Double pass extinction is a transmissometer type for visibility, dust and opacity sensors. These transmissometers measure the extinction coefficient of the atmosphere, and are used to determine visual range. With double pass extinction, the light source and detector are on the same side of the stack in a unit called a transceiver. A special reflector (the retro reflector) is installed where the photocell would be in a single pass unit. The purpose of the retro reflector is to send the light back along the same path it took when coming across the stack. A half silvered mirror is placed in the light's path to reflect half of the returning light from the retro-reflector onto the detector. Because the light traverses the smoke twice, the unit is called a double pass system.