LED filament bulbs are designed to replicate the visual look and emission pattern of incandescent lamps but use LED technology to deliver energy efficient lighting and long life, dependable operation. The history of electric lighting started out from incandescent lighting and has been punctuated by a series of major innovations, with the most prominent one being solid state lighting based on LED technology. Marrying together a vision of historical nostalgia and forward-thinking sustainability, LED filament bulbs are an inspiring union of timeless design and cutting-edge technology. These vintage-inspired LED bulbs are breathing new life into old-fashioned Edison style light bulbs. Their arrival dispelled people's concern over the extinction of ambience and aesthetics unique to incandescent lamps. No longer do nostalgists have to compromise on style to harvest the benefits of LED lighting.
The Everlasting Appeal of Incandescent Lighting
The rising popularity of LED filament bulbs is rooted in their nostalgic novelty. Tungsten filament bulbs remain a time-honored icon in lighting design. The classic cast of incandescent bulbs warms up dark spaces, creates shine and sparkle, and fosters a feeling of comfort and relaxation. The undeniable sense of nostalgia makes them the lighting candidates for retro- or industrial-inspired decors which can be found in a range of locations such as restaurants, hotels, bars, residential and retail spaces. There's no shortage of ways to dress up a space with Edison bulbs. Whether they're installed in pendants, chandeliers, wall sconces, or caged fixtures, it's hard not to be drawn in by the sight of glowing filaments through the glass envelops. The rustic simplicity delivered by bare or exposed lamping with Edison bulbs is tough to compete with. Infusing an iconic form and warm ambience, incandescent bulbs radiate an intriguing glow and a timeless flair that instantly enhances any space they resides in.
The Slipping Away of Quality Lighting
Most people love only the vintage look and warm ambience of incandescent lamps but have never realized these thermal radiators provide the highest quality of light, outshining all their successors so far. For the past two decades consumers have been misguided to embrace energy efficient lighting without being informed of the deterioration in light quality. The introduction of fluorescent lamps signaled the departure from high quality light that humans had enjoyed ever since the invention of incandescent bulbs by Thomas Edison. The poor color rendering, high flicker and excessively high color temperature of fluorescent lighting significantly affected the interaction between the human visual system and the lighted environment.
Humans have an inherent need to make sense of what they see. Incandescent lighting carries a spectrum of light that faithfully reproduces the colors of objects or scenes being illuminated. In contrast, the colors of objects illuminated by fluorescent lamps appear distorted. Abnormally high flicker is unacceptable because it can trigger a nervous system response and lead to visual perception problems and health consequences. Before the advent of fluorescent lighting light flicker is seldom an issue for incandescent lighting because thermal radiators have a relatively long persistence.
LED Filament Bulbs: Beyond the Vintage Look
The quality of light is worth taking seriously. When evaluating an LED filament bulb more emphasis should be placed on the light it produces. LED filament bulbs are designed to rival incandescents in looks whilst outperforming them in the aspects of energy efficiency and operational life. However, the key part of lighting is often ignored by both uneducated consumers, unethical lighting manufacturers, and ill-informed regulatory bodies. The assessment metrics of light quality for LED lamps followed the standards established for evaluating fluorescent lamps. As a result, the color quality of light (color rendering) and visual comfort of lighting (flicker control) are on the barely acceptable level. When you are buying an LED filament bulb, you are essentially buying the look of the incandescent bulb as well as the high quality of light indigenous to incandescent lighting. Despite being technologically advanced, the light quality of LED lamps remains a serious concern as with fluorescent lamps. It's a common practice in the lighting industry to sacrifice light quality for a higher luminous efficacy and lower cost. LED filament bulbs are no exceptions.
