What Is a High Mast Light​

A high mast light is an area lighting system that is raised high to minimize ground level obstructions and create uniform illumination across a large area. Throwing a controlled flood of light from above 15 meters (18 m - 55 m typically), high mast lighting systems are the functional extension of public lighting infrastructure which provides area and road illumination at pedestrian, urban or vehicle scale from below or at a height of 15 meters. The high mounting height combined with a multiple-luminaire configuration makes high mast lighting the most efficient and effective way of illuminating large areas. High mast lights are also the most heavy duty lighting systems that must possess the strength and resistance required to survive the most challenging outdoor environments.

Advantages of High Mast Lighting​

High mast lighting is primarily designed to provide an extended lighting coverage while minimizing shadows that may occur when shorter poles are utilized. Expansive light distribution from a single assembly of luminaires allows large area lighting with minimum poles. Wide pole spacing means less visual clutter and improved visibility. Low pole density creates less physical obstacles and hence leads to improved safety. Increased mounting heights allow far reaching illumination using high power luminaires, therefore high mast poles can be located away from traffic areas and high activity spaces. This results in fewer design conflicts with other elements and allows more efficient space utilization of the area being illuminated. The use of high mast lighting poles for co-location of other high mast equipment such as surveillance cameras and cellular antennas reduces the number of tall structures required in an area.

Typical Applications​

The ability to deliver large area illumination with maximum pole spacing lends high mast lighting to a multitude of applications. That being said, high mast lighting is an extremely versatile outdoor lighting solution in that it can provide accurate and comfortable visibility for massive outdoor areas during the nighttime and extend the usability of large-scale outdoor facilities into night. Among various types of outdoor lighting infrastructures, far-field, large area high mast lighting plays an exclusive role in facilitating nighttime travel and transportation, citizen engagement in social gatherings and sports/recreation activities, around-the-clock industrial production, safety and guidance of traffic, and security of commercial, residential and public facilities and properties.

Highways and interchanges​

High mast lighting provides a full panoramic view of roadways, intersections, roundabouts and crossworks as well as the areas immediately beyond the roadways. Both foveal and peripheral visions are critical to safe navigation of the road system. The ability to pull the roadside obstacles, fixed structures and approaching objects within the field of view of a driver provides greater visibility and visual comfort to traveling motorists, which translates to enhanced safety and security of road traffic.

Stadiums and athletic fields​

High mast systems provide a uniform luminous environment that contributes to the visibility of the playing target, the players and the surrounding backgrounds. There're a variety of sports held outdoors, which include but not limited to archery, baseball, bicycle racing, golf, motor racing, horse racing, football, soccer, skating, cricket, track and field. Luminaire location is a key consideration in the lighting design of outdoor sports venues. The lighting design is challenged by various factors such as light blockage, light distribution, visual comfort, and television broadcast. High mast lighting is the only way to address these challenges.

Commercial parking lots​

A high mounting height for area luminaires assists in reducing shadows between parked vehicles. Highly efficient lighting with minimal obtrusive light for drivers can be provided for very large parking areas by using high mast installations. In environments with multi-pole lighting installations, the area where the candlepower beams of adjacent luminaires meet usually has a low luminance that requires the visual system to adjust to the change in the light level. Wide coverage of light with high mast lights reduces pole density, which means reduced frequency of transient adaptation to different vision states. High mast poles can be placed along perimeters of the parking areas for a minimum of interference.

Airport aprons​

The aprons, including commercial aprons, general aviation aprons, cargo aprons and hangar aprons, require adequate illumination to ensure the parked aircraft are safely serviced. High mast lighting which allows a large setback distance of the poles can be easily coordinated with buildings, apron equipment and boarding bridges to avoid conflicts. Optimum visibility, uniformity, and glare control can be achieved by high mast floodlighting from multiple directions.

