The permeability of a pulsed laser beam into Satsuma mandarin, White grapefruit and Fuji apple was compared by time-of-flight near infrared (ToF-NIR) spectroscopy. In this study, a compact measurement system including a dye-doped plastic laser cavity with a wavelength of 800 nm and an avalanche photodiode were employed. The NIR radiation attenuance (i.e. optical density) in apple was higher than in the citrus fruit. The peak maxima of transmitted radiation of intact Satsuma mandarin increased 130 times more than that of intact grapefruit. Differences in diameter related directly to attenuance. The peak maxima of transmitted radiation of peeled grapefruit increased 250 times more than that of intact grapefruit; however, these increases were not found for Satsuma mandarin or apple. NIR radiation attenuance of the grapefruit peel was greater than for the peel of the other fruits. The incident pulsed laser beam was uniformly scattered in the citrus sample, whereas the scattering condition of the pulsed laser beam in apple might not be uniform because of the presence of core tissue. Thus, while the pulsed laser beam could be transmitted through citrus fruit and apple, the effect of fruit size and peel on the NIR radiation attenuance characteristically varied.

Keywords: time-of-flight, dye-doped laser, time-resolved profile, scattering condition, optical density, fruit, peel