Underwater video cameras used on Remotely Operated Vehicles (ROV), Autonomous Underwater Vehicles (AUV) and by Divers for inspection of underwater structures and pipelines must be chosen carefully to give the best of high-resolution, good contrast and low sensitivity to produce high quality, detailed underwater video images.
Colour zoom cameras tend to be the most popular for ROV inspection applications and colour with fixed focal lenses for Diver video inspection applications.
When choosing an underwater camera specifically for inspection purposes the following should be considered.
Considerations
Colour Will a colour image provide additional useful information? e.g. show rust deposits, help identify marine fouling etc.
Resolution Is a measure of the level of fine detail that the camera is capable of resolving. The higher the resolution the more likely it is to spot fine structural defects.
Zoom If it is impractical to get really close to the scene then zoom is a definite advantage if not a necessity.
Digital Imaging Gives the ultimate in fine detail, at near photo resolution a digital image can be further zoomed when captured to show detail that even the human eye may not normally see.
Control Methods Are your choices limited to those already available on an ROV? Would serial control of zoom and focus plus additional functions not otherwise available be useful? Would control of the iris to allow dealing with tricky lighting conditions be advantageous? Is it necessary to open the camera to set alternative control methods?
Spectral Response One of the fundamental physical properties of seawater is that it does not transmit all colours of the light spectrum equally. The maximum transmission in seawater occurs with light having a wavelength of 450 nanometres. This corresponds to blue/green on the colour spectrum. The relative transmission through one metre of seawater is around 98%. An important point to note is that the longer wavelengths of light (the red end of the colour spectrum) are more severely attenuated. At a wavelength of 650 nanometres (red) the relative transmission is around 73% and over a 4 metre distance this is reduced to around 30%. At 450 nanometres and 4 metres distance the relative transmission is still in excess of 90%.
The effect on colour camera performance is that as viewing range increases, the red wavelengths of light are lost from the image and the camera cannot maintain a natural colour balance and the camera image becomes more dominated by the blue/green wavelengths of light. In practical use it has been found that because of this effect, underwater colour cameras can only produce a natural colour balance within about 2 – 3 metres of the subject.
It is important to note that this phenomenon is independent of the light sensitivity or zoom capability of the colour camera or of the power of any artificial lighting used.
Light Sensitivity For underwater inspection applications this is an important parameter. Cameras with very good light sensitivity offer a number of advantages:
Long range viewing
Reasonable operation in turbid conditions
Maximise the opportunity to gain true colour reproduction at closer ranges
Reduced light power requirements
Lens A zoom lens is the most popular choice for the underwater inspection camera although a steady platform is essential when telephoto is activated. For this reason zoom is not that practical for diver operations unless the diver can use a cross member as a seat or steady thus reducing the need to “swim” the total structure.
Cameras used on ROVs and AUVs
