A gas measurement system of this invention includes a detector assembly having a beamsplitter adapted to separate infrared radiation into a first beam and a second beam and a mirror adapted to receive the first beam from the beamsplitter. The first and second beams are directed to first and second detectors that are disposed in a common plane. In one embodiment, the optical elements are provided on or in an optical block. In another embodiment, the gas measurement system includes a housing that contains an infrared absorption gas measurement assembly, a luminescence quenching gas measurement assembly, and a processor that is programmed to measure gas constituents of a gas flow based on an output of the infrared absorption gas measurement assembly and the luminescence quenching gas measurement assembly.