SIDEBAR // LUMEN AND LUX

LUMEN

Lumen (Lm) is a unit of light measurement otherwise known as luminous flux.

We use lumens to compare the total amount of light output from a light emitter. However, lumens will only tell you part of the story, because when it comes to evaluating usable light, lumens don’t provide enough information about how the light output is used. A highly specialized light-integrating sphere is used to measure lumens. A lumen is the total number of “packets of light” (or quantity of light output) produced by a light source. For example, a 100-watt incandescent lamp emits about 1,300 lumens. While useful, this does not tell us anything about the focus and/or usability of the light emitted.

LUX

Lux (Lx) is a unit of light measurement taking area into account; in other words, light intensity.

Lux is used to measure the amount of light output in a given area, where one lux is equal to one lumen per square meter. Lux is a great measurement for determining what we see as the brightness of a beam. If the light output is concentrated over a smaller area, we see this as very bright. If the light output is spread over a larger area, we see this as very weak. This is true even if we have much more light spread across the larger area. We normally use mirrors, reflectors, and optics to con trol the path of light and create the de sired beam pattern. Lux also determines the magnitude of light intensity traveling over distances. A light that is configured for high lux output will travel farther but will have a smaller footprint of light (e.g., lighthouse spotlight), and a low lux level will be configured to travel shorter distanc es but have a larger footprint (e.g., decorative down lighting or ambient lighting). by a reflector and focused to provide a beam usable to the diver. LED lights mostly project light from the emitter in a forward direction, and some lights also collect a small amount of side-spill with a reflector. The main effect of this design is that the light is more difficult to focus with a significant amount of light projected outward. This outcome means that divers should be aware of two measures in evaluating a LED diving light: lumen and lux. See page 57.

A device called a lux meter is used to measure lux at a specific location.

The lux is a ratio of illumination (or lumens) over a distance: 1 lux = 1 lumen per square meter.

Simply put, lux is different from lumens because lux takes into account the actual area where lumens are spread, while lumens simply represent the total quantity of light produced by a light source.

From a diving standpoint, lux is a more important rating than lumens, since the higher the lux rating is, the more intense the beam is. Beam intensity is probably the most important factor for a dive light because this determines the light punch through the water and the distance that the beam can be seen. Divers should be cautious when rating different lights, especially when comparing lumens, which tend to be less useful and often over-reported by most manufacturers.

Batteries

Most diving lights now use either nickel metal hydride (NiMh) or lithium-ion (Li-ion) battery packs. While it’s true that the latter provides more energy density, it is also true that they are more volatile. Lithium batteries now come in a variety of chemistries, with some appearing to be less volatile. Solid-state batteries, which replace the liquid part of a battery pack, also promise excellent energy density and greater safety. The landscape for batteries will likely change radically in the next decade, especially given all the development for electric vehicles and other battery-powered devices in common use.

Always independently burn test your use, burn times for rechargeable batteries will gradually lessen. It is therefore advisable to burn test your batteries at least a couple of times each season.

Backup lights

Backup lights are an essential piece of equipment. In the event of a primary lighting system failure, backup lights serve as a reliable source of illumination. They provide a vital layer of redundancy, significantly enhancing the safety factor during such dives.

When selecting a backup light, there are a few key factors to consider. First and foremost, opt for a backup light with simple and rugged construction. This ensures durability and reliability in challenging underwater conditions. Additionally, pay attention to the burn time of the light. It should be long enough to allow for a safe exit from the overhead environment. As a general guideline, evaluating backup lights with a burn time twice as long as anticipated is a good starting point.

Two backup lights should be carried. Each should have a bolt snap affixed with cave line. Use the bolt snap to attach one to each of the left and right chest D-rings. The body of the light should be captured under a rubber retaining band on the harness. Backup lights should be exposed so accidental ignition will be more obvious to the team. A backup light stowed inside a pocket or inside a BC can activate and discharge without the diver or team ever noticing.

The backup light is probably the most neglected piece of equipment that a diver wears. When you need to deploy a backup light, you really need it. A diver should take care to check the backup light prior to every dive and replace batteries as necessary. Even when not used, the batteries should be replaced at the start of every season and any O-rings lightly lubricated to ensure a good seal.

Advancements in battery technology have resulted in smaller battery packs, rendering corded lights unnecessary for shorter dives.

Powerful primary lights still serve a purpose, even on tropical dives with good visibility.

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Modern-day lights are almost exclusively LED and provide great energy efficiency and brighter light output with superior durability.