AC LED —AC LEDs were introduced by companies such as Seoul Semiconductor and Lynk Labs. They consisted of string of LEDs in parallel with an LED string of reverse polarity, packaged into a single device. High-voltage AC LEDs — with a Vf of at least 170V (which requires about 40-50 LED chips in each string) — can be placed directly across 120V 60 Hz line voltage. The end result is that AC LEDs use the LEDs themselves to rectify the AC line voltage. This approach is used in Seoul Semi’s first-generation AC LED Acriche line, but not its second-generation Acrich2. AC LEDs have problems with photoscopic flicker as well as an inability to dim off of a phase-cut triac dimer switch. (AC LEDs should not to be confused with newer AC drive LEDs.)
Ballast — A ballast is what the lighting industry calls the power supply that is in every fluorescent fixture. Fluorescent lamps require a different voltage from ac house voltage — just like your other household electronics, such as a desktop computer or video screen. The job of a ballast, or power supply, is to take the ac house power and convert and regulate it to a voltage and current that will cause the fluorescent lamp to turn on and continue emitting light. LED lamps also need a power supply, but they are built into the lamp itself, where they are referred to as the driver electronics.
Older ballasts on T12 bulbs were often magnetic ballasts, which are much less power-efficient than the more modern electronic ballasts.
Avoid magnet ballasts — which fortunately are hard to come by now.
Although the mechanical portion of the fixture is the same for a T8 lamp and a T12, the ballasts are different. If you simply want to upgrade your existing T12 fluorescent fixture so that it can operate with T8 lamps, you’ll still need to change the old ballast to a new T8 electronic ballast. But why not re-wire the fixture to remove the old ballast completely, and run retrofit LED lamps?
Chip-on-Board (COB) — a packaging technique resulting in a packaged array of LED chips that all share the same substrate. The phosphor covers the entire array of LED chips so the light-emitting area is not a point but a wide light source. The advantage is that the space is smaller, since each LED chip doesn’t require its own ceramic substrate, phosphor and lens. In addition, assembly costs can be lower, and electrical connection to the LED array can be from the top through a solderless plastic connector. The disadvantage is that heat sinking is crucial, and, because the light emitting area is so large, it’s difficult to focus the light; COB packages often don’t have any lensing. If you look closely at the CXA3070 pictured below, the faint lines you can see outline the positions of the LED die under the phosphor.
CRI (Color Rendering Index) — CRI measures of the quality of light: The light source’s ability to render color. The higher the CRI, the better the light source renders colors in the visible spectrum. CRI values can range from 0 —the worst— to 100 —(ideally) the best. To have what is generally considered good color rendering, a source must be 90 CRI or better. R values — sometimes called the Special Color Rendering Index (Ri), R values are a subset of the overall CRI score for a light, and measure the light’s ability to render the individual colors. The two most important R values are R9 (red), especially important in measuring skin tones and food, and R12 (blue).
Color Temperature — Color temperature, more completely Correlated Color Temperature (CCT), is a reference number for judging how warm or cold is the color of a white light source. CT is the color of light given off by a particular light source that most closely represents the light emitted from a perfect blackbody radiator when heated to a specific temperature. Incandescent bulbs are often 2700K, called a warm white light, while fluorescent lamps can have CCT’s varying from 3000K to 5000K, called a cool white light. 3500K is generally considered “neutral” white light. Today’s LED tube lights can have CCTs ranging from 2700K to 5000K. (K, or Kelvin, is used as the unit of measurement for temperature in the physical sciences. As a reference, 2700K is about 2427°C.)
Diffuser — Because LEDs are such an intense light source, lenses are often placed in front of them to spread the light out and these lenses are called diffusers. They often have a frosted appearance.
Diode — A basic component of electronic circuit design, diodes only allow current to flow one way. For current to flow, the diode must be positively biased, with a positive voltage on the anode side, and the negative voltage on the cathode. All diodes, including LEDs, have a forward voltage drop; Vf. Vf remains relatively constant even as the current run through it varies. (Within limits: If you run 10 amps of current through a diode rated at 100mA, the diode simply burns up.) Generic diodes are valued more for the amount of current they can carry and still withstand a reverse voltage. [Side note: Diodes have a common use in LED replacement bulbs: When arranged in a diode bridge, they block the negative half of the ac house voltage and rectify it, or turn it into a positive voltage so that it can be massaged by the LED driver into a well-behaved current source.] LEDs are special form of diode: As you run current through them, they emit light.
