Abstract: In the gas analyzer described in the specification, a test chamber housing for accommodating a volume of gas to be analyzed has a spherical shape and a spherical inner mirrored surface with windows to receive a radiation beam from a source and pass the beam to a detector. To provide different test distances through the chamber for the radiation beam, the source of radiation and the housing are movable relative to each other so that a reproducible change in the test distance can be achieved through multiple reflections on the inner mirrored surface of the housing.