Parking lot lighting​

Parking lot lighting is designed for illumination of outdoor parking areas where quality visual information is critical to vehicle wayfinding and pedestrian safety. A mix of pedestrians and vehicles traffic within an open area that often neighbors populous human settlements creates an environment that demands adequate, reliable lighting with reduced offsite visibility and on-site glare. The illumination of open parking lots requires matching the lighting to the task - communicating all essential information about the surrounding nighttime environment to drivers and pedestrians in the most cost-effective way while transforming the parking facilities into a true point of arrival.

Quality of light​

The design of parking lot lighting systems involves many variables, such as performance, energy efficiency, economics, aesthetics, reliability, maintenance cycles, and environmental impacts. The emphasis of parking lot lighting is placed on the minimal use of energy for maximal visibility that can give drivers adequate reaction time to stay clear of possible hazards. Therefore, it is the quality of light within the parking area, not quantity, that is more closely associated with the perceptions of a safe, comfortably lit area. The quality of light in this respect is about addressing the illuminance requirements of a parking facility which can be affected by the layout, vehicular speeds, plus the visibility and security needs of pedestrians walking to or from their vehicles. Energy considerations and high pole spacing requirement require more careful tailoring of illuminance to task needs.

The illuminance needed for a driver to detect an object within the field of view is dependent on the vehicular speed. Within a parking lot vehicular speeds are typically 8 km/hr to 24 km/hr. Adequate illuminance should be provided for the safety stopping sight distance of 20 meters (80ft) at 24 km/hr. A low maximum-to-minimum illuminance uniformity ratio must be maintained to ensure the object detection is not affected by illuminance differentials within the safety stopping sight distance. For parking facilities with concrete pavement, the minimum horizontal and vertical illuminance is 10 and 5 flux respectively. For parking facilities with asphalt pavement, the minimum point values of 5 flux horizontal and 2.5 flux vertical should be maintained. The maximum-to-minimum illuminance uniformity ratio should not exceed 15:1. While horizontal illuminance is the normal design criteria in photometric calculations, vertical illumination is an important part of parking lot lighting because it creates visibility of vertical objects such as pedestrians, site obstructions, oncoming vehicles, and other obstacles. The vertical illuminance is determined at a height of 1.5 meters above the floor.

Energy efficiency​

Many factors enter into design and specification decisions for parking lot lighting. Energy efficiency is obviously one of the top priorities in this application owing to the long operating hours and relatively high wattages typically involved. High intensity discharge (HID) lamps such as high pressure sodium (HPS) and metal halide (MH) lamps used to be the principal light sources for parking lot lights due to their relatively high luminous efficacy. However, the energy waste of these light sources are still enormous and there's no efficacy potential in these technologies. Another issue with HID lamps is their optical efficiency is extremely poor. Optical efficiency includes two aspects: luminaire efficacy and illuminance distribution. The omnidirectional emission pattern of HID lamps results in a significant optical loss during the regulation of luminous flux at luminaire level. Since HID lamps are near point light sources, there's a high flux density in the beam center and a lower flux density around the beam center. The high maximum-to-minimum illuminance uniformity ratio translates to a need for a high density of pole installations to meet the illuminance uniformity requirement. Tight pole spacing means an increased quantity of luminaires and consequently more energy consumption.

Light distribution​

Light distribution is another critical design consideration in outdoor lighting. The light distribution of parking lot lights is defined in three ways: (1) transverse light distributions in Type I, II, III, IV, V, and VS classifications; (2) lateral light distributions with maximum candela emission occurring at various vertical angles for short, medium and long distributions; and, (3) the IES Luminaire Classification System (LCS) which defines the distribution of light from a luminaire within three primary solid angles (forward light, back light, uplight). Transverse light distributions define the location of the poles (forward throw for perimeter lighting, and symmetric for the main parking section). The vertical angle of maximum candlepower determines the pole spacing. The LCS was developed to supersede the old IES cutoff classifications (full cutoff, cutoff, semi-cutoff, and non-cutoff) which serve to identify products with high angle brightness.

Parking lot lights must be designed to distribute light within the specified zones so as to not to produce obtrusive light. The BUG (Backlight, Uplight and Glare) rating system is based on the LCS zonal lumen classifications for luminaires. The B rating takes into account the amount of light leaving a luminaire in the opposite direction to the main aiming angle. The backlight solid angle is further refined into four secondary solid angle zones of low, medium, high and very high (BL, BM, BH and BVH). Light distributed in these zones is considered spill light. Spill light that falls onto neighboring properties is called light trespass. The U rating considers the total light propagating from the luminaire in a near-horizontal or above-horizontal direction, which causes the sky glow. The uplight solid angle is further refined into uplight low (UL) and uplight high (UH). The low angle uplight describes lumen distribution between 90 and 100 degrees vertical. The high angle uplight relates to luminaire lumen distribution between 100 and 180 degrees vertical. The G rating evaluates the fowardlight in the high and very high (FH and FVH) zones, which causes glare and spill light.

