Table of Contents Hide
- 1 How Do LEDs Work?
Advantages of LED Lighting
- 2.1 Energy efficiency
- 2.2 Optical delivery efficiency
- 2.3 Illumination uniformity
- 2.4 Directional illumination
- 2.5 Spectral engineering
- 2.6 On/off switching
- 2.7 Dimming capability
- 2.8 Controllability
- 2.9 Design flexibility
- 2.10 Durability
- 2.11 Product life
- 2.12 Photobiological safety
- 2.13 Radiation effect
- 2.14 Fire and explosion safety
- 2.15 Visible light communication (VLC)
- 2.16 DC lighting
- 2.17 Cold temperature operation
- 2.18 Environmental impact
- 3 Disadvantages and Challenges of LED Lighting
- 4 Trickiness of LED Lighting
The global lighting market has been undergoing a radical transformation driven by the massively growing adoption of light emitting diode (LED) technology. This solid state lighting (SSL) revolution fundamentally altered the underlying economics of the market and dynamics of the industry. Not only different forms of productivity were enabled by SSL technology, the transition from conventional technologies towards LED lighting is profoundly changing the way people think about lighting as well. Conventional lighting technologies were designed primarily for addressing the visual needs. With LED lighting, the positive stimulation of biological effects of light on people's health and well-being is drawing increasing attention. The advent of LED technology also paved the way for the convergence between lighting and the Internet of Things (IoT), which opens up a whole new world of possibilities. Early on, there has been a great deal of confusion about LED lighting. High market growth and huge consumer interest create a pressing need to clear the doubts surrounding the technology and to inform the public of its advantages and disadvantages.
How Do LEDs Work?An LED is a semiconductor package comprising an LED die (chip) and other components that provide mechanical support, electrical connection, thermal conduction, optical regulation, and wavelength conversion. The LED chip is basically a p-n junction device formed by oppositely doped compound semiconductor layers. The compound semiconductor in common use is gallium nitride (GaN) which has a direct band gap allowing for a higher probability of radiative recombination than semiconductors with an indirect band gap. When the p-n junction is biased in the forward direction, electrons from the conduction band of the n-type semiconductor layer move across the boundary layer into the p-junction and recombine with holes from the valence band of the p-type semiconductor layer in the active region of the diode. The electron-hole recombination causes the electrons to drop into a state of lower energy and release the excess energy in the form of photons (packets of light). This effect is called electroluminescence. The photon can transport electromagnetic radiation of all wavelengths. The exact wavelengths of light emitted from the diode is determined by the energy band gap of the semiconductor.
The light generated through electroluminescence in the LED chip has a narrow wavelength distribution with a typical bandwidth of a few tens of nanometers. Narrow-band emissions result in light having a single color such as red, blue or green. In order to provide a broad spectrum white light source, the width of the spectral power distribution (SPD) of the LED chip must be broadened. The electroluminescence from the LED chip is partially or completely converted through photoluminescence in phosphors. Most white LEDs combine short wavelength emission from InGaN blue chips and the re-emitted longer wavelength light from phosphors. The phosphor powder is dispersed in a silicon, epoxy matrix or other resin matrixes. The phosphor containing matrix is coated onto the LED chip. White light can also be produced by pumping red, green and blue phosphors using an ultraviolet (UV) or violet LED chip. In this case, the resulting white can achieve superior color rendering. But this approach suffers from a low efficiency because the large wavelength shift involved in the down-conversion of UV or violet light is accompanied with a high Stokes energy loss.
Advantages of LED LightingThe invention of incandescent lamps well over a century ago revolutionized artificial lighting. At present, we are witnessing the digital lighting revolution enabled by SSL. Semiconductor-based lighting not only delivers unprecedented design, performance and economic benefits, but also enables a plethora of new applications and value propositions previously thought impractical. The return from harvesting these advantages will strongly outweigh the relatively high upfront cost of installing an LED system, over which there is still some hesitation in the marketplace.
