An LED panel light is a low profile, fully luminous panel that utilizes edge-lit LED technology to deliver uniform, smooth and visually comfortable direct (downward) lighting. Functionally speaking, it is a flat panel troffer. Troffers are square, rectangular or linear light fixtures that are installed in the ceiling and distribute light downwards only. They are workhorses in offices, hospitals, schools and commercial facilities where overhead luminaires are the primary source of ambient and task lighting. The goal of lighting in these spaces is to enable occupants to see their visual tasks easily and comfortably while addressing economic and environmental concerns, and taking architectural considerations into account. For a long time however, this has been a mission impossible because of the inherent limitations of conventional lighting technologies.
The Dilemma of Direct Lighting with Conventional Optical Design
General lighting in commercial and institutional spaces is ubiquitously provided by direct-type luminaires which provide 90% to 100% downlight. In contrast to general diffuse and indirect lighting systems, direct-type luminaires are most efficient in delivering light to a horizontal task plane. Oftentimes they are the only option for spaces with low ceiling heights, which are found in buildings with suspended mechanical ceilings (i.e., drop ceilings). However, achieving quality lighting in task-laden spaces such as offices, classrooms and laboratories involves more than simply specifying the illuminance level. Mitigation of glare, shadows, and other undesirable visual effects is equally important. In interior spaces where people spend a long length of time working or learning, lighting is a critical element of the design that may enhance or degrade organizational productivity, task concentration, environmental and job satisfaction, social interactions, aesthetic perceptions, safety and security.
In the past, direct lighting with luminaires designed in a conventional way are challenged with improving uniformity and reducing discomfort glare. Various optical components, e.g. reflectors, diffusers, lens, and louvers, have been used to control glare from the unwanted angle of view or to reduce excessively high luminance of the emission interface. Optical systems for lamp-based fluorescent luminaries are rather bulky and inefficient. LEDs can be particularly challenging in designing direct-lit luminaires. By the very nature of their design and operation, LEDs are high flux density light sources that produce concentrated light output. Even with diffuse shielding, these pinpoint light sources can still create hot spots of focused light that would detract from the visual appeal of a fixture. High level of diffusion affects the LED light transmission due to large amounts of scattering loss. As such, conventional optical design of direct lighting involves various compromises.
Edge-lit technology capitalizes on light guide technology as well as characteristics unique to LEDs. An array of miniature LEDs are placed along two or four edges of a light guide panel (LGP). Light emitted by the LEDs is coupled into the LGP and transported within the light guide over a desired distance through total internal reflection (TIR). The light guide has interruption points which allow light captured by the light guide to escape. These light extraction points are distributed in a way to support uniform distribution of the escaped light behind a rear reflector. The light guide refracts the beams towards an opal diffuser which provides Lambertian distribution of the light. The optical combination of total reflection, refraction, and Lambertian scattering allows the high intensity light emitted from edge-mounted LEDs to be uniformly extracted and distributed across an emission surface.
With the advent of edge-lit LED lighting, there's never been a better time to ditch maintenance-heavy fluorescent troffers and also the visually unpleasant direct-lit LED luminaires. Edge-lit technology enables luminaire designers to create surface emission devices with LED point sources. Soft, pleasant luminance devoid of harsh shadows across the entire span of a light panel delivers unprecedented visual comfort that is impossible with conventional designs. Edge-lit LED panels produce extremely uniform light distributions that are highly desired in general lighting applications. The side-emission design allows for color mixing within the light guide, this addresses the color uniformity issue that can be evident in direct-lit LED luminaires when there're color deviations between LEDs.
In direct-lit LED luminaires that employ a diffuser, the LED module must be placed with a minimum distance away from the diffuser such as to avoid harsh hot spots of LEDs. Since edge-lit optical systems no longer need such a setback distance and the LEDs are side-mounted within the luminaire, LED panel lights can be made ultra-thin with a depth less than 10 millimeter. The ultra-thin profile allows installations in very shallow ceiling plenums.
An edge-lit LED panel light is comprised of a multi-layered optical assembly and an aluminum frame assembly. The multi-layered optical system typically includes a bottom diffuser, a light guide panel, and a white reflector. The optical assembly and a steel top backplate which protects the optical assembly are secured by a slotted aluminum frame. Inside the aluminum frame mounts linear LED modules with the light emitting surfaces of LEDs facing the entrance end of a light guide. The aluminum frame provides mechanical support for the optical assembly, accommodates the LED modules and shields the LEDs from direct view, and works as a heat sink to draw the waste heat away from the semiconductor junction of the LEDs.
