An absorption spectroscopy apparatus including a sample cell having a central axis, and a side wall coaxially positioned about the axis. The side wall defines a generally circular cross-section of the cell as taken along a plane extending perpendicular to the axis, wherein the generally circular cross-section of the sample cell has an average radius. The side wall includes a plurality of light reflective segments arrayed about the axis, wherein each reflective segment has a cross-section taken along a plane extending perpendicular to the axis of the cell that is concave with respect to the axis. The concave cross-section of each segment has an average radius that is unequal to the average radius of the generally circular cross-section of the sample cell. Among other aspects and advantages, the apparatus of the present disclosure is able to use incoherent, non-collimated light sources while maintaining high optical throughput efficiencies.