Energy efficiencyOne of the main justifications for migrating to LED lighting is energy efficiency. Over the past decade, luminous efficacies of phosphor-converted white LED packages have increased from 85 lm/W to over 200 lm/W, which represents an electrical to optical power conversion efficiency (PCE) of over 60%, at a standard operating current density of 35 A/cm2. Despite the improvements in the efficiency of InGaN blue LEDs, phosphors (efficiency and wavelength match to the human eye response) and package (optical scattering/absorption), the U.S. Department of Energy (DOE) says that there remains more headroom for PC-LED efficacy improvements and luminous efficacies of approximately 255 lm/W should be practically possible for blue pump LEDs. High luminous efficacies are unquestionably an overwhelming advantage of LEDs over traditional light sources—incandescent (up to 20 lm/W), halogen (up to 22 lm/W), linear fluorescent (65-104 lm/W), compact fluorescent (46-87 lm/W), induction fluorescent (70-90 lm/W), mercury vapor (60-60 lm/W), high pressure sodium (70-140 lm/W), quartz metal halide (64-110 lm/W), and ceramic metal halide (80-120 lm/W).
Optical delivery efficiencyBeyond significant improvements in light source efficacy, the ability to achieve high luminaire optical efficiency with LED lighting is less well-known to general consumers but highly desired by lighting designers. The effective delivery of the light emitted by light sources to the target has been a major design challenge in the industry. Traditional bulb-shaped lamps emit light in all directions. This causes much of the luminous flux produced by the lamp to be trapped within the luminaire (e.g. by the reflectors, diffusers), or to escape from the luminaire in a direction that is not useful for the intended application or simply offensive to the eye. HID luminaires such as metal halide and high pressure sodium generally are about 60% to 85% efficient at directing light produced by the lamp out of the luminaire. It is not uncommon for recessed downlights and troffers that use fluorescent or halogen light sources to experience 40-50% optical losses. The directional nature of LED lighting allows effective delivery of the light, and the compact form factor of LEDs allows efficient regulation of luminous flux using compound lenses. Well-designed LED lighting systems can deliver an optical efficiency greater than 90%.
Illumination uniformityUniform illumination is one of the top priorities in indoor ambient and outdoor area/roadway lighting designs. Uniformity is a measure of relationships of the illuminance over an area. Good lighting should ensure uniform distribution of lumens incident over a task surface or area. Extreme luminance differences resulted from non-uniform illumination can lead to visual fatigue, affect task performance and even present a safety concern as the eye needs to adapt between surfaces of difference luminance. Transitions from brightly illuminated area to one of very different luminance will cause a transitional loss of visual acuity, which has large safety implications in outdoor applications where a vehicle traffic is involved. In large indoor facilities, uniform illumination contribute to high visual comfort, permits flexibility of task locations and eliminates the need of relocating luminaires. This can be particularly beneficial in high bay industrial and commercial facilities where substantial cost and inconvenience are involved in moving luminaires. Luminaires using HID lamps have a much higher illuminance directly below the luminaire than areas farther away from the luminaire. This results in a poor uniformity (typical max/min ratio 6:1). Lighting designers have to increase fixture density to ensure the illuminance uniformity meets the minimum design requirement. In contrast, a large light emitting surface (LES) created from an array of small-sized LEDs produces light distribution with a uniformity of less than 3:1 max/min ratio, which translates to greater visual conditions as well as a significantly reduced number of installations over the task area.
Directional illuminationBecause of their directional emission pattern and high flux density, LEDs are inherently suited to directional illumination. A directional luminaire concentrates light emitted by the light source into a directed beam that travels uninterrupted from the luminaire to the target area. Narrowly focused beams of light are used to create a hierarchy of importance through the use of contrast, to make select features to pop out from the background, and to add interest and emotional appeal to an object. Directional luminaires, including spotlights and floodlights, are widely used in accent lighting applications to enhance the prominence or highlight a design element. Directional lighting is also employed in applications where an intense beam is needed to help accomplish demanding visual tasks or to provide long range illumination. Products that serve this purpose include flashlights, searchlights, followspots, vehicle driving lights, stadium floodlights, etc. An LED luminaire can pack enough of a punch in its light output, whether to create a very well defined "hard" beam for high drama with COB LEDs or to throw a long beam far out in the distance with high power LEDs.
