An Optical Imaging System for Capturing Images in Low-Light Aquatic Habitats Using Only Ambient Light.

Abstract

S1. DEPLOYMENT AND UNDERWATER OPERATION The optical imaging system (OIS) was deployed in the kelp forests on the northern edge of the South La Jolla State Marine Reserve (SMR; 32.8263°N, 117.2901°W) in approximately 14 m water depth from July 10 to 23, 2018 (Figure S1a). It captured 24 images every 12 minutes between 05:00 and 21:00 (i.e., at least one hour before sunrise and after sunset) to ensure that no potentially usable light conditions were missed for image acquisition. Images were captured under aperture priority mode (aperture: f2.8) as uncompressed, raw image files with a 16:9 aspect ratio. The focusing distance of the lens was set at 0.3 m in air so that a fish one meter away from the camera in water would be in focus. This was determined using equations in Jenkins and White (2001) to compute the focusing distance for thick lenses, and we have included a MATLAB R2016b (MathWorks, Natick, MA) script to calculate this in the GitHub repository (https://github.com/cpagniel/FishOASIS/blob/master/hardware/FishOASIS_lens_ focusing_dist_code.m). The ISO (International Standards Organization) sensitivity of each image was automatically set by the camera between 50 and 409,600. The white balance was also automatically set by the camera. The OIS was mounted by divers to an L-bracket (for landscape images) on a 1 m tall u-post set in a 50 × 50 × 10 cm concrete block (Figure S1b). It could also be mounted directly to the stand for portrait images. The battery pack was placed on the concrete block and cable-tied to two eyebolts set in the concrete block. A HOBO Pendant temperature/light 8K data logger (Onset Computer Corporation, Bourne, MA) was attached to the camera housing to measure seawater temperature (in °C) and ambient light (in lux). To correct for the ambient light spectra, a DGK Color Tools WDKK Waterproof Color Chart was deployed in the field of view of the camera at a distance of 2 m. To approximate the distance of fishes from the camera, distances to various stationary objects always visible in the images (e.g., rocks, kelp holdfasts, cinderblocks) from the camera were measured using a transect tape. To demonstrate the OIS’s ability to synchronize its clock with that of a passive acoustic recorder, the OIS was deployed alongside a SoundTrap ST4300 (Ocean Instruments, Auckland, NZ) four-channel acoustic recorder equipped An Optical Imaging System for Capturing Images in Low-Light Aquatic Habitats Using Only Ambient Light