## 1 Introduction  
Pathogens of fecal origin are the cause of most food-borne illnesses and fecal contamination of produce fields is a recognized food safety risk. An accepted practice is to survey fields for fecal contamination and signs of animal intrusion. Our laboratory has developed a number of optical techniques with the goal of augmenting such surveys. The most sensitive optical techniques rely on detecting fluorescence responses of materials in feces to UV or violet illumination. However, the low quantum yields of fluorescence responses means that the responses are generally masked by ambient illumination. The purpose of this study was to investigate the possibility of using a shroud to mitigate solar illumination and thereby facilitate detection of fluorescence responses using relatively inexpensive optical instrumentation. According to the Centers of Disease Control and Prevention (CDC), 128,000 Americans are hospitalized and 3000 die each year due to food-borne illness [1]. Leafy green produce is estimated to be the source of 2.2 million annual incidents of food-borne illness, or 22% of all incidents of food-borne illness [2]. Leafy green produce presents a particular problem with regards to food safety as it is commonly consumed raw, with no kill-step such as cooking or pasteurization. Produce fields are at the apex of the food distribution chain, and any mitigation at this level will thus have the greatest potential impact on reducing outbreaks of food-borne illnesses. A number of mitigation strategies are commonly employed at the field level, including routine surveys of fields for signs of animal intrusion and fecal contamination [3; 4]. Signs of animal intrusion are often used as part of surveysto help locate fecal materials left by animals. The importance of birds as a risk factor is receiving more attention as birds have been found to host an increasing number of human pathogens, and as birds often produce no signs of intrusion as they fly over and leave deposits behind. With no sign of intrusion, surveyors generally do not enter fields to check for such deposits. Mitigation strategies for birds include traditional use of scaerecrows, loud noises, and, more recently, use of predatory birds as a deterrent [5; 6]. An optical technique that allowed surveying entire fields for bird fecal contamination would be a significant improvement over current methods. Our laboratory has investigated using both fluorescence and reflectance imaging techniques for detection of contaminants. Fluorescence responses to UV excitation have been used to detect fecal contamination on apples [7; 8; 9], cantaloupes [10], and spinach [11]; and for development of a handheld system to monitor the efficacy of cleaning and sanitation procedures in food processing facilities [12]. A hyperspectral system that uses a laser for illumination to allow capture of time-resolved fluorescence responses was modified for use in produce fields [8; 13]. A particular advantage of the laser-based system is that the excitation energy is high enough so that ambient illumination is not an issue. The megawatt power of the expanded laser beam results in short term fluorescence responses that are magnitudes of order brighter than ambient sunlight. A system for use in produce fields that uses reflectance imaging as opposed to fluorescence imaging has also been developed [14]. The reflectance system utilizes images acquired at two different wavelengths. This system uses two cameras with a single lens to allow capture of registered images to facilitate use of transformations that include ratios or subtraction of images. There are concerns with these existing techniques. The laser-induced imaging system is costly and necessitates use of a high-power laser with corresponding safety concerns. The reflectance imaging system has lower sensitivity and has problems with false positives.