Mounting heights​

Unlike street lighting and pedestrian-scale lighting, parking lot lighting gives particular consideration to the location of poles. Parking lot lights should be strategically placed to minimize shadows between parked vehicles and reduce the probability of damage from moving vehicles. This often demands wide pole spacing for the purpose of creating maximum parking space. When the poles are placed along perimeters of parking areas a large setback distance of the poles is required so that there's no interference encroaching on parking space. Wide pole spacing and long setback distances result in high mounting heights for parking lot lighting. Parking lot lights have a similar minimum pole height with highway lighting systems (10 meters typical). However, in many applications they can have a mounting height greater than the typical maximum height (15 meters) of highway lighting and reach the scale of high mast lighting. Higher mounting heights allow for fewer poles because of maximized light coverage, but a higher lumen package is required to maintain minimum average illuminance.

Lighting technology​

Outdoor lighting applications are undergoing a massive migration from HID to LED. In contrast to conventional light sources, LEDs not only outperform in energy conversion efficiency and service life, but also offer impressive controllability of their light distribution. In outdoor settings, the ability to provide uniform illumination without light pollution is equally as important as long life cycles and high energy savings which stand out as the most appealing benefits of LED lighting. By the very nature of their design and operation, LEDs are a highly directional light source which has a high light intensity in the forward direction. This optical characteristic allows an efficient cutoff design to be created in order to comply with the challenging requirement on BUG ratings. Along with improved light distribution, LED parking lot lights provide uniform illumination from their low-profile, large-emitting-surface light engines which incorporate an array of LEDs. The illuminance uniformity, which is improved by more than a factor of two with the LED parking lot lights, contributes to enhanced visibility and maximized pole spacing.

Types of luminaires​

Parking lot lights are mostly pole-mounted area and roadway luminaires, although wall mounted luminaires are used in relatively narrow parking areas that are between or adjacent to buildings. These luminaires are available in a variety of lumen packages and light distributions that allow lighting to match the application. Parking lot luminaires are often interchangeable with roadway luminaires when they are equipped with type III, type IV or type V optics to provide area illumination. Traditionally, "shoebox" luminaires are also used to provide parking lot lighting. Today the LED successors to traditional "shoebox" luminaires have no difference in terms of construction with roadway luminaires except for that the former may come with house side shielding functionality for tight backlight control. Parking lot lights at the scale of high mast lighting typically use floodlighting luminaires which are designed to project a beam across a long distance and are usually capable of being aimed at any direction. Because of potential spill light and glare control problems, floodlighting may be the least appropriate solution for parking lots in close proximity to residential complexes or buildings and facilities that cannot tolerate obtrusive light.

Luminaire construction​

LED area luminaires for parking lot lighting are constructed with a heavy duty aluminum housing that provide structural strength along with resistance to corrosion and impact. High pressure die casting production allows precise tolerance control and repeatability in manufacturing for aluminum castings with complex shapes that facilitate thermal convection. The luminaire housing consists of an optical compartment that hosts the light engine and an electrical compartment that accommodates the LED driver and other electrical components. These compartments must be sealed from the outside environment for ingress protection against water and dust. The optical compartment should be engineered for a very high watertight integrity to prevent moisture and corrosive gases penetrating into LED packages. The luminaire housing should have a low effective projected area (EPA) to ensure optimized pole wind loading.

The light engine may be either of modular design or integrated into the optical compartment. Modular design allows a significant amount of options and customizations as well as effortless field maintenance and future upgrades. Nonetheless, integrated light engines usually receive more efficient thermal management as the entire housing serves as the heat sink while the modular light engines dissipate most of the heat through the self-contained heat sink. Integrated light engines are better protected in the compartment which is often sealed by a gasketed glass lens for a high IP rating. The enclosure integrity of modular light engines entirely relies on the polymer plastic lenses which are prone to UV and thermal degradation. The plastic lenses that are exposed to the air can accumulate dust and dirt, resulting in a higher rate of luminaire dirt depreciation (LDD). LED area luminaires with integrated light engines can be designed to provide a sleek look. Aesthetics of parking lot lights are typically an important design factor in many urban projects.

