Contrast enhanced imaging in the sea:
Application of the optical transfer function for image reconstruction
Frank Ahrenberg, Stefan Harsdorf, Joerg Niehues, and Rainer Reuter
Carl von Ossietzky Universität Oldenburg
26111 Oldenburg, Germany
email: {harsdorf / reuter} @las.physik.uni-oldenburg.de
Range gated imaging is a well-known technique to enhance the performance of underwater video imaging. In accordance with the concept of lidar a short laser pulse is used as the light source of a camera with an exposure time in the range of a few nanoseconds. A synchronisation of laser emission and camera gate-on time allows to suppress the backscattered light from the water column between detector and object and to record only the light backscattered by the object. This generally results in a contrast enhanced video image.
The image from the object is transmitted through a scattering medium, i. e. the water between object and detector. The undisturbed image of the object is there-fore convoluted with the optical transfer function (OTF) of the camera and the water. This results in an attenuation especially of high spatial frequencies and hence to a loss of contrast in the image. The detector can be regarded as a constant, but the OTF of the water is strongly dependent on its optical properties, mainly the amount of suspended particles.
The OTF of different water types has been measured in a 10 m laboratory tank using a range gated camera. The system consists of an Nd:YAG laser (532 nm, 4 ns pulse width) and an intensified CCD camera with a 5 ns minimum gate time.
A direct image deconvolution using the OTF is mostly impossible, since the measured OTF and video images are obscured by noise. To overcome this problem, two techniques for image reconstruction, i.e. the Monte Carlo approximation and the wavelet transformation and filtering, have been adapted, tested and compared using a computer simulation of image recording. Pre-liminary results from computer simulations and from experiments in the laboratory are presented