(Source: Cree LED)
Starting in the 1960s, light-emitting diodes (LEDs) performed in low-power applications such as indicator lights and watch screens. Initially available only in red and green, they lacked the ability to emit enough of the light spectrum to replace incandescent bulbs. However, breakthroughs in GaN epitaxial growth and doping enabled the first bright blue LED in 1993. These new LEDs used a phosphor coating to convert the blue wavelengths into a broader spectrum of wavelengths that appear white in our eyes. With the ability to emit bright white light, LEDs became a viable replacement for traditional incandescent lighting in almost every application, from household and handheld lights to streetlights and industrial lights.
The first commercially available LED fixtures in the early 2000s were more expensive than incumbent fixtures, reflecting the significant investments in recent research and development that had gone into them. But as LED manufacturing techniques scaled and improved, LEDs are now one of the lowest cost and longest-lasting items in the bill of materials. The price and performance improvements are largely due to breakthroughs in manufacturing the semiconductors that power LEDs, particularly the GaN in blue LEDs.
With LEDs becoming more advanced and prevalent in our world, this blog takes a closer look at what makes this technology so impressive, and highlights some of the latest LED products.
Understanding LED specifications is critical for finding the right solution for your application. Someone designing lighting for an office space might primarily be concerned with power efficiency, while someone designing a color-changing fixture will want to ensure the LEDs can accurately reproduce colors.
Below are some of the banner specifications for LEDs.
Efficiency and efficacy are often used interchangeably. And while the terms are related, they mean slightly different things. Efficacy quantifies the amount of light generated per watt, while efficiency gives a relative ratio of performance.
Luminous efficacy, expressed in lumens per watt (lm/W), measures the amount of visible light produced by a device with respect to the electrical input.
Luminous efficiency, expressed as a percentage, compares the efficacy of the device to the theoretical ideal efficacy for that device. For example, if an LED has an efficacy of 172lm/W, we can compare it to a known theoretical ideal of 683lm/W. Thus, 172lm/W over 683lm/W gives an efficiency of about 25 percent. The 683lm/W value used here is for an ideal green light source at 555nm – the wavelength the human eye is most sensitive to. Efficiency calculations may be adjusted depending on the light source; it makes more sense to compare a white LED to an ideal white LED than to compare it to a green LED.
Lumens measure luminous flux, or the total output from a light emitter. In simple terms, a light source with more lumens emits more light. Lumens help quantify how bright a light source is, but real-world performance will be determined by the light fixture or luminaire in which the emitter is packaged. A car headlight focuses the light into a forward-facing beam, while interior lighting spreads the light across a room. In both cases, some of the luminous flux of the light emitter will be lost inside the luminaire as the light bounces around before escaping through the lens. Therefore, understanding other specifications, such as lux and candelas, provides a more comprehensive perspective of a device’s performance.
Lux measures the amount of light hitting a surface, expressed in lumens per square meter. Understanding your lux target for a given area can help you calculate the amount of lumens you need for your space. The amount of required light for commercial applications like warehousing or other workplaces is often defined in terms of lux. Government agencies such as OSHA often set these standards to ensure safety and functionality. Typical offices or classrooms are between 300–1000 lux, with precision workplaces and laboratories being at the higher end of the range, while warehouses can be as low as 50 lux. For reference, full sunlight can be upwards of 100,000 lux. As the area you’re lighting increases, the amount of lumens required to maintain your lux target also increases.
The candela (cd) measures luminous intensity, which is luminous flux in a particular direction. Candelas quantify the amount of luminous flux in an area defined by a solid cone originating from the light source. To convert lumens to candela, we need to divide the overall light intensity by the angular span of the source in steradians, a unit of measuring angle in 3D space. For a full sphere (no direction), the angular span is 4pi, and therefore, one candela equals 12.57 lumens from an omnidirectional light source. As the name suggests, a candela is the equivalent light intensity of one candle. An ideal light source emits light in all directions, but we often don’t need light in all directions. Candelas are especially important in directional lighting applications such as spotlights or automotive headlights and are usually measured after an LED has been installed in a final fixture with associated optics. The candela measurement enables us to quantify how much light is in the space where we need it to be.
Color rendering index (CRI) compares how a light source reveals the colors of objects relative to if the objects were illuminated by sunlight. CRI is a scale from 0 to 100, with 100 being ideal sunlight. Sunlight contains the full light spectrum, so objects lit by sunlight reflect their colors accurately. If a light source does not contain equal amounts of the entire light spectrum, objects lit by that source may appear dull or washed out if their colors are not properly lit.
Approaching the ideal CRI in LEDs requires precision engineering, so designers should consider the CRI level needed for their application. A CRI of 70–80 is adequate for high-power outdoor lighting. For indoor lighting, a CRI above 80 is considered good, with many consumer-grade bulbs offering a CRI of at least 90. Applications where color is critical, such as art galleries and high-end retail, use advanced LEDs with a CRI of 98.
