Underwater video systems
Underwater video systems refers to the equipment used to capture video in water (or other liquids), and to playback the material. The equipment can be divided in components such as camera, housing, carrier (the means ov moving the camera around), transmission and/or recording, and finally equipment for viewing.
For a discussion on the uses of underwater video, se the article Underwater video, for an introduction to the video technology as such, se Video Technology.
Contents
Underwater video systems
A basic underwater video system consists of a camera in a watertight housing being moved around on a carrier, and a way to transmit and/or record the pictures to the viewer where they are reproduced. These components are discussed briefly below.
Camera
The video camera consists of a lens, a light-sensitive element and electronics. The camera is sometimes combined with a recording device, and is then called a camcorder, if not, the (electrical) video signal is transmitted to surface over a cable, sometimes called tether.
A video camera is characterized by its lens (focal length), the resolution (in pixels or TV lines) and its light sensitivity (lux), but also by the image sensor type and size.
The lens is typically wide angle for stand-alone cameras, while the standard zoom lens is used on camcorders. To achieve a wide field-of-view or close-up focus, a negative lens is often attached to the lens.
Today, the image sensor in the camera is almost always of the CCD (Charge Coupled Device) type, although some systems may have a CMOS (e.g. APS) sensor that is cheaper due to manufacturing costs. A CMOS sensor normally has low resolution and is less light sensitive, but is less sensitive to over-exposure (blooming) and can be chip-integrated with the drive electronics. It is also faster, and often seen in high-speed cameras.
Some low-light cameras may still have a so-called Silicon Intensified target sensor (SIT) but the performance of CCD cameras is today surpassing them.
Housing
Obviously, an underwater video device must resist water and pressure. Usually this function is achieved by putting the camera in an underwater housing. The housing is basically a container. The purpose of this container is to protect and support the components in the camera. As the camera (especially if it is of the camcorder type with an internal tape recorder) is fragile, it is essential that the housing is sturdy as well as resistant to pressure, as well as chemical and mechanical forces in the environment where it will be used. The housing always has a design depth, that should be respected.
Figure 1: A somewhat outdated video housing, intended for diver operated VHS camcorders. Note the levers and buttons penetrating it. Due to the volume, this housing requires extra weights, that are attached to the red base.
If the camera will be handled by a diver, it is also important that it is easy to operate (focus, tape control, viewfinder…), and that the weight can be modified to achieve the desired buoyancy. Other applications may put restrictions on size, weight etc.
If the camera is a drop camera, ROV-camera or similar, the housing usually contains a power regulating device and cable driver electronics to minimize degradation of the video signal. Sometimes the housing is integrated with motors or servos to move it without having to move the platform it is on. It may also contain illumination.
Figure 2: A very basic drop camera. The down-looking camera is the black cylinder, sitting in a simple frame that protects the camera. Protruding from the camera is the tether or the cable, from which it hangs.
The most simple way to protect a camera from intruding water is to put it in a sealable soft bag (essentially a plastic bag made of thick soft plastic) with a clear, flat window (port), often made of acrylic glass or glass, for the lens. Such a bag will provide good protection from rain, dust, salt spray and splashing water. It can (if the manufacturer says so) be used underwater down to a couple of meters, but the camera controls are often difficult to operate because of the water pressure on the soft bag, as you operate the camera by simply pushing through the plastic.
A more elaborate housing for a diver operated camcorder (a camera with an integrated recorder) is almost always customized to the particular camera model used. Today, mini-DV camcorders are frequently used, but there are of course housings for professional cameras available too. For small depths (<50 m) the housing is generally made of plastic, sometimes aluminum. Professional housings for small depths, and those for larger depths are made from aluminum, titanium, and/or other high strength materials.
Figure 3: A modern housing for a diver operated DV camcorder, front view (left) and back view (right). The camera is operated by buttons (electrical) which simplifies the design.
There are a number of companies providing underwater housings for video cameras. Very rarely, two camera models even from the same manufacturer share size, shape and locations of the controls, so a new adaptation of the housing has to be done for every model.
Usually it is molded to the shape of the particular camera model, and fitted with a number of mechanical buttons and levers that penetrate the housing, allowing the diver to operate the camera controls directly. Usually fitted with a flat port, there are also models that provide a dome port (se below).