The preference for color appearance of white light altered by fluorescent lighting can be more of a concern for human health. The color appearance of white light affects people's emotional response to how they experience spaces and perceive objects. Most significantly, it's indicative of the content of biologically effective light that influences circadian entrainment. Warm white light with high red component emitted from an incandescent lamp encourages relaxation and regeneration, while exposure to blue-rich cool white light emitted from fluorescent lamps suppresses the melatonin production. Nighttime suppression of melatonin production may disrupt circadian rhythms and impair the vital protective mechanism that suppresses developing cancer cells in our body.
LED filament bulbs have a construction very similar to incandescent bulbs in that they typically comprise a filament assembly mounted on a glass stem, a glass envelop, a gas fill, and a base. They only exception is that there is a driver circuit in the base of an LED filament bulb. The glass stem is mounted with multiple straight LED filaments or one flexible LED filaments. The filaments are arranged in a way that simulates the light pattern of tungsten filaments in incandescent lamps. The filaments are supported by lead-in wires which also deliver electric current to the filaments. The lead-in wire consists of three sections. The section that runs from the stem press to the filament is made of nickel or nickel-plated copper. The lead-in wires that run through the stem press are made of a nickel-iron alloy core with a copper sleeve (Dumet wire). This ensures that the wires have the same coefficient of expansion with the glass so that no air-leaking gap will be created within the wire passage when the stem press is heated. The section of lead-in wires that run from the stem press to the driver is made of copper.
The stem assembly is fused into the glass envelop. Air in the glass envelop is evacuated and a thermally conductive gas is introduced through the discharge tube of the glass stem which is then sealed by a flame. The driver is anchored to the stem via the lead-in wires and is electrically insulated from the lamp base with an insulation sleeve. Since there is usually not enough room in the lamp base to accommodate a driver that has a bulky component like a smoothing capacitor, a plastic extender is added between the base and the glass envelop to make room for the capacitor. The glass envelop and driver assembly are capped by a lamp base which is typically fabricated from nickel-plated aluminum, brass or iron. For use in wet locations, high purity copper (98% min. copper) or nickel-plated brass (64% min. copper) bases are required in order to resist corrosion from humidity. Typical filament bases include medium screw bases (E26, E27), miniature candelabras (E11, E12), and double contact bayonet bases (B22).
The Fundamental Differences between the Two Types of Filament Bulbs
Notwithstanding the similarity in construction, LED filament bulbs operate in a completely different way from incandescent bulbs. Incandescent bulbs are thermal radiators that emit optical radiation when the tungsten filament is heated to incandescence by the passage of electric current. LED filament bulbs are equipped with semiconductor devices which employ the principle of injection electroluminescence to generate light. Specifically, when electrical current is passed through the p-n junction of an LED in the forward direction, holes and electrons are injected into the active region where they recombine to release energy in the form of photons (packets of light). This mode of operation brings about a quantum leap in the efficiency of conversion from electrical to optical power. Compared with the efficiencies of incandescent bulbs that typically run in the single digits, LEDs have an efficiency approaching 50%.
Incandescent bulbs are simply voltage-driven devices that respond to only the change in supply voltage and can tolerate current fluctuations. The filament of an incandescent bulb is basically a resistor. In contrast, LEDs are current-driven semiconductor devices and extremely sensitive to both current and voltage fluctuations. They have fundamental challenges with line and load regulation. The light emitted from a tungsten filament is broad spectrum warm white light that is of very high quality and directly applicable to the human eye. Conversely, LED chips emit in narrow spectral bands having a typical wavelength distribution of a few tens of nanometers, resulting in a color light that cannot be used for general or task lighting. The narrow spectral emission of an LED therefore requires a wavelength converter, which partially or completely converts the electroluminescence to provide broad spectral bandwidth white light. As a byproduct of converting electricity into light, incandescent lamps radiate much of the heat in the form of infrared (IR) energy. LEDs are self-heating devices that dissipate all the thermal energy within the device packages. Lack of thermal management or inadequate thermal management will result in efficiency degradation and premature failure. For LED filament bulbs to be a dependable light source that produces high quality light and delivers all the benefits unique to LED lighting, many technical complexities need to be addressed and many trade-offs need to be balanced.