Light Guide Panel (LGP)
The light guide plays a key role in the photometric performance of an edge-lit LED panel light. It undertakes to capture and transport light emitted by the LEDs and then to extract it out towards the desired direction in a uniform beam matrix. For the maximal capturing efficiency of LED light, the light entrance end of a light guide should be designed with a coupling interface that matches the radiation pattern and package configuration of the mating LEDs. A common practice is to place non-lensed SMD LED packages in close proximity to a polished coupling surface on an LGP with a thickness at least the same with the LES of LEDs. The TIR efficiency of the light guide is governed by the material refractive index and reflectance of the guide boundary surface. The higher the refractive index and reflectance, the better the TIR efficiency. The most important element of a light guide is the optical pattern of light extraction points. Light extraction is the primary factor in determining the efficiency of the light guide, as well as light distribution of the LED panel. The optical pattern can be laser etched, thermally embossed, injection molded, or printed. V-cut grooves, etched dots, printed dots, and pixel-based elements are the commonly used light extractions patterns on LGPs.
LGP with LEDs pumping from the side (Image courtesy of Yongtek)
LGPs are made from optically clear polymers, such as polycarbonate (PC) or acrylic (PMMA). Polycarbonate offers superior thermal stability, ignition resistance and durability as compared to acrylic resins. However, acrylic is a leading material choice for LGPs because of its relatively low cost, high light transmittance and good UV stability. The downsides of acrylic are its higher tendency to discolor under the conditions of high operating temperature and high water absorption. While acrylic LGPs has a useful life of 4 to 8 years depending on the operating environment, LGPs made of polystyrene (PS) yellow in two years due to the poor photostability and thermal performance of the polystyrene polymer. Despite their high probability of rapid polymer discoloration that literally announces the end of luminaire life, PS LGPs are still widely used in LED panel lights made for the entry-level market simply because of their significantly lower cost as opposed to PC and acrylic LGPs.
Edge-lit LED panel lights use mid-power LEDs of various types, including SMD 2835, 3014, 4014, 3528, 5630, 2016, etc. These LEDs are plastic leaded chip carrier (PLCC) packages which, due to the inherent characteristics of the package platform, are of various qualities. PLCC packages typically have a high initial efficacy because the reflective plastic cavity and leadframe plating allow high efficiency light extraction. However, PLCC packages can exhibit fast lumen depreciation, especially considering LED panel lights oftentimes use medium or low quality LEDs as with other mass-produced interior lighting products. The packaging materials, such as polyphthalamide (PPA) or polycyclohexylenedimethylene terephthalate (PCT) for the reflective cavity, silver plated lead frame, phosphors and encapsulant, are prone to deterioration under high thermal and/or environmental stresses.
Lumen maintenance of an LED panel light generally depends on three variables: LED lumen maintenance, thermal management, and drive current. A long lumen maintenance of light source under the LM-80-15 test conditions (case temperature 55°C or 85°C) is the prerequisite to a long system life. Improved plastic resins such as epoxy molding compound (EMC) allow mid-power LEDs to operate at higher temperatures. Thermal management of the LEDs is determined by the conductive and convective cooling performance of the aluminum frame. The aluminum frame should have an adequate surface area to ensure its thermal transfer rate outpaces the load rate (at which thermal energy is introduced to the junction of the LEDs). The drive current should be properly managed to prevent thermal buildup as a result of overdriving the LEDs.
Compared to lumen depreciation, color shift is more of a concern for edge-lit LED panel lights. Thermal degradation, photo-oxidation and other deterioration mechanisms occur not only in the plastic LED packages, but also on the multi-layered optical system that is made of polymeric materials. Therefore, LED panel lights can undergo more deterioration mechanisms than other type of LED luminaries. Lumen depreciation and color shift are usually concurrent consequences of these failure mechanisms. While lumen depreciation is the gradual reduction in luminous flux over time, color shift can lead to significant discolorations that could render the light quality unacceptable.
The direction of the color shift can indicate the degradation/ deterioration mechanisms that are active. A shift in the blue direction can be related to discoloration of the plastic resin, loss phosphor quantum efficiency, operating the phosphor above the saturation flux level, settling and precipitation of the phosphor, mechanical damage such as cracks in the phosphor-binder interface. Photo-oxidation and thermal degradation of light guides, lenses and diffusers leads to a color shift towards the yellow direction. An increase in the efficiency of phosphor may also be accompanied by a color shift in the yellow direction. A green shift is an indication of chemical changes in the phosphor, such as oxidation of nitride red phosphor that shifts emission intensity to shorter wavelengths. Red shifts have some similarity with green shifts in that they can be attributed to spectral changes in the phosphor, possibly due to thermal ageing of silicone/YAG phosphor composite or the quenching of some phosphors.
