LED lighting a breakthrough for independent living and emergency preparedness Part 1: LED technology essentials By Tim Thorstenson
Issue #127 • January/February, 2011

LEDs—short for Light Emitting Diodes—offer significant advantages for certain types of lighting, such as flashlights and lanterns for camping, chores, and emergencies, or in bulbs and fixtures for the cabin or off-the-grid home.

However, LED technology is advancing rapidly, which means there are plenty of products on the market that use older and less efficient LEDs. There are also situations where other lighting alternatives may be a better choice. Purchasing LED lights without the proper background information could lead to an expensive investment in stuff that is not nearly as bright and efficient as it could be.

With this in mind, let's examine the things we should know when evaluating and selecting LED-based products. Our main emphasis will be on general principles rather than on details related to specific products. Such details can become outdated pretty rapidly, but a solid background on the technology itself is something that will remain useful even as the products continue to improve.

Our exploration will consist of three parts. In the first, we will focus on the LEDs themselves. In the second, we will examine different categories of LED-based products: flashlights, headlamps, portable lanterns, work lights, and, bulbs and fixtures for area lighting. The things to watch out for can vary quite a bit from one product category to the next. Finally, we'll explore the option of building your own LED-based lights and we'll also consider the important topic of delivering light efficiently. Making your own lights does not need to be complicated, and it can allow you to build the most efficient LEDs into finished lights that will work well for your specific needs.

Small LEDs like this can offer a huge efficiency advantage when compared to incandescent bulbs. But it is important to understand the details of this new technology.

LEDs vs. incandescent lights—efficiency is the key

Note: The reader who pursues more information on lighting will find that the term "efficacy" (rather than "efficiency") is often used to describe the performance of light sources. I elected to stick with a more familiar term for our visits.

Efficiency—making as much light as possible from a given amount of power—is critical for anyone interested in independent living or emergency preparedness. Flashlights and lanterns of practical size only have so much room for batteries and we want them to be able to run for a good long time without having to have a huge reserve of batteries on hand. An off-the-grid home will have access to a lot more juice than tiny flashlight batteries, but the power is still sharply restricted in comparison to that available in a conventional home. Some folks take a lead-acid battery to the cabin and then lug it back home for recharging. In this last case, efficient power use is a particularly weighty issue.

In all of these situations, the superior efficiency of LEDs can provide amazing benefits relative to incandescent light bulbs. With respect to flashlights and lanterns, LEDs can accomplish the "impossible" by allowing for products that are small, bright, and long running. In the cabin or home, LEDs can offer a choice of benefits. Depending on your goals, they can provide brighter lighting or a reduced power demand or some combination of the two. But to see the benefits and to decide whether the improvement is worth the effort, we need to first examine efficiency in more detail.

Understanding LEDs and comparing efficiency

It is their superior efficiency that makes LEDs worth considering, but what's so hot about them? Well, the answer is quite literally that they are "cool." Conventional incandescent bulbs use the brute force of electrical energy to heat a metal filament until it glows white hot. As a result, most of the electricity actually goes into making heat (invisible infrared radiation) rather than light. In contrast, the process used by LEDs generates a lot less heat. The conversion from electricity to light is still not 100%, but it is dramatically more efficient than the incandescent method.

To see the difference, we need to compare LED and incandescent light sources. To do that, we first need to consider how light is measured. You are probably familiar with the "candlepower" ratings found on things like spotlights, but candlepower measurements are dependent on how the light is focused. This means that the same light bulb can have either a high candlepower value (if placed in a reflector that focuses the light tightly) or a low one (if placed in a lantern that is designed to spread the light uniformly over a large area). The "lumen" is far more useful because it represents the total amount of light produced by a source and it is independent of how the light is focused.

Compact, efficient, and flexible: The LED flashlight at center can produce several times more light than the 2-D-cell incandescent at left. But it can also be turned down to roughly the brightness of the incandescent mini-light at right.