Spectral engineeringLED technology offers the new capability to control the light source's spectral power distribution (SPD), which means the composition of light can be tailored for various applications. Spectral controllability allows the spectrum from lighting products to be engineered to engage specific human visual, physiological, psychological, plant photoreceptor, or even semiconductor detector (i.e., HD camera) responses, or a combination of such responses. High spectral efficiency can be achieved through maximization of desired wavelengths and removal or reduction of damaging or unnecessary portions of the spectrum for a given application. In white light applications, the SPD of LEDs can be optimized for prescribed color fidelity and correlated color temperature (CCT). With a multi-channel, multi-emitter design, the color produced by LED luminaire can be actively and precisely controllable. RGB, RGBA or RGBW color mixing systems which are capable of producing a full spectrum of light create infinite aesthetic possibilities for designers and architects. Dynamic white systems utilize multi-CCT LEDs to provide warm dimming that mimics the color characteristics of incandescent lamps when dimmed, or to provide tunable white lighting that allows independent control of both color temperature and light intensity. Human centric lighting based on tunable white LED technology is one of the momentums behind much of the latest lighting technology developments.
On/off switchingLEDs come on at full brightness almost instantly (in single-digit to tens of nanoseconds) and have a turn-off time in the tens of nanoseconds. In contrast, the warm up time, or the time which the bulb takes to reach its full light output, of compact fluorescent lamps can last up to 3 minutes. HID lamps require a warm-up period of several minutes before providing usable light. Hot restrike is of much greater concern than initial start-up for metal halide lamps which were once the principal technology employed for high bay lighting and high power floodlighting in industrial facilities, stadiums and arenas. A power outage for a facility with metal halide lighting can compromise safety and security because the hot restrike process of metal halide lamps takes up to 20 minutes. Instant start-up and hot restrike lend LEDs in a unique position to effectively carry out many tasks. Not only general lighting applications benefit greatly from the short response time of LEDs, a wide range of specialty applications are also reaping this capability. For example, LED lights may work in synchronization with traffic cameras to provide intermittent lighting for capturing moving vehicle. LEDs switch on 140 to 200 milliseconds faster than incandescent lamps. The reaction-time advantage suggests that LED brake lights are more effective than incandescent lamps at preventing rear-impact collisions. Another advantage of LEDs in switching operation is the switching cycle. The lifespan of LEDs is not affected by frequent switching. Typical LED drivers for general lighting applications are rated for 50,000 switching cycles, and it's uncommon for high performance LED drivers to endure 100,000, 200,000, or even 1 million switching cycles. LED life is not affected by rapid cycling (high frequency switching). This feature makes LED lights well suited to dynamic lighting and for use with lighting controls such as occupancy or daylight sensors. On the other hand, frequent on/off switching may shorten the life of incandescent, HID, and fluorescent lamps. These light sources generally have only a few thousands of switching cycles over their rated life.
Dimming capabilityThe ability to produce light output in a very dynamic way lends LEDs perfectly to dimming control, whereas fluorescent and HID lamps do not respond well to dimming. Dimming fluorescent lamps necessitates the use of expensive, large and complex circuitry in order to maintain the gas excitation and voltage conditions. Dimming HID lamps will lead to a shorter life and premature lamp failure. Metal halide and high pressure sodium lamps cannot be dimmed below 50% of the rated power. They also respond to dimming signals substantially slower than LEDs. LED dimming can be made either through constant current reduction (CCR), which is better known as analog dimming, or by applying pulse width modulation (PWM) to the LED, AKA digital dimming. Analog dimming controls the drive current flowing through to the LEDs. This is the most widely used dimming solution for general lighting applications, although LEDs may not perform well at very low currents (below 10%). PWM dimming varies the duty cycle of the pulse width modulation to create an average value at its output over a full range from 100% to 0%. Dimming control of LEDs allows to align lighting with human needs, maximize energy savings, enable color mixing and CCT tuning, and extend LED life.