Moisture ingress can often be an accelerator of spectral changes in LEDs. Most LEDs use silicone binders which have high water permeability. When LED panel lights operate in environments of high humidity, moisture can diffuse inwards the silicone/YAG phosphor composites. Presence of moisture results in oxidation of nitride red phosphor and causes the color of warm white LED emission to shift towards the green spectral region. Moisture absorption are known to be the primary causes of interface delamination between the die and silicone encapsulant. The resulting air gap between the chip and the phosphor calls for additional down-conversion of the blue photons to longer wavelengths. This ends up with a color shift towards the yellow direction.
LED panel lights are used to provide general lighting that fills in shadows, provides orientation, and supports visual performance. The color of the light emitted by these overhead luminaires lays the foundation for the color scheme of a space. The color scheme in turn affects the pleasantness of the space and subjective interpretation of atmospheres. Good lighting not only fosters great atmospheres and creates visually pleasing environments, but has a positive biological effect and poses no photobiological hazard. All these design goals of lighting are closely associated with the correlated color temperature (CCT) of light. The use of cool white light is well justified for commercial, office, educational and retail applications for which LED panel lights are designed. However, uneducated consumers have been accustomed to the extremely cool white light provided by fluorescent lamps. Despite the fact that LEDs are highly flexible in spectral output, Asian manufacturers continue to sell high CCT products for cost and efficacy considerations.
Humans should not be exposed longtime to light that has a CCT exceed 5300 K. CCT is highly predictive of blue light content. Warm white light contains less blue wavelengths in its light spectrum. Cool white light is rich in blue content. White light on the cool side of the CCT scale (6000 K to 6500 K) poses no photobiological hazard under normal behavioral limitations (Risk Group 1). However, that does not mean that the safety of optical radiation is guaranteed. In the environment of excessively high intensity, high CCT lighting, some populations, such as infants that have not yet developed aversion responses, may be at the risk of blue light hazard.
The more practical concern of high CCT lighting is circadian disruption. People often work or study late into night. At night and under conditions of darkness, the pineal gland releases melatonin which is engaged in the body's metabolic processes. Cool white light with a very high percentage of blue suppresses the release of melatonin, thus disturbing the day/night rhythm and affecting metabolic functions. In fact, moderately cool white light (around 4100 K) has a blue content that is sufficiently high to maintain suppression of melatonin and reduce drowsiness during the day while encouraging the release of dopamine, cortisol and serotonin to improve performance, vitality and concentration.
Tunable White Lighting
The emerging studies on the physiological and psychological effects of light give unprecedented impetus to the design of CCT tunable light fixtures. Tunable white LED systems provide the variable color temperature controls from warm white to cool white light. With a tunable white solution, the concept of human centric lighting (HCL) can be implemented to support the health, wellbeing and performance of humans. The dynamic changes in light levels and CCTs of natural daylight are genetically registered in human biology as a system of internal clocks, which is known as the circadian rhythm. Disruption of the circadian rhythm will interrupt the biological processes in our body and result in negative health consequences. A continuously adjustable range of color temperatures from, for example, 2700 K to 6500 enables the creation of scenes that help synchronizes the human circadian rhythm with the natural course of the day. Tunable white lighting also allows the setting of specific ambience for different events or tasks and thus creation of psychologically stimulating environments. Tunable white lighting is achieved by color mixing of LEDs of different CCTs. The LEDs are operated by a multi-channel driver controllable by various protocols including DALI, DMX, or 0-10V.
The color rendering performance of LED panel lights is weighed against cost and efficiency based on the particular needs of the application. How accurately a light source shows the colors of an object compared to the natural light depends on its spectral power distribution (SPD). In order for the LEDs to produce light that reproduces colors faithfully, a large amount of short wavelengths emitted by the semiconductor die must be down-converted to longer wavelengths for rendering saturated colors. Wavelength down-conversion is accompanied with Stokes energy loss which ultimately compromises luminous efficiency. To deliver radiant power fairly broadly across the visible spectrum, more conversion phosphors must be used, and this adds up the cost of LED packaging.
