To only measure reflection from the front surface, the light passing into the material must be absorbed without reflecting from the second surface.

Matte black photography wrap is inexpensive works perfectly at absorbing any light passing through the material 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 practice a drop of water or isopropanol (evaporates after measurement) is sufficient to get accurate gloss results for most transparent samples.