Research on the major factor of hyperspectral image measurement method for tree leaves

Street trees are exposed to various stresses, thus the detection and management of stresses is significant for street tree vitality. Hyperspectral imaging is one of the non-destructive techniques to detect stress responses and vitality of trees. The spectral reflectance of leaves could change as the method for hyperspectral image measurement. We aimed to research the effect of distance and angle between the hyperspectral camera and leaves to recognize the factors for the optical method at the leaf scale. The target is tree seedlings for three major street tree species (Ginkgo biloba L., Chionanthus retusus Lindl. & Paxton and Prunus × yedoensis Matsum.). The hyperspectral images were collected at various distances (30cm and 80cm) and angles (90° (top), 60°, 30°) between the camera and leaves. When measuring, the halogen light was located at the same height as the camera and maintained a 45° angle with leaves. Also, hyperspectral images were collected at various light intensities (low, medium, high) with an 80cm distance and 90° angle between the camera and leaves. We analyze the change in reflectance in the spectral range of 400nm~1000nm as the various measurement methods. For all species, the effects of measurement methods on spectral reflectance were similar. The reflectance near infrared region increased at 30cm distance compared to 80cm distance. Also, the difference in spectral reflectance between 30cm and 80cm distance was continuously increased at over 800nm. However, there were no changes in the spectral response pattern among the light intensities. It means that the reflectance changes due to the distance between the camera and leaves might result from the physical distance, not the light intensity that can increase as the distance gets closer. Meanwhile, the reflectance value could change slightly, but the spectral response patterns were similar among angles between the camera and leaves. The wavelength in which the reflectance value changed differed for species. Therefore, the distance between the camera and leaves might be the major factor in setting the method. We select the distance between the camera and leaves as 80cm to measure the whole seedling crown. Then, we treated the salt stress (three times of 5% CaCl2 500ml with 2-3 days intervals) for G. biloba L. and C. retusus Lindl. & Paxton seedlings. The hyperspectral images were measured before and 7 days after salt treatment with an 80cm distance and 90° angle between the camera and leaves. The changes in the spectral index (R900/800) by the treatment compared between control and salt treatment. The stress responses were detected with the set measurement method; the spectral index increased higher in salt treatment than in the control.