Measurement of the dependence of ultra diluted gas transmittance on the size of the detector.

We show that measured optical transmittance of an ultra thin gas depends on the detector size. To this end we conducted an experiment that compares transmittances measured in parallel with a pair of detectors with different diameters ranging from 2 to 200 [Formula: see text]m. A Tunable Diode Laser Absorption Spectroscopy type system was used. Transmittance of [Formula: see text] 10[Formula: see text] mbar water vapor on NIR absorption line [Formula: see text] nm was measured using a 61.6 m long multi-pass cell placed inside the [Formula: see text] 300 l vacuum chamber. The result of the experiment shows higher transmittances when the measurement is performed using smaller detectors. The difference reaches as much as [Formula: see text]%, which is greater than 0 with [Formula: see text] statistical significance. Qualitatively it is in agreement with the recently developed model of thin gas optical transmittance taking into account the quantum mechanical effects of spreading of the wave functions of individual gas particles.