Table of Contents Hide
- 1 What Is an LED Light Bulb
- 2 New Technology in Old Package
- 3 A-type Bulbs
- 4 How LEDs Work
- 5 Technological Advantages
- 6 Miserable Reality
- 7 Design and Construction
- 8 Thermal Management
- 9 LED Driver
- 10 Dimming Control
- 11 Flicker
- 12 Light Source
- 13 Color Rendering
- 14 Correlated Color Temperature (CCT)
What Is an LED Light BulbAn LED light bulb is a solid state lighting (SSL) device designed as a replacement for a traditional incandescent or fluorescent light source. Since Edison's invention of the incandescent lamp, lighting had been exclusively delivered by light bulbs and all light fixtures had been designed to accommodate light bulbs. With the advent of LED lighting, the form of lighting has been fundamentally changed. The term "light source" has been redefined to include small LED packages, which are blocks of semiconductor devices. The unique optical, electrical and physical characteristics of LEDs kicked off a trend to directly integrate LEDs into light fixtures, rather than designing fixtures around a lamp or a group of lamps. As compared to lamp-based traditional light fixtures, integrated performance luminaires allow for more optimized light distribution, efficient thermal management, sophisticated lighting control, and inspiring or architecturally adaptive fixture design.
New Technology in Old PackageThe trend of designing LED luminaires towards integrated systems does not signal the end of light bulbs. Despite the phase-out of incandescent and fluorescent lamps, the form factor and light distribution pattern of legacy light sources are still influential today. A considerable number of light fixtures that were designed to use legacy light sources need to be upgraded to LED technology. To facilitate an easy and low cost lighting upgrade, the LED lamp retrofits must have exactly the same supply adaptor with as well as the physical configuration and light distribution as close as possible to the lamps to be replaced. LED bulbs are designed as a plug-and-play solution which allows conventional light fixtures to be upgraded to the latest LED technology without any need of electrical or structural modification to the fixtures. Aside from the retrofit application, many light fixtures that carry a classic design thrive on the use of light bulbs, and many utility lights still rely on the use of light bulbs to simplify user maintenance.
A-type BulbsMore often than not, the term "light bulb" refers to a very common general service lamp (GSL) or general lighting service (GLS) which is also known as an A-type bulb (“A” is the first letter of “arbitrary spherical tapered to narrow neck”). Even though gas-discharge CFL bulbs (compact fluorescent lamps) did not inherit the shape of incandescent and halogen light bulbs, A-shape bulbs remain incredibly popular to this day and the LED replacements for both incandescent and CFL lamps brought back the classic design of tungsten filament light bulbs. Light bulbs may come in other shapes such as bulged (B), conical (C), elliptical (E), flame (F), globular (G), mushroom (M), pear (P), reflector (R), straight-sided (S) and tubular (T). However, the A-type bulbs constitute the predominant part of lamps to be retrofitted in the residential sector. These bulbs find their use on table lamps, floor lamps, pendants, ceiling lights, ceiling fans, wall sconces, and even bare-bulb applications. Consequently, it makes sense to look into the design and engineering of A-type LED lamps.
The A-type light bulb is defined as a GSL bulb with a shape having a spherical end section that is joined to the neck by a radius. The radius has a center outside the bulb and a magnitude greater than radius of the spherical section. The radius is tangent to both the neck and the curve of the spherical end section. The A19 bulb (or its metric equivalent, the A60 bulb) is the most sold model among the A-series family which also includes A15, A17, A21 and A23 lamps. The number in each code refers to the maximum diameter of the bulb in eighths of an inch. An A19 lamp is 19 eighths of an inch in diameter at its widest point, which is 2-3/8" inches or 60 mm. The A19 bulb is approximately 4-3/8 inches or 110 mm in length. A one-inch long base allows the A-type bulb to be used in sockets commonly found in residential, commercial, and industrial fixtures. The E26 (26 mm in diameter) medium screw base is used in United States, Canada, Japan, Mexico, and most of the countries in Central America. The E27 (27 mm in diameter) medium screw base is designed for bulbs sold in China, Europe, the United Kingdom, Argentina, Russia, India, Australia, Brazil, many other countries in Asia, Africa and South America. The twist type B22 bayonet base is less commonly these days and finds its adoption in some former members of the British Empire, e.g. the United Kingdom, Australia.
