How it works
The Environmental Protection Department says it has obtained 100 per cent accuracy from its emission-detecting system by placing two sets of sensors 10 metres apart. The department said California had adopted the same method, which it calls a dual remote sensing unit, and put it into practice this year.
Senior environmental protection officer Yam Yat-shing said unstable computer systems in car engines sometimes triggered a sudden rise in emissions. But placing two sensors 10 metres apart will confirm whether the emission is continuous. The system works by directing an infra-red or ultra-violet beam through the exhaust of passing vehicles to a reflector on the other side of the road. Exhaust gases absorb energy in the beam. The system calculates the level of emissions by measuring the energy that has been absorbed from the beam. A camera will automatically take pictures of cars that emit excessive pollutants.