A non-dispersive single beam detection assembly in an infrared gas analyzer. The detection assembly comprises radiation source(s) providing infrared radiation, a measuring chamber, and a physical beam splitter for dividing said radiation beam into a reflected beam portion and a transmitted beam portion, or for combining a reflected beam portion and a transmitted beam portion into said radiation beam. A measuring detector receives one beam portions, and a reference detector receives another beam portion. Alternatively a measuring/reference detector receives both beam portions. Said transmitted beam portion has a first spectral intensity peak at shorter wavelengths with a first peak wavelength, and said reflected beam portion has a second spectral intensity peak at longer wavelengths with a second peak wavelength. There is a wavelength gap between said second peak wavelength and said first peak wavelength, which gap corresponds a wavelength shift of an optical interference filter with said second peak wavelength as tilted from its perpendicular position to an angled position. Said wavelength gap is at maximum 10% of the second peak wavelength, and at minimum 0.5% of the second peak wavelength.