Correlated color temperature (CCT), measured in Kelvin (K), specifies the color temperature of a light source on a scale from 1000K (candlelight) to 10,000K (blue sky). Light sources on the lower end of the scale (<3000K) are warmer-toned and have a softer glow. These lights are ideal for indoor home use or in places like restaurants where their warmer glow is comfortable and relaxing. Neutral whites around 3500K-4000K emulate daylight and work well in office and retail space. Bright whites at 5000K to 5700K emulate direct sunlight and provide bright, crisp light ideal for sports arenas, precision workplaces and warehouses. Cool whites at 6000K and above are used in indoor agricultural applications.
Thermal management is a crucial factor for designers to consider when using high-power LEDs. Half or more of the consumed current is turned into heat, which must be removed to ensure LED longevity.
Transients and surges also decrease the life expectancy of LEDs, so designers must use transient suppression techniques and devices on the LED driver board to block voltage and current spikes that can be damaging. Furthermore, drive current needs to be precisely controlled as well. Overdriving the LED can produce more heat than the thermal design can handle, meaning care must be taken when designing the LED driver board.
The operational environment needs to be accounted for as well. LEDs are used indoors and outdoors. Harsh environments need specialized designs and operating ambient temperatures and humidity can also affect performance and lifetime.
If not overstressed, the life expectancy for LEDs is much higher than filament bulbs. A typical filament bulb will last about 1000–1500 hours, whereas LEDs are rated for over 50,000 hours. Again, life expectancy will be significantly influenced by how well the driver board is designed to squelch spikes and surges, how well the heatsinks are drawing heat away from the LED component, and environmental factors such as ambient temperature.
In the face of numerous design challenges for LEDs, the Cree LED XLamp® XP-G LED set the new standard for breakthrough performance in 2009, producing up to 400 lumens at 1 Amp. This brightness and efficiency opened up large areas of lighting applications such as household lighting, track lighting, general commercial lighting and some recessed downlights with minimal heat and power requirements.
The XLamp® XP-G2 LED soon followed with a 20 percent higher lumen output, followed by the XP-G3 LED in 2016 with 31 percent higher lumens at 8 percent higher watts. The higher efficacies meant more light with less power and heat. This higher intensity makes these lights suitable for medical applications that need focused light at high intensity without much heat. Biomedical phototherapy applications can also take advantage of this compact intensity. On a larger scale, streetlights could replace costly gas-filled or halogen bulbs with LED equivalents. Stage lighting and spotlights could run cooler using less power. Improved lifetimes and reliability provided long-lasting performance for end users. The ceramic substrate and heatsink packaging made it easier for engineers to design with and manage thermally.
The newest, most modern family, the XLamp® XP-G4 LEDs (Figure 1), continue to push the envelope of performance and efficiency with 75 percent more lumens than the XP-G2 LEDs and an even lower thermal resistance that allows it to expel heat quicker. The superior thermal performance supports a current rating of 3A, compared to 1.5A for the XP-G2. The ability to safely and efficiently use more current enables designers to optimize for demanding applications, such as sports lighting.
Figure 1: Cree LED’s XLamp XP-G4 and XP-G4 High-Intensity White LEDs incorporate the latest advancements in high-power LED technology for improved optical performance while delivering industry-leading efficacy. (Source: Cree LED)
The XLamp XP-G4 LEDs emit light in a narrower, uniform cone, offering exceptional performance for directional lighting. A well-defined light emitting surface (LES) enables you to configure your optics to make the most of the light coming from the LED, reducing wasted lumens. With a wide variety of CRI options up to 90, the XP-G4 is suitable for a wide variety of applications, from handheld and indoor lighting to stadium lighting.
The agricultural industry can take advantage of the ability to finely tune light colors to create high-power grow light systems with targeted colors at higher intensities using less power and less heat. Specific lighting systems optimized for particular plants and crops can be made. The advanced heatsink technology and finely tunable spectrum provide a flexible upgrade over legacy grow lights.
By using the same package throughout the entire XP-G series, Cree LED has provided an effortless migration path for engineers to increase the capabilities of existing products and use them in new applications with enhanced brightness or spectral features. Alternatively, engineers can simply use the latest LEDs to make current lighting fixtures cooler at current brightness levels, thereby increasing overall reliability and life expectancy.
LEDs have come a long way from humble beginnings as indicator lights and calculator displays. With the ability to produce extremely bright light in a wide variety of colors, LEDs are a reliable and energy-efficient option for almost any lighting application. Cree LED’s expertise in LEDs brings you cutting-edge lighting technology as well as the knowledge of how to fit it into your application best and reduce your design cycle time.
After completing his studies in electrical engineering, Jon Gabay has worked with defense, commercial, industrial, consumer, energy, and medical companies as a design engineer, firmware coder, system designer, research scientist, and product developer. As an alternative energy researcher and inventor, he has been involved with automation technology since he founded and ran Dedicated Devices Corp. up until 2004. Since then, he has been doing research and development, writing articles, and developing technologies for next-generation engineers and students.
Cree LED, a Smart Global Holdings company, delivers best-in-class technology and breakthrough solutions for our focus applications: High Power General Lighting, Video Screens, and Specialty Lighting. Cree LED offers the industry’s broadest portfolio of application-optimized LEDs that lead the industry in lumen density, intensity, efficacy, optical control, and reliability.