  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 ${\mu}$m to 200 ${\mu}$m. A Tunable Diode Laser
Absorption Spectroscopy type system was used. Transmittance of $\sim$1e-2 mbar
water vapor on NIR absorption line $\lambda$=1368.60 nm was measured using a 60
m long multi-pass cell placed inside the 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 1.23 $\pm$0.1 %,
which is greater than 0 with >5$\sigma$ 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.