Optical engineering​

The optical system is usually what differentiates the good from the poor for area luminaires. Luminous flux from LEDs are regulated by secondary optics to facilitate wide pole spacing, excellent uniformity, and control of glare. LED lenses makes use of total internal reflection (TIR) to efficiently extract, diffuse and control all of the light produced by LEDs. TIR optics consist of a refractive lens which nestles in a reflector to collimate the rays emitting from an LED for maximized beam uniformity with no striations. TIR optics can be sculpted to produce a precise beam pattern that is essential in obtrusive light control. An extensively used polymer plastic in LED lens is polymethyl methacrylate (PMMA) which is favored for its clarity, UV stability, high transmissivity, and low manufacturing cost. Some parking lot luminaires change the distribution, regulation or distraction of luminous flux using convex lenses or metalized reflectors. These optics exhibit poor controllability in beam patterns but provide a low cost optical control solution. A house side shield can be field or factory installed to provide maximum control for backlight and corner applications.

Thermal management​

Most failure mechanisms in LED systems are thermally induced. When selecting light sources for parking lot lights, lighting manufacturers have a strong preference for plastic leaded chip carrier (PLCC) mid-power LED packages which have a high initial efficacy and attractive price point. However these SMD LED packages are constructed from a plastic housing which is prone to thermal degradation under high temperature conditions. As such, efficient thermal management is the key to maintaining consistent light output over the rated product life and preventing the LEDs from premature failures. Thermal management of outdoor LED luminaires relies on the natural thermodynamics of conduction and convection, rather than on fan-assisted or other active cooling methods. The passive cooling system comprises a heat sink in physical contact with the heat source through additional thermal interfacing layers such as a metal core printed circuit board (MCPCB) and thermal interface material (TIM). The die cast luminaire housing which provides heat sinking for the LEDs must provide a sufficient physical volume to ensure a high latent heat capacity. Also creating an aerodynamic heat sink with more surface area will dramatically increase convective heat transfer from the heat sink to the ambient air.

Drive current​

An LED must be driven by a DC forward current that is properly regulated. An LED driver provides a designated supply of current to the LEDs regardless of the supply voltage fluctuations and changes in other operating parameters. Overdriving what the LED is rated for will cause current crowding, increased junction temperature and the premature aging of the LEDs. Catastrophic failure mechanisms such as electromigration can be triggered. Electromigration is electrically induced movement of the metal atoms. Migration a significant number of metal atoms far from their original positions can contribute to short circuit failures.

The weak current-carrying capacity of the LED chip and internal interconnections makes LEDs especially vulnerable to electrical overstress (EOS), electrostatic discharge (ESD) and voltage spikes. EOS usually causes bond wire fracture. ESD may result in an immediate open circuit failure in LEDs. All necessary precautions should be taken in the design of LED drivers to protect the LEDs from EOS/ESD surges, short circuits, and reverse power connections. The LED drivers should also be able to handle tough transient environments in outdoor applications and allow the LEDs to survive high voltage spikes due to lightning or adjacent inductive loads.

Interconnect reliability​

The reliability of an LED lighting system is determined by the reactions of all its components under environmental or operational stresses. For outdoor LED luminaires that use mid-power PLCC LED packages, solder joint reliability is a particular concern. Repeated temperature cycling in outdoor environment can produce a high thermomechanical stress than drives the interconnect-related failure mechanism due to recurrent thermal expansion and contraction. Therefore, a high coefficient of thermal expansion (CTE) match between the LED package and the circuit board is required to reduce the thermomechanical fatigue of solder joints to a minimum. The reliability of solder interconnects is also affected by environmental loads such repetitive vibration stresses that often occur in areas of heavy vehicle traffic. The small solder joint area of SMD LED packages demands high metallurgical bonding performance of the solder alloy.

Light source​

As previously noted, mid-power LEDs are favored by lighting manufacturers for their cost and efficacy advantages. But nevertheless, these plastic packages suffer from a number of failure mechanisms that are very likely to cancel out their advantages. This type of LED packages is very tricky to source as many cheap products may use low quality materials to build the plastic housing, lead frame and bonding wire. On other side, ceramic-based high power LED packages are more expensive but offer fundamentally improved reliability. They eliminate the use of plastic housing and come with a high efficiency thermal path as well as electrodes with enhanced interconnect reliability. These advantages allow high power LEDs to survive high drive current operation and high temperature conditions which are typical in high wattage LED luminaries. Excellent lumen maintenance and high package reliability lend high-power LEDs the ability to drive a significantly better return on investment (ROI) than mid-power LEDs.

Lighting control​

A wide variety of lighting controls can be integrated into parking lot lights for controlling a number of lighting features from simple on/off switching and dimming operated by photocontrols and/or time clocks to occupancy-based adaptive lighting and network-based remote control and monitoring. Wireless control over a radio-frequency (RF) network may enable a distributed intelligence architecture that facilitates advanced light management for maximized energy savings.