This project focused on the idea that shallow water benthic features in natural environments, both biological and human constructed, often fluoresce when illuminated with blue light and that fluorescence signals contain information regarding objects of interest, such as obstacles to navigation and ordnance as well as marine organisms important for remediation efforts. The project objectives were to examine the sensitivity and noise attributes of the imager and to test the Active Fluorescence Imaging (AFI) approach within an experimental water tank under conditions of variable water clarity.
The AFI approach, meaning that the imaging approach included a dedicated illumination source to excite fluorescence and a sensor capable of recording the resulting fluorescence signals in high spectral resolution. In this initial laboratory test of the AFI concept, the light source was a dive light equipped with high intensity light-emitting diodes emitting at 464 nanometer (nm) and the sensor was a line imager that records the complete visible spectrum (400 – 720 nm) at high spectral resolution (ECOTONE UHI; www.ecotone.com).
The AFI concept was shown capable of imaging fluorescence from submerged targets through turbid water across attenuation lengths, defined as one-way transmission between the camera and target and measured at the illumination wavelength (464 nm), ranging between 2.8 (weak fluorescence) and 6.4 (strong fluorescence). Unlike reflectance signals, fluorescence is unaffected by path radiance (light backscattered by hydrosols), resulting in greater imaging distance and higher target contrast. A Monte Carlo radiative transfer model was developed to simulate the complete experimental set-up. Model simulations are in excellent agreement with experimental results spanning a range in water turbidity serve to validate the AFI concept.
The next step for the project is to configure the AFI components for deployment and testing in natural environments.