It can be problematic to measure the gloss of transparent sheeted material because light is reflected from both the front surface and internally from the second surface.
To only measure reflection from the front surface, the light passing into the material must be absorbed without reflecting from the second surface.
Environmental setup to measure transparent materials with defined backing
An inexpensive standard background to use would be matte black photography wrap that also works almost perfectly at absorbing any light passing through the material.
As any air between the transparent material and the foil will cause the second surface to reflect light, a liquid must be used to optically bond the transparent material to the black foil.
To completely eliminate second surface reflection a liquid with similar refractive index to the test material should be selected. In common practice a drop of water (readily available) or isopropanol (evaporates after measurement) are sufficient to get accurate gloss results for most transparent samples.
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