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 does LED 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 Lighting

The 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.

1. Energy efficiency

One 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).

2. Optical delivery efficiency

Beyond 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%.

3. Illumination uniformity

Uniform 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.

4. Directional illumination

Because 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.

5. Spectral engineering

LED 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.

6. On/off switching

LEDs 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.

7. Dimming capability

The 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.

8. Controllability

The 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).

9. Design flexibility

The 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.

10. Durability

An 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.

11. Product life

Long 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).

12. Photobiological safety

LEDs 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.

13. Radiation effect

LEDs 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.

14. Fire and explosion safety

Fire 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.

15. 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.

16. DC lighting

LEDs 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.

17. Cold temperature operation

LED 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.

18. Environmental impact

LED 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.