In stores, you will find that many light bulbs—and even some flashlights and lanterns—have lumen ratings on the packages. You probably know what a 60-watt incandescent bulb can light up in practical terms. So if you file away the fact that this same bulb produces about 840 lumens of light, you have a mental reference point that can be used to estimate what other light sources will be able to do based on their lumen ratings.

To evaluate efficiency, we need to consider the power used as well as the light produced and we already have the numbers we need for our 60 watt incandescent light bulb: if we divide the 840 lumens of light produced by the 60 watts of power consumed, we get 14 (rounded off), which is the number of lumens produced for each watt of power used. If we now look at a very different incandescent bulb, we find something rather interesting. A 2-D-cell flashlight with a standard bulb and fresh batteries will produce much less light (about 20 lumens) and it also uses much less power (around 1.5 watts). But if we divide the lumens by the watts, we get a very similar efficiency, about 13 lumens per watt. This may seem surprising at first, but it actually makes sense because there is a fundamental limit to incandescent efficiency which isn't greatly influenced by the brightness of the bulb. To be fair, we should understand that this can be improved somewhat: "halogen," "xenon," and "krypton" bulbs include chemicals that allow the filament to be run at higher temperatures without burning out quickly. With these strategies, incandescent efficiency can be increased to about 20 lumens per watt or a little better.

Now that we have a baseline for what ordinary light bulbs can do, let's consider the LEDs. A readily available LED that is widely used in flashlights can produce up to 230 lumens of light while using about 3.7 watts of power. Dividing this out gives us an efficiency of a little over 60 lumens per watt. This is about three times more efficient than the best incandescent bulbs and it illustrates the potential value of LEDs. But consider this: the same LED maker has just released a newer LED that can produce more light (about 340 lumens) while using less power (only about 3.3 watts). That calculates out to around 100 lumens per watt and it represents a huge increase in efficiency relative to the older LED. Buying a couple of flashlights without knowing this would probably not be the end of the world, but equipping an entire off-the-grid home with LED bulbs before discovering that newer and better LEDs are available could be pretty darned irritating. Depending on your preferences, the newer technology could allow for a lot brighter lighting or a significant reduction in the size (and expense) of your charger and battery system.

Raw efficiency is very important, but there are also several other essential things to consider. In the rest of this article, we'll look at efficiency in more detail, but we'll also identify and examine other important variables.

Other benefits of LEDs

In addition to efficiency, there are some other potential benefits to LEDs that should be mentioned up front. First, they can be very durable and long-running. In a properly designed flashlight or lamp, they can operate for tens of thousands of hours while being very resistant to impact, vibrations, and other stresses that can easily break the filament in an incandescent bulb. LEDs can also be "dimmed" quite easily. You can also dim incandescent bulbs, but the light will change from white to yellow or even orange as the power is reduced. In contrast, LEDs can be run at reduced lumen outputs while still producing white light. This allows you to easily tailor the brightness of the lighting to the needs at hand. Perhaps more importantly, LEDs actually become more efficient as the power input is reduced, so they can provide for effective energy management in the event of an emergency or if the wind and sun are not cooperating with a charging system.

Efficiently adjustable brightness is a major benefit of LEDs. These beams are from identical flashlights, but the beam at left is the lowest of three available outputs, while the one at right is the highest. Runtime ranges from 45 minutes to dozens of hours.

The nuts and bolts details

Now that you have a general idea about LEDs and their potential advantages, let's consider a number of technical details.

Not all LEDs are created equal.

