Homogenization Optics to Improve Detectability of Fluorescence Response to a Single Laser Pulse: Detection of Feces on Apples

Abstract

<abstract> <bold><sc>Abstract.</sc></bold> Fecal contamination of produce in fields is a known food safety risk. It is theoretically possible to enhance the current practice of visually inspecting fields for fecal material prior to harvest by using imaging to detect the fluorescence response of fecal material to UV excitation. For field application, an expanded-beam pulsed laser and a gated-intensified camera can be used for imaging to negate problems due to reflected ambient light masking fluorescent responses. A particular problem when using an expanded laser beam for illumination is that temporally inconsistent and spatially non-uniform energy distributions can produce false positives. This technical note describes an optical system that expands and homogenizes a high-energy laser beam using a lightpipe. To validate the homogenizing optics, images were acquired using both homogenizing optics and a simple optics system that did not incorporate a lightpipe. Sequential images acquired using uniform fluorescent targets demonstrated that temporal variability in illumination intensity for individual pixels was reduced when using the homogenizing optics. For single images, the Gaussian energy distribution seen when using simple optics was eliminated when using the homogenizing optics. To test how these technical advantages might affect the quality of acquired images in practice, apples artificially contaminated with cow feces were imaged using a multispectral adapter with 678 and 500 nm filters. Threshold detection rates for 1:1000 dilutions were 50% and 15% when using the homogenizing and simple optics, respectively. In addition, for 1:2 and 1:20 dilutions using 678/500 nm ratio images, the relative magnitude of the measured response was greater when using the homogenizing optics. It is recommended that homogenizing optics be used to expand pulsed laser beams that are used to excite fluorescence responses.