## 3 Results and Discussion
### Using Spectra for Detection  
For a detection system to be commercially viable, cost is an overriding consideration. Cost considerations suggest that the optical system use at most one or two wavelengths for imaging and that the system include only limited computational capabilities. Cost considerations also require essentially no false positives due to the potential expense of the unwarranted condemnation of a large area of a produce field. Thus, only simple, robust, detection schemes needed to be considered. Fortunately, results demonstrated that bird droppings could be readily detected against a background including soil and plant materials using images acquired at a single wavelength and a threshold filter. As can be seen, this detection scheme is robust because it could utilize any of the wide range of wavelengths. Because of the simplicity of the detection scheme, the use of statistical methods to identify appropriate wavelengths is not necessary. Furthermore, an attempt to statistically validate the detection scheme is not warranted as the equipment used in this study is not appropriate for commercial use. Field studies of the validity of the detection scheme should use a cost effective optical system that includes a monochrome camera and a single wavelength interference filter. Previous study results suggested that for fluorescence imaging, 525 nm would be a suitable wavelength for the detection of dairy manure against a background matrix that included produce and soil [17]. Given this finding, measured responses at 525 nm were used to test system settings for saturation prior to initiating acquisition of hyperspectral data sets. Fluorescence spectra for the three species tested are somewhat dissimilar; representative spectra are shown in Figures 2-4. However, all species showed broad elevated responses that peaked around 640 nm for dove and around 550 nm for pigeon and gulls. Responses for produce were minimal below 650 nm, and responses for soil were minimal at all wavelengths. Some individual dirt clods did show low levels of response in the blue and green regions that were well below the relative response intensities of bird droppings. The response peak at around 635 nm for excrement in Figure 2 was unique across tested droppings for all bird species, as well for over ten additional dove droppings that were not included in the data sets selected for detailed analyses. It was assumed that this peak was due to something unusual in that bird's diet. Figure 2 was selected for presentation as an example of the need to examine a large sampling of droppings to fully elucidate potential confounding factors. Produce showed the expected peak due to chlorophyll and its by-products at around 685 nm [24]. However, the magnitudes of the peak varied from leaf to leaf, with a tendency for a greater magnitude of response from romaine leaves compared to spinach leaves. It was also noticed that responses seemed to be greater in fresh leaves compared to leaves harvested earlier in the day. An implication of this finding was that fluorescence studies that used store-bought produce may underestimate responses that might be encountered in field measurement situations. Figure 3: Fluorescence spectra for a pigeon dropping, romaine leaf, spinach leaf, and soil. Note that data from the image at 465 nm were excluded as the image was found to have been corrupted. Figure 2: Fluorescence spectra for a dove dropping, romaine leaf, spinach leaf, and soil. As mentioned, a prior study determined that 525 nm was a suitable wavelength for detecting dairy manure using fluorescent imaging, and dairy manure had similar fluorescence response characteristics as manure from other 4-legged animals [24]. The 525 nm responses for birds are close in magnitude to peak responses; thus, 525 nm is probably a good choice for designing optical instrumentation to use fluorescence to detect all types of animal exceta in produce fields.