Freight terminals​

High mast lighting can provide effective large-area illumination for outdoor logistics activities in airports, seaports, railroad terminals, and trucking terminals. Freight terminals are often filled up with stacked cargo containers. High piles of stock require a raised light source to provide adequate vertical illuminance and reduce shadows. High mast lighting systems can be located remotely or used in confined spaces with no or minimal interference encroaching on adjacent cargoes or the loading and discharging equipment.

Industrial facilities​

Petroleum refineries, chemical plants, drilling rigs, oil depots, open pit mines, and other manufacturing and industrial facilities utilize high mast lighting to illuminate large production areas for around-the-clock operation.

Structure​

A high mast lighting system consists of three main sub-assemblies: mast structure, headframe, and luminaires. The headframe can be further divided into the fixed and mobile types. In a system with a mobile headframe, the mast structure has a winch mechanism which allows the headframe to be lowered to ground level for luminaire maintenance. The mobile headframe comes with a latching mechanism that holds the entire weight of the headframe and luminaires. In fixed headframe systems, the mast either has rungs/ladders or a motorized lift system via which the maintenance platform can be accessed. The mobile access system can be a mobile step system gliding on two rails fixed to the mast, or a power lift consisting of a cage, a guidance system, an elevator winch system and a safety braking system.

The headframe where a cluster of luminaires are mounted comes in variety of forms. The fixed equipment includes crossarms, rings, and frames. Crossarms such as U-bolted straight crossarms and curved crossarms are designed for lightweight lighting assemblies that use a limited number of luminaires. Rings and frames host more luminaires. A maintenance platform is usually fixed in close proximity to the crossarms, rings or frames to enable convenient servicing. The platform may also be structurally integrated with rings or frames to serve as a headframe assembly. A tilted frame with a central shaft is designed to accommodate a huge number of luminaires for single-side floodlighting. This type of headframes is most often found in stadiums where an exceptional uniformity of illuminance is critical. The mobile headframe is typically a ring structure which is suspended by stainless steel cables passing downwardly from pulleys on top of the mast to a winch in the base of the mast.

The masts are made of high tensile steel plates confirming to BS EN 10025 or equivalent standards and are hot dip galvanized after fabrication. They are continuously tapered and come in a round-conical or polygonal cross section. The mast is delivered to site in sections which fit into one another to achieve a desired height. Masts may come with a hinge which allows to bring the headframe down to human heights for ease of maintenance. The hinged mast has a fixed mast base and an upper part hingeable at either mid-level or the bottom. A weather and vandal resistant door is provided in the base of the mast to provide access to the base compartment equipment like winch, motor, cable. A flange plate, which is welded to the base of the mast, allows the mast to be secured to the foundation by anchor bolts bonded in the foundation block.

Lighting Technology​

Large area lighting is a drain on energy because high mast luminaires must output a substantial volume of lumens to cover an expansive zone from a long distance. High mast fixtures for road, area and industrial lighting applications also operate for significantly longer periods than lighting products in other categories. As a result, there's a never-ending quest for new lighting technologies that comply with ever-changing energy codes. To stay in line with the budget and energy constraints, the outdoor lighting market had long tolerated the terrible color rendition (22 CRI) and low mesopic luminous efficacy (S/P ratio 0.65) of high pressure sodium (HPS) lighting. Since a number of outdoor applications, such as sports lighting and industrial lighting involving color-critical operation, require a decent color rendition, metal halide (MH) lamps had been used to as a supplement to HPS lighting. Although MH lamps deliver a 65 CRI, their luminaire efficacies are significantly low (40 - 50 lm/W). In fact, even the luminaire efficacy of HPS lamps at a typical 61 lm/W is far from perfect. Useful life is another downside of traditional lighting. High-intensity discharge (HID) lamps, including HPS and MP, have only two- or three-year maintenance cycle. Short lifespan severely affects the ROI of HID lamps.