How LEDs WorkA light-emitting diode has a p-n junction sandwiched between two oppositely doped layers of semiconductor material. When the p-n junction is forward-biased, electrons from the n-junction will drop down from the conduction band and move across the boundary layer into the p-junction. Holes from the valence band of the p-junction migrate across the junction in the opposite direction. Electrons and holes recombine in the active region of the diode, releasing energy in the form of photons. This effect is called injection electroluminescence. The photons produced through electroluminescence have typical bandwidths of a few tens of nanometers and thus appear a single color. The monochromatic light must be partially or completely phosphor-converted to broaden the bandwidth of the emitted light which is perceived as white light by the human eye. The highest efficiency LEDs today are made from indium gallium nitride (InGaN) which has an external quantum efficiency (EQE) as high as 60%. Most white LEDs are therefore InGaN-based blue pump LEDs which are capable of delivering luminaire efficacies greater than 200 lm/W.
Technological AdvantagesLED technology itself is unquestionably superior to conventional lighting technologies. The transition from traditional lighting to LED lighting is a huge endeavor, but a number of features unique to LED lighting will justify the benefit of this transition.
- In contrast to the miserable efficiencies of incandescent (1.9-2.6%, typically) and halogen (2.6-3.5%) blubs, and the relatively low efficiency of CFL bulbs (8-11%), the wall plug efficiency (WPE) of 40% to 50% makes LEDs particularly appealing.
- The rated service life of LEDs last from 30,000 to as long as 100,000 burntime hours, which is considerably longer than that of conventional light sources. Because the filament heated to a sufficiently high temperature by an electric current passing through it, incandescent and halogen lamps have short lamp lives (1,000 - 5,000 hours). The lifespan of compact fluorescent lamps is limited by the electrodes which are used to excite a gaseous medium. These lamps have a rated life of around 10,000 hours.
- Tailoring the spectral quality of white light became more convenient with LED technology. The spectral composition of light is a key component that goes into the design of interior lighting. The spectral power distribution (SPD) of white light determines the color of light as well as the ability of a light source to faithfully reproduce the colors of various objects. Traditional light sources have very limited spectral tunability, while LEDs can spectrally tuned to produce light in any correlated color temperature (CCT) and meet any color rendering requirement.
- LEDs designed for general lighting applications produce light only in the visible spectral region. No infrared (IR) radiation and no ultraviolet (UV) emission, LED lighting is well suited for use by people with a specific sensitivity for UV radiation and does not pose retinal thermal hazard. Incandescent lamps radiate a large amount of heat in the form of infrared light, fluorescent lamps emit a small amount of ultraviolet light.
- The semiconductor nature of LEDs allows instant on/off control and considerably more switching cycles throughout the rated life. In contrast, traditional lighting does not provide instant brightness, and frequent on/off switching shortens the lifetime of lamps. The excellent controllability of LEDs is also manifested in their dimming performance. LEDs can be controlled to provide variable light output using phase control dimmers, analog dimming or digital dimming circuits.
- The solid state nature of LEDs allows LED lamps to provide greater resistance to shock, vibration, and wear. No filament, glass or tube to break, enhanced durability increases the lifespan of LED lamps significantly.
Miserable RealityUnfortunately, the benefits of LED lighting cannot be reaped with ease. There is a fundamental tradeoff between the cost and operational reliability for LEDs. There is also an intrinsic tradeoff between color quality and luminous efficacy for LEDs. An LED lighting system is interdependent upon the thermal, electrical, and control system applied. Therefore, an LED lamp is a multidimensional engineering work that demands a holistic approach. LED bulbs are commodity products that are sold to general consumers. Most consumers are uneducated and thus they're unable to gauge the quality of LED bulbs. The consumer market is highly sensitive about the price, especially considering light bulbs are mass produced to the point that low prices became a common practice. As a result, lighting manufacturers try to compete on price, instead of selling value.
