Figure 2.

(a) Schematic drawing: Diverging LED light within a non-optimized resonant cell similar to that proposed in Ref. [5]. The light hits the inner walls at the red dashed line, causing the background signal. The lower signal to offset ratio results in a worse limit of detection and stability problems. (b) Concept for use of LED light. The T-cell consists of an absorption chamber that enables the non-contact feedthrough of the beam. Perpendicular to it there is the resonance cylinder, where the standing wave pattern is formed.