Light Emitting Diodes

LED Power Dissipation

The most critical consideration in light emitting diodes selection is power dissipation. Component datasheets provide power dissipation in watts (W) or milliwatts (mW), where 1000 mW is equal to one watt. Datasheets corresponding with a typical 5mm through-pin LED light will state a power dissipation of approximately 100 mW. Newer LED technology such as the "high-power" LED lamp may contain ratings as high as 5 watts or greater. This important characteristic shares a positive correlation with luminous output, heat dissipation, and cost. Typically, LED emitters with elevated power ratings offer an increased photometric output in lumens, greater heat dissipation, as well as a higher overall cost. A clear understanding of this relationship becomes critical while selecting optoelectronic candidates for a new design. It is also important to remember power dissipation normally represents an absolute rating. When not offered directly by corresponding datasheets, multiplying the maximum forward current rating by maximum forward voltage rating results in calculated power dissipation for the device. This calculation reminds us that power dissipation provides the maximum overall power rating of the LED.

LED Optical Wavelength

Another important consideration is optical wavelength, measured in nanometers (nm). Light emitting diodes are commonly available within the electromagnetic range of 470 nm (blue) to 630 nm (red). Exceptions include colors falling below the common range such as purple and ultra-violet, as well as those above the common range such as infrared. Perhaps optical wavelength is an effortlessly determinable characteristic for new lighting projects. However, this point in the design process may be an appropriate time to calculate for radiometric flux. If not for any other reason, to monitor the possible risks associated with human eye safety. A green LED lamp with an optical wavelength of 555 nm will dissipate 1 watt of radiometric flux while generating 683 lumens. Compare this to a ultra-violet LED lamp with an optical wavelength of 400 nm, which also dissipates 1 watt of radiometric flux while generating only 0.27 lumens. The human eye safety factor becomes apparent since the ultraviolet LED lamp produces an equal amount of total light energy, but with only a fraction of the visible light equal to 1/2530.

LED Beam Angle

Yet another optical characteristic is beam angel, given in units of angular measure equal in magnitude to 1/360 of a complete revolution (0 and 360). The majority of all light emitting diodes fall between 10 degrees and 180 degrees. To be more specific, most through-pin LEDs feature a beam angle between 20 - 50 degrees while most surface-mount light emitting diodes contain a beam angle between 100 - 130 degrees. Recent advances in optoelectronics lead to new SMT light emitting diodes with narrow beam angles, less than 100 degrees. Calculating the most efficient beam angle for a specific design application can require complex mathematical formulas and trigonometry. Often times the calculated results call for a rare beam angle lacking a corresponding optoelectronics component equivalent. In many applications, an exact match or external optic is just not feasible. For this reason, a developed market familiarization becomes a valuable asset during the selection process.

LED Physical Package

Today, light emitting diodes are readily available in numerous styles ranging from traditional through pin to modernized surface mount packages (SMD). As through pin technologies grow obsolete, advances in surface mount technology continue to progress beyond the capabilities of most traditional components. Several primary advances in optoelectronics associated with surface mount LEDs include increased efficiency during assembly as well as numerous advantages from a design aspect. Surface mount LED lights can typically dissipate heat more efficiently. Additionally, since the surface mount LED lamp is mounted on the surface of the printed circuit board, the opposing LED circuit board layers are conserved for additional components or copper traces. However, a handful of applications can still benefit from traditional through pin optoelectronics. These instances occur primarily when a relatively narrow beam angle is required, and an external optic is just not feasible due to space restrictions. It is very important to recognize common characteristics associated with various package styles. For example, through pin LEDs tend to offer a narrow beam angle, decreased luminous output, and reduced component cost. Assembly costs tend to be higher. On the other hand, surface mount LED (SMD) packages tend to offer a wider beam angle, increased luminous output, and increased component cost. In addition, assembly costs tend to be less expensive. Ultimately, many LED lighting applications are limited to a specific package style due to beam angle restrictions.

LED Manufactures

Committing to a specific LED manufacture based solely on finical related benefits can lead to imminent failure. One must consider all aspects presented by numerous manufactures rather than focusing strictly on costs. The quality of customer service provided by the LED manufacture can play a critical role in the success of a new LED lighting project. Overseas LED manufactures tend to offer extremely competitive pricing. However, extensive lead-times or poor customer service can easily impair the success of any project. One example includes failed optoelectronics components, in which case tracing lot numbers is required to pinpoint a potential manufacturing issue. In this case, a dedicated manufacture should be willing to cooperate fully. Another extremely important aspect to consider when initially selecting an LED manufacture is the availability of lifespan data associated with the light emitting diode. A high percentage of LED manufactures do not offer sufficient LED lifespan graphical data. Instead, they may state the longevity rating is simply 100,000-hours. This information is useless without knowing the lumen maintenance for this rating. Proper graphical data should compare hours of accumulative operation with the luminous output, at least out to the point where luminous output degrades to 50%. Determining whether an LED manufacture offers such data is always an essential worth researching.

LED Binning

Perhaps LED binning is the most commonly overlooked and misunderstood aspect of the LED lamp selection process. During the LED manufacturing process, unique lot numbers assigned to the components provide some means of traceability. Due to typical variations during manufacturing, each light emitting diode will possess and exhibit a unique set of characteristics. Subsequent to testing, optoelectronics with closely related characterizes are grouped into specific bins with pre-defined bin identifiers. Any components that do not meet criteria of a specific bin number result in discard. LED manufactures normally bin according to three primary characteristics. The intensity bins segregates components in accordance with luminous output. Color bins provide separation for variations in optical wavelength or color temperature. Voltage bins divide components according to variations of their forward voltage rating. Some optoelectronics manufactures provide the convenience of allowing the customer to select from specific bins. However, they may not offer an inventory guarantee for some specific bins, leading to a potential trap! For example, a prototype contains light emitting diodes from the highest intensity bin, and yields extremely favorable results. During initial production several months later, yields from the original intensity bin are not sufficient to satisfy a high volume production run. As a last resort, the optoelectronics manufacture must provide a much dimmer LED from a lower intensity bin.

The information contained within this article is for general educational use. Individual LED lighting projects can vary dramatically. For more information about the specifics that apply to your custom LED lighting project, please contact our engineering department with your complete project details. Lunar Accents Design Corporation has access to many new and upcoming LED specifications, in many cases prior to the component's public introduction. We encourage you to take advantage of these new advances in optoelectronics.