  The paper describes a study of wet foams in microgravity whose bubble size
distribution evolves due to diffusive gas exchange. We focus on the comparison
between the size of bubbles determined from images of the foam surface and the
size of bubbles in the bulk foam, determined from Diffuse Transmission
Spectroscopy (DTS). Extracting the bubble size distribution from images of a
foam surface is difficult so we have used three different procedures : manual
analysis, automatic analysis with a customized Python script and machine
learning analysis. Once various pitfalls were identified and taken into
account, all the three procedures yield identical results within error bars.
DTS only allows the determination of an average bubble radius which is
proportional to the photon transport mean free path $\ell^*$. The relation
between the measured diffuse transmitted light intensity and {$\ell^*$}
previously derived for slab-shaped samples of infinite lateral extent does not
apply to the cuboid geometry of the cells used in the microgravity experiment.
A new more general expression of the diffuse intensity transmitted with
specific optical boundary conditions has been derived and applied to determine
the average bubble radius. The temporal evolution of the average bubble radii
deduced from DTS and of the same average radii of the bubbles measured at the
sample surface are in very good agreement throughout the coarsening. Finally,
ground experiments were performed to compare bubble size distributions in a
bulk wet foam and at its surface at times so short that diffusive gas exchange
is insignificant. They were found to be similar, confirming that bubbles seen
at the surface are representative of the bulk foam bubbles.
