Thailand – Australia – United Kingdom

CAVE DIVING: The Ins and Outs of HID lighting

by Marius Clore
Equipment and Technology Chair, NACD

Cave diving is critically dependent on lighting, hence the absolute minimum requirement of one primary light and two backup lights. The primary light must be sufficiently powerful both to signal appropriately and to view the cave, while the backup lights, which must be very reliable, need be only sufficient to follow the guideline out of the cave.

The advent of HID lighting has led to considerable improvements in both the quality of light and burn time over the older halogen lights. HID stands for High Intensity Discharge, and HID bulbs consist of two electrodes a short distance apart in a gas-filled chamber. A high voltage (low current) pulse across the electrodes creates an initial spark that results in the formation of a small plasma arc which produces a very broad band of high intensity light extending from the infrared to the ultraviolet.

In this brief article, I will summarize the basic features of HID lights, and compare HID lights in the 18/21W range from a number of manufacturers, including Salvo, Halcyon, Dive Rite and Sartek (see Table for summary of features and specifications).

In terms of components, an analogy between a HID light and a tank and regulator is useful. Every HID light comprises a battery canister, a battery, a ballast and a light head which are analogous to the tank, air supply, regulator 1st stage and regulator second stage, respectively. It is also worth considering that HID light usage should be considered in the same vein as air usage. Just as one turns a dive on 1/3rds, one should never plan a dive where the total possible duration of the dive is going to exceed half the burn time. Moreover, one should be conservative in this estimate since, in contrast to one’s air supply where one has a direct read out of remaining air pressure, it is impossible to know exactly what the total burn time is going to be, since this is obviously dependent on how well the battery has been charged (see below).

Given that HID lights are built from standard components, the variations in design are rather limited, and consequently, the choice of light is to a large extent a personal one based on preferences relating, for example, to the size of the canister, the packaging of the light head and the materials employed.

The canister. The canister should be robust and water tight. Salvo and Halcyon use Delrin, Dive Rite uses PVC and Sartek uses acrylic. Delrin is a more reliable material under extreme conditions, is not susceptible to cracking when dropped, and doesn’t become brittle in extreme cold. This may be important when diving in the Arctic, but under the environmental conditions found in Florida and Mexico, there is little to distinguish between the three materials. The dimensions of the canister are governed by the choice of battery pack configuration. The dimensions of the Salvo and Halcyon 9 Ah canisters are identical, while Dive Rites’ is wider, and Sartek’s is both wider and shorter. Indeed, the dimensions of Dive Rite’s wreck canister is the same length as the Salvo and Halcyon’s 13 Ah canister and only minimally narrower (3.5″ versus 3.75″).

The batteries. Modern HID lights are generally powered by nickel metal hydride (NiMH) rechargeable batteries. NiMH batteries provide the same energy (measured in Wh) as lead acetate batteries in a much smaller package (about half the weight and size for equivalent power capacity), they have a relatively long life with no cell memory, and withstand high charge/discharge currents. Generally, the nominal voltage of the pack is 12V and the total energy is given by the capacity in Ah times the nominal voltage. Thus a 9Ah 12V battery pack has a total energy of 108 Wh. If the ballast consumes 24 W (for an output of 18 W dues to losses), the total burn time can be estimated at 4.5 hours. However, the capacity of NiMH batteries is usually 10% lower than the rated capacity, yielding an actual burn time of about 4 hours. A 9 Ah pack can be built from either twenty 4.5 Ah 4/3 Fat A batteries comprising two strings in parallel, each comprising 10 batteries in series which easily fits into a 2.75″x10.5″ canister, as in the case of the Salvo and Halcyon lights. Alternatively, a more reliable pack in terms of obtaining a full charge (see below) can be built from a string of ten 12V, 9Ah D batteries in series, but the dimensions of this pack are quite a bit larger and can therefore only fit in either the 13.5 Ah Salvo and Halcyon canisters (3.75 x 10.75″).

