SCUBA Equipment Guide: Dive Lights

An Introduction to Dive Lights

In the interest of full disclosure please know upfront that I sell Green Force dive lights.

The best [fill in the blank]?  Well, that depends… is the only honest answer to such a broad topic.  This is the same immediate answer I have given to countless divers, instructors, and consulting clients over the years.  I have always preferred to inform and educate so that they can determine the best [insert item] for themselves.  Surprising to most people is how complex lighting can be.  I’ll try to be thorough yet brief and plain spoken.

Types of Dive Lights

A popular online retailer lists 5 types of lights; Backup, Emergency, Primary, Tactical, and Video.  Other than “tactical” this is common way for divers and dive centers to categorize lights.

Backup lights are generally handheld, pocket sized lights that have a much lower output then primary lights.  These tend to hold few cells and the output may not be advertized.   This type of light are so called because, as the name implies, they are used by advanced divers as a backup to their larger, more powerful, primary light.  So, backup and primary are really relative terms depending on what type of diving you are talking about.

Emergency lights typically have a strobe or flashing mode, are small, designed to work for 6-20 hours, and often produce a red or green light.  Of course, any light can be used in an emergency; it’s just that flashing lights usually get more attention.

Primary lights are getting harder to define.  Once upon a time it was a giant beast with a separate battery pack and had what looked like a motorcycle lamp.  A few years ago it might have been anything over 10-watts (10W).  Today you can get handheld ‘backup’ lights that have better output than a 10 year old ‘primary’.  As a wreck diver and cave diver, when someone says ‘primary’ light I think of a ‘canister’ light system (battery pack + umbilical + lighthead). However, to an advanced or night diver it may be a pistol grip or lantern grip light that uses as few as 4 C batteries.  Training agencies and instructors have specific requirements regarding what is or is not a primary light and even that may change depending on the specific class your taking.

Video lighting is a separate level of hell and will not be specifically covered in this article because the lighting is only one part of a video system and it depends on a number of things including the camera and subject matter.  However, topics covered here should be a benefit to anyone interested in lighting.

Light Basics

What we humans call light is a specific range of radiant energy.  This band of visible wavelengths is between ultraviolet light (UV) and infrared energy (heat).  The visible spectrum for the average human eye runs from violet at 390nm to red at 750nm.  “White” light is an even balance of these wavelengths.

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We do not actually ‘see’ light; rather we see what is reflected or emitted.  When we see a color it is actually our eyes receiving the wavelengths reflected off an object and the object has absorbed most of the other wavelengths. 

One of the first lighting issues is that only a very few and expensive lamps currently produce an even balance of wavelengths (white light).  If the lamp does not produce a color then it cannot be reflected by the object and it will seem to be a different/wrong color.  Sylvania has a booklet titled “Color is How you Light It”. 

The second big problem divers have is that water absorbs light unevenly.  Water absorbs the longest wavelengths first.  So, as you may recall from your Open Water and or Deep Diver training, we lose red first, followed by orange, yellow, green, etc.  Red is absorbed after roughly 5m (15ft).  That may not sound like much until you consider that if you are just 8 feet away from something the light must travel 16 feet.  Remember, the light starts from your light, travels 8 feet, then must travel back 8 feet.  8 + 8 = 16, and all the red light has been filtered out!  Underwater photographers soon learn that even 5 feet of water can create a noticeable loss in reds.

Light Metrics

As you may now understand lighting can be a little complicated, so it is usefully to have a few ways to measure and describe light.  Basic measurements attempt to describe the quantity, quality, and efficiency of the lamp.  The lamp’s performance will also be impacted by the other parts of the light system (batteries, circuit boards, ballasts, etc). 

Quantity

Which is brighter, a 10W lamp or a 20W lamp?  It depends… The watt is unit of power and only tells us how much energy the lamp uses, and nothing about what is coming out of it.  In the begining when then was only one time of lamp it implied a level of performance just like the Horsepower rating of a car or truck engine, but today it can be extremely misleading.

Lumen (lm) is a popular unit of measurement for Luminous Flux or the measure of the perceived power of light.  I often explain lumens as gallons of water coming out of a hose.  Submersible dive lights are now often advertized by watts or lumens.   Like car engines many halogen and HID lamps will have decrease performance as they age. 

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Lux (lx) is a unit of measurement of light’s intensity perceived by the human eye.  Lux = lumens per square meter.  Again with the water hose; Lux would be similar to the water’s pressure.

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Lumens and lux often confuse people.  The important difference between lumen and lux is that lumen is the raw output with no concern about direction or distance, while lux measures how much energy is delivered to a specific area.  By focusing or changing the distance one can change the lux measurement.

Quality

Generally when we talk about the quality of a lamp it relates to our ability to see objects in there ‘correct’ color(s).  However color is a human perception construct; it depends on the light source, the object’s reflectance, and the human eye.  Common light quality attributes include Spectral Power Distribution (SPD), color temperature, and Color Rendering Index (CRI).

Spectral Power Distribution (SPD) is measure of the strength of a light source across the visual spectrum.  Natural light is relatively even across the spectrum, but artificial light is usually uneven with spikes and sometime gaps in the spectrum.   If a light source is weak for a given color objects of that specific color will not be seen ‘correctly’.

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 If there are gaps or large variations in the SPD of a light source, there is a potential for confusion between the apparent colors of objects.

