Horizontal visibility of an underwater low-resolution video camera modeled by practical parameters near the sea surface

Abstract

Underwater video cameras can be used for fisheries, aquacultures, and marine ecology to monitor the existence of fish. In order to assess marine structures, monitoring is also important to understand the effects on the local fish community. Underwater vision is sometimes degraded due to the condition of the water and sunlight. In particular, horizontal visibility is not easily obtained with regard to water quality. So far, an empirical relationship with horizontal visibility has been described according divers' vision, and it states that divers' horizontal visibility is approximately equal to 0.7 times the Secchi depth (Wright and Colling, 1995). If the video camera has a high resolution, visibility may be similar, however, if the camera has a low resolution, detectable distance can deteriorate. For practical usage, a video camera with low resolution has the advantage of reducing storage of data and it is easy to transmit video remotely, in other words, real-time monitoring. Real-time data will provide a prompt understanding of aquatic environmental changes. In order to obtain such data, a low-resolution video camera is still required for usage in a field. There are few models that describe horizontal visibility using a camera in water, therefore this study aims to model one. This paper showed a new approach to model horizontal visibility by water depth and Secchi depth, which are practical and obtainable parameters for underwater monitoring. In addition to this, global solar radiation is adopted for modeling. The model is expressed as relative contrast between target and background. Based on two observations near the sea surface (less than Secchi depth), the model is created. The model showed plausible horizontal visibility using an underwater video camera for real-time monitoring of fish and marine environment.