Dear Sir. Concerning your issue about the unit of opacity of a biopolymeric film. The opacity of the films was determined according to the procedure of Gontard et al. (1992) and the ASTM D 1003-00 (ASTM, 2007) recommendations. The films prepared with different MMT concentrations and MMT dispersion methodologies were studied. Film samples were cut into rectangles and placed on the inside of the spectrophotometer cell. The absorbance spectrum (400–800 nm) was recorded for each sample using a 10UV Spectronic Unicam Genesys (USA) spectrophotometer. The film opacity was defined as the area under the recorded curve, determined by an integration procedure. The opacity was expressed as absorbance units x nanometers (AU nm). Samples were previously conditioned at 53% RH and 25 °C and the values reported were averages of at least four readings. I think the following below links may help you in your analysis:

http://www.scielo.br/pdf/bjft/v15n3/aop6411.pdf

Thanks

Hi

the opacity is defined the value of light absorbance at 600nm divided into film thickness (mm). the unit of opacity is A/mm.

doi:10.1111/jfpe.12305