Canister dive lights - a quick introduction
Canister dive lights are unfamiliar to most recreational scuba divers and for good reason - they are typically used only by exploratory divers such as cave divers.
Most canister dive lights are primary lights which feature long battery life and a bright, concentrated beam. A canister dive light design usually consists of a cylindrical battery canister (now you know how they got their name), a small light head, and an umbilical/power cord that connects the two. This design offers two primary advantages over handheld flashlights: an extremely long battery life and a compact, lightweight light head. The Bigblue TL3P is an example of a modern canister dive light.
In the early days of scuba diving, a long burn time (battery life) required big heavy batteries. Holding a dive light with such heavy batteries over a long dive was impractical, so the batteries were separated from the light head and stored in a battery canister which was mounted to the diver's belt or scuba tank. This approach distributed the battery weight more evenly and allowed the diver to carry only the light head by hand.
Further design innovations were the Goodman handle (or Goodman glove) which allowed the diver to carry the light head on the back of their hand and still have the use of both hands.
Though technical divers still rely on canister dive lights, battery and lighting technology have evolved to the point that handheld flashlights like the Bigblue TL1800 are an option for some types of technical dives.
(Image Source: Bigblue dive lights - TL3P Canister Light with Goodman glove)