Optical path structure for open path emissions sensing with spinning filter wheel

An optical system for a gas component analysis, comprising:

• a first emitter located on a first side of a vehicle path for emitting a first light beam having a first spectrum across the vehicle path;
• a first receiver for receiving the first light beam;
• a spinning filter wheel that filters the beam from the first emitter before the beam crosses the vehicle path;
• a second emitter located on the first side of the vehicle path for emitting a second light beam at a second spectrum across the vehicle path;
• a second receiver for receiving the second light beam;
• a third emitter for emitting a third light beam;
• a third light receiver for detecting the third light beam, wherein the third light beam travels along a third path, and at least a portion of the third path overlaps with at least a portion of the second path;
• and
• a reflector that is a retroreflective assembly having at least three reflective faces, and wherein at least one of the beams travels across the road at a first height above the road, and returns across the road at a second height above the road different from the first height.