Diagnostics of Mechanical Fruit Damage Using Static Speckle Interferometry

Abstract

The epidermal layer of fruits plays a key protective role, and its damage leads to rapid microbial contamination and loss of merchantability. Early detection of these defects during harvesting or prior to storage is of critical importance. We propose a method for the express diagnostics of mechanical abrasive (scuffing) damages to the fruit skin, based on the evaluation of the spatial coherence of the speckle structure of reflected laser radiation (static speckle interferometry). It is demonstrated that the intensity correlation radius of the reflected multimode semiconductor laser beam decreases significantly in areas of abrasive damage (by a factor of 1.4 to 1.8) compared to the intact skin. Scanning areas devoid of skin also yields a significantly lower variance of the correlation radius, which serves as a convenient secondary criterion for defect identification. The observed differences are attributed to a substantial increase in surface roughness and the effective size of the illuminated area within the damaged region. The method was tested on apples (Granny Smith) and pears (Osennyaya Yakovleva). It demonstrated high statistical significance of the differences. The method is robust against dynamic changes caused by the metabolic activity of tissues, it does not require complex optics or reconfiguration for different fruit types and can be implemented using inexpensive laser modules and web cameras. The application of fast Fourier transforms to calculate the correlation characteristics enables real-time surface integrity assessment, making the method promising for automated quality control on sorting lines.