ControllabilityThe digital nature of LEDs facilitates seamless integration of sensors, processors, controller, and network interfaces into lighting systems for implementing various intelligent lighting strategies, from dynamic lighting and adaptive lighting to whatever IoT brings next. The dynamic aspect of LED lighting ranges from simple color changing to intricate light shows across hundreds or thousands of individually controllable lighting nodes and complex translation of video content for display on LED matrix systems. SSL technology is at the heart of large ecosystem of connected lighting solutions which can leverage daylight harvesting, occupancy sensing, time control, embedded programmability, and network-connected devices to control, automate and optimize various aspects of lighting. Migrating lighting control to IP-based networks allows intelligent, sensor-laden lighting systems to interoperate with other devices within IoT networks. This opens possibilities for creating a wide array of new services, benefits, functionalities, and revenue streams that enhance the value of LED lighting systems. The control of LED lighting systems can be implemented using a variety of wired and wireless communication protocols, including lighting control protocols such as 0-10V, DALI, DMX512 and DMX-RDM, building automation protocols such as BACnet, LON, KNX and EnOcean, and protocols deployed on the increasingly popular mesh architecture (e.g. ZigBee, Z-Wave, Bluetooth Mesh, Thread).
Design flexibilityThe small size of LEDs allows fixture designers to make light sources into shapes and sizes suited for many applications. This physical characteristic empowers the designers with more freedom to express their design philosophy or to compose brand identities. The flexibility resulted from direct integration of light sources offers possibilities to create lighting products that carry a perfect fusion between form and function. LED light fixtures can be crafted to blur the boundaries between design and art for applications where a decorative focal point is commanded. They can also be designed to support a high level of architectural integration and blend in any design composition. Solid state lighting drives new design trends in other sectors as well. Unique styling possibilities allow vehicle manufacturers to design distinctive headlights and taillights that give cars an appealing look.
DurabilityAn LED emits light from a block of semiconductor—rather than from a glass bulb or tube, as is the case in legacy incandescent, halogen, fluorescent, and HID lamps which utilize filaments or gases to generate light. The solid state devices are generally mounted on a metal core printed circuit board (MCPCB), with connection typically provided by soldered leads. No fragile glass, no moving parts, and no filament breakage, LED lighting systems are therefore extremely resistant to shock, vibration, and wear. The solid state durability of LED lighting systems has evident values in a variety of applications. Within an industrial facility, there are locations where lights suffer from excessive vibration from large machinery. Luminaires installed alongside roadways and tunnels must endure repeated vibration caused by heavy vehicles passing by at a high rate of speed. Vibration makes up the typical working day of work lights mounted on construction, mining and agricultural vehicles, machinery and equipment. Portable luminaires such as flashlights and camping lanterns are often subject to impact of drops. There are also many applications where broken lamps present a hazard to occupants. All these challenges demand a rugged lighting solution, which is exactly what solid state lighting can offer.
Product lifeLong lifetime stands out as one of the top advantages of LED lighting, but claims of long life based purely on the lifetime metric for the LED package (light source) can be misleading. The useful life of an LED package, an LED lamp, or an LED luminaire (light fixtures) is often cited as the point in time where the luminous flux output has declined to 70% of its initial output, or L70. Typically, LEDs (LED packages) have L70 lifetimes between 30,000 and 100,000 hours (at Ta = 85 °C). However, LM-80 measurements that are used for predicting the L70 life of LED packages using the TM-21 method are taken with the LED packages operating continuously under well controlled operating conditions (e.g. in a temperature-controlled environment and supplied with a constant DC drive current). By contrast, LED systems in real world applications are often challenged with higher electrical overstress, higher junction temperatures, and harsher environmental conditions. LED systems may experience accelerated lumen maintenance or outright premature failure. In general, LED lamps (bulbs, tubes) have L70 lifetimes between 10,000 and 25,000 hours, integrated LED luminaires (e.g. high bay lights, street lights, downlights) have lifetimes between 30,000 hours and 60,000 hours. Compared with traditional lighting products—incandescent (750-2,000 hours), halogen (3,000-4,000 hours), compact fluorescent (8,000-10,000 hours), and metal halide (7,500-25,000 hours), LED systems, in particular the integrated luminaires, provide a substantially longer service life. Since LED lights require virtually no maintenance, reduced maintenance costs in conjunction with high energy savings from the use of LED lights over their extended lifetime provide a foundation for a high return on investment (ROI).