The process of forming the little "chip" of light-emitting material in an LED is rather touchy. This means that individual LEDs can vary considerably in efficiency from one to the next within the same product line. If you ran into handgun ammo that varied in velocity by 20% from cartridge to cartridge, I bet you'd find a different supplier. But with LEDs, this is simply the way it is. Fortunately, the LED manufacturers don't make their customers play roulette. Rather, the LEDs are tested and sorted into different "bins" based on their efficiency. As an example, a company called Cree makes a line of LEDs called the "XR-E." This is a high quality LED that is suitable for a number of purposes and, until recently, it was about as efficient as you could get. Now the best readily available bin of the XR-E (which is labeled "Q5") can boast an efficiency of 92 lumens per watt when run at low power. But the lowest bin of this same LED (called the P4) has a notably lower efficiency—only about 70 lumens per watt. Not surprisingly, the best bins command a premium price while the less efficient ones are cheaper.

A company making LED-based bulbs or flashlights could honestly state that it uses a certain brand of LED that is known for good efficiency. But the maximum possible efficiency of that specific product will depend on exactly which "bin" of the LED is being used. Makers of certain better-quality products will tell you exactly which bin they are using, but most consumer quality products will not. In most cases, consumer products will use less efficient bins to keep the production costs down. Such products are not necessarily a bad deal (and there is nothing wrong with lower bin LEDs in terms of their mechanical quality or reliability), but this is certainly a good thing to understand before making a purchase.

There is a law of diminishing returns.

The brightness (lumen output) of an LED can be varied over a wide range by adjusting the power input. Thus, the same compact flashlight that can belt out a couple hundred lumens for security or for checking cattle can be dialed down to a couple dozen lumens for routine tasks or even down to just a few lumens to provide electric candlelight for camping or a long-duration emergency. Fixed lighting in a cabin or home could be adjusted in a similar fashion to accommodate different lighting requirements and to conserve power when needed.

Cool white LEDs (left) are more efficient. But a warm white output (right) can be more pleasant and provide better color perception.

LED makers specify the maximum power that their products are designed to handle. But they often measure performance at a lower power input and the difference in efficiency at these two points can be considerable. For instance, a Cree XR-E LED from the Q5 bin will produce around 230 lumens at maximum power with an efficiency of about 62 lumens per watt. At the lower power level used during testing (which is about 1/3 of maximum), only about 107 lumens of light is produced, but efficiency climbs to about 92 lumens per watt. In other words, one must roughly triple the power input in order to double the light output. The numbers will vary with different LEDs, but the general principle is the same—efficiency decreases as the power input is increased.

With flashlights, it is nice to be able to crank the LED up to a high power level because, in some situations, you can use all the light you can get. But the ability to reduce the brightness is not only useful for preventing glare and temporary blindness during close-range work, it is also extremely valuable for battery conservation during emergencies. With a properly designed LED flashlight, a disproportionate power savings can be gained as the brightness is reduced. As an example, a company called Fenix is well regarded for making good quality LED flashlights and their "LD20" model will produce 180 lumens for up to two hours from a pair of rechargeable AA batteries. But if the light is switched to its lowest output of 9 lumens (about the brightness of a good penlight), runtime stretches to 71 hours. Here, the light output is being reduced by a factor of 20. But in exchange, the runtime is extended by a factor of 35. Proper electronics are essential to actually getting this sort of performance, so careful flashlight selection is essential.

Somewhat different (but related) considerations apply to bulbs and fixtures. The cheapest way to make a bulb of a given brightness would be to select a suitable LED and run it at maximum power. But a much more efficient approach would be to use two or three of the same LEDs and run them at a more conservative power level. It is certainly reasonable to suspect that bargain basement products will use the cheap approach rather than the efficient one. As we will see, the "cheap" strategy can have other serious drawbacks in addition to reduced efficiency.

Beware of rosy numbers.

The lumen outputs and efficiencies are values for the LEDs themselves. But the overall efficiency of an actual bulb or fixture will always be somewhat lower, for a number of reasons. First, electronic controllers are usually used in LED-based lights and these consume some power, too. Second, any real-life lighting product requires "optical" components like reflectors, lenses, and diffusers, and even the best of these will scatter and absorb some light and keep it from getting to where it is needed. Third, virtually all real life products will run the LEDs at temperatures which are somewhat higher than ideal and this causes output and efficiency to decline somewhat. As a result of these factors, even a carefully designed product will generally have an overall efficiency that is no more than about 75% of the published efficiency of the LED itself. With inexpensive, mass-produced products, it is reasonable to expect even a little lower performance. But a poorly designed product could have an overall efficiency that is much lower yet.

