Cave diver with head-mounted light
|Uses||Providing light underwater for safety, navigation, communication, facilitating work or restoring colour|
A dive light is a light source carried by an underwater diver to illuminate the underwater environment. Scuba divers generally carry self-contained lights, but surface supplied divers may carry lights powered by cable supply .
A dive light is routinely used during night dives and cave dives, when there is little or no natural light, but also has a useful function during the day, as water absorbs the longer (red) wavelengths first then the yellow and green with increasing depth. By using artificial light, it is possible to view an object in full color at greater depths.
Water is responsible for the attenuation of light due to absorption so use of a dive light will improve a divers underwater vision. As the depth increases, more light is absorbed by the water. Color absorption depends on the purity of the water - pure water is most transparent to blue frequencies, but impurities may reduce this significantly. Color vision is also affected by turbidity and larger particulate matter.
Early underwater lights were fixed electric flood lamps or portable lamps with dry batteries for use by divers in standard diving dress. Siebe Gorman & Company Ltd developed a model of portable lamp with a parabolic lens and attached to the divers corselet via a ball and socket joint allowing the diver to use both hands while working.
In 1906, the tungsten filament was introduced and used to produce 200 to 3,000 candlepower dive lights. The Siebe Gorman & Company introduced a 250-watt mercury vapor hand lamp in 1919 that could produce as much as 18,000 candles. For diving in turbid water, 45-watt Sodium hand lamps became the preferred choice. These early lights had to be turned on underwater to avoid cracking the heated glass as it entered cold water.
The first lantern available to the US Navy had 150 candlepower. The early testing showed a need to increase the length of cable on the US Navy Standard Lantern from 125 feet to 250 feet in 1915 to allow for greater operational range. The United States Navy Experimental Diving Unit continues to evaluate dive lights for wet and dry illumination output, battery duration, watertight integrity, as well as maximum operating depth.
There are several options when it comes to light head/bulb types:
Xenon (incandescent) - emit a warm natural light, and tend to be the less expensive, however, offer less brightness and have a shorter battery life than LED or HID lights.
LED (light emitting diode) - are very durable, efficient and powerful.
HID (high intensity discharge) - are extremely powerful and have a nearly white beam. On the downside, HIDs are very delicate and quite expensive. 
A modern dive light usually has an output of at least about 100 lumens. Bright dive lights have values from about 2500 lumens. Halogen lamps provide this light at over 50W power consumption. High-intensity discharge lamps (HID) and Light-emitting diodes (LED) can provide similar output for less power.
The light source and the power supply are housed in the same water and pressure resistant housing. The light is switched on and off by a mechanism built into the housing
Canister lights originated when the battery size required for sufficient burn time of a high output light was too large to be conveniently carried in the diver's hand. The work-around was to use a light-head with the light source, which is small and light enough to easily carry in the hand or on a head mount, and provide power through a cable from a waterproof battery canister, which is carried on the diver's harness.
Head mount lights are used by divers who need to use both hands for other purposes. This can be underwater work by a commercial diver, or getting through tight restrictions for a cave or wreck diver. With a head mount there is a greater risk of dazzling other divers in the vicinity, as the lights move with the diver's head, and this arrangement is more appropriate for divers who work or explore alone. Helmet mounts are common for commercial work which is often monitored by a helmet mount closed circuit video camera.
Both one piece lights intended to be hand held, and light heads powered from canisters or surface supplied cables are used for head mount lights. There are also a few one piece lights specifically made for head mounting, and a full face mask made by Ocean Reef which has integrated lighting.
Caving head lights are sometimes usable as head mount lights for shallow cave dives, as they are waterproof, but they are generally not designed to be used under much pressure.
If the diver is supplied with breathing gas from the surface, it is convenient to supply power for lights from the surface through the same umbilical. An extra cable is generally used for this purpose. The light head is functionally the same as for a canister light, and is usually mounted on the helmet.
Several materials are used for housings. Injection moulded plastics are most popular for the low end and some of the more powerful lights produced in large quantities. Low volume products and home built lights are generally machined from aluminium alloy or high grade engineering plastics such as acetal (Delrin), or occasionally stainless steel, brass or bronze. Lenses (ports) are usually high grade transparent plastic or toughened glass. O-ring seals are standard for sealing joints and connections and for sealing penetrations through the watertight housing for switch controls.
Voltage depends on the requirements of the specific light source and usually ranges from 1.2V for a single NiMH cell, to 12V for a lead-acid accumulator. Surface powered helmet lights may use 24 or 36V power supplies. High voltage is not generally used except in the final stage of the HID circuit, for safety reasons.
Lights which must be switched on or off underwater require a switch which is unaffected by the pressure and is waterproof.
Technical divers often use a Goodman type handle to carry a light on the back of the hand, so the hand can be used without dropping the light. This handle type was originally rigid, and used to carry a canister light head, but is now also used for small one piece lights. Derivatives use bungee to hold the light in place
Narrow beam (spotlight) vs wide beam (floodlight), vs focusing (by adjustable lens or reflector):
Beam angle is chosen to suit the conditions and activities of the planned dive. Cave divers frequently choose narrow beam lights when diving in good visibility as these provide good illumination over relatively long distances, which is useful when navigating into an unfamiliar space where it is actually possible to see a fairly long distance ahead. Some feel that in poor visibility, or at short range, a narrow beam is not much use, and a wider beam spreads the light over a more useful area. Others feel that in low visibility settings a wide beam angle is distracting as it tends to reflect back to much light from floating particulates (back-scatter). Video lighting usually requires a wider beam as the view angle of the video camera must be accommodated for good results. In the case of very wide angle video, such as the GoPro and similar, an extremely wide angle beam is necessary.
Beam angle may be as tight as 8 to 10 degrees in extreme cases, or as wide as around 140° with a plain flat glass port. Even wider angles are possible (exceeding 180°) with a domed port on the light, or a "test tube" light without reflector. LED lights usually use an internal "lens" to produce a focused beam. This actually focuses the beam by internal reflection on a parabolic surface. Several lens options may be available for different beam angles for a given LED, but most models of light use only the one chosen by the manufacturer.
A few underwater lights have a focusing facility by which the beam angle can be adjusted. There are two ways this can be done, depending on the basic emission pattern of the light source. HID lights, which generally produce non-directional light, are usually focused by a reflector, which can be slid longitudinally over the light. They produce a beam with a central bright area and a diffuse general illumination around it. This is good for illuminating a combination of near and far areas at the same time, but not good for wide angle video, as the hot-spot will confuse the automatic exposure software and the result is generally overexposure in the hot spot and underexposure around it.
Other lights use a lens system over the front of the light, which may be focused by axial movement of the lens, which may be on a screw thread for fine control.
Primary, backup, video light
Usually a powerful light suited to the planned activity of the dive, with sufficient power and suitable beam angle. The beam angle and light output required depend on the objectives of the dive.
Backup lights are carried in case the primary light fails during the dive. This is particularly important in cave and wreck penetration diving, where the light is required to facilitate navigation out of the enclosed space. Cave divers are trained to carry three dive lights as a result of an accident analysis by Sheck Exley published in 1977.
Video lights are a special application. They are generally not important for dive safety, but are required for the video camera to get an acceptable image quality, either for video recording, or for the surface team to monitor the work done by the diver. Modern underwater video lights are now relatively small, have run times of 45–60 minutes and output 600-8000 lumens. These LED lights are powered by Lithium-ion batteries and usually have a 5600K (daylight) color temperature.