## 3 Results and Discussion
### Using Spectra for Detection
Figure 5 shows the raw image of a gull dropping at 525 nm and the same image scaled to emphasize a lower intensity response to allow the produce leaves to be identified in the image. The 525-nm image is for the same gull dropping used to produce the spectra in Figure 4.  
The patterns of measured reflectance spectra were similar across bird species (Figures 6-8). For bird excrement, ROI were selected that encompassed the brightest areas of droppings; in general, spectral intensities increased with increasing wavelength. Spectra for soil showed a similar pattern to droppings, but at lower relative intensities. Spectra for romaine and spinach were similar in pattern with a peak around 560 nm, a valley around 640-670 nm, and a sharp rise starting at around 650-680 nm. For all droppings, the measured intensities for droppings exceeded the intensities of soil and plant materials for wavelengths below 640 nm. Regions where intensities were high for bird droppings and where relative differences from soil and plant materials were consistently greatest  
Figure 4: Fluorescence spectra for a gull dropping, romaine leaf, spinach leaf, and soil. Figure 5: Fluorescence images of gull dropping, romaine leaf, spinach leaf and soil at 525 nm. Image (**a**) is the raw image, and image (**b**) is the image filtered using intensity scaling to emphasize low intensity aspects of the image so that the spinach leaf (above) and romaine leaf (below) are more easily visible. were around 500-530 nm and 610-640 nm. In reality, any wavelength 640 nm or less could be used to discriminate bird excrement from the background matrix. Figure 8: Reflectance spectra for a gill dropping, romaine leaf, spinach leaf, and soil. Figure 6: Reflectance spectra for a dove dropping, romaine leaf, spinach leaf, and soil. Figure 7: Reflectance spectra for a pigeon dropping, romaine leaf, spinach leaf, and soil. In a prior study examining the detection of dairy cow feces on spinach leaves, images at 690 nm and 710 nm were used to construct ratio images with a threshold used for detection [18; 22]. Bird droppings contain both urine and feces components; the urine component results in the \"white\" aspect of droppings and the feces is responsible for the \"dark\" aspect of droppings. The spectra for droppings shown in Figures 5-7 are for white areas. In many of the samples of droppings, there were little or no dark areas; thus, attempting to use the prior findings for feces as a basis for detection of bird droppings was not feasible. Furthermore, the measured intensities for romaine and spinach at both 690 nm and 710 nm often showed maximums similar to those measured in white areas of droppings. Thus, to be able to reliably detect both bird droppings and feces from other animals using reflectance imaging would require taking images at the 690 nm and 710 nm wavelengths used in the prior study and, as results of this study suggest, at a third wavelength below 640 nm. Alternatively, it might be feasible to use two wavelengths. Examination of the principal component data presented in the background study that was used to select the 690 and 710 nm wavelengths suggested that it might be feasible to replace the 690-nm wavelength with a wavelength around 650 nm [16]. The 650-nm wavelength would be suboptimal for detecting both droppings and feces; however, further testing might demonstrate that this wavelength would be adequate for reliably detecting both. Unfortunately, the acquired reflectance images in the current study in the red region often had areas of saturation that made formal testing of this hypothesis using data untenable. However, the potential difficulty of using 650 nm to detect bird droppings was evident for one of the pull sample data sets. Figure 9 shows reflectance images at both 525 nm and 650 nm. At 525 nm, a simple threshold filter could be used to discriminate the dropping.