The main starting point for considering outdoor lighting is to establish the main design goals. Traditionally, street lighting has been a fundamental part of outdoor lighting. However, in modern urban environments, it is mainly post top pole lights that defines the visual environment, provides humane lighting, creates the ideal ambience, provides a sense of security, and is able to perform a variety of outdoor tasks at night. Street lighting is designed to provide an enhanced visual environment for people to safely use the road system, while area lighting emphasizes creating visibility of areas within specific boundaries, such as car parks, pedestrian streets, downtown plazas, office buildings, exhibition areas , residential areas, sports fields, company grounds, campuses, parks, airports, toll plazas, railway yards, loading docks, and surrounding buildings.
In many cases, street lights can be used as area lights as long as their light distribution patterns meet the requirements of area lighting. Area lights are more versatile in terms of light output, optical pattern, mounting height, fixture style, aesthetic features, quantity and quality of lighting. Area lighting systems are designed with practicality for outdoor facility lighting in industrial, transportation, sports, parking lots and large open areas. These lighting systems must accommodate vehicular and pedestrian traffic in the most economical way, produce sufficient light output, and withstand harsh operating environments. Area lights for pedestrian areas often provide functionality while aesthetically matching the architectural theme of the space. Their daytime appearance infuses an organic feel to the urban environment. Pole heights and luminaire dimensions must be in perfect proportions for effective pedestrian lighting and aesthetics. In addition to providing functional lighting and enhancing the built environment of parks, squares and other open pedestrian areas, this type of luminaire also assumes the role of street lighting for pedestrian paths, cycle paths and narrow roads in private spaces.
Post top lights
The post top lamp is a pedestrian-only lamp, and the installation height of the lamp varies from 3 meters to 9 meters. In addition to lighting performance and optical control, aesthetics, scaling and styling are other top priorities in every post top fixture design. These street lights come in a variety of traditional and modern forms and blend well in many scenarios, including downtown streetscapes, city parks, residential areas, sidewalks and bike paths. The traditional aesthetic is expressed in acorn and lantern style lamps, which are mounted on recessed and/or tapered rods with decorative bases. The contemporary design utilizes smooth lines and clean geometric shapes to ensure an architecturally harmonious appearance with the surrounding structures. Proper balance should be placed between the pole and the light fixture to ensure that the entire light fixture does not look out of proportion and mismatch.
Luminaire Construction
Considering the energy efficiency, optical control, spectrum management, and lifecycle advantages of LEDs, LED luminaires are more challenging to design and engineer than traditional lighting systems. A robust LED lighting system requires the relevant integration of thermal, optical, electrical and mechanical engineering to ensure that all components operate within given parameters. An LED is a semiconductor device in which light is generated by the radiative recombination of electrons in the conduction band of the negatively charged electrode and holes in the valence band of the positively charged electrode in a p-n junction (positive and negative junction). These solid-state light sources are very sensitive to operating temperature and drive current. Also, LEDs are point light sources, and their flux is concentrated in a small light emitting surface (LES), which looks harsh and adds to the luminaire’s glare profile.
Thus, the systems engineering of LED luminaires provides a tightly controlled environment for optimal operation of LEDs, while aesthetically modifying the appearance of the light source and controlling the distribution of emitted light with minimal optical loss. At its minimum, an LED area light fixture consists of an LED module, an LED driver, a thermal management system and secondary optics. LED modules typically integrate an optical lens on the LED assembly, but reflectors, refractors, diffusers, or other optical components can also be used to control the distribution of light. Since LED lighting systems require convective heat dissipation with the surrounding air, luminaires are often architecturally integrated with the heat sink of the thermal management system. The LED driver regulates the power of the LED and can be configured to respond to a control signal provided by a sensor or light controller.
Light Source
LEDs produce white light through phosphor conversion, which involves the use of phosphor coatings. The phosphor down-converts part of the electromagnetic radiation from the semiconductor diode into shorter wavelength light, which mixes with the unconverted blue light to produce white light that the human eye can perceive. This process is performed in the package architecture. The LED package also provides an interface for the LED chip to make thermal, electrical and mechanical contact with the operating environment. A third purpose of LED packaging is to protect the exposed die from physical damage and environmental contamination. The performance, efficiency and reliability of LEDs depend on the epitaxial structure and wafer material of the LED chip (semiconductor chip), as well as the package design and packaging materials.
Generally speaking, the vast majority of LED manufacturers use semiconductor molds of the same epitaxial structure and wafer material to produce their products. Only a few LED manufacturers have taken a different route. Cree LEDs, for example, are packaged with the company’s proprietary GaN-on-SiC semiconductor chips, rather than the most common, but less efficient and less reliable GaN-on-Sapphire devices. Therefore, the quality and performance of LEDs on the market largely depend on the packaging design and packaging materials of the product.
High power LEDs offer reliability that medium power PLCC LEDs cannot. Metallized ceramic substrates provide an efficient, highly reliable thermal path for high flux density operations. The package design eliminates the use of plastic materials and lead frames, which greatly reduces the failure factor of LEDs. Compared with medium-power LEDs, high-power LEDs have relatively low luminous efficiency, but have a longer life and more stable output. However, high-power LEDs struggle to compete with mid-power LEDs because not all users put long-term interests ahead of immediate interests.
Chip-on-Board (COB) LEDs have larger LESs and are provided by an array of semiconductor chips to provide uniform illumination for high lumen applications. The LED die is directly mounted on a ceramic substrate or metal core printed circuit board (MCPCB) so that the heat extracted from the LED junction can be effectively dissipated. COB LEDs are typically used in light fixtures that require a wide light source with high uniformity over a small light source area. However, COB LEDs are not commonly used in area lighting applications because they require a very large, expensive optical component to control the beam angle.