LED bulbs currently available on the market can't be worse. If you're looking for a light bulb that is to be used on a table lamp, a floor lamp or an overhead light fixture, as an industry insider I would rather suggest you to buy an incandescent bulb. Notwithstanding a low energy efficiency, the technically simple incandescent lamps can definitely give you a peace of mind. In fact, incandescent lamps had been an affordable luxury in the history of artificial lighting. Incandescent lighting provides a spectrum of light that exhibits qualities closest to natural daylight. Fluorescent lighting deprived the human world of the welfare to enjoy fantastic color experiences. LED bulbs, which should have done better in this respect, are deficient in key wavelengths that are important to bring out saturated colors. Incandescent lamps do not produce flicker, whereas fluorescent and LED lamps have been challenged to eliminate flicker and this challenge is often not well addressed in cheap products. Lighting manufacturers also cut corners on thermal management which is critical to the long life and high drive current operation of an LED bulb. This means the lifespan and efficacy benefits of LED lighting are significantly compromised in LED bulbs.
U.S. President Donald Trump complained that energy-efficient light bulbs make him look unnatural. His administration rolled back the efficiency standards that would have phased out incandescent light bulbs. Opponents criticized the rule change, arguing that the reversal could lead to higher electric bills and increased greenhouse gas emissions. These critics have never realized that among all lighting products developed using LED technology, LED bulbs are literally rubbishes when it comes to light quality and system life. Energy incentive and rebate programs such as the Design Lighting Consortium (DLC) and Energy Star place their value purely in the luminous efficacy, while ignoring critical factors that contribute to quality lighting and the sustainability of lighting. These factors include flicker, color rendition, lumen maintenance, color stability, system reliability, and even safety. For LED bulbs, virtually all parameters in these regards are on the barely acceptable end.
What's more, the efficacy advantage of LED bulbs over their predecessors is not as apparent as that of other types of LED systems. The use of cheap driver electronics results in a poor circuit efficiency. The fairly large power conversion loss, combined with the thermal constraint and optical diffusion loss, translates to an extremely low efficacy of LED bulbs. The typical luminous efficacy of currently available LED bulbs is 90 lm/W, which is only slightly higher than that of CFL bulbs (46-87 lm/W). In contrast, industrial and outdoor LED systems usually have an efficacy greater than 140 lm/W thanks to the synergistic combination of high quality light source, high efficiency power regulation, and effective thermal management.
Another serious concern is that many of the LED bulbs sold on the market come with a high color temperature. This is designed to maintain a high luminous efficacy. A high CCT generally corresponds to a relatively high proportion of blue wavelengths in the visible spectrum. Nighttime exposure to blue-enriched white light leads to circadian disruption which has a negative influence on health.
Design and ConstructionA typical A-type LED bulb uses a curved "snow cone" design in which the "cup" serves as the housing of the bulb. A printed circuit board (PCB) slides into a plastic carrier at the foot of the housing. The driver board connects to the LED assembly through two tabs which are soldered. The circuitry driving the LEDs is comprised of various discrete components mounted on two sides of the circuit board. The PCB may be potted into the housing to provide protection against mechanical stresses, to support the screw base of the bulb and the contact foot, and to improve conduction of heat generated by the key power components to the surface of the housing.
A critical role of the housing is to provide heat sinking and draw heat away from the LEDs. The housing can be made from aluminum, ceramics, or plastics. Most LED bulbs come with a polycarbonate (PC) housing which has an aluminum lining designed to increase the heat spreading area. A disc-shaped aluminum heat sink sits atop the conic aluminum lining and competes the heat sink assembly of the light bulb. The PC/aluminum construction is a result of cost reduction in the driver circuitry and thermal management system. The plastic housing is primarily designed to provide electrical insulation. In low-cost designs, the driver circuit does not include galvanic isolation from the input circuit. When the output circuit is not galvanically isolated from the mains, touching a metal housing may result in a lethal electrical shock hazard.
