Originally Posted by glad rag
Hmm would a colour camera/lighting system be of any use in that depth/darkness?
It's not so much the depth and darkness as the basic properties of water. Water absorbs (or scatters, or both) light at the red end of the visible spectrum, and to a lesser extent at the blue end. This leaves a transmission peak for green light, with the overall effect that color pictures, of sunlit scenes, taken at depths of more than a few meters have a strong blue-green tone. (I suppose modern CCD cameras may be able to color correct that to some extent.) This effect is modified by the presence of other things in the water, such as algae and plankton at temperate and polar latitudes. In the tropics, ocean water, viewed from above, looks very blue due to scattering, I think, and is often nearly free of marine organisms. Deep sea water is usually very clear, also.
Electromagnetic absorption by water - Wikipedia, the free encyclopedia
So, in answer to the question, deep sea photos require artificial light, as all sun light has been absorbed. That light is often white, as is the xenon strobe on the REMUS 6000 vehicle, or it could be green (thallium-iodide arc lamp, green LEDs, etc.) to take advantage of the transmission peak and scattering minimum. When photos are taken at close range (both the light and camera close to the subject), as is generally the case for ROV and manned submersible work, color cameras are normally used and the colors look natural, or close to it. When the range is greater, the light that travels from the source to the subject and back to the camera becomes so dominated by blue-green that a color camera adds very little. Furthermore, a B&W camera is usually more sensitive (works with lower light) than a color camera using similar technology, so operation from greater range is enhanced. Modern CCDs have become very sensitive, but in the earlier days of underwater video imaging, intensified cameras were sometimes used, and the intensifier was applied to a monochrome camera.
REMUS would fly at 5-20m (typically 10m) above the bottom for photo work, depending on water clarity and height of possible obstructions. The two-way path length is thus quite long and there is little usable color. If any ROV images or video are released, they are likely to be in color.
The ultimate range of underwater imaging is usually limited by back-scatter from the water and the particles in it, which fogs the image. That is a large topic, not related to the question, but I thought I should mention it.
As a side note, I expect that the visible spectrum (spectrum of light visible to humans) is so narrow, being less than one octave from about 400-700nm, exactly because of the absorption properties of water. Either because our retinas are sensitive to the light that gets through our water-filled eyes, or possibly because the type of eyes we have evolved in water dwelling lifeforms. There is probably known science on this, I'm just not familiar with it.