Chapter 4. The Bulb

There isn’t a whole lot to the typical LED bulb. In addition to the standard gear—the base, lens (optics), and/or reflector—there is a driver, which is explained in more detail below; PCB, LEDs, an encapsulant over the LEDs, a phosphor, and usually some sort of a heat sink. The ideal scenario (or the goal for some manufacturers, at least) is for the LEDs, hardware (cooling materials, etc.), and driver to each make up for one third of the cost of the bulb, but that’s essentially just a rule of thumb—real-world demands and technology issues often get in the way of such ideals. As with any other manufactured product, it’s all about tradeoffs. By investing more in the cooling, it’s possible to run more power through the LEDs, which means you can use fewer of them to get the same amount of light. Similar tradeoffs must be made, for example, between the quality of the light and the efficiency of the bulb.

The driver is essentially the lamp’s power supply, but because this is a modern light source, there is some intelligence built in as well. The driver, which usually lives at the base of the bulb, is able to not just transform the energy the bulb takes in to what the LEDs need, but also to do things like throttle the power being sent to the LEDs if it senses that the unit is too hot. This will not only prevent possible damage to the surrounding area but it will extend the life of the LEDs.

The driver is one area where bulb manufacturers—who often use LEDs from the same providers—can compete with one another in terms of size and electrical conversion efficiency. Drivers can be AC or DC, both of which have their advantages, and they can be as complex an exercise in integrated circuits as any electronic product in this price range. For example, AC systems do not use as much power as DC ones, but they are able to transfer less to the LEDs because of the inherent properties of the alternating current. AC-AC power supplies use less parts and are cheaper to produce than AC-DC ones, but often result in a less efficient bulb, when comparing the total amount of power taken in to the lumens it puts out. As with other parts of the lamp, there is a constant push to lower cost, which often means using fewer components.

Because LEDs do not run off of the same level of current that is provided to buildings—120V in the United States—the driver must perform a power conversion. Many LEDs are low voltage devices (at least, relative to standard line voltage), so a decline must take place. High voltage LEDs do exist, but those in many lamps might need just 3.3V or so. This requires the use of small, dependable electronics that will be able to power the bulb as long as the LEDs inside it will live. Components like electrolytic capacitors and MOSFETs must be used in order to deliver consistent power drive the LEDs. Add in the dimming (possibly through pulse width modulation [PWM]) hardware and an intelligent LED controller and you have a complex piece of electronics that replaces an incandescent…which had no problem using the main’s voltage.

One of the conundrums of today’s LED bulbs is that because bulbs are often oriented upside-down and in areas with limited air circulation (like a ceiling can), the trusty A19 shape can become a serious problem. Because the bulb is oriented upside-down the driver is at the top, which is where the bulb’s heat flows. Special precautions have to be taken when designing a bulb so that the driver does not overheat. If a bulb were to overheat it would initially reduce the lumen output, but in extreme cases it could damage the LEDs as well as the interior circuitry, possibly creating a fire hazard.

The heat sink, the metal structure coming up from the base of most bulbs, is one of the most noticeable parts. It is designed to dissipate heat from the LED out to the surrounding area. Thanks to the LED’s high efficiency, not much power is turned into heat but the cooler LEDs run the longer their lives are (to a point), so it makes sense to include these. All high-quality, brand name LED bulbs will have some sort of passive cooling as it’s an important part of transferring heat away from the sensitivity components.

But why does an LED bulb need a heat sink when none of the other major bulb types require one? The main reason is that LEDs don’t give off heat in the form of infrared radiation. This means cooling must be handled through other means, such as conduction through a heat sink. Interestingly, consumer LEDs don’t give off ultraviolet (UV) light either. This happens to have a very useful side effect: LED bulbs don’t attract insects, which are drawn to UV light.