The LED assembly is a circuit board of SMD LEDs. The circuit board on which an array of discrete LEDs are reflow soldered is very often a metal core printed circuit board (MCPCB). The MCPCB consists a copper trace layer, a dielectric layer, and an aluminum substrate. MCPCBs offer high through-board thermal conductivity while providing dielectric isolation. The LED board is attached to the aluminum disk via a thermal interface material (TIM) which is designed to maximize heat transfer between two mating surfaces. A domed PC diffuser distributes luminous flux from the high intensity LEDs evenly to all directions thus eliminates bright spots of the light source and reduces glare. However, the diffusion will lead to a 15% optical loss. LED bulbs are not as omnidirectional as incandescent bulbs as the housing blocks light transmission. These bulbs usually distribute light in a beam angle less than 330°.
Thermal ManagementLED bulbs have plummeted in price and the ex-works prices cost as little as less than a dollar. The drop in cost comes largely at a at the expense of shortened lifespan and compromised light quality. One of the key engineering points that has been sacrificed in LED bulbs is thermal management. LEDs are self-heating devices. Currently, the efficiency of energy conversion from electrical power to white light is less than 45%. This means more than a half of the electrical power fed to the LEDs is not utilized, but converted into heat. Since LEDs do not radiate heat in the form of infrared energy, any heat generated by a LED must be dissipated through the device package itself.
What makes thermal management in LED bulbs particularly challenging is that a significant portion of products use linear power supplies. Linear power supplies are a low cost driver solution but can cause heat dissipation problems because of low efficiency. They are typically implemented as a driver-on-board (DOB) solution. The driver components are mounted onto the same circuit board with the LEDs. Therefore, the waste heat generated from linear power regulation introduces an additional thermal stress to the co-located LEDs. Since linear power supplies operate with a less-than-85% efficiency, the amount of heat that has to be dissipated from the LED board is substantial.
The ability of an LED bulb to draw heat away from the LED junction is critical to achieving the reliability expected from LED lighting. Overheating of the LEDs may accelerate nucleation and growth of dislocations in the active region of the LED, resulting in efficiency degradation. Thermal buildup can cause the materials in the package can discolor, which is a dominant chromaticity shift and lumen depreciation mechanism in mid-power LED packages.
LED bulbs used to be equipped with very large and heavy aluminum heat sinks which provide effective heat dissipation but at a high material cost. As such, the heat sink became one of the cost optimization targets in LED bulbs and this is accompanied with impaired thermal management. To squeeze every penny possible out of the cost structure, the size of aluminum heat sink used in LED bulbs has been reduced to a point that the lamp life has been severely affected. Not only the aluminum heat sinks became more lightweight than ever, in some cheap LED bulbs even the aluminum lining that helps spread heat from the LED assembly is eliminated to reduce cost. The use of plastic housing further compromises thermal management efficiency. For a thermal management system to perform to its full capacity, the heat sink design needs to be optimized to provide high efficiency thermal conduction and convection. The plastic housing, which has a very low thermal conductivity, impedes the conductive and convective transfer of heat away from the LEDs.
LED light bulbs that are being sold as commodity products pose significant design problems in managing thermal equilibrium. The bulb form factor provides limited space for the heat sink. These entry-level products provide very little investment in heat sinks. The increasing use of low efficiency linear power supplies with a DOB design means that the LEDs are stressed by a higher-than-normal thermal load. The thermal transfer rate (conduction and convection) falls behind the rate at which thermal energy is introduced to the LEDs. As a result, LEDs used in light bulbs experience high thermal degradation, which leads to poor lumen maintenance and color stability.