The raw lumen and efficiency values provided by LED makers such as Cree are invaluable for comparing different LEDs to each other. But when it comes to comparing products, we need to have honest lumen measurements and efficiency ratings for the final product. This is generally not a big deal with things like flashlights and camp lanterns, but it is critical if you will be buying a number of (potentially expensive) bulbs or fixtures for lighting up a cabin or off-the-grid home. It is also useful to understand that this same concern applies to other lighting technologies, too. For instance, currently available "compact fluorescent" (CFL) bulbs are capable of certain efficiencies, but this is not a guarantee that any given CFL bulb will actually provide this level of performance.

Regardless of the type of product or the technology being considered, the most reliable way of dealing with this issue is by getting reliable numbers from a reputable manufacturer. But it should be noted that the required testing can be prohibitively expensive for a small company. For instance, there are several makers of some pretty decent quality flashlights that use excellent electronics and top-grade LEDs, but they do not conduct actual lumen output measurements because the required equipment is touchy and expensive. I mention this for an important reason: as LED technology continues to improve, the big companies will likely focus on bulbs and fixtures designed for use with 120 volts AC. But a smaller company can focus on specialty applications and quickly incorporate the newest and most efficient LEDs. Thus, small manufacturers may offer some very good products for off-grid applications, but they may not be able to provide the sort of testing and "independent certification" that we would expect from an outfit like G.E. or Sylvania. A good understanding of the technology can help with a sensible evaluation of products like these.

Electronic controllers (or drivers) are usually used to power LEDs and they provide several benefits. Drivers can be purchased as separate components by those interested in building LED-based lights.

Heat is a concern.

There is a diminishing return with LEDs—as power input is increased, efficiency will decrease. But a higher power input will also cause the LED to produce more heat. If this heat is not removed, it will cause the LED to operate at a higher temperature with a couple of negative consequences. First, the efficiency and lumen output of the LED will decrease as the operating temperature increases. Second, LEDs usually fail by slowly deteriorating rather than "burning out" quickly like an incandescent light bulb. This process happens more quickly at a higher temperature, so an increased operating temperature will result in a shorter service life. Because good quality LEDs are rather expensive, getting a long service life is important.

For these reasons, LEDs must be mounted to a metal surface that will conduct the heat away from the LED. For things like flashlights, the metal mass of the light itself is often an adequate "heat sink." But bulbs used for area lighting are often operated for extended periods, so sheer metal bulk may not do the job. If the heat is not transferred to the surrounding air, it will build up in the metal and the temperature of the LED will increase over time. Here, surface area is more important than sheer bulk and "cooling fins" are the key. These are occasionally used in flashlights, but they are especially critical with area lights. For making your own lights, aluminum "heat sinks" can be purchased for a reasonable price and these provide heat removal that is far superior to what can be had from a simple block or chunk of metal. "Active cooling" (using a small fan) can provide extremely effective heat removal, but it is often impractical for lighting applications.

When it comes to buying products, it can be difficult to determine whether the design includes good "heat management" just by looking at the product. This heat issue also reveals that there are some situations where LEDs can be less than ideal. "Recessed cans" and other enclosed fixtures may prevent good cooling and result in higher operating temperatures and shorter service lives. Many flashlights are designed with the assumption that the product will not routinely get used for long, uninterrupted periods. If you intend to habitually use a flashlight as an "electric candle" it would be a good idea to select one with adjustable brightness settings and dial it back somewhat during such use.

One note: This discussion of heat may make it sound like these LEDs are veritable blast furnaces. While this heat is an important technical issue so far as good LED performance is concerned, it is only a fraction of what is generated by an incandescent light of equivalent lumen output.

