Primary Dive Lights
Primary dive lights can be used for daytime or nighttime dives. They are typically larger and brighter, come in pistol grip or lantern grip and both Xenon and eLED® versions. A primary light should be very bright, have a long run time and be very durable. For primary lights, the more power the better. This is doubly true for night diving, when a bright light is the best choice for proper underwater lighting.
Secondary or Back-up Dive Lights
The main purpose of a secondary, or back-up dive light is to protect the diver during a nighttime dive in the event the primary dive light fails. Secondary dive lights are typically more compact, lightweight, and feature a narrower beam.
Underwater Photography and Video Lights
If you intend to do underwater photography or videography, a bright wide beam light is a necessity to adequately light your subjects. These use ultraviolet light to entice many marine animals to light up in beautiful shows of color, making a spectacular effect.
Disposable versus Rechargeable Dive Lights
Many frequent divers prefer Rechargeable Dive Lights, which typically cost a little more initially, but ultimately save money on batteries and are more environmentally-friendly. Rechargeable batteries can provide more power and are necessary for any light that produces over 10 watts. However, since rechargeable batteries go bad over time, occasional divers storing their lights for long periods between uses should use disposable batteries which they install before going diving.
Divers are advised to carry at least two lights when diving at night: a primary dive light to illuminate your dive and a secondary dive light, or back-up dive light, in the event your primary dive light fails. It is also advantageous to use a mark light —a small light attached to your tank valve or snorkel that helps a diving partner to locate you.
THINGS TO CONSIDER
Brightness : When it comes to underwater lights, the general rule is the brighter the better. This is particularly true in night diving, when a bright light with a wide beam, for maximum visibility.
Beam angle : Depending on what kind of diving you do, a tight beam or a wide flood may each be appropriate. A tight bright spot is useful for spotting, for looking into crevices and under ledges, and a must for limited visibility diving in murky water.
A tight beam with no spill will cut through the murkiness and allow you to see further, when a wide beamed light would reflect back from the particulate and make it more difficult for you to see.
Tech divers tend to prefer narrow beam lights because they allow you to see further down long passages or into wrecks.