It's a classic scene from many a war movie – a submarine's presence is given away by its periscope protruding through the surface of the water. If submariners want to see what's up there, however, they really have no choice ... although that may be about to change. Scientists at the Technion-Israel Institute of Technology have devised a system that allows an underwater camera to look up through the surface from below, with a minimum of distortion.
The system is called Stella Maris, which is short for Stellar Marine Refractive Imaging Sensor. It works by digitally altering the images from the camera, in order to compensate for the visual distortions caused by waves moving through the water's surface. Given that waves move in a random pattern, however, how can it know what sort of alterations are required?
Well, although the waves are random, the position of the sun in the sky at a given time and location is a constant. In Stella Maris, the sun's rays shine through an underwater pinhole array (a thin metal sheet perforated with a grid of laser-cut holes) and down onto a glass image plane.
When the surface of the water is completely flat, the result will be a grid of evenly-spaced points of light on that plane. As waves cause the rays to be refracted in a chaotic manner, however, the points of light on the plane will likewise move around relative to one another.
The system's computer uses a sensor camera (as distinct from the primary imaging camera) to monitor those movements. Since it knows what the grid of light points is supposed to look like, it can tell exactly how the wave-altered pattern is deviating from that model. It applies that information to the picture from the primary camera, altering it in the same way that the jumbled pattern of light points would need to be altered in order to make it uniform again.
It's the same principle used by the Shack-Hartmann astronomical sensor, which compensates for turbulence in the Earth's atmosphere when viewing celestial bodies through a telescope.
Needless to say, its usefulness on submarines looks like it would be limited to situations in which crews were trying to view ships or other objects that were quite close to the vessel. The scientists believe that it could have other applications, however, such as on marine biology research platforms in which cameras need to see both above and below the surface.
More information on how it works is available in the video below.
Source: Technion-Israel Institute of Technology