Driver — LED lamps’ internal drivers function as a power supply, taking the power — which is either from the fixture ballast in a replacement LED lamp, or is the ac house power directly wired into the fixture — and convert and regulate it to power the lamp’s internal LEDs. As you can read in the LED definition, LEDs are current-controlled devices, so LED drivers are current regulators. (Most electronics are voltage controlled.)
Efficacy/efficiency — How does “efficacy” differ from “efficiency”? In the lighting world, efficiency refers to the lighting fixture, which is the device that holds the lamps. Fixtures reflect the light out from the lamp onto the area to be illuminated. No fixture is perfectly efficient and some light is lost due to reflection and absorption. If a fixture has a 100 lm lamps but only emits 70 lm, then it is 70% efficient.
Use the term “efficacy” when the units of the input differs from the units of the output. In lighting, we are interested in the amount of light, measured in lumens, produced by an amount of electrical power, measured in watts. If a lamp consumes 9 W to produce 800 lm, then its efficacy is 89 lm/W.
Fixture — the metal device, mounted on the ceiling, that holds the lamp(s). In the world of fluorescent lighting, the fixture also has the ballast. When you buy a fluorescent light fixture at a store such as Home Depot, the ballast is always included as part of the fixture.
Fluorescent light — A low pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. The glass tube with phosphor deposited on its glass wall is filled with mercury vapor. A high voltage is applied across each end of the lamp, causing an electric current in the gas. The current excites the mercury vapor which produces ultraviolet (UV) light When the UV photons hit the phosphor coating on the glass wall, the phosphor absorbs the UV photons and then emits photons as light in the visible region. Fluorescent lighting requires the use of mercury, a hazardous material, and requires that fluorescent lamps be disposed of separately as hazardous waste.
Internet of Things (IoT) — A network of physical objects that contain embedded technology to sense, interact, and communicate within their physical environment. Although “Internet” is part of the name, the network could also be a private or closed network. An example of how the IoT might work within the lighting world is through smart lights, which have the ability to communicate wirelessly with a wider network, that can sense the presence of humans through sensors such as IR detectors, RFID tags, and cameras, and adjust the lighting accordingly with no human intervention. The resulting cost savings in energy, as well as a performance boost through adjusting light color temperature, will hopefully more than offset the march towards Big Brother-type surveillance.
Lamp — This term is used differently in the lighting world than in the regular consumer world, where lamps are devices that sit on your end table and have one or two bulbs in them. The correct lighting terminology for the fluorescent tube is a fluorescent lamp, which fits into a fixture (or sometimes luminaire). Lamps are always replaceable light sources units, separate from the fixture.
LED — A light-emitting diode (LED) is a semiconductor junction: Two dissimilar compounds that don’t do a particularly good job of conducting electricity – until you apply a forward voltage, Vf, across the junction, at which point current flows just fine. In an LED, in addition to current flowing across that junction, the junction also emits photons, or light. LEDs emit light in only one wavelength, or color. The most common colors of LEDs are red, yellow, green, and blue.
White light is the presence of all wavelengths, so making a white LED is more complex. The most common and inexpensive way is to take a blue LED and cover it with a phosphor. When the blue light strikes the phosphor some of the light passes through as blue, and some excites the phosphor which then emits its own light — mostly yellow, but some red in there two. The combination of the three colors of light is a white light.
The components we usually think of as LEDs, such as the low-power indicator LED or the mid- or high-power LEDs used in bulbs and luminaires, are packaged LEDs; The bare LED itself, before it’s packaged, is called a chip.
Vf is typically 2-3V. A big advantage of packaged LEDs is that all the LED chips are connected together internally to the package in a string so that their Vf add up to a much higher voltage: An LED package with 10 LED chips inside, all with a Vf of 2.25V, will have a Vf for the LED package of 22.5V. And 22.5V is much easier for a LED bulb designer to work with than 2.25V.
High-power (1w – 3W and higher) diagram
Example: Cree XLamp XML
Mid-power (.1 – .5W) diagram
Example: Philips 5630
Indicator (also referred to by its diameter: 5mm) – draws up to 20mA.