Commodity-grade LED bulbs have a very limited operational life when compared with commercial, industrial and outdoor LED systems which have a lifespan typically well over 50,000 hours. The nominal life cited by manufacturers is typically 10,000 to 25,000 hours. In real life applications, the lifespan of LED bulbs may be much shorter than what lighting manufacturers claim. Don't believe in 5-year or 10-year warranties, even if they're offered by big brands. These meaningless warranties are based on the limited hours of operation each day (e.g. 2 - 3 hours of operation per day). Heavy duty use (longer time operation) will accelerate the kinetics of thermal degradation and often result in premature failures.
Constant current LED drivers are typically designed as switched mode power supplies (SMPS) which may operate by using either pulse-width modulation (PWM) or pulse-frequency modulation (PFM) control for switching regulation of the power switching transistor(s). High conversion efficiency, high power quality, high flexibility output control, wide input voltage range and the ability to provide I/O circuits isolation are the main reasons for designers to use SMPS drivers. SMPS drivers are designed in a single-stage or double-stage configuration depending on the integration of power factor correction (PFC) and DC/DC converter circuit. In cost sensitive, space constrained retrofit lamp applications, the single-stage design is favored because it saves 20-50% of the circuit parts, size and cost, as opposed to the two-stage solution. Single-stage LED drivers may employ circuit topologies such as flyback, buck converters, and buck/boost.
The use of switching power supplies, however, must take into account the electromagnetic interference (EMI) filtering and screening because of the high-frequency switching noise. The lifetime of a switching driver circuit is heavily dependent on the electrolytic capacitor which is used as an energy storage component. The electrolytic capacitor is often the first component to fail in an LED driver because the electrolyte inside the capacitor evaporates fast under high operating temperature, which will cause the rise of equivalent series resistance (ESR) and the fall of capacitance. There're high operating temperature capable electrolytic capacitors, which may have a lifetime of 10,000 h at 105°C and 40,000 h at 85°C, for example. But yet cost is always the issue.
Designing an SMPS driver for LED bulbs can be challenging due to the cost and space constraints. Linear power supplies, therefore, take the stage because they are a simple and cheap solution, and require considerably less components than SMPS drivers. A linear regulator operates in the linear region in which a series transistor works as a variable resistor, adjusting its resistance to maintain a set current. Linear regulation does not generate high frequency pulse noise and thus does not require additional circuits or complicated circuit design to filter out EMI radiation. Unlike SMPS drivers that count on large, reactive components and require a dedicated FR4 PCB board, linear circuits make use of compact integrated circuits (ICs) and discrete devices which can be mounted onto an MCPCB and thus share the circuit board, thereby saving on PCB cost.
Linear power supplies are inherently a choice as a result of low cost outweighing efficiency, performance, output quality, and even safety. There are many downsides with using linear power supplies. A linear power supply regulates power by dropping the input voltage down to the desired output voltage. The efficiency is low because the dropout voltage is usually high. Poor efficiency causes thermal management issues as the excess electrical power is dissipated in form of heat. This demands additional cooling capacity to dissipate heat out of the lamp, which is particularly important for DOB-based LED bulbs. However, it is inconceivable to equip a one-dollar lamp with a high performance heat sink. Linear power supplies step down a higher voltage on the input to a lower voltage on the output and cannot compensate for power that drops below the output voltage. For this reason they do not have universal input voltage capability. Another problem with linear regulation is that galvanic isolation between the input and the output circuits cannot be implemented with this driver solution. Hence, extra caution should be exercised to electrically isolate all metal contacts.
