Non-contact measurement of firmness properties of fruits and vegetables using parabolic-reflector airborne ultrasonic transducer and laser Doppler velocimeter

The requirement of high-quality fruits and vegetables have been increased. Sorting and grading of them in non-contact way is important, and firmness is an important criterion. In this study, we developed a non-contact measurement system for firmness of fruits and vegetables by using airborne ultrasonic wave and parabolic reflector. Focused airborne ultrasound produced vibration on the surfaces of samples and the vibrations were detected by a laser Doppler velocimeter to characterize elastic properties. First, we designed and fabricated an offset-type parabolic reflector. Then, the elastic parameters (maximum displacements, center frequencies, and damping factors) were determined based on Kelvin-Voigt model. Finally, individual differences of elastic properties of 12 kinds of samples and temporal changes in maturity state and elastic properties of three kinds of samples were observed. From the results, we confirm the developed system could be potentially used as evaluation system of firmness of fruits and vegetables.