The heat sink shouldn’t be written off as just another part of the bulb: cooling is a critically important part of the design. We know heat limits LED life, but that’s not the only reason to keep cool. There is a short term impact to heat as well: hot LEDs produce fewer lumens.

In addition to the LED’s sensitivity to heat, the aforementioned drivers are often designed to throttle back light output when too much heat is detected in order to cool the lamp and preserve its life, and to prevent parts from being damaged. But as destructive as heat is on the LEDs, there is another important reason to limit heat build-up: the Underwriters Laboratories (UL) will not approve any lamp that goes above 90° Celsius, as per the UL 8750 safety standard.

LED bulb manufacturer Switch Lighting has taken an extra step with cooling by filling their bulbs with liquid. Their design not only has metal heat sinks on the interior of the bulb but the use of a special liquid promotes cooling through convection as well as conduction. This further dissipates heat, giving Switch better cooling which, in turn, means the company has been able to design brighter bulbs than some of their competitors, including 75W- and 100W-equivalent bulbs (1180 and 1750 lumens, respectively). Better cooling also means that a manufacturer could theoretically use fewer LEDs and push them harder in order for a bulb to produce the same amount of light, but would have to trade off lifetime for price.

The inability of LED bulb makers to produce high-lumen (essentially higher than 60W equivalents) bulbs has been problematic. It has created the perception among consumers that LED bulbs are unable to produce the equivalent amount of light as a 75W or 100W bulb. This has, so far, limited LED bulb applications to standard task, ceiling, and floor lighting. As designs improve and LEDs become more efficient, higher wattage-equivalent bulbs will become available, though their success as products will be limited by their price. As with anything else, these prices will drop eventually, but it will take time before a 75W incandescent can be affordably replaced by an LED bulb.

One question that is invariably asked when it comes time to buy an LED bulbs is, why are some of them yellow? (Or, more often, why would I buy something that looks like a bug lamp?) This happens with models like the popular Philips AmbientLED, which has an aluminum heat sink crowned by a deep yellow, plastic-y bulge that is split into thirds. The packaging makes it abundantly clear that while the top is yellow, the light it emits is the standard white that you’d expect from any household bulb. But why the yellow and why the bulge?

It turns out that the latter is the easy part to explain. The extension of the light-producing bulb elements means that the light that leaves the bulb is able to reach a larger amount of its surroundings. This is to say that the bulb is less directional than it otherwise would be, and less directional than an older style of LED bulb. If you were to look at the pattern of the light the bulb throws, you would see that it offers better coverage to the area behind the base than a typical LED bulb does.

And what about that odd, off-putting yellow? That’s because of the bulb’s phosphor (not phosphorus). A phosphor is a general term for any material that radiates light without having to be heated up to do so. A glow-in-the-dark toy uses one type of phosphor, but so does something as simple as petroleum jelly, which will glow when held under a black light (an ultraviolet light).

How this ultimately works is that the LEDs inside bulbs are often blue (using a semi-conductor like indium-gallium-nitride [InGaN]), not white. When the blue light contacts a phosphor, the result is white light. Some bulbs use LEDs that are coated in a phosphor; in this case, Philips has put the phosphor on external components, creating what is known as a remote phosphor. The blue light contacts the phosphor, and the mix of blue and yellow produces white light.

Of course, not just any white light is OK—people want a particular color temperature for their environments. This is usually 2700K–3000K for an American home, but preferences can vary in different countries. In Asia (Japan especially), higher color temperatures are more popular. In other parts of the world, people prefer the nearly amber light of 2500K bulbs. Because the phosphor is the largest single factor when it comes to color temperature and CRI, it’s the most important decider when it comes to a critical part of the lighting discussion: light quality.

Light quality is one of the reasons early LED bulbs didn’t take off and why CFL adoption was slow. Buyers, especially consumers, have a set of expectations for their lighting purchases, which takes into account brightness, price, temperature, and CRI. Maybe they don’t know the terms, and maybe reading the back of the box won’t help much, but once the light is home and operating, you can be be sure a lot of people were not happy with that blue-tinted CFL (which probably also took a long time to warm up and didn’t work with their dimmer).

