## 3 Results and Discussion
### Spectra  
The measurements made after sunset represent the best case concerning effects of mitigating solar illumination and results for the three heights of the drapes were very similar. Presented are the spectra for the lowest and highest camera gains tested for the 250 cm drape condition (Figure 3). The gains are the two extremes of the range of gains tested and the 250 cm condition is theoretically the condition that would allow the most ambient light penetration. The two corresponding spectral graphs demonstrate the predominant spectral characteristics that, considering prior research, might be expected to be used to detect fecal material in a field of leafy greens (Figure 4). Past studies have shown that spectral characteristics of fluorescent responses for fecal materials are similar across animal species and that the optimal illumination wavelength for illumination is around 415 nm [16]. The 405-nm LEDs used for illumination in this study were selected as their emission peak was the closest available to 415 nm. The most obvious spectral characteristic in the fluorescence responses are the broad peaks in the red region. As expected, the response for the spinach leaf was greater than the response for the manure. However, the rise of the response begins at a slightly lower wavelength for the manure as compared to the spinach leaf. This difference has been identified previously and could be used in attempts to develop detection algorithms [16]. Another difference between spectra for manure and spinach is a relatively higher measured response of manure compared to spinach in the blue-green region. The maximum difference was at 525 nm. This also is a recognized phenomenon that has been used to develop very effective detection algorithms [11]. These effective detection algorithms also utilized  
Figure 2: Insolation by time of day Beltsville, MD. responses measured in the red region. As the response of manure is higher in the blue and lower in the red compared to spinach, ratios of responses in the blue to responses in the red will amplify response measures for manure compared to spinach. To use the ratio method would require two cameras with appropriate filters. Results by gain suggest that a high gain be used for measurements at 525 nm and a lower gain be used for measurements for a wavelength in the red region. The lower gain in the red region is necessary so the saturation of the detector does not effectively eliminate the differential response of manure compared to spinach. Overall, results demonstrate that measurements at a single wavelength (e.g., 525 nm) might serve as the basis for developing a commercial system to survey produce fields at night. For measurements made near sunset, results were comparable to results at night except for the addition of a spectral response for soil (Figures 5 and 6). Images at 525 nm demonstrated that intensity measurements varied across the soil; consequently, a bright area of soil was used when calculating spectra. The response characteristics of soil could complicate detection schemes. For example, if ratios alone were used for detection, areas of soil would have the highest calculated response. Thus, to successfully use ratios for detection of manure it would be necessary to eliminate soil areas from ratio calculations. Elimination could be accomplished using a mask that was created by setting a threshold in the red region where responses of both manure and spinach are greater than responses for soil. Figure 4: Spectra at gain 20 (**left**) and gain 32 (**right**) of soil, spinach, and dairy manure created using the images in Figure 3. Image was acquired at night with the drape raised 250 cm. Figure 3: Gain 20 (**left**) and gain 32 (**right**) images at 525 nm of manure on a spinach leaf. Images were acquired at night with the drape raised 250 cm. For measurements made in full daylight, results were less positive compared to the other lighting conditions (Figures 7 and 8). For the 250 cm drape condition, images at higher gains were generally saturated.