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

The requirement for high-quality fruits and vegetables have been increased. The sorting and grading of them in a non-contact way is important, and firmness is an important criterion. In this study, we developed a non-contact measurement system for the firmness of fruits and vegetables by using airborne ultrasonic waves and a parabolic reflector. Focused airborne ultrasound produced vibration on the surfaces of samples and the vibrations were detected by a laser Doppler velocimeter to characterize the 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 the Kelvin⎯Voigt model. Finally, individual differences in the elastic properties of twelve kinds of samples and temporal changes in the 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 an evaluation system for the firmness of fruits and vegetables.