Photobiological safetyLEDs are photobiologically safe light sources. They produce no infrared (IR) emission and emit a negligible amount of ultraviolet (UV) light (less than 5 uW/lm). Incandescent, fluorescent, and metal halide lamps convert 73%, 37%, and 17% of consumed power into infrared energy, respectively. They also emit in the UV region of the electromagnetic spectrum—incandescent (70-80 uW/lm), compact fluorescent (30-100 uW/lm), and metal halide (160-700 uW/lm). At a high enough intensity, light sources that emit UV or IR light may pose photobiological hazards to the skin and eyes. Exposure to UV radiation may cause cataract (clouding of the normally clear lens) or photokeratitis (inflammation of the cornea). Short duration exposure to high levels of IR radiation can cause thermal injury to the retina of the eye. Long-term exposure to high doses of infrared radiation can induce glassblower's cataract. Thermal discomfort caused by incandescent lighting system has long been an annoyance in the healthcare industry as conventional surgical task lights and dental operatory lights use incandescent light sources to produce light with high color fidelity. The high intensity beam produced by these luminaires delivers a large amount of thermal energy that can make patients very uncomfortable.
Inevitably, the discussion of photobiological safety often focuses the blue light hazard, which refers to a photochemical damage of the retina resulting from radiation exposure at wavelengths primarily between 400 nm and 500 nm. A common misconception is that LEDs may be more likely to cause blue light hazard because most phosphor converted white LEDs utilize a blue LED pump. DOE and IES have made it clear that LED products are no different from other light sources that have the same color temperature with respect to the blue light hazard. Phosphor converted LEDs do not pose such a risk even under strict evaluation criteria.
Radiation effectLEDs produce radiant energy only within the visible portion of the electromagnetic spectrum from approximately 400 nm to 700 nm. This spectral characteristic gives LED lights a valuable application advantage over light sources that produce radiant energy outside the visible light spectrum. UV and IR radiation from traditional light sources not only poses photobiological hazards, but also leads to material degradation. UV radiation is extremely damaging to organic materials as photon energy of radiation in the UV spectral band is high enough to produce direct bond scission and photooxidation pathways. The resulting disruption or destruction of the chromophor can lead to material deterioration and discoloration. Museum applications require all light sources that generate UV in excess of 75 uW/lm to be filtered in order to minimize irreversible damage to artwork. IR does not induce the same type of photochemical damage caused by UV radiation but can still contributes to damage. Increasing the surface temperature of an object may result in accelerated chemical activity and physical changes. IR radiation at high intensities can trigger surface hardening, discoloration and cracking of paintings, deterioration of cosmetic products, drying out of vegetables and fruits, melting of chocolate and confectionery, etc.
Fire and explosion safetyFire and exposition hazards are not a characteristic of LED lighting systems as an LED converts electrical power to electromagnetic radiation through electroluminescence within a semiconductor package. This is in contrast to legacy technologies which produce light by heating tungsten filaments or by exciting a gaseous medium. A failure or improper operation may result in a fire or an explosion. Metal halide lamps are especially prone to risk of explosion because the quartz arc tube operates at high pressure (520 to 3,100 kPa) and very high temperature (900 to 1,100 °C). Non-passive arc tube failures caused by end of life conditions of the lamp, by ballast failures or by the use of an improper lamp-ballast combination may cause the breakage of the outer bulb of the metal halide lamp. The hot quartz fragments may ignite flammable materials, combustible dusts or explosive gases/vapors.