A critical aspect of any battery is the ability to easily and reliably obtain a full charge. The simplest and most compact charger, in my opinion, is the multi-current Universal Smart Charger (which comes with the Salvo light but can be bought from under $30 from a third party supplier such as or equivalent (e.g. sold with the Dive Rite and Sartek lights). These chargers automatically detect the battery pack’s voltage and are automatically cut-off by a negative delta V when the battery pack is fully charged. Halcyon employs a more cumbersome, bulky and, in principal, more sophisticated charger which also makes use of a thermistor to cut-off the charger should the battery pack overheat. For a 12V battery pack, however, a thermistor is completely unnecessary and unfortunately can result in the unfortunate circumstance of the charger being shut off before a complete charge is obtained. This, of course, is potentially highly problematic in the context of cave diving if, for example, a situation arose in which the battery pack were only half charged and one went diving on the expectation of a burn time of say 4 hours when the available burn time was actually only 2 hours!

One problem associated with the series/parallel design commonly employed by HID dive light manufacturers is that one string can take all the charge leaving the other string uncharged or only partially charged. This will occur when the impedance of the two strings is no longer close to identical. This will invariably occur with age as the capacity of the individual cells falls off at slightly different rates. One solution would be to charge the two strings separately, but the way the packs are put together doesn’t allow this and, moreover, an external connection between the strings would probably introduce an undesirable failure point. Alternatively, one can directly monitor both the voltage and the capacity during the charge by means of a mini-power analyzer (e.g. Watt’s Up which can be purchased for around $50 from which monitors energy (Wh), charge (Ah), power (W), current (A) and voltage (V). Given the importance of knowing that one’s battery pack is fully charged, I personally consider the mini-power analyzer as an absolutely essential piece of gear that should always be used when charging one’s battery pack. Another very useful tool is a battery burn tester such as the CBAII West Mountain Radio computerized battery analyzer, which performs a constant current capacity test that can be directly visualized on a PC.

In terms of battery capacity, I would personally advise going with a 13.5 Ah battery pack rather than a 9 Ah one. This isn’t an issue if one is only going to do a single 60-90 minute dive per day, but for visitors to cave country who want to do two or more dives a day, the additional 50% burn time afforded by the larger battery pack gives piece of mind. At this time, 13.5 Ah battery packs are offered by both Halcyon and Salvo and are built from 3 strings in parallel, each comprising ten 12V 4.5Ah 4/3 Fat A batteries. These fit into a canister 3.75″ x 10.75″. Although the diameter of the 13.5 Ah canister is about 30% wider than the 9 Ah canister, the difference in size does not result in any increase in drag and is completely unnoticeable under water.

The canister/battery pack is connected to the ballast by a cable, typically around 40″ in length. The connections to the canister and the ballast should be equipped with strain relievers. Robot grade cable which is flexible and resists ware well is desirable. However, if damaged, water will be forced down the robot grade cable and can potentially damage the whole system. To prevent this, Salvo has recently introduced a completely sealed dry lid that prevents water intrusion into the canister should the cord or switch boot be damaged. More reliable cable may be afforded by submarine grade waterblocked cable which has even greater damage tolerance, but is less flexible.

The ballast. The purpose of the ballast is to ensure a constant, high voltage, power supply to the bulb. (The typical voltage required to start a HID light is between 1000 and 2000 V). As the voltage of the battery decreases, so the amps drawn increases. The two main manufacturers of HID bulbs and ballasts are Welch Allyn (US) and Brightstar (Taiwan). The Welch Allyn ballast is about 75% efficient, while Brightstar’s ballast is greater than 85% efficient. That is 25% of the input power in the case of the Welch Allyn ballast, and 15% in the case of the Brightstar one is dissipated in the form of heat. What this means in practice is that for 18W of output from the ballast, the input power required by the Brightstar ballast is significantly less than that for the Welch Allyn ballast (about 21W versus 24W). This directly translates into a proportionately longer burn time for the Brightstar compared to the Welch-Allyn ballast/bulb system (i.e., for a 9 Ah battery pack and a HID bulb powered at 18W, 5 hours versus 4 hours, respectively). In addition, because the Brightstar ballast is digital it allows for a hot re-strike at an interval of less than 1 second compared to over 8 seconds for the Welch Allyn ballast. From an ergonomic perspective, there are also differences in the form of the ballast. The Welch Allyn ballast is a rectangular block while the new Brightstar one is cylindrical (see below).

The HID bulb. In considering the HID bulb itself, several factors need to be taken into consideration. The intensity of the beam, both perceived and real, is governed not by the wattage, but by the color temperature (measured in Kelvins), the luminosity (measured in lumens), and the ability of the reflector to redirect and focus the light into a narrow beam.

