The SERDP Munitions Response Program Area addresses the problem of detecting and identifying unexploded munitions submerged in a wide variety of aquatic environments such as ponds, lakes, rivers, estuaries, and coastal habitats. Focused investments seek to identify cost-effective technologies that can rapidly assess benthic features and ascertain the distribution and concentration of unexploded munitions in areas where the bottom depth is less than 5 meters.
A research team lead by Dr. Steven Ackleson at the Naval Research Laboratory (Project Overview) is investigating the application of active fluorometric imaging (AFI) for detecting and identifying benthic features of interest. Any object immersed within a shallow aquatic environment will rapidly become colonized by organisms, both plants and animals, many of which fluoresce due to the presence of biological pigments. Similarly, many munitions include identifying markings and lubricants for moving parts. Often, these features will fluoresce in unique ways. Thus, AFI can potentially provide information for detecting and identifying benthic objects, especially when combined with other complimentary optical and acoustical imaging technologies.
In this proof-of-concept project a solid-state non-scanning solution is examined using high intensity LED illumination in the blue portion of the spectrum (peak illumination at 464 nm) for fluorescence excitation energy and a CMOS-based spectral line imager (ECOTONE UHI, https://ecotone.com) to measure reflectance at the excitation wavelengths and fluorescence signatures across the remaining visible spectrum. The AFI concept is being tested under controlled laboratory conditions using a 500 L water tank containing submerged targets of variable fluorescence efficiency and emission spectra (Figure 1). Initial measurements indicate that the UHI sensor is highly sensitive and can easily detect signals from fluorescent targets that are typical of naturel coral substrates (Figure 2).
In addition to laboratory experiments, the AFI concept is simulated using a 3-D Monte Carlo radiative transfer model, Coupled Land-Ocean Vector Rendering (CLOVER). The model includes a complete description of the AFI system components and the environment in which it is deployed, including water column optical properties, imaging distance, and the reflectance and fluorescence spectra of benthic targets and features (Figure 3). CLOVER will be used to verify and examine the laboratory results and investigate system modifications that can improve performance. In addition to the test scenarios, CLOVER will be used to investigate a wide variety of environmental conditions not included in the laboratory experiments, such as AFI operations with and without background solar illumination, i.e., daylight versus night operations, and longer imaging distances in order to better understand the limits of application.
Figure 1. The laboratory set-up using a 500 L water tank and a rotating drum fitted with various fluorescing targets (yellow boxed areas, enlarged and presented below the tank images). The UHI imager is equipped with a low-noise, high sensitivity CMOS detector array (right side of each top image) and is positioned at the side of the tank at 1 m distance from the rotating drum. Illumination is provided as white light (left panels) or high-intensity blue LEDs (right panels). While this example represents clear water, turbidity is adjusted by adding suspensions of test clay particles to the water.
Figure 2. This figure illustrates UHI images collected in clear water at a distance of 1 m from the submerged targets. The light source is an array of high intensity LEDs emitting at a peak wavelength of 464 nm. The images on the left side represent reflectance at the peak illumination wavelength and the images on the right represent fluorescence measured at 538 nm. The two florescence samples (spc1 and 4d1) represent the upper and lower bounds of fluorescence efficiency observed within natural coral reef ecosystems. Both samples appear bright in the fluorescence image while a circular reflectance reference (95% reflectance across the visible spectrum), clearly visible within the reflectance image, does not fluoresce and, therefore, does not appear within the fluorescence image. Note that several pigments used in the video resolution chart (top portion of each image) do fluoresce slightly. The center plots illustrate the combined reflectance/fluorescence spectrum of the reflectance reference (blue curve) and the two fluorescent samples (spc1 as the red curve and 4d1 as the green curve). Both plots show the same data, but the intensity scale of the lower plot has been decreased in order to highlight the lower intensity fluorescence signals.
Figure 3. Initial simulations of the AFI concept using the Coupled Land-Ocean Vector Rendering (CLOVER) radiative transfer model. Viewing through 1 m of moderately turbid water, the reflectance image (left panel) indicates a progressive increase in backscatter from the intervening water as the pathlength increases with view angle from nadir at the center of the target to 65o at the edge. The Fluorescence image (right panel) does not suffer from path radiance and is thus sharper compared with the reflectance image. Fluorescence efficiency increases within the grid pattern from right to left and CLOVER clearly captures the resulting increase in contrast in the same direction. Maximum contrast occurs at nadir (image center) where the imaging pathlength is shortest and, therefore, image attenuation is smallest. The fluorescence pattern disappears along the right side of the image where efficiency is zero.