2.5.3. Color Appearance and Opacity

The colors of the bi-layer films were measured using a portable scanner (G4050, HP, USA) and then expressed as L\*, a\*, and b\* values. The opacity of the films was recorded with a UV-vis spectrophotometer at 200 to 800 nm. The opacity formula was as follows [11]:

$$Opacity = \frac{Abs\_{600}}{d} \tag{3}$$

where *Abs*<sup>600</sup> is the absorbance at 600 nm, and *d* is the average thickness (mm).
