Hyperspectral, time-resolved, fluorescence imaging system for large sample sizes: Part I. Development of high energy line illumination source

Authors: TEWEY, KEVIN - University Of Maryland; Lefcourt, Alan; SHILTS, PATRICK - University Of Maryland; TASCH, URI - University Of Maryland; Kim, Moon

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/23/2017
Publication Date: 4/1/2018
Citation: Tewey, K., Lefcourt, A.M., Shilts, P., Tasch, U., Kim, M.S. 2018. Hyperspectral, time-resolved, fluorescence imaging system for large sample sizes: Part I. Development of high energy line illumination source. Transactions of the ASABE. 61(2):381-389. https://doi.org/10.13031/trans.11570.
DOI: https://doi.org/10.13031/trans.11570

Interpretive Summary: Contamination of produce with pathogens prior to harvest is a recognized food safety concern. In an attempt to reduce the potential for contaminated produce from entering the food supply, produce fields are visually inspected for possible sources of fecal contamination and identified problem sites are not harvested. In an effort to increase the effectiveness of these existing inspections, our laboratory developed an imaging system with the goal of using the system in produce fields to increase the effectiveness of current inspections. The imaging system consisted of a gated, intensified, camera; a spectral adapter; and a “Powel” lens with a pulsed UV laser providing illumination. The UV illumination elicits fluorescence responses that are captured by the camera in a nanosecond time frame. The system was tested with apples and spinach leaves that had been inoculated with small amounts diary feces. Our tests demonstrated that the use of the “Powell” lens to expand the laser beam allowed for 28.5% of the energy emitted by the laser to be captured in the imaging field. By selecting appropriate exposure settings for the camera, it was possible to create images of the contaminated apples or spinach leaves where fluorescence responses from contamination sites were still evident, but responses for normal surfaces were effectively extinguished. These results demonstrate that the system has potential to be used to detect sites of fecal contamination in produce fields. Given that produce accounts for the majority of outbreaks of foodborne illnesses and that field inspections for fecal material occur at the beginning of the produce supply chain, even a small increase in the effectiveness of produce field inspections could have a material impact on risks of foodborne illness. This information should be useful to other scientists, the produce industry and regulatory agencies.

Technical Abstract: To reduce the risk of foodborne illnesses, produce fields are visually surveyed prior to harvest for signs of fecal contamination. In an attempt to improve the efficacy of these surveys and consequently to reduce risks of foodborne illness, a hyperspectral, line-scan, laser-induced fluorescence, imaging system was developed. The eventual goal is to incorporate the imaging system into a field deployable apparatus to survey produce fields. The system includes a gated intensified camera, a prism-grating-prism spectral adapter, a frequency-tripled (355 nm) Nd:YAG pulsed laser, and a Powell lens that is used to expand the laser beam into a line-illumination source. Software was developed to facilitate precise alignment of the Powell lens with the laser beam and of the resulting line-illumination profile with the line-imaging field. To test the uniformity and efficiency of the expansion optics, comparisons were made with previously developed simple and homogenizing full-field expansion optics. Spatial and temporal uniformity measures for regions within illumination profiles that roughly corresponded to the actual imaging area were similar for Powell and homogenizing optics, and both were better than for simple optics. However, total efficiency was better for Powell compared to homogenizing optics at 28.5% and 3.0%, respectively. After creating a linear spectral calibration equation using expanded 532 and 650 nm laser pointers, theoretical and measured spectral peaks of 5 fluorescent standards were identical. Images of apples and spinach artificially contaminated with dilutions of dairy manure demonstrated high contrast. By selecting appropriate gate timing parameters , it was possible to create images where responses from contamination sites were still evident but responses for normal surfaces were effectively extinguished. These results demonstrate that the system has potential to be used to detect sites of fecal contamination in produce fields.