Visible light communication (VLC)LEDs can be switched on and off at a frequency faster than the human eye can detect. This invisible on/off switching ability opens up a new application for lighting products. LiFi (Light Fidelity) technology has received considerable attention in the wireless communication industry. It leverages the "ON" and "OFF" sequences of LEDs to transmits data. Compared current wireless communication technologies using radio waves (e.g., Wi-Fi, IrDA, and Bluetooth), LiFi promises a thousand times wider bandwidth and a significantly higher transmission speed. LiFi is considered as an appealing IoT application due to the ubiquitousness of lighting. Every LED light can be used as an optical access point for wireless data communication, as long as its driver is capable of transforming streaming content into digital signals.
DC lightingLEDs are low voltage, current-driven devices. This nature allows LED lighting to take advantage of low voltage direct current (DC) distribution grids. There is an accelerating interest in DC microgrid systems which can operate either independently or in conjunction with a standard utility grid. These small-scale power grids provide improved interfaces with renewable energy generators (solar, wind, fuel cell, etc.). Locally available DC power eliminates the need for equipment-level AC-DC power conversion which involves a substantial energy loss and is a common point of failure in AC powered LED systems. High efficiency LED lighting in turn improves the autonomy of rechargeable batteries or energy storage systems. As IP-based network communication gains momentum, Power over Ethernet (PoE) emerged as a low-power microgrid option to deliver low voltage DC power over the same cable that delivers the Ethernet data. LED lighting has clear advantages to leverage the strengths of a PoE installation.
Cold temperature operationLED lighting excels in cold temperature environments. An LED converts electrical power into optical power through injection electroluminescence which is activated when the semiconductor diode is electrically biased. This start-up process is not temperature-dependent. Low ambient temperature facilitates dissipation of the waste heat generated from LEDs and thus exempts them from thermal droop (reduction in optical power at elevated temperatures). In contrast, cold temperature operation is a big challenge for fluorescent lamps. To get the fluorescent lamp started in a cold environment a high voltage is needed to start the electric arc. Fluorescent lamps also lose a substantial amount of its rated light output at below-freezing temperatures, whereas LED lights performs at their best in cold environments—even down to -50°C. LED lights therefore are ideally suited for use in freezers, refrigerators, cold storage facilities, and outdoor applications.
Environmental impactLED lights produces notably less environmental impacts than traditional lighting sources. Low energy consumption translates to low carbon emissions. LEDs contain no mercury and thus create less environmental complications at end-of-life. In comparison, the disposal of mercury-containing fluorescent and HID lamps involves the use of strict waste disposal protocols.
Disadvantages and Challenges of LED LightingDon't get excited over the wealth of benefits offered by LED lighting. While this technology is definitely a landmark achievement in the history of electric lighting, it raises problems of its own. The lighting industry is facing a challenge in a magnitude it has never had to deal with before. Solid state lighting altered the philosophy of design and engineering. Lighting systems are no longer dumb illuminants, they have evolved into power electronics. In other words, the design of lighting systems is unprecedentedly complex. LEDs are self-heating, current-sensitive and luminously intensive semiconductor light sources. This gives rise to the largest concern of LED Lighting—the performance and reliability of an LED system heavily rely on a multidimensional work. The LED package metrics are just one aspect of the holistic design and systems engineering of an LED lighting system. Many other interdependent factors come into play, including thermal management, drive current regulation, and optical control.