Technically, the color temperature refers to the temperature to which one would have to heat a theoretical black body source to produce light of the same visual color. Typical color temperatures are 1500 K for candle light, 3400 K for a tungsten lamp, 5500 K for sunny daylight around noon, and 6500-7500 K for an overcast bluish sky. Color temperature is not measured quantitatively but by human perception comparing the light source to a true black body source. Hence the numbers quoted by different HID manufacturers are not strictly comparable. That being said, the the old 21W Brightstar bulb (5500 K) appears white, the new 18W/21W/24W Brightstar bulb appears distinctly bluish at 18W (7000K), as does the 18W Welch-Allyn (6500 K). Moreover, when driven by an 18W ballast the new Brightstar HID bulb is clearly bluer than the 18W Welch-Allyn one. When the same Brightstar bulb is driven by a 21W ballast, the color temperature is virtually identical to that of the Welch-Allyn bulb, but the overtones around the outer edges of the beam are greenish versus reddish in hue. The latter are irrelevant since they cannot be seen under water.

Lumens are a measure of total light output at the source, but it is important to stress that one cannot directly compare the lumen ratings from one manufacturer to another. This is because visible light ranges from 400-700 nm, but HID lights produce a lot of invisible light in both the infrared and ultraviolet, and the spectral power distribution of the different HID lights is not the same. Thus, for example, although the 18W Brighstar is rated at 1300 lumens, the light produced does not appear to be quite as intense as a brand new 18W Welch-Allyn which is rated at 1100 lumens (note the intensity of the Welch Allyn bulb tends to decrease with age and usage). However, the exact same Brightstar bulb driven by a 21W ballast, which is rated at 1500 lumens, is essentially identical to the naked eye in intensity (perhaps a little brighter) and color to the 18W Welch-Allyn bulb. Since the Brightstar ballasts are more efficient than the Welch-Allyn one, the burn time of the new 21 W Brightstar system is the same as that of the 18W Welch Allyn.

The reflector. The reflector takes the light generated by the HID and redirects and focuses it into a tight beam. The narrower the beam, the less the scatter, and the further the throw. Thus a floodlight, which is useful for videography, will produce a lot of illumination but only extends over a short distance, whereas a narrow beam, which is essential for signaling, will extend over a much longer distance. It is therefore desirable to have a focusable light. This is easily obtained by a design in which the position of the reflector relative to the bulb can be easily adjusted. The beams produced by the Salvo, Halcyon and Dive Rite lights are all quite comparable. The Salvo and Halcyon reflectors are identical and personally I prefer their design to that employed by Dive Rite. (In addition, the Salvo and Halcyon beams are a little narrower than Dive Rite’s). An even more narrowly focused beam can be obtained with the EKPP reflector but this comes at a cost. The diameter of the EKPP reflector is 27% larger (3.25″ versus 2.75″).

The Goodman handle assembly. Effective use of a HID light in cave diving requires a means to hold the light head assembly while still permitting one to use one’s hands for other things (e.g. running a line, pulling oneself along, etc….). This is accomplished by a Goodman Handle. My preference is for a metal (generally aluminum) Goodman handle which brings me onto the feel/ergonomics of the light in one’s hands. If one just looked at the Salvo and Halcyon 18W HIDs, the only noticeable difference would be the shape of the ballast. The Brightstar ballast is cylindrical, whereas the Welch Allyn one is a rectangular block. The complete light head used by Dive Rite is also cylindrical but of larger diameter than the Salvo light. Personally, I prefer the way the Salvo cylindrical ballast and light head lies on one’s wrist, but this is very much a personal opinion.

HID durability. In considering HID lights, one other important consideration needs to be borne in mind, namely the sturdiness of the bulb to the sort of abuse that is likely to be encountered by a diving light (both in the water and on land). HID bulbs are expensive (upwards $200) and therefore not throw away items. The design of the Welch-Allyn bulb is such that it is very easily broken by shear forces. This is not an issue in the water unless one were to smash one’s light with considerable force against a rock, which is probably quite a lot harder to accomplish than one might imagine, given the viscosity of water. However, it is a very significant problem on land. Both the old 21W Brightstar bulb used by Dive Rite and the new 18/21/24W Brightstar bulb used by Salvo are very resilient to sheer. Thus, one can drop a Salvo or Dive Rite light with impunity and the HID bulb will not be damaged in any way, but the Welch-Allyn bulb employed by Halcyon will very likely break. Likewise one can smash the Salvo HID light on a table again and again with no ill-effect. This is absolutely not the case for the Halcyon HID bulb which will break immediately upon such harsh treatment.

