Underwater lighting is driven by the viewing system, which is made up of one or more television, video or still cameras. To properly design an underwater viewing system, the understanding of several relationships is essential.

Both absorption and scattering present difficulties when optical observations are made over appreciable distances in water. Dissolved matter increases the absorption, and suspended matter increases scattering. Scattering is the more troublesome, as it not only removes useful light from the beam, but also adds background illumination. Compensation for the loss of light by absorption can be made by the use of stronger lights, but in some circumstances, additional lights can be degrading to a system because of the increase in backscatter. These circumstances are analogous to driving in fog; the use of "high beam" headlights in most cases causes worse viewing conditions than "low beam" headlights.

Keeping unnecessary light out of the water between the object being photographed and the camera can reduce the background illumination caused by scattering. This can be accomplished by separating the light and camera, and in very turbid water using two or three lower powered lights positioned efficiently instead of one higher-powered light helps the situation.

Objects a few meters from a camera can be clearly imaged in ocean water, but unlike air, even in the best ocean water the clarity is sharply reduced for distances even as small as 5 to 10 m. In some coastal water the effect of backscatter can reduce visibility or useful photo range to only a meter or two. Keeping the light source away from the front of the camera helps the situation.

In addition to the basic effect of light intensity reduction in water due to absorption, the matter is further complicated by the fact that absorption is a function of color. Red light is absorbed approximately six times faster than blue-green light in water. This is why long distance underwater photographs are simply a blue tint without much color. The graph shows the severe attenuation of red light (7000 A) compared to that of blue-green and violet.

Even in the clearest surface water, reds are virtually non-existent in ambient light beyond 4 to 5 m depth. This situation is greatly improved in underwater photography by using powerful strobe lights with the camera to get more "red" light to the subject and thus yielding a more color balanced photograph.

The color temperature of a lamp is measured in degrees Kelvin (K). Most underwater lights use tungsten halogen incandescent lamps with a color temperature of 2,800-3,400 degrees K. The dominant wavelengths at this color temperature are red and light at this color temperature comprises primarily red wavelengths. As previously discussed, red is rapidly absorbed in water, reducing penetration of the light and also the range at which true color imaging is achieved. This is fine for some ROV applications where the intent is merely to navigate or produce videotape of close-in work for documentation. Higher color temperature light also reduces backscatter, particularly at longer distances underwater as the ratio of near scattered light (more red) is lower relative to light coming in from the object of interest farther away.

For professional video imaging applications, lamps such as DeepSea Power and Light’s HMI® lamps provide the excellent illumination. DSPL’s 400 W HMI SeaArc2 (shown on the left) is an arc discharge lamp in which the luminous arc burns in a dense vapor atmosphere comprising mercury and the rare earth halides. HMIs have a ''daylight'' color temperature around 5,600 degrees K, similar to natural sunlight. Light produced by HMIs has a higher color temperature (longer light wavelengths), and thus penetrates further, providing greater true-color illumination over a wide area, making them an ideal illumination source for filming wrecks.

HMIs are also more efficient than incandescent lamps, producing 3-4 times more lumens per watt. Since there is no filament to break, they are less sensitive to shock and vibration. A separate electronic ballast regulates power input. Primarily used for documentary expeditions (such as the movie Titanic), HMI lights are gaining acceptance in the general ROV market.

Complementing HMI lights are metal halide high intensity discharge (HID) lights. HID lamps are arc lamps, just like HMIs, but use a magnetic ballast instead of an electronic one. HID lamps can also be doped to produce different colors of the spectrum. DeepSea offers a choice of Daylight, Thallium-Iodide (TI), and ultra-violet. Daylight lamps produce a color temperature essentially the same as HMIs, 5,000-6,000 degrees K. TI lamps produce green light, which penetrates the furthest underwater, and are the best lamp for long-range piloting. Ultra-violet (UV) lamps used in conjunction with UV filters are useful for finding oil leaks. HID lights are useful for applications where long lamp life is important, or lamps are left on continuously without off/on cycling. DeepTow photo survey vehicles, long duration ROVs and tourist submarines are examples.