General lighting products commonly carry a mediocre color rendition, and LED panel lights are no exception. A color rendering index (CRI) of 80 is typical of LED panel lights. This color rendering performance is sufficient for performing tasks that're not color critical. However, many tasks require a high color rendition of the light source. The 80 CRI LEDs can often cause color distortion because of the missing or an inadequate amount wavelengths at the saturated color zones. For a space to look pleasant and for colors to appear natural, LED panel lights with a CRI of 90 or greater should be employed. The rendering performance of saturated colors (R9 to R14), which is not reflected in the general CRI, should also meet the minimum requirements.
When LED panel lights are installed in large volumes in a single project, color variations from luminaire to luminaire should be factored into luminaire designs. To ensure no noticeable color differences across multiple luminaires, LEDs used in all luminaires installed within a space are binned for their chromaticity (color temperature), and sometimes their luminous flux and forward voltage. A 5 to 7 MacAdam ellipses (5 - 7 SDCM) is currently representative of the color variation tolerance in general lighting applications.
Since LED panel lights have a large emission surface, the luminance at all viewing angles near horizontal is as high as looking straight up at the luminous panel. In a large office, this will result in discomfort glare as well as possible reflections in specular VDT screens. To address this problem, a micro-prismatic diffuser is added to the multi-layer optical system. The micro-prismatic diffuser features geometric structures such as pyramids, hexagons, and triangular ridges. Prismatic configurations make it possible to shield glare from the field of view at higher angles. When high visual comfort is paramount, LED panel lights are engineered to deliver a Unified Glare Rating (UGR) of 19 or less.
UGR < 19 LED panel light (Micro prismatic diffuser)
Image courtesy of Powersave Solutions Italia
LED panel lights are powered by an external driver which provides constant current output through a switching mode power supply (SMPS). In a typical driver configuration, a bridge rectifier converts the incoming AC power into DC power. The residual of the AC input that appears on the output as a variation or ripple is smoothed by capacitor. An active power factor correction (PFC) circuit is placed at the output of the bridge rectifier to correct phase errors and reduce harmonics. A switching regulator provides tight regulation and control on the current output provided to the LED load using a topology such as buck, boost, buck-boost, flyback, or SEPIC. Switching regulation produces electromagnetic interference (EMI) that has to be suppressed by additional circuits and careful circuit board design.
SMPS drivers are designed as either as cost-effective single-stage systems or state-of-the-art double-stage systems. Single-stage drivers combine the function of PFC and DC-DC converter in one circuit. Two-stage drivers include two separate circuits for AC-DC/PFC and DC-DC regulation, respectively. Single-stage circuits are simple but usually suffer from the large current ripple. The two-stage design is challenged with high component counts, circuit complexity and manufacturing cost. However, drivers of this type are capable of delivering to its load a precisely regulated DC voltage having very small ripples and handling larger fluctuations in the incoming AC power.
Continuous dimming of LED panel lights is typically accomplished using constant current reduction (CCR) dimming, also known as analog dimming. The CCR method adjusts light output by varying the drive current fed to the LEDs. The dimming circuitry is often controlled through the 0-10V protocol. 0-10V controlled drivers generally provide smooth dimming down to 10%. For applications that require a consistent CCT over the full dimming range, pulse-width modulation (PWM) is a viable approach. PWM drivers provide digital pulses of different widths to dim the LEDs.
LED drivers must be designed to not only operate with a high efficiency but also generate minimal ripples in the output current supplied to the LED load. Residual ripple is the cause of light flicker at a frequency of twice the power line frequency (e.g., 120 Hz or 100 Hz). Prolonged exposure to flicker will strain the human eye, reduce visual task performance, and even trigger symptoms such as headaches, migraines and epileptic seizures in some populations. Large current ripple usually occurs in outputs produced by low cost single-stage drivers due to incomplete suppression of the alternating waveform after rectification. Despite the fact that cost often outweighs light quality, light flicker should be tightly controlled, especially considering LED panel lights have a lager emission surface that is often in the field of view. For a smooth light output, the current ripple value should be reduced to a minimum (less than ±10%). When the luminaire is operated on 120 Hz frequency, the percentage flicker (flicker modulation) should be less than 10%, and preferably less than 4%.
Size and Installation
Most edge-lit LED panel lights designed for installation in drop ceilings have a nominal size of 2' x 2' or 600 x 600 mm. 2' x 4' or 600 x 1200 mm is another commonly offered size. The actual luminaire size is slightly smaller. LED panel lights come with mounting options for integration onto T-grid ceiling systems or for installing into drywall or plaster with accessory flange kits. These low profile luminaires can also be surface mounted using accessory frame kits or suspended using aircraft cable mounts.