LEDs often claim a lifetime of many thousand hours, but it is important to understand what is meant by the claim. The numbers will vary from one manufacturer to the next, but here's an example. The Cree XR-E LEDs mentioned earlier claim a service life of 50,000 hours. But LEDs do fade in brightness with use and 50,000 hours is the point at which lumen output will have fallen to 70% of its original value. Very importantly, this lifespan assumes that the LED is being operated at a specified temperature, so service life in the real world can vary considerably. With a combination of a conservative power input and good heat removal, the LED will operate at a much lower temperature and tens of thousands of hours of operation is possible with little or no deterioration in brightness. On the other hand, conditions that lead to a higher operating temperature can reduce service life and accelerate the deterioration of the LED by quite a bit.

For items like flashlights and headlamps, service life is generally not a major issue because such products rarely get tens of thousands of hours of use. But when it comes to bulbs or fixtures for a well-used room in the home, it doesn't take too long to pile up a lot of hours. A fixture or bulb maker can parrot the service life claims of an LED manufacturer like Cree. But unless the product is properly designed, its real operating life can be much shorter. It can be virtually impossible to assess whether a given product is well designed just by looking at it. Thus, reputable manufacturers are an important consideration because it is often necessary to rely on their claims.

"White" LEDs are actually available in different color tints, ranging from "cool white" to "neutral white" to "warm white" and many LED-based products are available in color tints. It turns out that cool white LEDs are generally best in terms of raw efficiency and things go downhill from there. For instance, the 92 lumen per watt efficiency quoted earlier for the Cree XR-E LED was for the best bin in cool white. But in neutral white, the best available efficiency is only 81 lumens per watt. In warm white, it drops even further to 70 lumens per watt.

It can be very tempting to select cool white products for efficiency's sake. With flashlights or portable lanterns that will be used for things like maintenance, security, and emergencies, this may be a sensible strategy. But many users state that the neutral or warm LEDs provide better color perception and they are equally or more effective in the field, even though their lumen outputs may be lower. And when it comes to area lighting in a cabin or home (or even a tent), a lot of folks will find the harsh moonlight color of cool white to be ghostly, sterile, and perhaps even a little unnerving. On the other hand, "warmer" tints are found to be more pleasant and this may be worth the tradeoff in some settings. Neutral white may be a good compromise in certain situations and, in some LED product lines, the efficiency of cool and neutral white are very similar or even identical. The ideal approach would be to try out the different color tints before purchasing a large number of LEDs or bulbs.

A multi-die LED provides a greater lumen output by packing several light-emitting chips into a single LED housing. The one shown here contains four chips.

Electronics

To control the power flowing to an LED, electronic regulators (often called "drivers") are usually used. Commercial products will incorporate the driver right into the product, but they can also be purchased as separate components for making your own LED-based lights.

Most LED manufacturers specify operation in terms of electrical current. This is the actual flow of electrons through the LED and it is usually measured in "milliamps" (mA). The published efficiencies and lumen outputs of many small LEDs are reported when the electrical current is 350 mA, but most of them can handle higher inputs of 700 or 1,000 mA. If we think in terms of water, the electrical current (milliamps) flowing through the LED would be equivalent to a water flow measured in something like "gallons per minute." Voltage is the pressure behind the electricity and pushing more current through an LED will require more voltage. As an example, it takes about 3.3 volts to run a Cree XR-E LED at 350 mA. But if we want to operate it at maximum (1,000 mA for cool white XR-Es), the voltage (pressure) must be increased to around 3.7 volts. As you may expect, the required voltage will vary from one brand of LEDs to the next. But it will also vary a bit with individual LEDs from the same product line due to the afore-mentioned touchiness involved in the chip-forming process. Because of this variation and because a small difference in voltage can produce a large change in current flow, trying to power LEDs by adjusting the supply voltage to some measured value is not the best approach.

