Lighting

Lighting is used underwater to provide illumination for divers, for submersible pilots and passengers, and for cameras . Three general classes of lights exist: incandescent, arc, and LEDs.
Incandescent lights work by heating a filament to the point where it radiates light. Quartz halogen lights are the most commonly used incandescent. The name derives from the quartz glass bulb used that is filled with a halogen gas which is used to redeposit evaporated filament materials back on the filament. Incandescent lights are inexpensive but bulbs are fragile and have relatively short life times.
Arc lights , or gas discharge lights, utilize electrodes to heat a mixture of gases until a luminous plasma is produced. Depending on the gas mixture, different qualities of light can be produced. Examples are HID, HMI, and Xenon. Since gas discharge lights have no filament to break, they are more robust then incandescents and usually have longer burn life. Additionally, they are much more efficient at converting electrical power into light, an important consideration if utilizing battery power. Power limiting electronics called ballasts are required. These can be housed with the light fixture or remoted to a separate electronics bottle. In addition to the drawback of having a ballast, some arc lights require several minutes to warm up to full power and need to cool down before re-starting.
LEDs are small, low-cost, solid-state devices. They have very long life times and have no filaments to break nor do they require a ballast. They are available in a variety of color outputs including white. Because of their small size, they can be easily incorporated directly into a camera housing.