Over the past decades, solid state lighting (SSL) has been making breathtaking progress that is profoundly changing the way light was generated. Outdoor lighting applications are rapidly migrating to LED light sources which provide overwhelming advantages over HID lamps with regards to efficacy, lifespan, light quality, optical performance, and environmental sustainability. While the initial capital costs are higher than traditional lighting systems, LED lighting provides a significantly better return on investment when overall energy and maintenance savings are taken into account. It's no surprise that LED luminaires can achieve a system efficacy way over 150 lm/W, which results in an accelerated payback. A properly designed and engineered LED luminaire can last over ten years, which translates to a significantly longer gain from the investment. High mast luminaries are energy intensive systems. High energy and maintenance savings with high power LED luminaires over a long period provides a sound justification for investing in LED technology.

Beyond financial returns, LED lighting performs a lot better than HID systems in terms of light quality and optical performance. The scotopic/photopic (S/P) ratio is used to evaluate light source performance in mesopic vision which is the typical vision state in street and roadway lighting. An S/P ratio of up to 2.0 with LED lighting contributes to improved visibility in low light levels and consequently traffic safety. When it comes to outdoor lighting, LEDs have an inherently high color performance. The 70 - 80 CRI is only of low end colorimetric quality for LEDs. Yet this color performance is sufficiently good for outdoor lighting applications, including sports lighting which requires an accurate color reproduction of the illuminated object in order to meet the visual requirements of players, spectators and television broadcasting. HID luminaires cause the area where the luminaire aims to have much higher illuminance than areas around the beam center, which leads to a poor uniformity ratio (6:1 typical). In contrast, the uniformity ratio was improved by more than a factor of two with the LED luminaires. Good uniformity ensures minimal distortion of the visual perception, helps create comfortable visibility, and maximizes pole spacing.

Luminaire Construction​

High mast LED luminaires are high power lighting systems that usually consume hundreds of watts and produce tens of thousands of lumens. The construction of LED luminaires varies depending on type of light sources used, optics design, driver design, and thermal design. The luminaire comprises essentially an LED assembly, an LED driver, a die cast housing, and oftentimes an additional electrical compartment. The LED assembly comes in either an integrated design in which an LED board is thermally interfaced with the luminaire heat sink and protected by the luminaire housing and optical lenses against dust and moisture, or a modular configuration in which the use of self-contained, waterproof light engines eliminates the need for the whole luminaire to be disassembled when there is a need for a modification or upgrade. The LED assembly typically comes equipped with secondary optics to regulate the distribution of luminous flux at package-level, although there're some products, e.g. LED floodlights, using external reflectors to control beam spread.

The luminaire housing often serves the dual purpose of environmental protection and thermal management for the LEDs. Die cast aluminum construction provides strength and durability as well as thermal conduction and convection. LED luminaires used on high mast installations must be reliable and dependable in the most extreme environmental conditions. The aluminum housing is powder coated with a UV stabilized, high corrosion resistant polyester paint, laboratory tested for superior weatherability, cracking and fade resistance. The optical chamber of the integrated-type luminaire is completely sealed with a one-piece extruded silicone gasket placed between the housing and optical lens. High ingress protection (IP) rating ensures absolute resistance to the intrusion of water, insects and dust. A membrane breather equalizes pressure differentials within sealed enclosure to avoid premature seal failure and thus to preserve the integrity of the enclosure.

Thermal Management​

One misconception of LED lighting is it can last for a considerably long time, such as 10 years or beyond. While long lifecycle is certainly an intrinsic advantage, there are failure points. Most of the degradation and failure mechanisms that rule the performance and lifetime of an LED luminaire are caused by inefficient thermal management. LEDs convert only a small part of electrical energy into light and the rest is converted into heat. As a byproduct of operation, heat is produced as a result of non-radiative recombination in the LED junction and Stokes shift in the phosphor layer. At high power, higher current density operation which is typical in high mast lighting, a substantial amount of heat is generated. If this heat is not dissipated properly, thermal buildup within the LED causes lumen depreciation due to degradation and quenching of phosphors, die cracking, bond wire fracture, solder joint fatigue, carbonization of the encapsulant, etc.

