Combining multispectral reflectance and fluorescence imaging for identifying bruises and stem-end/calyx regions on Golden Delicious apples

As a part of the project for detecting bruises on Golden Delicious apple using vision system, the present paper shows a method that could separate the stem-end/calyx regions from the true bruises by combining the information of hyperspectral reflectance and fluorescence images. The images were scanned between 400 nm and 1,000 nm with a hyperspectral imaging system. Different light sources were constructed for capturing the reflectance and fluorescent images. Compared to the reflectance signal, the fluorescence signatures are much less intense, so that only the fluorescence of chlorophyll waveband was further examined (i.e., 685 nm). The analysis showed that the Principal Components scores images, which were based on the reflectance images, can be used for separating the bruised areas as well as the stem-end/calyx regions from the sound apple tissues; whereas only the stem-end/calyx was able to be recognized from the fluorescence images. For the samples investigated in this study, no stem-end/calyx regions were misrecognized as bruises; however, about 12% of bruised surfaces were misclassified as stem-end or calyx regions. All of the healthy tissues were correctly recognized as non-stem-end/calyx regions. The classification results indicated that combining multispectral reflectance and fluorescence imaging may help to identify the stem-end/calyx regions from the true bruised tissue and therefore to improve the accuracy for bruise detection on Golden Delicious apples.