# Detecting Green Mold Pathogens on Lemons Using Hyperspectral Images
## 2 Materials and Methods
### Experimental Set-Up for Taking Spectral Images and Their Intensity Distributions  
As earlier mentioned, each color of an object corresponds to a specific wavelength in near-UV (ultraviolet) to near-infrared wavelength regime. This means that if there is a spectral filter which passes only a narrow wavelength range within the above wavelength regime, it will be possible to take an image of the object, which has the specific color corresponding to the wavelength range. When a hyperspectral filter is represented by its center wavelength as in this paper, it indicates that the filter can transmit the light in the wavelength range of \(\lambda_{k}\lambda_{k}\)- 5 nm to \(\lambda_{k}+\) 5 nm. In the spectral imaging, it is important to know that the spectral intensity distribution of the illumination source and the spectral transparency of the filter because the intensity distribution of each spectral image is linearly dependent on them. Figure 3 shows the experimental set-up to take the hyperspectral images of lemons. It consists of a camera (2), a halogen lamp (4), two polarizers (5,6), a power meter (7) with a detector (8), and 16 hyperspectral filters in the spectral range of 500 nm to 650 nm for 10 nm interval (9). Nikon D810 with the detector resolution of 7360 \(\times\) 4912 [27] is used as the camera. Each spectral filter has a diameter of 25 mm and a spectral bandwidth of 10 nm, and is produced by Asahi spectra [28]. The transmittance  
Figure 2: RGB histogram in 3-dimensional color space: (**a**) image 1 (**b**) image 15, and (**c**) image 20 of Figure 1.  
of the filters in the spectral range of 500 nm to 650 nm is in the range of 79% to 80.5%. The images with the spectral filters with less than 500 nm and greater than 650 nm have much lower brightness than those in 500 nm to 650 nm. This is why they are not analyzed in this paper. Each spectral filter (9) is placed in front of the camera objective with its center matched on the camera optical axis. The objective is opened only through the filter. For each object, the spectral filters are changed 16 times. We used the data of the halogen lamp spectral power distribution [29] for calibrations of the optical system. Light intensity can be calibrated with the spectral power of the lamp for the lamp's spectral wavelengths. The accuracy of calibration can be in the range of 2% to 7% depending on the wavelength. The angle of the lamp beam to the normal direction of the optical table is 30\({}^{\circ}\) (Figure 3). The illumination direction of the lamp is adjusted in such a way that the beam center is directed to the mid position of the object (1). Each of the two polarizers is located in front of the spectral filter. The filter is for taking only the surface reflected lights from the object [30]. Therefore the polarizer in front of the camera works as an analyzer. The object is at \(\sim\)350 mm distance from both polarizers. It is located so that its image appears at the mid area of the camera's viewfinder. The power meter is New Port 1830C [31], which has full-scale accuracy of 99.6%. In this experiment, the illumination and photographing conditions are kept the same for each object. As described in Section 2.1, fully infected lemons are a source of green mold infection. As the lemon shape is changing as the infected area grows, the objects' positions are carefully adjusted to be located at the mid-position of the viewfinder of the camera to keep the illumination condition the same for all cases. In the experiment, 16 objects with different degrees of infection are investigated. The objects can be 17 when the fresh lemon is included. For each object, 16 different spectral images are taken to observe the spectral property changes of each object's skin due to the pathogen infection. The spectral property is represented by the intensity average of each spectral image of the same object. Figures 4-7 present the 16 spectral images of the objects shown in samples numbered 1, 12, 14, and 20 of Figure 1, respectively.