Dimming ControlIt is often desirable to dim the LED bulb to any desired brightness. LED bulbs, as a retrofit solution, are frequently required to work with phase cut dimmers which include leading edge (TRIAC) dimmers and trailing edge (ELV) dimmers. However, conventional dimmers are designed to drive resistive and inductive loads. In contrast, LED drivers that employ an SMPS present a reactive load to the dimmer. As a result, dimming LED lights with phase cut dimmers may incur a number of problems such as low end drop out, TRIAC misfiring, minimum load issues, dead travel, light flicker, and large steps in light output. Therefore, SMPS drivers must be designed to be compatible with resistive loads. Linear circuits, on the other side, work with conventional dimmers as they are variable resistance devices. The level of compatibility such as smoothness and dimming range (the range between minimum and maximum phase angles of a dimmer) for both switching and linear driver circuits may differ from dimmer to dimmer, depending on various factors, such as the model and/or type of dimmer. Despite the inherent compatibility with phase control, SMPS drivers can be designed to support analog and PWM dimming, allowing lighting to become more adaptive to user needs and integrated with sensors and processors more easily.
FlickerFlicker is a significant concern in LED bulbs because the dirt cheap price of these entry-level products is obtained by sacrificing quality of light, efficiency and reliability of the lamp. LEDs emit photons (packets of light) only when the p-n junction is forward biased and an electrical current flows across it. To create a steady and continuous source of light, the power fed to the LED load should not be interrupted. This means it is the power supplies that cause the flicker in LED lamps. The low cost requirement for LED bulbs force a fundamental compromise of power quality. Many cheap driver solutions fail to remove the residual sinusoidal waveform of the alternating mains voltage which falls below the forward voltage of LEDs within each half-wave of the sinusoidal mains voltage. As a result, the LED bulb is switched off at a frequency at twice the line frequency. The human eye perceive this phenomenon as flicker.
In general, visible flicker (amplitude modulation of light at frequencies below about 80 Hz) is not common in LED lamps, but invisible flicker which occurs at a higher frequency (e.g., 120 Hz or 100 Hz) can still produce a nervous system response and cause mild to severe health related issues. Blurred vision, eyestrain and compromised visual task performance are direct consequences of exposure to flicker. There is also a special at-risk population for which flicker is more than just annoying but can trigger symptoms such as headaches, migraine, and epileptic seizures. Both single-stage SMPS and linear driver circuits integrated in LED bulbs are usually inadequately designed in ripple suppression. LED bulbs with their power regulation provided by linear power supplies can exhibit a very high percentage of flicker due to due to incomplete suppression of the alternating waveform.
Standards for flicker for different applications and populations are yet to be established. That's why lighting manufacturers dare to sacrifice this specification during driver design. For mains voltage with a sinusoidal 60 Hz frequency, the percent flicker (120 Hz flicker) produced by LED bulbs should be less than 10%. The percent flicker (100 Hz flicker) shall not exceed 8% for LED bulbs powered by mains voltage with a sinusoidal 50 Hz frequency. 4 percent flicker or less at 120 Hz or 3 percent flicker or less at 100 Hz or is considered safe for flicker-sensitive populations.
Light SourceThe A-type LED bulbs generally use mid-power SMD LEDs which are low-cost, plastic leaded chip carrier (PLCC) packages, although there're vintage style light bulbs that use LED filaments to mimic the decorative look and omnidirectional light pattern of incandescent tungsten lamps. The most popular form factor of LED packages used in LED bulbs is 2835 in various power options. As expected, the mid-power LED packages used in light bulbs are typically of very low quality. Together with inadequately designed heat sinks and driver circuits, they contribute to the abnormally low cost as well as the crappy quality and short life of LED bulbs. The initial efficacy of the PLCC-type packages can look attractive as highly reflective materials are used in the packages to maximize light extraction efficiency. However, these LED packages can exhibit fast lumen depreciation under high temperatures. That is because mid-power LEDs based on the PLCC architecture have a less robust construction than ceramic-based high-power LEDs or chip scale packages (CSPs).
