Location and Number of Sampling for Optical Reflectance Measurement of Chinese Cabbage and Kale Leaves

Abstract

Non-destructive sensing of plant growth and components has been investigated recently, and sampling scheme affects accuracy and reliability of the approach. Objective of the paper was to determine location and number of sampling points for optical reflectance measurement of Chinese cabbage and kale plant leaves. Chinese cabbage and kale were grown in a plant factory, and 9 leaves for each cabbage and kale were sampled and optical reflectance spectra were measured with a UV/VIS/NIR (ultra-violet, visible, and near-infrared) spectrometer (Jaz instrumentation, Ocean Optics, FL, USA). Reflectance was measured in a range of 250~1,100 nm at 30 points (15 points on each vein and blade areas) over the entire leaf area. To determine optimum location and number of sampling, reflectance difference and cross correlation among the reflectance spectra were computed and compared. Reflectance difference was computed by subtracting averaged spectrum from each spectrum. Cross correlation method was used to calculate degree of similarity among spectra at different sampling locations. Four pre-processing methods were tried to reduce the spectral noise and to compare reflectance difference of cabbage and kale leaves. Reflectance of the blade part was greater and more reliable than that of the vein part for both of the cabbage and kale leaf samples. Reflectance measurement at 12 and 9 locations over the upper leaf part was considered as preferable for rapid and stable sensing of cabbage and kale leaves, respectively.