This study presents an effort to use SWIR hyperspectral imaging to detect sour skin, one of the most serious onion postharvest diseases. The spectral characteristics of the infected onions were distinct from those of the healthy onions, and the differences were more pronounced in the neck area than in the bulb area. The log-ratio image utilizing two optimal wavelengths (1070 nm and 1400 nm) was proven to be effective in magnifying the spectral differences of the two classes of onions. Although the SVM classifier with three image features as input variables achieved 87.14% classification accuracy, the result could be further improved by reducing the false positives caused by the influence of the surface fleshy scales of onions. Moreover, the orientation of onions should be considered in future studies. The final algorithm proposed by this study involved three image features extracted from the log-ratio image of the onion bulb, which was calculated from the 320 Ч 256-pixel images at two bands. The algorithm suggested a two-band multispectral imaging system for detecting sour skin-infected onions on classification lines. In sum, this study demonstrated the efficacy of the hyperspectral imaging technique for onion sour skin detection and the results of this research can be utilized to develop a multispectral imaging system for rapid and nondestructive detection of sour skin-infected onions on packing lines.
Figure 1. Diagrammatic illustration of the parts of the onion bulb: neck dry tissue (a), outermost scale/skin (b), fleshy scale (c), root cap (d), outer root (e); neck area (A), shoulder/upper half of the onion bulb (B), and lower half of the onion bulb (C), and root area (D). II. Examples of sour skin infected onions at the outer scales (1) and the inner scales (2); the infected areas are marked by arrows.
Figure 2. Spectral images of an onion at five selected wavelengths. Images were taken on the same onion before inoculation (healthy) and five days after inoculation (sour skin-infected), respectively.
Figure 3. Illustration of the log-ratio images of an onion sample before inoculation (healthy) and 5 days after inoculation (sour skin-infected).