Most significantly, the packaging materials for LEDs used in LED bulbs are selected to meet the cost target, rather than the reliability and color quality criteria. The high efficacy of mid-power packages is the result of using a plastic housing with reflective sidewalls and a lead frame plated with a reflective metal. A major issue that accompanies this design is that problems of deterioration can arise with these materials. The LED housing is usually injection molded from resin-based materials such as polyphthalamide (PPA) or polycyclohexylenedimethylene terephthalate (PCT) which have poor thermal stability and photo-stability. At high temperatures and long operating times, the resin can discolor, crack, or delaminate, ultimately causing lumen depreciation and color shift. Plastic resins such as epoxy molding compound (EMC) and silicone molding compound (SMC) have an improved resistance to discoloration at higher temperatures, but they come with a higher cost. The corrosion of lead frame plating is another lumen depreciation and color shift mechanism in LEDs. Silver plating can interact with corrosive gases such as hydrogen sulfide (H2S) and lose reflectivity.
When thermally constrained LED packages operate in a thermally stressed environment, the result is obvious. Except for an electrical open circuit (e.g. electromigration, bonding wire breakage, electrostatic discharge) or an electrical short circuit, there is a very small probability of abrupt failures in LEDs. While the gradual reduction in lumen output is annoying, color shift can be frustrating. PLCC packages have a variety of color shift mechanisms. Many of them result in color shifts in the blue direction. An excessively high percentage of blue wavelengths in the light spectrum can pose photobiological hazards and cause disruption to the body's circadian system.
Color RenderingThe quality of the light emitted by an LED bulb affects the perceived colors of an illuminated object or scene. Color rendering describes how well a light source reveals the colors of various objects. There're many systems available to assess color rendering. The color rendering index (CRI) is universally employed to indicate the color reproduction performance of a lamp. However, the CRI calculations do not take into consideration the ability of the light source to faithfully reproduce highly saturated colors, commonly referred to as R9 through R14. Eight samples of low to medium chromatic saturation are employed to calculate the general color rendering index Ra. Being a flawed system, the CRI value is still a useful reference for the general consumers.
As previously suggested, an incandescent lamp, instead of an LED bulb, should be your choice if the quality of light is the priority. Being a thermal radiator incandescent lamps produce minimal flicker, as with sunlight. In a similar fashion incandescent lamps produce light with a spectral power distribution (SPD) that closely replicates that of sunlight. Sunlight has the highest CRI of approximately 100 and incandescent lamps have a CRI of greater than 97. This means with incandescent lighting the colors of objects that fall into everyone's field of view are accurately rendered. It is ridiculous that ever since fluorescent lighting was introduced to the market, high color fidelity lighting became a thing of the past and the 80 CRI was accepted to be the standard color rendering metric in interior lighting for domestic applications. Not only the average color fidelity of eight low-chroma reference colors dropped steeply, the SPD of the majority of 80 CRI light sources literally contains no wavelengths for rendering saturated colors. When illuminated by these low CRI light sources, colors of the objects in our sights appear distorted.
LED technology provides the opportunity to render all objects in a pleasant and natural way. LEDs can be packaged to produce broad-spectrum white light with color quality exactly matching that of natural daylight. Unfortunately, the color metrics of LED lamps still follow the standards established for fluorescent lighting. The color rendering performance of residential LED lights is as poor as fluorescent lamps. LED bulbs sold on the consumer market have only the minimum acceptable CRI and simply take no consideration for rendering saturated colors. Why do lighting manufacturers still produce low CRI LED bulbs while it's technically feasible to allow faithful color reproduction with LED lighting? The culprit is the push to boost energy efficiency. There's a trade-off between color rendering and lamp efficiency. However, this should not be an excuse for compromising color quality. Even with a minimum CRI of 90, the luminous efficacy of LED lamps is still much better than the 80 CRI CFL bulbs. Moreover, residential lighting products consume significantly less electrical power than commercial and industrial lights as they usually operate for a few hours, while commercial and industrial lighting lights have a considerably long length of running time. To sacrifice light quality for a slightly higher energy efficiency in household lighting applications is a cart-before-horse thinking. When people retreat to recharge and relax in their houses after long hours of work, they deserve high quality lighting.
