Near-infrared light penetration depth analysis inside melon with thick peel by a novel strategy of slicing combining with least square fitting method

The light penetration depth in melon was explored by near-infrared (NIR) diffuse reflectance spectroscopy (500-1,100 nm) combining a novel strategy of slicing and least square fitting. The depths of peak wavelength and effective waveband (720-880 nm) based on three source-detector distances (d = 10, 20, and 30 mm) were calculated and compared by least square fitting. The results indicated that the obtained depth were similar both slicing method and least square fitting. The depth values of melon fruits were 12.3, 14.9, and 19.0 mm for peak wavelength, and the depth value ranges were 11.7-12.4, 14.2-15.8, and 16.8-21.0 mm for the effective waveband of 720-880 nm when d was 10, 20, and 30 mm, respectively. The overall results demonstrated that least square fitting method could be accurately applied to calculate the light penetration depth inside melon and the depth value increased with increasement of d. And 20 mm source-detector distance could be the optimal selection. This study can provide the valuable reference for development of spectral instrument used to effectively detect the internal quality of fruits, especially for fruits with thick peel. Practical application The light penetration depth in fruit tissues was very important for obtaining the more effective spectral information used to accurately and robustly assess the internal quality of fruits. Currently, there was no effective and rapid technology to measure the light penetration depth of fruits, especially fruits with thick peel. Hence, in this research, we provided a novel method of slicing and least square fitting to calculate the light penetration depth in melon based on different source-detector distance. The result indicated that the proposed method could be applied to calculate the penetration depth in melon and provided a valuable reference to develop the spectral equipment.