An LED filament is a linear LED package that is designed to produce an emission pattern resembling that of a tungsten filament. The package architecture of the LED filament is similar to chip-on-board (COB) packages which consist of a large array of small LED dies (chips) mounted onto a metal-core printed circuit board (MCPCB) or a ceramic substrate. However, LED filaments typically use a transparent substrate such as a thin glass or sapphire in order to allow light to be radiated in all directions. As such, these products are often referred to as Chip-on-Glass (COG) packages. A strait LED filament is typically about an inch long. The linear array of low-power LED chips are connected in a series configuration in order to achieve an identical current flow in each LED and uniform light output across the length of the filament. Once the LED chips are connected together in series and bonded to a substrate, the linear assembly is coated with a phosphor-impregnated polymer matrix which diffuses light and provides down-conversion. Different formulations of phosphor coating give distinctive color characteristics (color rendering and color temperature) to the light. Electrical pads are fastened to the ends of the substrate, one on each end, to complete the electrical path to the string of LED chips.
Image courtesy of LEDVANCE GmbH
Two types of LED chips are used to fabricate LED filaments: conventional LED chips and flip chips. Conventional LED chips are GaN-on-Sapphire chips with laterally spaced electrodes. Wire bonding is used to apply a bias across the epitaxially grown GaN layers and connect the string of LED chips. Flip chips are designed with the sapphire substrate side facing up and the P-type epitaxial layer is the bottom layer. The positive and negative electrodes which also serve as thermal conduction pads are located on the bottom of the LED. The flip chip technology offers tremendous advantages over conventional wire-bonded chip technology. Wire bonding of lateral chips may cause current crowding, which can affect the injection efficiency and pose risks of thermal runaway. The amount of heat transfer that takes place via the bond wire is significantly limited. By comparison, mounting both P-type and N-type contacts on the bottom of the LED allows flip chips to be assembled directly on SMT lines for higher throughput and eliminates the need of wire bonding which is the weakest point in conventional chips. The use of large-surface electrode pads which are in direct contact with the epitaxial layers allows for homogeneous distribution of current density and high efficiency thermal transfer. Thus flip chips can be driven at higher currents and deliver higher light output than conventional chips.
The filament substrate can be made with sapphire, glass, ceramic, or a metal such as aluminum or copper. Ideally the substrate should be highly thermally conductive and transparent. Sapphire and glass have high transparency but have their own trade-offs. Sapphire is single crystal aluminum oxide which is colorless and optically clear. It's thermally stable and thermally conductive (46 W/mK). Mechanically, sapphire has high wear and scratch resistance, high modulus of elasticity and high tensile strength. These qualities make sapphire the benchmark substrate material for LED filaments. However, there's a cost challenge with sapphire.
Glass is used as a low cost alternative to sapphire substrates. Nevertheless, the poor thermal conductivity (1.4 W/mK) of silicon dioxide (SiO2) glass makes the thermal path along the substrate virtually of no use. Additionally, glass is fragile and cannot be processed to have a thin profile. The glass substrate usually has a thickness of 1 mm to 2 mm, whereas sapphire substrates can be made very thin (0.8 mm to 1 mm). Filament substrates can also be made from transparent ceramic which excels in thermal conduction but is challenged with cost and processability. Metal substrates are less commonly used in LED filaments due to their opacity. However, they have compensating advantages such as high thermal conductivity, high ductility, and low manufacturing costs. These substrates usually have a hollow structure which allows light to leak through the substrates.
Flexible LED filaments use thin strips of a flexible electrically insulating material, such as polyimide (PI), polyether ether ketone (PEEK), polyester (PET), polyethylene napthalate (PEN), polyetherimide (PEI) as the substrates.
