Underwater optical imaging

Underwater optical imaging has many interesting and beneficial attributes for underwater vehicle navigation, as well as its ability to open up a wealth of understanding of the underwater world. However, it is not an ideal environment for optical imaging as many of its properties inherently affect the quality of image data. While image quality is a pertinent issue for vision system performance, other difficulties are also encountered such as the lack of distinguishable features found on the seafloor and the need for an artificial light source (Matsumoto & Ito 1995). For most UUV applications (below 10 meters) natural lighting is not sufficient for optical imaging so artificial lighting is essential. Light is absorbed when it propagates through water affecting the range of vision systems (Schechner & Karpel 2004). Many variables can affect the levels of light penetration including the clarity of the water, turbidity, depth (light is increasingly absorbed with increasing depth) and surface conditions (if the sea is choppy, more light will be reflected off the surface and less light transmitted to the underwater scene) (Garrison 2004).

Underwater optical imaging has four main issues associated with it: scattering, attenuation, image distortion and image processing. Scattering is as a result of suspended particles or bubbles in the water deflecting photons from their straight trajectory between the light source and the object to be viewed. There are two different types of scattering; backscatter and forward scatter (see Fig. 1). Backscatter is the reflection of light from the light source back to the lens of the camera. This backscattering can result in bright specs appearing on the images, sometimes referred to as marine snow, while also affecting image contrast and the ability to extract features during image processing. Forward scatter occurs when the light from the light source is deflected from its original path by a small angle. This can result in reduced image contrast and blurring of object edges. The affect of forward scatter also increases with range.

The rapid absorption of light in water imposes great difficulty in underwater imaging. This attenuation necessitates the use of artificial lighting for all but the shallowest of underwater missions (less than 10m, dependent on water clarity). The visible spectrum consists of several colours ranging from the red end of the spectrum (wavelength of <780nm) to the blue (wavelength of >390nm). Water effectively works as a filter of light, being more efficient at filtering the longer wavelength end of the visible spectrum, thus absorbing up to 99% of red light by a depth of approximately 4m in seawater (Garrison 2004). Absorption intensifies with increasing depth until no light remains (see Fig. 1). The effects of absorption discussed apply not only to increasing depth but also to distance.

Due to the extreme pressures associated with deep-sea exploration there is need for high pressure housing around each sensor. In the case of a camera a depth rated lens is also required. Imperfections in the design and production of the lens can lead to non-linear distortion in the images. Moreover, the refraction of light at the water/glass and glass/air interface due to the changes in medium density/refractive-index can result in non-linear image deformation (Garcia 2001). To account for this distortion the intrinsic parameters of the camera must be found through calibration and using radial and tangential models the lens distortion effects can be compensated for. The characteristics of the underwater environment not only create issues for collection of clear and undistorted images but also affect the subsequent image processing. Due to the severe absorption of light and the effects of scattering (marine snow etc.) it is essential to decrease range to the objects being viewed in order to obtain higher resolution clearer images. This has the consequence of limiting the field of view (FOV) of the camera and thus not allowing for wide area images of the seafloor while also challenging the assumption that changes in floor relief are negligible compared to camera altitude.

The motion of the artificial light source attached to the vehicle leads to non-uniform illumination of the scene thus causing moving shadows which makes image to image correspondence more difficult. The lack of structure and unique features in the subsea environment can also lead to difficulties in image matching. While terrestrial applications can make use of man-made structures, including relatively easily defined points and lines, the subsea environment lacks distinguishable features. This is in part due to the lack of man-made structures but also due to the effects of forward scattering blurring edges and points. An issue that affects all real-time image processing applications is whether the hardware and software employed are capable of handling the large amounts of visual data at high speed. This often requires a trade-off in image processing between the frame rate and the image resolution which can be detrimental to the performance of the application.

Fig. 1. Scattering and light attenuation (left), colour absorption (right) (Garrison 2004).
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