Lighting in Cave Diving: Primary Lights, Backups and Redundancy

Beyond the reach of daylight, an underwater cave is completely dark. Every diver who goes past the cavern zone depends on artificial light to see the guideline, read instruments, watch teammates and avoid damaging the cave. Lights are therefore one of the core pieces of cave equipment, and the way cave divers think about them, as a redundant system rather than a single tool, says a lot about the discipline as a whole.

Lights as a backup system

Sheck Exley's booklet Basic Cave Diving: A Blueprint for Survival, first published by the Cave Diving Section of the National Speleological Society (NSS-CDS) in 1979, made an argument that still shapes training: the guideline (the continuous line laid from open water into the cave) is what gets a diver out, and lights are a backup to it ^[1]. Following a line without light is possible but slow and gas-hungry, so losing all light turns a routine exit into a serious problem ^[1]. Exley also stressed that lights are how divers talk to each other and keep track of a partner in the dark ^[1].

The rule of three

The fifth of Exley's ten recommendations for safe cave diving is to carry at least three lights per diver ^[1]. He noted that the National Speleological Society had already advocated three independent light sources for caving for nearly forty years ^[1]. His reasoning was statistical: even well-maintained lights fail occasionally, and across hundreds of dives the chance that every light a diver carries fails on the same dive drops sharply with each independent light added. Using an assumed failure rate of one failure in 50 dives, he showed that a diver with a single light would very probably be left in the dark at some point over 100 dives, while three independent lights made total darkness over 1,000 dives very unlikely ^[1].

Modern training standards keep the same structure. The NSS-CDS equipment list for its Apprentice Cave Diver course calls for one primary light and two backup lights per diver ^[2]. Global Underwater Explorers (GUE) specifies one primary and two backup lights in its rebreather and sidemount configurations and in its cave course equipment lists ^[3]. The common term is a primary light, used throughout the dive, plus two backups that are smaller, independent units carried in reserve.

Burn time: what the number means

Burn time is how long a light runs on one charge or set of batteries. Exley's principle was that every light should be able to last at least as long as the planned dive, and that a diver whose lights could not do so should carry more of them ^[1]. Today the NSS-CDS asks for a primary light with a rated burn time of 150 percent of the expected dive time ^[2]. DAN (Divers Alert Network) makes the same general point: the intended use governs the burn time needed, and a cave diver needs far more than someone making a short daytime reef dive ^[4].

A rated burn time is not the same as a full-brightness burn time. Under the ANSI/PLATO FL 1 flashlight standard, which some manufacturers of portable lights follow voluntarily, run time is measured from shortly after switch-on until output has fallen to 10 percent of its initial value ^[5]. A light can therefore still be "running" near the end of its rated time while producing only a small fraction of its starting output. Many lights also have several power settings, and higher output shortens run time. Not every dive-light maker states which test method its figures use, so a published burn time is best treated as a claim to be understood rather than a guarantee.

Batteries also age. DAN recommends a periodic burn test, timing how long a fully charged light actually runs, every six to twelve months so that a decline in capacity is noticed before it matters ^[4]. DAN has also noted that lithium-ion packs typically last three to five years and lose capacity even when unused ^[6].

Beam, brightness and team matching

Two different measurements describe a light. Lumens describe total light output, while lux describes how much light falls on a given area ^[4]. The FL 1 standard adds peak beam intensity, reported in candela, and a beam-distance figure calculated from it ^[5]. For cave diving, beam shape matters as much as raw output. DAN points out that a tighter beam makes light signals easier to read, which is especially important where there is no ambient light ^[4]. Cave primaries therefore tend to have a focused beam that can be seen and interpreted by teammates.

More light is not automatically better for the team. Exley argued that it matters more for all divers in a team to have lights of roughly similar brightness than for any one light to be as bright as possible, because the human eye adapts to the available light and one very bright light can spoil everyone else's adaptation ^[1]. DAN's guidance on light etiquette points the same way: avoid shining a beam into other divers' eyes, move the light slowly and steadily, and consider having the lead diver dim their light so that followers' signals stand out ^[4]. Erratic light movement can be mistaken for an emergency signal ^[1][4].

Form factors and mounting

Two broad designs are common. A canister light keeps a large battery in a separate housing worn on the harness, connected by a cable to a light head held in the hand. DAN notes that technical and cave divers often prefer canister lights for their longer burn times, and because the head can be let go without losing the light ^[4]. Handheld lights keep the battery in the same body as the head; some recent cordless designs have specifications approaching those of canister lights ^[4].

How a light is carried is part of its design. A Goodman handle (a rigid or soft strap that holds the light head on the back of the hand, leaving the fingers free) is one widely used option ^[3][4]. GUE's configuration, for example, calls for a rechargeable primary with a Goodman-style handle and, where an external battery is needed, a canister on the right hip; it calls for backup lights that run on alkaline batteries, have few protrusions and a single attachment point at the rear, use a twist switch, and are stowed on chest D-rings except in sidemount ^[3]. Other agencies and divers arrange things differently, but the shared aim is the same: a light that does not snag, stays out of the way until needed and can be found by touch. Exley made a similar point in 1979, warning that large lights snag in narrow passages and are hard to handle in current, and that lights should be small and close to neutrally buoyant ^[1].

Batteries and care on land

Most modern primaries use rechargeable lithium-ion batteries. DAN's guidance on these packs explains that a short circuit can heat a pack very quickly in a reaction called thermal runaway, which can lead to fire ^[6]. It advises never charging or using a pack that has been wet, charging on a non-flammable surface where smoke alarms work, staying present while charging rather than leaving packs overnight, covering terminals on spare packs, and stopping if a pack or charger becomes unusually warm ^[6]. These points matter for cave divers because primary-light packs are large and are often charged in vehicles, dive-centre rooms and accommodation.

Routine care is part of reliability: seals cleaned and inspected, connectors kept dry, burn tests repeated, and the whole light system checked by the diver and the team before entering the water. Exley described a pre-dive check in which team members confirm each other's primary and backup lights are working ^[1].

A short history

Early cave divers in Florida often had to build their own primaries. The NSS-CDS credits Frank Martz with beginning to manufacture the first dependable bright underwater lights, along with guideline reels ^[7]. Exley's 1979 booklet observed that most lights sold by major diving manufacturers at the time were fine as backups but few were suitable as primaries, and that cave divers in north Florida were making better ones ^[1]. Today DAN describes light-emitting diode (LED) lights as increasingly popular for their powerful beams, while noting that some divers prefer high-intensity discharge (HID) lights for murky water ^[4].

The bigger picture

Lights in cave diving are best understood as a layered system: a primary sized for the dive with margin, at least two independent backups, a team whose lights are compatible, and divers who are trained to keep the guideline within reach so that light loss never becomes line loss. Training agencies treat loss of light as a core skill area; GUE, for example, lists managing loss of lights among the critical skills its courses practise ^[3]. How that is practised belongs in a course with an instructor, not in an article.