To change tape and/or charge the battery pack on a camcorder the housing has to be opened. If possible, avoid opening the camera at all while in field, where water spray can reach it – always work on a clean, dry place if you can.
Whenever a camera housing is opened, note that different models may require special procedures to be followed when the housing is dis- or re-assembled, i.e. levers in a certain position, cables removed in a particular order, etc. Read the manual before you start! You can never overstress the fact that most floods are caused by mistakes or lack of due care when the housing is assembled, in particular related to the O-rings, the sealings that constitute a barrier between the camera and the water.
Dome port and flat port
The clear glass- or plastic window in front of the camera lens in an underwater housing is called the (lens) port. There are basically two types, the flat port or the dome port.
Due to differences between the indices of refraction for light for water and air, a flat port will increase the focal length of the lens by approximately 25%. It will also distort and/or blur the image, more near the borders of the imaged area. This radial distortion is more pronounced for large apertures and short focal lengths (wide angle lenses). Sometimes a phenomenon called chromatic aberration is seen as a loss of sharpness and/or color fringes on edges in the picture; again more pronounced for large apertures and short focal lengths.
To overcome these imperfections, the dome port was invented, The dome port is shaped as an arch, rotated 360 degrees on its vertical axis. This cupola shaped optical window will, due to the refraction at the water/optical window/air interfaces, make it act as a diverging (negative) lens, that not has the distorting properties of the flat port.
Figure 4: A simplified ray diagram of a flat port (left) and a dome port (right). FOV means Field Of View, which is the angle of inclusion, seen by the video camera.
The curvature of the dome results in a virtual image being created just in front of the dome port. The camera inside must focus on this virtual image (a few centimeters away), not on the object itself, and the camera lens is almost always fitted with close-up lenses to facilitate this.
However, flat ports are much easier to manufacture, and thus cheaper, and the limitations are for that reason tolerated in many applications.
Carrier
A video camera can of course be fixed to an object or sitting on the seabed on a frame, but more often it is moved around by a carrier. The carrier can be a diver using a helmet camera or a camcorder. For the cases where a diver is impractical or impossible to use, there are a number of more or less successful carrier designs.
The camera can sit on a frame that temporarily is placed on or just above the bottom. If the camera hangs in a wire it is often called a drop camera ; sometimes such a camera is remotely controlled (PTZ – Pan Tilt Zoom), sometimes not. Cameras can also be put on poles or frames attached to a ship or float, or of course attached directly to a surface or submarine vessel.
For transect studies the carrier can be a platform towed after a ship. It can be towed either as a sled on the seabed, or ”flying” in the water [1] [2] [3]. The towed platform can be actively steered or just act as a hydrodynamic depressor. In the latter case, depth is controlled by paying out or hauling in on the tow line.
The carrier can also be a ROV – Remotely Operated [4]. Apart from a video camera being used for documentation and inspection, a ROV can be equipped with other tools, such as manipulator arm, sensors, etc. that may require realtime vision. ROVs therefore often have several cameras.
An emerging class of carriers are AUVs – Autonomous Underwater Vehicles – that are similar to ROVs, but are able to operate more or less unsupervised to create for example mosaic images [5] .
Somewhat out of the line are the so-called crittercams, attached to animals like seals and whales, fish, and turtles [6] . Maybe the animals in these cases can be classified as carriers.
Transmission and Recording
The video material captured by the camera can be transmitted over a cable or fiber-optic link or over an acoustic link, or recorded. Radio waves are not efficient underwater.
For cable transmissions, twisted pair copper cables or coaxial cables dominate. Long copper cables will degrade picture quality, although this to an extent can be compensated for by electronic circuitry. If several video signals are transmitted through a (multi-conductor) cable, they can interfere with each other (crosstalk). Again, careful design of the system can minimize these problems. Modern video transmission system, in particular in the upper price range in the ROV business, use fiber optics which are less sensitive to signal degradation and crosstalk.