The rate at which the LED filament will age is highly dependent on the temperature at the p-n junction and the phosphor coating. Continuous operation of LED filaments at elevated temperature dramatically accelerates growth of defects in the semiconductor crystalline structure, phosphor thermal degradation, carbonization of the polymer material on the chip surface, encapsulant yellowing, etc. These failure mechanisms result in lumen depreciation and color shifts, which end up in shortened lifetime.
The thermal path from the semiconductor chips of LED filaments to the ambient environment typically has a higher thermal resistance compared with the thermal path of other types of LED lamps. Conventional LED bulbs mount SMD LEDs on a printed circuit board which is in thermal contact with a metallic heat sink. LED filament bulbs by their nature do not include a heat sink in the design. The heat generated by the LED filament has to be conducted by air within the glass envelop. Air is a very poor conductor of heat. LED filament bulbs are usually filled with a highly thermally conductive gas to facilitate thermal conduction. Helium is the most commonly used gaseous thermal conductor for its higher thermal conductivity compared to that of other common gases such as nitrogen, neon, argon, or krypton. The use of helium allows the glass envelop itself to serve as a heat sink.
However, the inherent thermal design limitation of LED filament bulbs makes it challenging to drive the LED filament to its maximum current limit. Inadequate thermal management leads to a reduction in the operational life of LED filament bulbs. Compared with other LED products, LED filament bulbs typically have a lower luminous efficacy (around 100 lm/W) and shorter lifespan (10,000 - 20,000 hours).
The driver that is located in the base of the LED filament bulb is physically small compared to the circuits that drive other types of LED lamps. Accordingly there is a need to provide an LED driver that is small in size and has a low component count. Three types of driver circuits have been used to operate LED filament bulbs: switching power supplies, linear power supplies, and capacitive power supplies. Among them capacitive power supplies or capacitive droppers are the lowest cost and simplest driver solution with the least component count. This type of drivers utilize the capacitive reactance of a capacitor to step down the mains voltage to a lower voltage. A linear power supply is a constant current device that adjust its resistance to maintain a set current. A pass transistor operates as a variable resistor in the linear region to drop the input voltage down to the desired output voltage. The switching circuit or switching mode power supply (SMPS), on the other hand, regulates a constant current output by varying the frequency or duty cycle of a saturated power switch. Constant current circuits for LED filament bulbs often use integrated circuit (IC) chips to reduce the footprint.
Both capacitive and linear circuits are step-down drivers that require careful matching between input and output voltages to keep power loses minimized. Despite its popularity in low cost designs, the capacitive circuit is being phased out mainly because of its poor transient response and output quality. Linear power supplies find a very strong niche in LED bulbs for their ability to produce constant current output without using bulky and expensive reactive components. As with capacitive droppers linear circuits are lower in power conversion efficiency (80 - 85%) and thus dissipate a large amount of heat. Although there's some improvement in output quality when compared with capacitive droppers, the residual ripple in the output current of linear circuits can cause invisible flicker in the 100-120 Hz range. Linear circuits generate no electromagnetic interference (EMI), which makes it possible to reduce both the circuit cost and size. These resistive circuits are compatible with resistive loads run from phase-control (leading/trailing edge) dimmers.
Switching circuits deliver high efficiency power conversion, tight current regulation, and excellent transient suppression. These drivers can be designed to generate minimal ripples in the output current provided to the LED load, which allows for low flicker lighting. Light flicker is one of the major light quality issue in LED bulbs. The cost-driven design and space constraint of LED bulbs frequently lead to compromised flicker control. Flicker mitigation involves the use of expensive, bulky capacitors to the smooth out the large output ripple. While visible flicker is rarely an issue in LED bulbs operated by linear drivers, invisible flicker generated from these bulbs is also undesirable. The invisible flicker at a frequency above 80 Hz can trigger nervous system responses in a small percentage of the population and cause migraine, headaches, or visual impairment.
