Beam splitter detectors are multi-channel detectors with only a single aperture, in which an array of a plurality of micro mirror surfaces works as an internal beam splitter. The division of the beam into two or four spectral channels is done only within the detector. The aperture with a diameter of 2.5 mm allows for the use of cells with very small dimensions for NDIR gas analysis. This result is very low dead volume of the measurement system. Mechanical expansion in the optical system of the non-dispersive infrared gas analyzer, aging effects or eventual smudging can effect detectors only before the beam splitting occurs and thus impact equally to all channels. So the signal ratio between the individual channels remains permanently stable.