## 3 Results and Discussion
### Discrimination of Fecal Matter on Soil and in Mixtures with Soil  
Pathogenic microbial contamination of agricultural products by feces may occur in fields during planting, growing, or harvesting as a result of wildlife intrusion, immature compost used as an amendment for soil quality, or irrigation water contaminated by feces of either wildlife or livestock.  
To explore the potential identification of the species origin of animal feces, the hyperspectral images of animal feces and soil samples shown in Figure 3 were subjected to PCA. The sample materials exhibited dynamic fluorescence characteristics, and PCA can reduce high spectral dimensions into a few significant principal components. The results showed that the first five principal components accounted for 99.8% of the spectral variation found in the samples. Figure 3a shows, from top to bottom, the first to fourth principal component (PC) score images. Note that the other PC score images were omitted for brevity. The first PC score image exhibited the relatively high emission features of the chicken feces followed by pig feces throughout the wavelength region under investigation. The second PC to fourth PC images highlighted differences between the individual feces samples.  
Figure 3b illustrates the distributions of individual feces and soil pixels in a scatter plot of the PC1, PC2 and PC3 domains. Chicken feces pixels were the most scattered, yet are clearly separated from other samples. Soil with minimal fluorescence emission intensities exhibited the least scattered grouping. The pixels for cow and sheep feces were districted in a close proximity in the PC1 to PC3 domain, as suggested by the spectral similarity observed in Figure 2b. Figure 3c is an RGB-composite image created using PC2, PC3, and PC4 score images (in Figure 3a). Individual feces samples exhibited unique color schemes suggesting that the identification of animal-fecal species in an animal feces-soil matrix may be possible using hyperspectral fluorescence imaging techniques.  
Figure 2: (a) Color photo of feces samples from four animal species and one soil sample; (b) Representative fluorescence spectra of animal fecal and soil samples. Hyperspectral fluorescence images of the samples acquired at (c) 520 nm and (d) 676 nm.  
As shown in the color photo in Figure 4a, small feces particles present on top of or mixed in with soil cannot be easily distinguished by the naked eye, except sometimes for chicken feces particles because they tend to be much lighter in color compared to the adjacent soil particles. Figure 4b shows a hyperspectral image at 676 nm (animal fecal peak wavelength). In this image, spatial placement of the fecal samples on or mixed in the soils can be seen clearly due relatively high fecal emission characteristics. On the basis of a single animal fecal emission band, a qualitative assessment of fecal presence is feasible--an automated threshold of the 676 nm image allows the detection of feces pieces in the samples (Figure 4c). Subjecting the hyperspectral images to PCA showed that the identification of individual animal-fecal species is also feasible in a soil-feces matrix, as illustrated in the RGB color composite image of the PC2, PC 3, and PC4 score images (Figure 4d). Individual PC score images were not shown for brevity. The upper-right Petri dish with the feces-soil mixture exhibited the four distinct color schemes of the feces for the four animal species. These results suggest that a simpler single-band imaging system can provide the detection of the presence of feces on or in soils, while a HSI technique is needed to determine the animal species of fecal materials.  
Figure 3: (**a**) First to fourth principal component (PC) score images of animal feces and soil samples; (**b**) Scatter plot of individual feces and soil samples pixels in PC1, PC2, and PC3 domains; (**c**) RGB color composite image obtained from a combination of PC2, PC3, and PC4 score images.