flow, thereby resulting in said electrical signal varying in amplitude and frequency over time; and a control device for receiving said electrical signal and controlling said drive unit, said control device comprising a processor for analyzing said electrical signal to determine a value determined by said number of maxima or minima light spots detected by said optical sensor over a time period, and controlling said drive unit in response to said value.