Color temperature is the temperature of an ideal black-body radiator that radiates light of comparable hue to that light source.  The idea is that we can heat a block of tungsten until it glows (emits) a red light, then continued heating it until it glows blue or white,  we could take it’s temperature and relate the color of the glow to a specific temperature.  Temperatures below 3,000K are described as warm and temperatures above 5,000K are considered cool.   For lights with a filament the stated temperature is the actual working temperature of the filament.  Lamps based on other methods have a correlated temperature.   Anyway, it is a reference scale for the coolness (blueness) or warmth (redness) of a light and summer sunlight is considered 5,500K. 

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The Color Rendering Index (CRI) is intended to measure the light source’s ability to reproduce colors in comparison with an ideal or natural light source.  The CRI scale is 0-100 with 100 being perfect.  The index does have critics and the common incandescent or halogen light bulbs with a 3200K temperature are rated at 100.

Efficiency

Efficacy is a simple calculation if you have the data; just take the lamps lumens and divide by rated watts and you will have the lamp’s lumens per watt (LPW or lm/W).  Higher is better. 

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Lamp manufacturers publish average wattage rating for their lamps.  Note, HID lamps require a ballast and therefore will reference one or more ballasts when publishing performance data.

Light Sources

The two most common ways we produce light is either passing current through an filament (incandescence & halogen) or through a gas until it becomes excited and glows (fluorescents, high-intesity discharge (HID).  The Light Emitting Diode (LED), that is becoming more popular, creates light from a special semiconductor by a process called electroluminescence.

Halogen lamps are an incandescent lamp with a tungsten filament contained within an inert gas and a small amount of a halogen such as iodine or bromine.  Halogens have an efficacy of 10–30 lm/W. These generally provide a warmer light, 3,700K, but also emit a lot of heat which is waste for our purposes.

High-Intensity Discharge or HID Lamps can have any variety of gas in the bulb but it is often Metal Halide or similar.  One popular lamp, the Welch Allyn 10W SEL HID Metal Halide Lamp, is spec as 6,500K, and 530 lumens from 10 watts.  The higher efficacy, 53 vs 30, generally makes HIDs preferred over Halogen lamps except for those needing a warm light source.  One common downside is that some of the lamps seem too fragile for the average diver.

LEDs are a small sturdy source of light.  However, until just a few years ago common LEDs only had an efficacy of 18-22 LPW so they had not matched the efficiency of HID lamps and were thus relegated to being backup lights at best.  More recently it is common to see 65 LPW and on February 3, 2010 Cree Inc announced they had a prototype LED producing 208 LPW!  It would appear that LEDs are set to be the new standard and the foreseeable future lamp of choice.

Other System Components

A lamp is not useful on its own; it is only one part of the light system so let us consider some of the other components.

Reflectors can make or break a light system and literally control how the light is emitted.  Remember, the lamp is just the source even though it may be permanently attached to the reflector.  More than likely the reflector was manufactured by a separate company and someone at the light system company decided to match the specific lamp and reflector.  It is the reflector that determines whether the torch produces a spot light or a flood light and therefore a significant impact on lux.  Unfortunately consumers don’t get much input on the reflector.

Batteries are the power source for the whole operation and there are 3 basic options; Alkaline, NiMH, and Lithium.

Alkaline batteries are the basic disposable battery.  They have a nice shelf life but once you use them they start to deteriorate and once depleted they are garbage. 

NiMH is the current standard rechargeable battery.  NiMH does not have a memory, that was NiCd.  Newer NiMH batteries are fairly stable and can hold a charge for months.  There is a lot of debate regarding the best way to maintain NiMH so check with the manufacturer for guidance.  Heat is one of the biggest killers of rechargeable batteries, so don’t leave them in your hot car.  Using a quality, intelligent recharger, will help minimize overcharging (overheating) the batteries.

There are actually several varieties of lithium based batteries, both disposable and rechargeable.  The disposable batteries are popular where long-life is desired, some are used in pacemakers.  I like and recommend long-life lithium batteries for backup lights.  Rechargeable lithium batteries offer a more compact size and may be the future but they have a risk of overheating, catching on fire, and exploding (when trapped in a sealed dive light).  Remember all those laptop battery recalls…   On the other hand, just about every modern rechargeable device (notebooks, cell phone, etc) now uses a lithium based battery.

Myths and Mistakes

Judging a light on the surface, above water, is not the same as judging the light in the water.  Okay, I’ll admit I’ve done it and it can be fun but it is no way to decide on a dive light.  Remeber that part above about water absorbing light waves unevenly?

“Light Comparison Photos”.  This is just a variation of surface comparisons with the added errors from camera settings and computer settings.  So, even if you take the photograph underwater you now have the camera skewing the colors and intensity.  If you get the camera setting perfect you have the color settings on the computer to overcome and there are very few monitors that accurately present all colors.  If the you are comparing lights in different photographs, your just wasting time.

X source of light penetrates better than Y.  Sorry, the laws of physics do not support this type of statement.  If both sources emit the same SPD, the same temperature, lumens, lux, etc it does not matter what the source is (HID, LED, or burning cat).  It is more likely that there is some other significant difference.

Another issue/trick is whether the published specs are for the complete light system or just the lamp manufacturer’s specifications.   Not all systems are created equal.  If the system builder takes a few shortcuts the lamp’s performance can be significantly reduced.

Hope this helps you decide which light is the best for you!