Analysis of light penetration depth in apple tissues by depth- resolved spatial-frequency domain imaging

Abstract

Spatial-frequency domain imaging (SFDI) has been developed as an emerging modality for detecting early-stage bruises of fruits, like apples, due to its unique advantage of depth-resolved imaging feature, in comparison with the conventional imaging techniques under uniform or diffuse illumination. This paper presents theoretical and experimental analyses to determine the light penetration depth in apple tissues under spatially modulated illumination. First, light penetrating capacity of the demodulated direct component and amplitude component images was investigated to prove the performance of the constructed SFDI system. Simulation and practical experiments were then carried out to explore the maximum light penetration depths in ‘Golden Delicious’ apples, in terms of two critical parameters, i.e., image contrast, and ratio of peak-to-valley intensity. Finally, apple experiment for early-stage bruise detection using the estimated reduced scattering coefficient mapping was conducted to validate the results of light penetration depths. The results showed that the simulations produced comparable or a little larger light penetration depth in apple tissues (~ 2.2 mm) than the practical experiment (~ 1.8 mm, or ~ 2.3 mm). Apple peel further decreased the light penetration depth due to the high absorption properties of pigment contents. The apple bruise, located beneath the surface peel with the depth of about 0-1.2 mm, could be effectively detected by the SFDI technique. This study, to our knowledge, made the first effort to investigate the light penetration depth in apple tissues by SFDI, which would provide useful information for enhanced detection of early-stage apple bruising by selecting appropriate spatial frequency.