LED (Light-Emitting Diode) Lights
These rely on a semiconductor microchip to produce light when an electric current passes through it. For LEDs to emit enough useful light, they must be assembled into an array controlled by a driver (HID lights are controlled by ballasts, similar to those that control fluorescent lights). A half-dozen to a dozen LEDs are strung together, similar to Christmas-tree lights. Today, high-power LEDs represent the cutting edge of this technology, improving at about the same rate as computer-memory and speed capabilities.
Two big players in LED underwater-light technology include Aqualuma Underwater Lighting and Ocean LED USA (find some excellent video of fish attracted to lights on Ocean LED's website).
Here comes the "but": Techies refer to HID as a mature technology, meaning on a developmental-completion scale of one to 10, HID lighting rates a nine. Conversely, LED lighting is a developing technology - and make no mistake, it's developing rapidly.
Is one better than the other? I'll leave that to someone far more knowledgeable than I to say.
Mark Olsson, president and founder of DeepSea Power & Light, has been involved with lighting for some 30 years, and his lights (he personally designed them) were used on Stephen Low's 1992 IMAX film Titanica, which begat James Cameron's Oscar-winning Titanic. Both used footage from the real RMS Titanic done with HMI lighting (a spinoff of HID lighting) at depths between 12,000 and 13,000 feet. Olsson knows lights and considers things to be moving and changing rapidly in the world of lighting.
"We are in the midst of a revolution in lighting, and that revolution is LEDs," says Olsson. "I expect that the efficiency of readily available power LEDs, which has just recently doubled, will double again in the next 12 months. Does that mean that LED lighting is better than HID? Not necessarily, and in my opinion, not yet. However, the gap is indeed beginning to close."
From someone who makes HID lighting for a living, I'd call that an honest appraisal. Meanwhile, though most present equipment is HID, LED lights are rapidly becoming more common in all facets of boating, including underwater lighting. Virtually every manufacturer of HID lighting has some form of LED products.
There's one other bit of terminology appropriate for a discussion of lights: light ratings beyond wattage. The most frequently used terms are lumens and lux. Simply put: The measure of the quantity of light seen by the human eye is lumens - twice as many lumens equals twice as much light; it's a measure of quantity.
For a variety of reasons, lumens are difficult to measure consistently and properly from light to light. An easier-to-determine measurement is lux, but it turns out that measurement may not be quite as useful in actually determining how much overall "light" there is. Regardless of which measurement is given, make sure - when comparing lights without actually seeing them in action - that you compare apples to apples.
So which lighting do you want? Other factors come into play regarding light ratings, such as how much light you lose in the housing, what cone angles (same as sonar) the lens creates and more. The bottom line remains: In order to see if a light does what you expect of it, you must see it in action.