Thermal management is a critical part of the design and engineering of LED luminaires. Thermal equilibrium in an LED system is broken by conditions that reduce an efficiency of heat dissipation. The goal of thermal management is to build a thermal path along which the thermal resistance of the components is minimized to a required level, while maximizing the effective area of the thermal path and minimizing the length of the thermal path. SSL thermal management consists of two sections: thermal conduction and convection. Thermal conduction copes with maximizing thermal conduction capacity of the heat sink, thermal interface material (TIM), metal core printed circuit board (MCPCB), and interconnects between the LED packages and MCPCB. This part of thermal management also includes minimizing difference in coefficient of thermal expansion (CTE) between the components along the thermal path. This is extremely important as outdoor luminaires can undergo repeated temperature cycling which can compromise the integrity of the thermal path.

The removal of waste heat by thermal convection is dependent upon the flow rate of the ambient air and the surface area around which air is circulating. Since there is an abundant availability of air flow around the luminaire in outdoor environments, high mast luminaires utilize natural convection to dissipate heat into the air. As a general rule of thumb, the heat sink is designed with a large surface area and an aerodynamic geometry to ensure effective air circulation.

Power Supply​

LEDs are complex semiconductor devices whose intercorrelated electrical and thermal characteristics should be factored into system design. As current-driven devices, LEDs must operate under constant current regulation in order to maintain their consistent output. Every LED, however, has a maximum rated current. Overdriving what the LED is rated for will result in irreversible performance degradation and shortened lifespan. As the current density is increased beyond a certain threshold, the internal quantum efficiency (IQE) is dropped. The reduction of quantum efficiency at high operating currents is called efficiency droop. A loss in efficiency means an increase in waste heat production. The forward current across the semiconductor junction of the LED can rise above the maximum allowed limit when there's an overvoltage event, or a failure of another LED string connected in parallel configurations.

An LED driver which regulates the power to the LED array of a high mast luminaire is designed as a switched-mode power supply (SMPS). SMPS drivers use a switching regulator to transform power rectified from AC mains supply into a pulsed waveform, which is then smoothed using an energy storage device. Switching power supplies are the only viable option for high power applications as they are very efficient, allow advanced dimming control, and have universal input voltage capability. In particular, the efficiency of an SMPS LED driver can be as high as 97% which is way much better than linear power supplies. Linear regulators have the advantages of low cost, driver-on-board (DOB) capability, and absence in electromagnetic interference (EMI). These driver circuits are found in some low-end products. However, this type of driving mechanism requires an input voltage at least some minimum amount higher than the desired output voltage. The minimum voltage differential between the input and output required for regulation is simply thrown away as waste heat, which not only leads to a significant power loss of around 20% but also produces substantial thermal stresses to co-located semiconductor components.

Switched-mode LED drivers are technically complex in that they use reactive components, such as oscillating coils and electrolytic capacitors in order to convert and store the electrical energy. Switching regulation generates high frequency noise that has to be suppressed by EMI filters. EMI filters also use reactive components such as filtering coils and high voltage capacitors. Flicker can be a problem in sports lighting applications and outdoor nighttime events where television recording and broadcasting take place. A ripple suppressor can be added to the driver circuit to reduce the output current ripple so there are no stroboscopic effects caused by flicker from the light source as well as no perceived flicker at high camera frame rates. Another essential requirement for line-operated LED drivers is power factor correction (PFC) which shapes and time-aligns the input current into a sinusoidal waveform in phase with the line voltage. The PFC is also used to suppress total harmonic distortion (THD) caused by non-linear electrical loads.

An LED driver executes a number of sub-tasks sequentially or in parallel, including but not limited to overcurrent protection, overvoltage protection, over-temperature protection, zero-current detection (ZCD) and handling, peak-current detection and handling, analog or digital voltage compensator, and constant light output (CLO). High mast luminaires are exposed to transient overvoltages caused by lightning, industrial and switching surges, or electrostatic discharges (ESD). A single-pulse event will cause an immediate catastrophic failure of the LED. Accordingly, a surge protective device (SPD) should be used to suppress excessive surges.