Light quality is a characteristic where lamps just need to reach a certain point: essentially, an acceptable color temperature and then 80-90 CRI. After that point, you’re off in the land of specialty bulbs, where choosy buyers can have a color temperature they prefer or an artist wants 95+ CRI bulbs to produce top notch color accuracy.

And why not just manufacture all bulbs at a high CRI level? The answer is just what you’d expect: cost. Proper color rendering isn’t as easy as just using the right components; it requires research and high quality materials. There is also a documented tradeoff between color rendering and efficiency (in terms of lumens-per-watt). With today’s manufacturers almost completely focused on making consumer bulbs cheaper to produce, and factoring in the never-ending push to maximize efficiency, color quality can fall by the wayside.

At some point, everyone interested in solid-state lighting probably wonders why some opt to use a blue LED—known as the emitter—and a remote phosphor. It’s all about efficiency, grabbing up to a 30% gain according to one source, albeit the CEO of a remote phosphor-focused company. It’s best to break down claims and design choices like this into their component parts, the first being, why use a blue LED in the first place? This is a fundamental concept of today’s LEDs; blue LEDs are just more efficient. This can be explained with scientific terms like “quantum-confined Stark effect,” but the larger point is that blue LEDs are just more efficient, enough so that it makes more sense to use them and a phosphor as opposed to trying to product white light.

And why place a phosphor apart from the LED as opposed to just combining the two? Not all companies use a remote design, so while this not is necessarily better, there can be advantages. One advantage has to do with LED placement. Since LEDs need to be kept as cool as possible for them to run efficiently, by placing them against the body of the bulb, they can use the heat sink to be cooled via conduction. Better cooled LEDs can mean cheaper lights—if fewer LEDs are used and they are pushed harder—or brighter bulbs, if they are simply pushed harder. With the LEDs against the heat sink and the light becoming diffused through the remote phosphor, it’s possible to design an omnidirectional bulb any number of different LED placement patterns.

The remote phosphor can control light quality, as well as act as a diffuser for the light, optimizing the pattern and filling a room better than the highly directed LEDs alone. It can also reduce the appearance of bright spots on the bulb, as well as make it easier for manufacturers to manage the purchase of LEDs (the remote phosphor can be changed to get to a desired CCT or CRI without having to be concerned with the LEDs themselves).

Currently, remote phosphor is a very good option many manufacturers use. Some of these bulbs, like those from Philips, allow the user to see the remote phosphor—that dark yellow plastic at the top of the bulb that immediately turns off some buyers. Luckily for companies that have opted to go the remote phosphor route, this isn’t as much of a problem as it might seem. First of all, it’s entirely possible to cover up the phosphor with a another diffuser. This will limit the brightness of the bulb to some degree (it’s akin to going from a clear bulb to a frosted one) but the gains of a high quality remote phosphor are often enough to outweigh this. For an example of this, look for one of the new CFLs on the market; there is a good chance that the helical glass tube will be placed inside a frosted bulb. Another point, raised by a representative of Intematix, is that the phosphor doesn’t have to be yellow, it can be a lighter shade, even khaki. These muted color options might not make for the best possible efficiency, but if the buyer doesn’t need a top CRI score or a perfectly dialed-in color temperature, there is some room for flexibility.

One interesting point with the phosphor is that today’s materials completely lack an important characteristic: persistence. So when you turn off an LED it stops producing light immediately (about 20 nanoseconds). As noted before, LEDs are highly sensitive to power inputs, and they are often designed to operate at 120Hz, basically flickering on and off faster than our eyes can recognize. How does this all tie together? This explains why LED lamps need good drivers and are sensitive to their power conditions. Even the briefest lapse in power will cause a flicker (they can react in mere nanoseconds) while on an incandescent, the lapse wouldn’t matter because the filament will retain heat for that brief period of time (it has persistence). If one day phosphor with a high level of persistence were developed, it could be a big step forward, possibly enabling better lights and cheaper drivers.