This is where the driver comes in. Most drivers are designed to prevent problems by maintaining a fixed flow of electrical current under varying voltage conditions. There are two general classes of drivers: "boost" drivers take a voltage which is too low and increase it to push the needed current. "Buck" drivers, on the other hand, can take a voltage which is too high and reduce it to keep the current flow at a specified level. Some very cheap flashlights and other products use a simple resistor to restrict the flow of current to the LED, but this inexpensive strategy will not compensate for a changing source voltage and it may not be very efficient. In contrast, a driver can control things on the fly and the benefits are well worth the expense in most cases. The electrical current delivered by many drivers can be manually adjusted by the user, so the driver can also serve as the means of adjusting brightness when that capability is desired.

A common annoyance with incandescent flashlights is that their brightness fades as the batteries discharge. But an LED flashlight that uses a well-designed driver can maintain almost full brightness until the batteries are nearly dead. In addition, the right driver design can allow a flashlight to use several different battery options that provide different voltages. Proper drivers can offer a similar benefit in a home or cabin equipped with storage batteries and a wind or solar charger. The driver will not only maintain full brightness as voltage drops during battery discharge, it will also protect the LEDs from excessive current when the charger is running and voltage increases. An important precaution here is to make sure that the drivers can handle the maximum voltage produced by the charger (this can be up to several volts higher than the "nominal" 12 or 24 volts at which the system is rated). If you are thinking about making your own, you can get drivers with pre-connected wiring harnesses and versions that allow the brightness of the LEDs to be adjusted simply by turning a knob.

Drivers do consume power and this can have a significant impact on the overall efficiency of a real life flashlight, fixture, or bulb. A good driver can be very efficient (90% or better) but a poor design can greatly reduce the overall efficiency of an LED-based product. An honest and accurate efficiency value from a reputable manufacturer will include the power used by the driver.

LEDs are ideal for use in weapon-mounted lights like the Streamlight "TLR-1" at left. Safariland's "Rapid Light System" (RLS), at right, can be had with an included flashlight, but the handy RLS mounting bracket can also be ordered alone and mated with a number or readily available "tactical" flashlights.

Some real-life LEDs

There are a few distinctly different types of LEDs and each has different benefits and drawbacks. As the technology continues to advance, the specific LEDs cited here (along with the efficiencies and lumen outputs) will become outdated, but the general principles will remain useful. Most manufacturers specify some "tolerance" (such as +/- 5%) when it comes to the performance of individual LEDs within a given bin, so the numbers presented here are subject to some variation. If you want an idea of what real life products are capable of offering, you can generally deduct about 25% from the raw lumen outputs and efficiencies that I cite to allow for various sources of efficiency loss.

Pin type LEDs: These little LEDs look like small plastic bulbs or bubbles and they are usually connected via a couple of small metal wires (i.e. pins) that stick out of the bottom of their plastic housings. While some of them can be fairly efficient, they are not very bright. Typically, anywhere from a few to several dozen of these LEDs are ganged together to produce a reasonable amount of light output. These little guys DO have their uses, but they are definitely not in the same class as the high-power, high-efficiency LEDs that are our primary interest. The nature of their design prevents good heat-sinking and this can lead to problems when they are used in applications that involve extended operation.

"Single die" LEDs: I will refer to the small LEDs suitable for applications like flashlights as "single die" LEDs. The Cree XR-E LED that we have used as a working example is an LED of this type and, until recently, it represented about the best efficiency you could find. Once again, a best bin Cree XR-E LED in "cool white" (the Q5 bin) can produce about 107 lumens with an efficiency of 92 lumens per watt at low power. Run at maximum power, an output of about 230 lumens is possible, but efficiency falls to around 62 lumens per watt.

