The TunnelCraft 3 CO/NO/VIS Air Quality Monitor (AQM) uses both infrared light and visible light channels to measure carbon monoxide, nitric oxide and visibility in a single compact instrument. The AQM consists of a transceiver that projects infrared beams to a reflector unit mounted 3m away. The reflected light is received by the transceiver and the specific absorption is measured to determine the visibility dimming coefficient, carbon monoxide and nitric oxide concentration in the tunnel atmosphere.