Uv-vis

Uv-vis is for solution samples, not for films.

No. UV-Vis can be used on films. They need to be free-standing or deposited onto a transparent substrate.

@Oswald: agreed, only the sample holder should be different and the spectrum of the substrate should be taken into account (you may use pure substrate as comaprison in the second ray path - only it has to be of identical thickness)

True. you always need a reference. I often fixed my samples with some tape just above th hole to the detector. Sample holders are overrated for such questions.

GISAXS is the well suited technique to survey the transparency of films.

GISAXS stands for "Grazing Incidence Small Angle X-ray Scattering".