Similar LEDs are also available from other companies, including Phillips (maker of the "Luxeon" brand) and Seoul Semiconductor Corporation (or SSC). But Cree blew everyone's doors off recently (including their own) with the release of the XP-G LED. At low power, the best version (the R5 bin) can generate 140 lumens with an efficiency of 130 lumens per watt. At maximum power, it can produce 340 lumens while retaining an efficiency of about 100 lumens per watt. Right now, it is available only in "cool white," but it is likely that warmer color tints (along with even more efficient cool white bins) will become available with time. Several flashlight makers have already incorporated the XP-G LEDs into at least some of their products. Unfortunately, the gears are turning more slowly when it comes to bulbs and fixtures for area lighting.

"Multi-die" LEDs: If more light is needed than one "single die" LED can provide, a viable strategy is to simply use two or more such LEDs. For area lighting, this is often the best approach because it allows both the light and the heat to be spread out over a larger area. But this can also be impractical in some situations, especially when space is limited. Because of this, another option called the "multi-die" LED is also available. We can understand both the name and the operation of this LED by simply imagining several (usually four) individual chips of light-emitting material packed into a single LED housing.

Example LEDs in this category include the "MC-E" from Cree and the "P-7" from Seoul Semiconductor Corporation (SSC). The best bin P7s are a little brighter, but the MC-E provides a little more flexibility with respect to how it can be connected and powered. Fed at low power, best bin versions of these LEDs can produce 400 to 500 lumens with efficiencies of around 90 lumens per watt. At maximum power, around 700 to 900 lumens is possible with efficiencies of 70 lumens per watt or better. Although the total lumen outputs are higher, the efficiencies here are quite similar to those of single-die LEDs like the Cree XR-E. This reflects the fact that multi-die LEDs use basically the same technology. Sticking several light-emitting chips into one package can provide more lumens, but it will not increase efficiency. Of course, advances in single-die LEDs could certainly be incorporated into multi-die units.

Multi-die LEDs are a viable alternative for brighter area lights, but heat management becomes especially important because they produce a comparatively large amount of heat which is concentrated in a small area. These LEDs are also used to make bright portable flashlights.

Different design strategies can be used to turn a given LED into very different final products. Even if you are not specifically interested in flashlights, this is a useful lesson that applies to other LED-based products as well:

First, a flashlight capable of running these LEDs at maximum power for any significant period of time will need to be somewhat bigger than the compact alternatives which use single-die LEDs. Even so, such products can still be smaller and lighter weight than a metal 2-D-cell incandescent flashlight, while belting out as much light as a small plug-in spotlight and running for an hour or more on self-contained batteries.

A second strategy is to put a multi-die LED into a more modest-sized package and run it at low to medium power. Initially, this may seem like a waste of the LED's potential, but it can actually be a very sensible approach that makes use of the relationship between power and efficiency: a multi-die LED at low power can produce more light than a single-die LED at high power, but the power demand and heat production remain modest enough to allow for a reasonably small package.

Finally, the multi-die LED is again built into a fairly compact flashlight, but the electronics now allow operation at maximum (or close to maximum) power. The caveat here is that full-power operation is restricted to only a few minutes at a time—either by the electronics or by the user. A perfect application for this design is self-defense (armed or otherwise): the flashlight is small enough to allow convenient on-person carry, but its ability to belt out 700 lumens or more for a few minutes can be very useful in an encounter with a weirdo. Lower output settings can be included in such a design to allow extended operation for other uses.

Bigger single-die LEDs: Multi-die LEDs present some technical limitations. Most notably, they can be difficult to focus tightly and uniformly when placed in a flashlight reflector. But there are also LEDs becoming available that use a single, larger chip of light-emitting material to produce multi-die levels of output (and more) without the need for multiple chips of emitting material. A very new and good example is the "PhlatLED" LEDs produced by a company called Luminus. Their "SST-50" LED is pretty comparable to the multi-die alternatives in terms of raw performance (lumen output and efficiency), but it can produce a tighter and/or more uniform beam in flashlight applications. Thus, the SST-50 could be especially useful in any of the flashlight design strategies that were just discussed in the multi-die category. The SST-50 has already been incorporated into a few flashlights.