Newer developments.
Switchable power HID lights. I didn’t test the old Sartek 18W HID. While the design is a little different from that of the Halcyon and Salvo HIDs (the rectangular block-shaped ballast on the Sartek model is mounted at right angles to the light head), the old Sartek 18W HID makes use of the Welch Allyn ballast and bulb, and consequently it’s performance is basically the same as that of the Halcyon HID. Sartek, however, are coming out with a new HID light which is switchable between 10W and 21W. The size of the light head and ballast assembly is similar to that of the Halcyon and Salvo lights, although it is made of aluminum instead of Delrin. The ability to switch between two power levels may at first appear to be something of a gimmick. However, power switchability may have some significant advantages. For example, most of the time the power of an 18/21W HID may not be required and one would only switch to higher power when one wanted the beam to project further, for example, for signaling or to see the depths of the cave. In addition, when visibility is reduced by particulate matter (e.g. murky water), dimming the light may actually yield better visibility by reducing the backscatter (equivalent to driving with fog lights or dimmed headlights in fog or snow).

Li-ion battery packs. Li-ion rechargeable batteries offer increased capacity in an even smaller package than NiMH batteries. For example, a 14.4V 9.6Ah Li-ion pack can readily fit into the 9 Ah Salvo and Halcyon canisters, offering close to 50% more burn time, relative to a 12V 9.5Ah NiMH battery pack. (i.e. for the Salvo 21W HID, the burn time is increased from 4 to 6 hours). Salvo has recently introduced a series of 21W HID lights powered by various Li-ion packs of different capacities, using the same Brightstar bulb and ballast employed for their NiMH based lights. Although these Li-ion packs employ a parallel/series design, the use of IC chips ensures that all strings in the pack can be fully and reliably charged. In addition, Li-ion batteries exhibit a much lower self-discharge rate than NiMH batteries (about 5% per month compared to 30% month). The disadvantage of Li-ion batteries, however, is that their capacity decreases over time from the date of manufacture, irrespective of usage, the extent of which depends upon storage conditions: at 100% charge about 20% per year at room temperature but only 4-6% per year at typical refrigerator temperatures; at 40% charge, however, these numbers are significantly reduced to 4 and 2%, respectively. In other words, do not fully charge up a Li-ion battery pack after a dive, but wait until the night before one’s next dive, and keep the pack in a refrigerator when not in use.

LED primary lights. A more revolutionary development is the use of LED lights. The problem with LEDs is that while they produce a lot of light, the light is difficult to focus and therefore, until recently, of little use in diving. A new company, Solus in Ireland, has succeeded in designing an LED array with high power optics of sufficient intensity to be suitable for a very nice primary wreck diving light. The Solus LED canister light generates light only in the visible range (400-750 nm) with an intensity that is claimed to be comparable to an 18W HID (although direct comparison with the Salvo 21W HID, incorporating the new Brightstar bulb, indicates that it is actually quite a bit less intense). The current Solus light design is not focusable, but the beam is still quite narrow (12 degrees), although nowhere near as narrow as those produced by the Salvo, Halcyon and EKPP reflectors (6-8 degrees). In addition the intensity of the LED light is switchable between two power levels. LED lights are virtually indestructible, extremely reliable since they employ solid state electronics, can be switched on and off instantly, just like a regular LED flash light, and their expected life time is about 30 times greater than that of a HID light (30,000 hours versus 1000 hours), which is probably more than enough for a lifetime of diving. Further refinement of the LED technology to generate narrower (6-8 degrees) and more intense beams clearer represents the wave of the future.

Conclusions. Salvo, Halcyon, Dive Rite and Sartek all make very high quality HID lights and provide outstanding after-sales support should things go wrong. Any one of these HID lights makes an excellent primary light for cave diving. Thus, choice comes down to personal preference and cost. With respect to the HID lights specifically listed in the Table, my own choice, taking into account value for money, performance, durability, quality of design and ergonomics, would be for the Salvo 21W HID. It should be noted, however, that Dive Rite has discontinued its 21W HID and is in the process of finalizing the design for a new 24 HID. Likewise, as noted above, Sartek is coming out with a new HID light that is smaller than their older 18W HID and has the added advantage that the light beam can be switched on the fly from low (10W) to high (21 W) power.


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