# Detecting Green Mold Pathogens on Lemons Using Hyperspectral Images
## 2 Materials and Methods
### Experimental Set-Up for Taking Spectral Images and Their Intensity Distributions
In Figure 4a, the number in each spectral image represents the central wavelength \(\lambda_{k}\) of the hyperspectral filter used. In images in Figures 4b and 5, Figures 6 and 7, each  
Figure 3: **(a)** Experimental set-up: **(1)** Object (Lemon), **(2)** Nikon Camera D810, **(3)** Spectral Filter, **(4)** a halogen lamp, **(5)** two polarizers, **(6)** analyzer, **(7)** a power meter, **(8)** a detector and **(9)** hyperspectral filters in the spectral range of 500 nm to 650 nm for 10 nm interval; **(b)** fully rotted lemons as source of green mold infection in special chamber. -lemon follows the spectral image orders as in Figure 4a. This means that the 1 st row images are in the blue-green color region (500-530 nm), the 2 nd row--in the green color region (540-570 nm), the 3 rd row--in the yellow to orange color region (580-610 nm), and the 4 th row--in the orange to red color region (620-650 nm) from left to right. Figure b of Figures 4-7 represents the grayscale version of the spectral images in the same order as the spectral images. When Figure a of Figures 4-7 is converted into a 16-bit image in grayscale using the program OriginLab 2017, they are shown in Figures 8-11. The intensity value of each pixel of the image corresponds to the total light intensity passed through a spectral filter after reflecting from the lemon surface that is imaged to the pixel. The color scale shows the intensity level of each pixel. The gray scale images are intended to normalize the brightness of the spectral image because eye response to color brightness is different for different colors [32] These images inform that the fresh lemon, i.e., the leftmost image at 3 rd row has the highest brightness at 580 nm and other yellow-orange images of the spectral range 590 nm to 610 nm look brighter than other spectral range images. Figure 5 distinguishes the yellow-orange colors. However, for the sample 12 as shown in Figure 6, the highest brightness still appears at 580 nm. However, the images at the 2 nd and the 3 rd in the 2 nd row, which correspond to 550 nm and 560 nm, respectively, become brighter than other images in the 3 rd row. Now the darkest images are in the 4 th row. For the sample 14 as shown in Figure 7, the 2 nd row images are brighter than other rows images and the 1 st row images become brighter than those in Figure 6. The darkest images are still in the 4 th row. For the case of Figure 7, the brightest images are the 2 nd and 3 rd ones in the 2 nd row. The 1 st row images look brighter than the images in the 3 rd and 4 th rows. These spectral images indicate that the skin color of the lemon turns more green as the pathogen infected areas increase, and the green becomes the dominant color of the lemon, when the skin is completely covered by the pathogens. The blue-green colors are also more enhanced than yellow to red colors. Therefore, the spectral image clearly informs the presence of green and blue molds. Figures 5 and 6 clearly inform that the infected lemon can be fully characterized by the green colors, even the infected area is too small to be neglected. To see more clearly the brightness distribution of the skin surface and brightness changes in different spectral ranges due to the infection, the intensity distributions of the grayscale images in Figures 5-8 are plotted in a 3-dimensional space for the quantitative comparison between them. Figures 8-11 represent the grayscale plotting of gray scale images in Figures 4-7, respectively, by considering lamp spectral intensity distribution, camera's spectral response, and the transparency of the spectral filters. The distribution represents only the lemon surface. As the lemons are having an oval shape, the intensity distribution over the lemon surface will not be uniform because of different heights. The \(x\)- and \(y\)-axes in Figures 8-11 correspond to images in columns and rows of Figures 4-7, respectively.