Armchair experts often compile a long list of disadvantages for LED lighting. And to make the story sensational they would never forget to mention that LED lighting can induce blue light hazards. White light is basically a mixture of wavelengths from different color bands. All whites with the same color appearance, regardless of the light sources from which the light is emitted, have roughly the same proportion of blue wavelengths across the visible spectrum. The color appearance of white light may be characterized as having a correlated color temperature (CCT). The blue content of a light source generally corresponds to its CCT. The higher the CCT the higher the proportion of blue wavelengths. Under the same luminance and illuminance conditions, blue radiation from a 3000 K LED product is as low as that from a 3000 K incandescent lamp, and blue radiation from a 6000 K LED product is as high as that from a 6000 K fluorescent lamp. As with other light sources, the blue light hazard is rarely a concern for white LEDs. The ability to engineer the spectral composition of white light is a huge advantage of LED technology. With LED lighting, any spectral composition of light that positively contributes to human health and well-being can be produced. Human centric lighting, a major technology trend that is driving the growth of the lighting industry, harvests the CCT tuning capability of LED systems to adjust the amount of blue radiation for a healthy spectrum of white light.
In fact, LED lighting has only a few intrinsic disadvantages.
The most well-known weakness of LED lighting is that LEDs produce a byproduct—heat. LEDs are called sell-heating devices because they generate heat within the device package—rather than radiating heat in the form of infrared energy. Around half of the electrical energy fed to an LED is converted into heat, which must be conducted and convected through a physical thermal path. Failure to maintain the device junction temperature below a set limit may accelerate the kinetics of failure mechanisms such as atomic defect generation and growth in the active region of the diode, carbonization and yellowing of the encapsulant, and discoloration of plastic package housing. Beyond the maximum rated junction temperature, the service life of an LED will be reduced by 30% to 50% for every 10 ° C rise in junction temperature.
The most unknown, and also the biggest weakness of LED lighting is that LEDs are delicate power electronics. They are extremely picky about their food—drive current. For LEDs, their high sensitivity to forward current is a double-edged sword. It gives lighting systems superior controllability but also makes drive current regulation enormously challenging. A very small change in drive current will cause the light output to fluctuate. LEDs are DC-driven devices, however they often have to be fed with an AC power source. Incomplete suppression of the alternating waveform after rectification may result in a residual ripple (residual periodic variation) in the current output from the driver to the LEDs. This ripple causes the LEDs to flicker at twice the frequency of the incoming line voltage, i.e. 100Hz or 120Hz. The electrical and thermal interdependence of LEDs also adds complexity to load regulation. As junction temperature rises, forward voltage decreases, the electrical power delivered to the LED is also reduced. On the other hand, the higher the drive current the greater the waste heat generated at the semiconductor die. Overdriving what an LED is rated for may lead to early failure of the LED due to thermal runaway. Nevertheless, the most damaging threat to LEDs comes from electrical overstresses (EOS). An EOS occurs when the drive current or voltage exceeds the component maximum rated values. There are many possible sources of electrical overstresses, which include electrostatic discharge (ESD), inrush current, or other types of transient power surges. The vulnerability of LEDs to various types of electrical stresses therefore necessitates tight regulation of the drive current.
A third disadvantage is that LEDs have a high flux density. The concentrated light sources of directional light can potentially create glare. High luminances in the field of view interfere with seeing (disability glare) or cause a sensation of irritation or pain (discomfort glare). Additional optics to mitigate glare can be incorporated into the luminaires design, but they often result in high optical loss.
The last but not least, increased complexity of system design leads to a higher first cost of LED products when compared with legacy lighting products. This makes cost optimization an important part of the luminaire design process. When the cost pressure outweighs performance and reliability of the products, a stream of problems will arise.
Trickiness of LED LightingWith cost being the ever-present concern, no LED lighting solution today is without compromise. The fact that LED products are built on a holistic framework heightens the trickiness in the lighting market. The design and engineering of LED systems, in a way, are about coping with trade-offs between different aspects of lighting (e.g. cost, efficiency, lighting quality, lifespan). While ethical lighting manufacturers address or reduce these trade-offs through innovative design and advanced engineering, there are a number of unethical players that cut corners and game the technology.