Thermal Management
Longevity is a key selling point for LED lamps. But without thermal management, LEDs can have as short a lifespan as incandescent bulbs. Light-emitting diodes are an automatic heating device, and more than half of the electrical energy it consumes is thrown away as waste heat. Only a small fraction of the electrical energy is converted into light energy. The waste heat must be extracted from the LED junction or die-related, package-related and interconnect-related failure mechanisms will be initiated. Typical problems caused by overheating of semiconductor junctions and surrounding structures include thermal phosphor degradation, mold cracking, bond wire fracture, solder joint fatigue, encapsulant carbonization, discoloration of plastic resins, electromigration of metal atoms in metallization layers, and more.
Thermal management of LED lighting systems is designed to minimize thermal resistance and improve the heat dissipation efficiency of components along the thermal path from the semiconductor junction to the surrounding environment. For area lighting systems, the reliability of the thermal path is as important as the heat transfer capability. These systems are subject to shock and vibration, which can mechanically stress the thermal gap connected between the LED package and the thermal components of the system. Outdoor products require additional environmental considerations, and temperature cycling is a major cause of interconnection failures in outdoor lighting systems. Therefore, high thermal expansion coefficient matching between thermally conductive elements is critical in thermal engineering. Thermal path integrity in lighting systems using mid-power LEDs is a major concern, as the solder joints between the LED package and the MCPCB are particularly susceptible to mechanical and thermal fatigue.
While LED luminaires rarely fail catastrophically, a large number of inexpensive products are prone to accelerated lumen decay. This is because cost optimization for these products is most likely achieved by cutting corners on the heatsink. The heat sink provides thermal conduction, transferring the heat from the LED junction to the boundary and then to the surrounding air by convection. Therefore, the design of a typical heat sink considers maximizing the thermal conductivity of the material and the effective area of the thermal path for efficient heat conduction, and optimizing the boundary conditions for effective thermal convection by increasing air flow and maximizing the total area of contact with the environment. Most luminaire heat sinks are made of die-cast aluminum, which provides good thermal conductivity and provides design flexibility for aesthetics and thermal convection purposes. Watch out for products with lightweight or plastic heatsinks.
LED Driver
The driver is an electronic device that converts AC power to DC power to activate current-driven LEDs. In addition to regulating the DC output under supply voltage or load changes, the LED driver must be configured to compensate for changes in the LED forward voltage. The forward voltage of the LED is easily affected by the junction temperature of the LED itself, resulting in a large change in the forward current and fluctuations in the light output. LED area lights are equipped with constant current LED drivers. Rather than regulating the voltage, the LED driver controls the DC current into the LED and ensures that the LED does not receive more current than its rated current. Overdriving the rated performance of LEDs can cause current crowding and lead to a high probability of localized overheating of the epilayer and thermal runaway.
Like light source efficiency, the efficiency of the LED driver can significantly affect the total system power consumption. Modern LED drivers are essentially based on switch-mode power supplies (SMPS), which regulate the output of LEDs through high-speed switching operations. smps based LED drivers are very efficient in power conversion. They can be designed to provide isolated and regulated DC output power with good power factor correction (PF) and low total harmonic distortion (THD). However, electromagnetic interference (EMI) generated by switching regulators must be suppressed through careful circuit board design, shielding, and filtering to meet electromagnetic compatibility (EMC) requirements. Additional EMI suppression circuitry can greatly increase overall cost and add bulk to an already bulky and expensive LED driver.
LED drivers used in street and area lighting systems must be able to handle the harsh transient environments they are exposed to. Some LED drivers have inherent lightning protection, providing differential-mode and common-mode immunity to the connected LED modules. Depending on local conditions, the AC input of the luminaire or LED driver must provide additional protection against excessive transient overvoltages to absorb high surge energy. Lightning arresters can be Metal Oxide Varistors (MOVs), Gas Discharge Tubes (GDTs) or Transient Voltage Suppression (TVS) diodes.
Lighting Control
LED drivers play a vital role not only in the operation of the LEDs, but also in controlling the light output of the LEDs. With LED drivers with pulse width modulation (PWM) or constant current reduction (CCR) dimming capability, a variety of lighting control strategies can be implemented on outdoor luminaires. LED area lights can be automatically time-based, ambient light, and/or occupancy-based dimming and on-off control, using time clocks, photocells, and/or motion sensors. Networked lighting control via a Centralized Management System (CMS) allows luminaires to be positioned and controlled individually or in groups. Adding Internet of Things (IoT) capabilities to area lighting fixtures opens up numerous opportunities for advanced lighting management and smart city services.
Light Distribution
Area lights have a variety of uses and have different requirements for light distribution. These systems must be designed to distribute light over an area generally distributed by its lateral (along the road) and lateral (cross road) directions.
Lateral light distribution with maximum candela emission occurs at higher vertical angles allowing longer luminaire spacing. The luminaires that produce lateral distribution, whose distance is 3.75 times to less than 6.0 times the installation height, belong to long-distribution luminaires. The lateral light distribution of maximum candela emission with lower vertical angle has shorter luminaire spacing but significantly reduces system glare. When the horizontal light distribution length of the lamp is 1.0 to 2.25 times the installation height, it is a short configuration. When the lateral length of the light distribution of the luminaire is 2.25 times to less than 3.75 times the installation height, it belongs to the medium distribution luminaire.