L70 — the number used to estimate LED lifetime, often on the order of 50,000 hours or almost 6 years of continuous use. The L70 number for an LED is the number of hours of operation after which the LED will have lost light output down to 70% of its original brightness. 70% is used because that’s the level of light loss that most people can recognize. Note that this “failure” is different from that of an incandescent or fluorescent lamp, which usually fails completely and burns out.
New LED components are being introduced each year, with ongoing advances in materials and manufacturing. It is difficult to extrapolate failure from shorter tests. The only real way presently to determine the lifetime of LEDs is to run them until they drop, but lighting designers can’t wait 6 years to find out exactly what the L70 number is for a new LED component.
Lumens — Lumens are a measure of how much light an object emits that can be seen by the human eye. I emphasize this last bit because it tells us a lot about why colder-CT LEDs output more lumens than warm-ish LEDs. The human eye is most sensitive to green light, specifically, light at 555nm. If a device produces 1W of light at 555 nm (green), it will be a maximum of 683 lm. Of course, few people want an intense green light – we want white light for illumination, and white light is made up of many different frequencies — a whole rainbow of them. The human eye isn’t as susceptible to these other frequencies though, and perceives them as being less bright, or fewer lumens. . This is why the first, inexpensive LED lights had a bluish cast, which is close to green — this was a practical way to make the lights appear brighter to humans.
Lumens are measured in a spherical pattern around a lamp. This is appropriate for omnidirectional light sources such as incandescent bulbs and fluorescent tubes, where the light coming out opposite the work surface usually reflects off of a lampshade (in a table lamp), the ceiling, or the top of the fixture (in a fluorescent light). It’s not the way LEDs work, however, because they are single-direction point sources of light. Since all of the light produced by the LED tube lamp is directed down onto the work surface, it takes fewer lumens to light up the work surface equally well. This is why a 1900 lm LED tube lamp produces the same useful light as a 2300 lm T8 fluorescent lamp.
Lux – A measure of illuminanace, or the amount of light that falls on an area. The light is measured in lumens and the area in square meters, so the units are lumens per square meters (lm/m2.) You measure lux with a lux meter; I use a Dr. Meter Digital Illuminance/Light Meter LX1330B which can be had for about $40.
Phosphor — Both LED components and fluorescent lamps make use of phosphors. These phosphors are a chemical blend, which when excited with a single wavelength of lights (blue in the case of LEDs; UV in the case of Fluorescents) emits all different colors of visible light, which the human eye perceives as white light. Usually yellow in appearance, you can see the phosphor in the photo of the mid-power LED component above.
Photon — elementary particle of light.
Pixelated — Because LEDs are such an intense single-point light source, lamps without diffusers allow the LEDs to appears as intense dots rather than a smooth even glow. This point-source look is called pixellation.
Render — “To generate.” How well a light renders a certain color is a measure of how well it generates the color which we perceive.
Replacement LED T8 lamp — Replacement lamps are a direct replacement for an existing fluorescent lamp. You pull out the old fluorescent lamp from the fixture, and in goes the LED lamp. This seems like an ideal solution, but its drawback is that the lamp must be able to work with the fixture’s existing ballast. This can be tricky because there are many, many different fluorescent ballasts out there.
A replacement LED lamp in a ballasted fixture will have two separate power supplies — its internal driver as well as the fixture ballast — and each power supply introduces power losses. Keeping the original ballast means an unnecessary power inefficiency. Plus, fluorescent ballasts generally have a shorter lifespan than LED lamps, so you may be dragging a ballast along that is inefficient AND will die before the LED lamp does.
Retrofit LED T8 lamp — Retrofit lamps require that the fixture be modified to remove the ballast, allowing a direct wiring between the lamp in the fixture and the house ac power. The obvious disadvantage is that it requires the cost, effort and knowledge to rewire the fixture. The advantage is that the light pays no efficiency penalty since it only has its own lamp driver and will cost you less in electricity costs, and you’ll never have to replace the fixture ballast. In addition, retrofit lamps are often less expensive than replacement lamps.
T8 tube lamp — A linear lamp, of which the most popular version is the fluorescent tube light. The “T” identifies the shape, which in this case is “tubular”, while the “8” indicates the diameter in eighths of an inch: 8 eighths, or one inch. Since fluorescent tube lights are the most popular lamp in the US – primarily in the commercial, industrial, and institutional space — being able to replace them with LED replacement lamps using the same fixture is a primary goal of the LED lighting industry. The most common version of the fluorescent T8 lamp is four feet long, consumes 32W, and emits 2800 lm.