Luminus also makes a bigger "PhlatLED" LED (the SST-90) and this is a good example of what is becoming possible in area lighting for the home. Its efficiency at low power is close to 100 lumens per watt (in cool and neutral white). Now this is not remarkable in and of itself, but the ability of the SST-90 to maintain this efficiency while providing a big lumen output is definitely noteworthy: at low power, the SST-90 can produce about 1,000 lumens. LEDs like this make bright fixtures and bulbs feasible without the need for a multiple number of LEDs and the potential applications for the cabin and home are pretty self-explanatory. At this writing, these LEDs are still quite expensive and I am not aware of any commercial bulbs or fixtures that have incorporated them. That, however, is likely to change with time. By the way, the SST-90 can produce about 2200 lumens when run up to maximum power. Efficiency here drops to about 70 lumens per watt and considerable heat-sinking is needed, but it could prove very useful in applications like car- or battery-powered spotlights and emergency work lights. At least one better-quality flashlight maker is already offering a product that uses this LED to its full effect.

There is a dizzying array of LED-based flashlights available. Selecting the right ones can be a challenge, so we'll explore the details in our next visit. For size comparison, a 2-D-cell incandescent flashlight is shown at the far right.

A bit of philosophy

For those who would like to light up a cabin or an off-the-grid home from batteries and a solar or wind charger, LEDs are a very promising technology. For anyone who just wants a flashlight that is super-bright, very-long-running, and/or extremely compact, the darned things are also pretty neat.

But the rapid and continuing advance in technology can also be rather frustrating and—potentially—disappointing. This advance will not continue forever because there is a scientific upper limit on how many lumens can be produced from a watt of power, but LEDs are likely to continue to improve in both brightness and efficiency in the foreseeable future. In addition, it takes some time for the newest LEDs to get incorporated into actual products. This process happens pretty quickly with things like flashlights, but it has been slower when it comes to products like bulbs and fixtures for area lighting. These factors lead to an obvious problem: even if you are armed with all the facts and do the best possible research, it is still likely that products you purchase will be outclassed by ones that become available in the future.

To avoid irritation, confusion, and bad purchases, here's a practical way of looking at it: when it comes to things like flashlights and headlamps, the LED-based choices which are already available can kick the pants off of incandescents in terms of efficiency, so they are an excellent choice for many needs. It still might be best to ease yourself into them instead of immediately outfitting the whole family, all the cars, and all the survival kits. Something "better" is almost guaranteed to turn up in the months following your purchase. A related problem with flashlights is that there are a lot of good options available (along with some junk!), so weeding through the choices to fit the products to your needs can be a challenge.

When shopping for fixed area lights for the cabin or home, things are a little different and much more caution is required. Because space, weight, and durability are not critical limitations as they are with flashlights, other technologies are practical and these include fluorescent, "metal halide" and, in a few applications, yellow "sodium vapor." Until recently, these alternatives have been able to beat the efficiency of LEDs and this has likely slowed the development of LED-based fixtures and bulbs. The newest LEDs will be able to equal or exceed these other choices with regard to efficiency, but most of the LED bulbs and fixtures available at this writing do not use these newest LEDs. Even once they do, there is always the possibility of even more efficient LEDs (or other technologies) coming down the pipe.

In addition, we also need to consider factors like cost. If things like microwaves, cellular phones, and televisions are any indication, it is quite likely that LED-based fixtures and bulbs will start out very expensive and become much cheaper after they have been around for a while. When considering fixed area lighting, it is important to look at all the options and compare not only efficiency, but also things like total lumen output, purchase price, and service life. The "best" choice will depend on individual needs and it will also change with time as the technologies evolve. Unfortunately, there is no perfect answer here, but we'll try to get a better handle on the issue in Part 2: LED technology essentials.

The author welcomes comments and feedback. You can reach him at timthorstenson@yahoo.com

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