Almost always, underwater video is recorded for archival purposes. Recording has until recently been done almost exclusively on magnetic tape, and this medium is still widely used. Following the general development of video and electronic equipment, it is today becoming increasingly common to record (digital) video on hard disks in computers or in dedicated recording devices, on optical disc storage media (e.g. DVDs), or on other non-volatile memory such as solid-state memory cards.
Viewing
No video system is complete without a method to recreate the moving pictures. This is done on a monitor (or VDU – Video Display Unit)that is adapted to the type of video used in the system. The still common CRT – Cathode Ray Tube – monitors are notorious for their bad performance in sunlight, but indoors or under a cover they still perform well in terms of picture quality. They are however heavy and bulky, and thus not very practical, at least for use onboard a ship.
The CRT has been the standard for monitors from the start, but it is rapidly being replaced by the TFT LCD – Thin Film Transistor Liquid Crystal Display.
A (color) CRT creates an image by firing a scanning beam of electrons at tiny red, green and blue phosphor dots on the inside of the screen. By turning the electron beam on and off, the dots can be made to glow or not, and from a distance they will create a picture.
LCDs use a backlight (or sometimes incident light) as the light source, and the picture is created by controlling how much of this light is allowed to reach the colored dots by selectively blocking or open the path of that light. This “light valve” is made possible by the Liquid Crystals, that remain transparent unless a voltage is placed across them.
While CRTs have issues with size and power consumption, TFT LCDs have other issues with for example resolution, response time, viewing angle, contrast/brightness and a few more.
The monitor resolution is rarely a problem for (standard) video applications, but sometimes when a monitor is used for video, the number of pixels on the screen do not match the video resolution, creating a blurry picture and/or artifacts. Another issue is that a TFT LCD does not refresh as fast at a CRT which leads to a smearing of the picture (ghosting) and sometimes jagged pixel effects; the response time is higher.
CRT monitors are viewable from almost any angle, while TFT LCDs only produce a good image from inside a certain arc of angles. As the technology improves, this arc is being increased.
Contrast is the range in which brightness can vary between the darkest and the lightest area on the screen, expressed as a ratio ( i.e. 800:1). The higher this ratio is, the better the image quality will be.
Brightness of a monitor, measured as luminance, is the amount of (visible) light leaving the surface of the monitor in a given direction. The light leaving the surface can be due to emission, reflection or transmission. The SI unit of luminance is candela per square meter (cd/m2), sometimes called nits, from Latin nitere, to shine. The greater this number, the brighter the display is capable of being and thus more visible in bright light, e.g. outdoors.
Note that some LCD monitors take advantage of the incident light by reflecting and using it and are not as affected by sunlight, and that a high brightness level will make the monitor consume more power, introducing a tradeoff between brightness and power consumption, and sometimes create a cooling problem.
References
- ↑ Smith, C. J., Papadopoulou, K.-N. (2003); Burrow density and stock size fluctuations of Nephrops norvegicus in a semi-enclosed bay; ICES Journal of Marine Science; 60, pp. 798–805
- ↑ Rosenkranz, G. E., Byersdorfer, S. C. (2004); Video scallop survey in the eastern Gulf of Alaska, USA; Fisheries Research; 69:1, pp. 131-140
- ↑ Coleman D. F., Newman J. B., Ballard R. D (2000); Design and implementation of advanced underwater imaging systems for deep sea marine archaeological surveys; OCEANS 2000 MTS/IEEE Conference and Exhibition;1, pp. 661-665
- ↑ Moser, M. L., Auster P. J., Bichy, J. B. (1998); Effects of mat morphology on large Sargassum-associated fishes: observations from a remotely operated vehicle (ROV) and free-floating video camcorders; Environmental Biology of Fishes; 51, pp. 391–398
- ↑ Sakai, H., Tanaka, T., Ohata, S., Ishitsuka, M., Ishii, K., Ura, T. (2004); Applicability and improvement of underwater video mosaic system using AUV; IEEE OCEANS '04; 2, pp. 659-664[29]
- ↑ Reina, R. D., Abernathy, K. J., Marshall, G. J., Spotila, J. R. (2005); Respiratory frequency, dive behaviour and social interactions of leatherback turtles, Dermochelys coriacea during the inter-nesting interval; Journal of Experimental Marine Biology and Ecology; 316:1, pp. 1-1
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