Lighting Control​

One of the significant advantages of LED lighting technology is the ability to work with solid state circuits and control the light output in a very dynamic way. High mast lighting systems can incorporate multiple control strategies for automated or remote switching or dimming operation. Lighting controls, including occupancy controls, photocontrols, time clocks, and energy management systems, are often installed at the circuit or luminaire level. LED drivers are configured to interpret control signals to dim or switch the LEDs. The control signals can be communicated to the luminaires using a variety of wired and wireless protocols, such as 0-10V, DALI, and ZigBee. Both local and centralized control systems can be integrated into high mast lighting systems. Luminaires or fixtures can be assigned to different zones, or areas of controls to maximize the flexibility of lighting control. Networked control systems combine software and hardware to provide a wide range of options for more adaptive lighting control and sophisticated user interactivity.

Light Source​

Recently, high mast luminaires that incorporate mid-power LEDs have been creeping into the market. What make mid-power LEDs appeal to lighting manufacturers are their low price points and high luminous efficacies. The problem is, mid-power LEDs are plastic leaded chip carrier (PLCC) packages that are prone to package material deterioration and rapid performance degradation in high power operating environments. The high efficacy of mid-power LEDs is founded on the high reflectivity of the plastic cavity and plated lead frame. At high temperatures and intense light levels, irreversible thermal oxidation and photodegradation in plastics, in particular polyphthalamide (PPA) and polycyclohexylenedimethylene terephthalate (PCT), can occur. The epoxy molding compound (EMC) has an improved thermal stability, but only to a limited extent. Silver plated lead frame which is exposed to the micro climate containing sulfur compounds will corrode. All these lead to a significant drop in light extraction efficiency. Not only do mid-power LEDs have a poor lumen maintenance and color stability, their reliability is a serious concern in outdoor environments. Leadframe corrosion can lead to an open contact because a mechanical separation between the bond wire and the connecting lead. The bonding wire that connects the lead frame to the LED electrodes can break due to internal stress, environmental vibration, thermal cycling, and electromigration.

For dependable lighting with high mast lights, high power LEDs deserve their prices. The ceramic-based LED packages are unencumbered with thermally unstable packaging materials. Unlike plastic-based mid-power LED, high power LEDs have high drive current capability and can survive a significantly higher operating temperature without compromising luminous efficiency and lifespan. The high power family also includes chip-on-board (COB) packages which are multi-die LED array typically used in applications needing a high lumen package from a light emitting surface with high emission uniformity. In addition to their high thermal performance, both high power ceramic LEDs and COB LEDs provide highly reliable interconnectivity between the package and MCPCB. The reliability of the interconnects between the LED package and printed circuit board is very critical in ensuring the overall reliability of an LED luminaire.

Optical Engineering​

High mast LED luminaires generally utilize integrated optical control components to produce useful beams and patterns without using any auxiliary optical control. Total internal reflection (TIR) optics use a refractive lens inside a reflector to control the entire initial distribution from the LEDs. The injection molded polycarbonate, PMMA or silicone lenses provide precise direction of the light to intended location. Glass refractors and glass convex lenses maximize light distribution and light uniformity. Aside from high efficiency extraction of luminous flux from the LEDs and uniform distribution of illuminance for maximum visual comfort and pole spacing, the optics must produce precise light patterns to facilitate glare control while reducing the growing concern with spill light or light trespass.

While plastic optics can be designed to provide the most accurate light distribution with minimal light loss, they have high moisture and gas permeability, and are susceptible to chemical ageing. Another major concern is exposed polymer optics can result in 3% output loss per year due to luminaire dirt depreciation. Glass is less attractive to dust and dirt accumulation. Therefore, in outdoor environments the LED luminaires should be preferably sealed with a flat tempered glass to exclude dirt and moisture and protect the plastic optics from environmental contaminants.