Light emitting diode (LED) technology has developed quickly over the last decade. Once only the familiar red dot indicating power was “on,” LEDs can now replace traditional incandescent and fluorescent lighting as our primary source of illumination.
In this role, energy efficiency and extremely long life are the key attributes. Additionally, LED’s high light output, compact size, and color variety can enable artistic expression formerly impossible with other light sources.
Looking beyond the more traditional uses, their unique spectral properties are providing research opportunities ranging from the plant growth systems being studied as part of NASA’s Advanced Exploration Systems, to compact ultraviolet disinfection, and the treatment of seasonal affect disorder. But with ever-increasing device capabilities comes an accompanying need for more complex controls, drive electronics, and heat management solutions. New LED technology requires a system perspective to maximize performance and utilize its full potential.
Control of LED systems often centers on the human perception of light and color. We perceive color through the combined stimulation of the cone cells in our eyes. The three types of cells present (sometimes referred to as red, green and blue receptors) are each sensitive to a particular range of colors (i.e. wavelengths of light energy). The perception of virtually any color can be achieved through the individually controlled emission of red, green and blue light (as from LEDs). This is the same principle used by each pixel in a television or monitor to display color images. When all three colors are present in the proper ratios, the perception of white light can be created. Smart control electronics can dynamically change the intensity of a system’s LED elements to create a variety of effects, moods, intensities and illumination colors.
Electronics may also control less obvious, and often invisible, aspects of system performance. Pulsed patterns of infrared LED light provide the data communication in most television remote controls and many short-distance fiber optic communication links. System interlocks may turn off ultraviolet emissions during certain operations to prevent human exposure to potentially harmful light output. A system could also actively monitor the temperature of a high-powered LED to prevent damage to the device.
A major trend in LED evolution has been the development of devices with increasingly higher light output.
More light, more power
A major trend in LED evolution has been the development of devices with increasingly higher light output. With increased output comes the need for correspondingly higher power input. To maintain system energy efficiency, an important advantage of LED-based lighting, more sophisticated switching converter drive electronics are often used to deliver the power required. This carries with it greater electronics size, complexity and cost, as well as the potential for product-level radio frequency emissions which must be controlled to meet FCC or European standards.
Unlike incandescent light sources, which operate by literally heating a filament white hot, LEDs emit light through the recombination of charges within a semiconductor junction which is easily damaged when subjected to high temperatures. LEDs are more energy efficient than many sources, but still only a portion of the power applied is converted to light (on the order of 20 percent). The rest becomes heat which, if not managed, can destroy the device. In a large LED panel, this can amount to hundreds of watts of heat that must be removed. The cooling task is made more challenging by the small semiconductor volume in which the heat is being generated. Thermal system design will generally involve heat sinks utilizing convection, forced air, or sometimes even fluid cooling, to keep the LEDs within safe operating temperatures. Additionally, heat management solutions must also consider the power dissipated in the circuitry used to drive the higher-intensity devices.
Iterate early and often
Prototyping of the product design in the early stages of development is strongly encouraged. Testing can confirm analyses or uncover problems while still relatively easy to fix in the many nuts-and-bolts electrical and thermal design details previously discussed. More importantly, early evaluations can be critical in refining other aspects of the system that may not lend themselves to the rigor of engineering calculations. Lighting system performance is often a subjective assessment, especially when related to color or other aesthetics, due to human perception and preference. Optical element design (i.e. the lenses, diffusers, light pipes and other mechanisms used to deliver and modify the light) also benefits from early prototyping and experimentation. Light can be tricky. Shadows, hot spots, light leaks, and non-uniformity are some of the common issues which might surface when the first systems are built.
LED technology continues to advance rapidly, most notably in the range of power and spectral outputs available. The design of new products striving to capitalize on these new capabilities needs to consider system integration implications far deeper than the simple selection of a new part.
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