The technology is advancing each year, so gains will surely be made on this front, with great strides having already taken place in the last few years. A final factor to keep in mind is that the most important buyer for early LED bulbs—businesses—usually don’t care what color the exterior of the bulbs happens to be, as bulbs are often enclosed and, more importantly, because they look totally “normal” when running. While the average consumer is said to operate their lights for three hours a day, the average business runs them for ten. This means that there will probably be no point when a customer walks in and sees a strange yellow bulb, because when customers are present, the light will be operating and they won’t appear out of the ordinary.

The alternative to using a blue emitter plus a remote phosphor would simply be to use an LED that produces white light. These LEDs sometimes combine colors (like RGB, for the primary colors red, green, and blue) to create that white light. While many producers are happy to go the blue emitter route, some proclaim the benefits of white mixing colors when it comes to color rendering, control over color temperature, and even efficiency. Another option is to use RGBA, the “A” being amber.

What this chapter has covered so far includes merely the basics of LED bulb design. There are many other questions that could be asked about any particular product, including: Does it use high-voltage LEDs? Does it have a few powerful, expensive LEDs, or does it use many cheap ones—sometimes over 100—to produce light? How is the light distributed? Is it directional, or will it give full coverage to a reasonably sized room if placed in the center? There is also the matter of the printed circuit board (PCB), if it has one. That’s an important part of many bulbs, as is the specific substrate the LEDs use. These are all important characteristics that, while fascinating, won’t affect buyers unless they lead to a particularly important advancement.

One example of such an advancement was developed by a company called Soraa, which is making a significant bet on “GaN on GaN” (gallium nitride on gallium nitride) substrates in their LEDs. The progress comes in the form of the technology’s efficiency over other methods like GaN on sapphire, or GaN on silicon carbide. Soraa’s research and development has led to its claim that they can use ten times the current as an alternative material. This increase translates to ten times more brightness and what the company believes will be an industry-changing advancement.

What’s the point to all this? LEDs and SSL can be just was complex as you want to make them, especially if you have a thing for semiconductors or photometry. It’s one thing to understand the lighting in terms of heat sinks and LEDs, but it’s entirely another to get deeply involved in the electrical engineering, physics, and material sciences that allows these products to improve from one year to the next. Many of these elements are outside of our scope and are fully explained in white papers, journals, and textbooks, if you’d like to dig more.

Before moving forward, there is one more thing we non-doctorates can do to understand how LED bulbs work. Here are a few pictures of the inside of two self-ballasted LED bulbs currently on the market: one from Lighting Science Group and one from Philips. While there is a lot to take away from these photographs, the most surprising fact is there are is no empty space in either bulb. Both are filled with a potting compound that makes them hard to take apart (not that they are user-serviceable) and protects the internal components.

An examination of each bulb gives us some insight into the design philosophy of Philips and LSG as well. Don’t miss the high quality connectors and ceramic LED boards, which are backed with a thermal interface material (TIM) in order to best transfer heat to the heat sink. The LSG Definity—A19 Omni V2—Dimmable bulb (Figure 4-4) isn’t as nicely constructed as the Philips 409904 Dimmable AmbientLED 12.5-Watt A19 (Figure 4-1), its materials don’t feel as nice, the bulb isn’t as atheistically pleasing, and the top pieces don’t fit together and come apart as the Philips does. (See Figure 4-2, Figure 4-3, and Figure 4-5 for photos of these deconstructed bulbs.) The LSG model has fewer components, which is an important factor, but it actually costs more then the Philips: $27.85 versus $24.34 (at the time of this writing). Both bulbs produce just about 800 lumens, according to their packaging.