System efficiencyWhen thermal management is effectively applied, the system efficiency of an LED lighting product is the cumulative efficiency of its LEDs, driver and optics. The efficacy of LED packages should not be equated with the efficacy of an LED luminaire. There can be cases where LEDs have a luminous efficacy of 200 lm/W but the luminaire has a luminous efficacy of 100 lm/W only. Such a high efficacy discrepancy between the light source and lighting system can be attributed to inefficient power conversion, ineffective light delivery, or a combination of both. Increasing power conversion efficiency of the AC-DC driver and optical delivery efficiency is, therefore, another way to improve lighting efficiency. The use of low cost driver circuits is the primary cause of abnormally low system efficiency. Linear power supplies, for example, have been tremendously favored by manufacturers of entry-level products. These driver circuits have significantly less circuit parts count and thus a considerably lower cost in comparison with switching power supplies. One of the problems with a linear regulator, however, is its low power conversion efficiency because it operates by dissipating excess power as heat.
The efficacy of an LED lighting system can undergo a rapid drop due to the use of low-performing LEDs, inadequate thermal management, overdriving, or a combination of them. Rapid lumen depreciation often occurs in lighting systems that uses mid-power plastic LED packages. These light sources have a high initial efficacy thanks to their high reflectivity plastic cavities and leadframe plating. But the plastic housing made of synthetic resins such as PPA and PCT will discolor when exposed to high temperatures. Operating these LEDs at elevated junction temperatures due to overdriving and/or inefficient heat dissipation will accelerate thermal degradation in packaging materials and lead to a permanent reduction in light output.
System reliabilityThe reliability of an LED system will be determined by all its constitutive parts and their ability to survive environmental or operational stresses. While the majority of parametric failures of LEDs such as lumen depreciation and color shifts of are temperature-dependent, the majority of catastrophic failures of LEDs are driver-dependent. An LED system is only as good as its weakest link, and the driver is usually this link. The driver is the heartbeat of an LED system as it undertakes to execute many sub-tasks sequentially or in parallel. Among these sub-tasks protecting LEDs from power surges and poor incoming power quality is especially important since the catastrophic failures of LEDs are often caused by EOS events. LED drivers typically make use of electrolytic capacitors to absorb the surge energy, smooth out the large output current ripple, and filter EMI. The lifespan of electrolytic capacitors is heavily dependent on the operating temperature and the ripple current flowing through it. This makes the driver itself to be frequently the first component of an LED system to fail because low cost drivers rarely use high operating temperature capable capacitors. Linear power supplies operate without using electrolytic capacitors and hence have a higher circuit reliability. However, the LEDs operated by these circuits are subject to electrical overstresses.
Lighting qualityAmong the variables that contribute to the quality of lighting, flicker control and color quality are often traded for lower cost system designs. As previously discussed flicker occurs when there are large ripples in the DC current. Low cost driver solutions such as single-stage SMPS circuits or linear regulators generally do not perform well in ripple suppression. The lighting industry also indulges in offering lighting products that have a low color rendering index (CRI) and high color temperature. This is due to the presence of the trade-off between luminous efficacy and spectral quality. To deliver high color rendering lighting, the light source must spread radiant power uniformly across the visible spectrum. This involves down-conversion of a large amount of short wavelength photons (e.g. blue photons). The larger the amount of short wavelength photons is converted the higher the Stokes energy loss. This results in a lower efficacy for LEDs with a higher color rendering performance.
Despite suffering from additional energy loss associated with wavelength conversion, the efficacy of LEDs with a high CRI is already sufficiently high to strike a balance between energy efficiency and color rendering. Most LED products designed for interior lighting are still being offered with an underperforming color quality. The light produced by these products is good at rendering medium-saturated colors but deficient in wavelengths for reproducing saturated colors.
Similarly, for an LED to produce warm white light a considerable amount of short wavelength light emitted by the LED chip needs to be down-converted into longer wavelength light. This causes phosphor converted LEDs with a cool white appearance to deliver a higher efficacy than those with a warm white appearance. Lighting products with a high color temperature (e.g. 6,000 K - 6,500 K) are heavily pushed in markets where consumers are less aware of the biological effect of blue-enriched cool white light. Nocturnal melatonin suppression due to exposure to high CCT light can disrupt circadian rhythms and results in negative health consequences.