Aside from high efficiency and high quality output, SMPS drivers offer a variety of features not available in capacitive and linear circuits. Common switching regulator configurations include buck, boost, or a combination of buck and boost (buck-boost). Unlike linear and capacitive circuits which cannot compensate for power that drops below the output voltage, switching circuits that use the buck-boost topology have universal input voltage capability for both high input voltage operation and high input current operation. Switching circuits may be designed with a dimming functionality to provide variable light output over an entire dimming range. However, the compatibility between SMPS drivers and conventional dimmers may be a problem unless the drivers are designed to recognize and respond to the voltage signals from phase control dimming circuits.
The major challenge of using switching circuits is the trade-off between performance variables (efficiency, output quality, EMI filtering, input range, galvanic isolation, etc.), size and cost. SMPS drivers are more expensive compared with other solutions and may require an additional housing (such as a plastic extender) to shield the cumbersome circuit components i.e. capacitors. The lifetime of the driver is heavily dependent on the reliability of electrolytic capacitors. The capacitor life is inversely correlated the operating temperature and the ripple current flowing through it.
One of the hallmark features of incandescent lighting is its excellent color rendering performance. Light emitted by hot filaments delivers radiant power fairly broadly across the visible spectrum. The continuous spectrum of wavelengths contains all the emissions required to render a wide range of colors. Incandescent and halogen lamps carry a minimum color rendering index (CRI) of 97 and a strong R9 (a saturated deep red color). By comparison, LED filament bulbs generally underperform the tungsten filament bulbs with regards to color rendition. Most LED filament bulbs have a mediocre CRI (Ra 80) and their spectral power distribution (SPD) is deficient in wavelengths for rendering saturated colors (R9 - R14).
High color rendering lighting with LED filament bulbs is not an unaffordable luxury. The reason behind the low color quality of mass-produced LED filament bulbs is that there's a trade-off between color render performance and luminous efficacy. To achieve a color quality comparable to that of incandescent lamps, a significant portion of blue emission from the InGaN dies of blue pump LEDs needs to be down-converted to longer wavelengths by a phosphor mixture. There is a considerable amount of energy loss (Stokes loss) occurring during this wavelength conversion process, which is called photoluminescence. The low eye sensitivity over the down-converted wavelengths compounds the loss in luminous efficacy.
Correlated Color Temperature (CCT)
When the incandescent lamp is operated at full rated power, its light exhibits a correlated color temperature (CCT) in the range of 2700 K to 3300 K which is referred to as a "warm white" appearance. The warm glow that is reminiscent of the sunset or a flame is best suited to restaurants, hospitality and residential spaces wanting to create an inviting, restful and intimate environment. Since LED filament bulbs are designed as incandescent replacements, they are typically designed to have a CCT falling in the same range. LED filament bulbs with an excessively high CCT (e.g. 4500 K or above) are not recommended, although they have a higher luminous efficacy as compared with lower CCT light bulbs. Higher color temperatures generally indicate there's a more significant blue component in white light. Nighttime exposure to bright light with a high blue component can suppress the release of melatonin, a hormone produced by the pineal gland that promotes a better quality of restorative sleep. Suppressing melatonin production in the evening and at night or a change in the timing of melatonin secretion can lead to circadian disruption and subsequent health effects.
The glass envelop of LED filament bulbs come in a variety of shapes:
- A - Arbitrary spherical tapered to narrow neck (A15, A17, A19, A21, A23)
- ST - Straight-tipped shape (ST15, ST18, ST19, ST20, ST52, ST58, ST64)
- G - Globe shape (G14, G16, G16.5, G19, G25, G30, G40, G50, G63)
- R - Reflector (R50, R63, R80)
- T - Tubular shape (T10, T14, T19, T20)
- B - Bullet shape, blunt tip (B8, B10, B11, B13)
- BA - Bulged with angular (bent) tip (BA10, BA11)
- CA - Candle shape with bent tip (CA5, CA7, CA8, CA10, CA11, CA17)
- BT - Bulged tubular (BT15, BT28, BT37, BT56)
- PS - Pear shape with straight neck (PS25, PS35)