A gas-measuring arrangement (1) with an open optical measuring section (7) is formed by a measuring device (5) with an array of lenses (10, 12, 19, 24), a phase mask (22), an optical path mirror (16) positioned obliquely, and a reflecting mirror (3) located outside the measuring device (5) at the end of the open measuring section (7). The light is decoupled to a detector (26) via a phase mask (22) and the optical path mirror (16). The phase mask (22) diverges a light intensity spot and the light intensity ring is decoupled by an obliquely positioned optical path mirror (16) towards a detector (26).