The light sources of LED bulbs are in most cases blue pump LEDs which utilize blue InGaN LED dies to pump phosphors within the device package. Part of the short wavelength light emitted from the blue LED is down-converted into longer wavelength light. The unconverted blue light mix with the down-converted light to produce the desired white light. An LED with high color rendition delivers radiant power uniformly across the wavelength range of the visible radiation spectrum. This requires a large portion of blue light to be down-converted by a mix of phosphors. The wavelength down-conversion involves a Stokes energy loss. The higher the portion of blue light to be converted, the higher the Stokes energy loss. Along with the increase cost of using broadband phosphors and the reduced eye sensitivity over the SPD, high color rendering lighting does come with certain disadvantages, which include higher cost and lower luminous efficacy. White light can also be created using violet pump LEDs which involve full wavelength conversion of all the emitted short wavelength light. Violet pump LEDs are very expensive and are only used in high end products designed for applications where high color rendition is of particular importance.
Despite a higher light source cost and lower luminous efficacy associated with high CRI lighting, a minimum CRI of 90 should be considered when the light bulbs are installed on table lamps, floor lamps, pendant lights, or any light fixtures designed for visually intensive tasks or color-critical applications. Because the CRI does not reflect the rendition of saturated colors. R9, the special color rendering index for a deep red sample, should be taken note of. An R9 value of 25 is considered as acceptable, and R9 values above is considered excellent.
Correlated Color Temperature (CCT)The correlated color temperature (CCT), measured in degrees Kelvin (K), is a metric that relates to the appearance of light produced by a light source. White light exhibiting a CCT in the range of 2700 K to 3300 K are commonly referred to as warm white light. White light with a CCT between 3500 K and 4100 K is classified as neutral white light. White light with a CCT above 4100 K is referred to as having a "cool white" appearance. The perception of warmness and coolness of light colors affects people’s subjective assessment of an interior space, and this usually governs the selection of color temperature for a light source. In North America and most part of Europe, interior lighting for residential spaces uses light sources of 2700 K to 3300 K mostly because of that the "warm white" appearance of light from candle lights and incandescent lamps has been rooted in their culture. In many Asian, African, and South American countries, cool white is the usual choice of color temperature for interior lighting. CCT selection for the general consumers is usually driven by psychological concerns that are primarily cultural and possibly climate-related.
While there're no strict rules with regard to CCT selection. There is science behind the appearance of light. The color of light not only affects our emotions, moods, perception and visual performance, but also physiologically influences the functioning of human body. In the course of human evolution, the human body has developed the circadian system under the influence of the natural sequence of day and night. The circadian rhythm is the 24-hour biological process that regulates the production of crucial hormones. Light is the stimulant that signals day and night to the suprachiasmatic nucleus (SCN) in the brain via ipRGC photoreceptors. The SCN is master circadian clock which coordinates the biological process. Specifically, this stimulation is governed by the total dose of blue light (around 480 nm) reaching the ipRGCs. A high dose of blue light signals the brain to release dopamine, serotonin and cortisol while suppressing the production of melatonin. This programs the body for day mode. On the other side, the SCN will respond to darkness or blue-depleted light by signaling the release of melatonin, which allows the body to regenerate and repair.
Exposure to cool white light, which contains a large amount of blue wavelengths, can acutely suppress melatonin production during the nighttime. Repeated exposure to bright light with a high CCT will disrupt circadian rhythm. Circadian disruption is linked to a variety of negative effects on human health. In contrast, warm white light carries a very low amount of blue wavelength in the spectrum. This can minimize disruption to the body's circadian system. Therefore, warm white light should be the choice of CCT for residential lighting.
Again, there's a trade-off between low CCT light and lamp efficiency. A large amount of blue wavelengths need to be down-converted to longer wavelengths so that the light has a relatively reddish tone for a warm white appearance. That's why lighting manufacturers heavily push to uneducated consumers the sale of light bulbs with an extremely high CCT of 6000 K to 6500 K. Exposure to white light with a CCT in this range is highly disruptive to the human circadian rhythm.