Parking garage lighting​

Parking garage lighting is designed to provide adequate illuminance for the safe movement of both vehicular and pedestrian traffic in parking facilities that are roofed over and at least partially enclosed. Also called parking structures, parking decks, parking ramps and parkades, parking garages include above-ground and underground facilities which usually require supplemental daytime illuminance and 24-hour lighting respectively. Parking garages are classified as indoor spaces, but they combine elements of both indoor and outdoor environments. Therefore, parking garage lighting is essentially treated as a low-mounting-height area lighting application which needs to address some additional challenges presented by the distinctive environment.

Seeing the unseen​

Parking garage lights serve to assist in the timely detection of moving vehicles, pedestrians, and any obstacles that may pose a potential safety hazard. Parking garages often have a high incidence of pedestrian/vehicle conflict which creates a variety of complicated visual tasks requiring adequate visibility. The human visual system is not an expert at spotting or tracking moving objects, in particularly when the movement is unpredictable. It is therefore important to render objects more visible with more contrast or luminance. Meanwhile, there're perception-reaction distances for vehicles and pedestrians. The moving speeds of vehicles and pedestrians decide the distance at which a potential hazard may be detected. And there's a safe stopping sight distance for the vehicle and a safe reaction sight distance for a pedestrian to execute the evasive action.

The typical vehicle speed within a parking facility is approximately 15 km/hr and the safe stopping distance at this speed is 15 meters. The average pedestrian walking speed is 4.7 km/hr and the perception-reaction distance is approximately 2 to 3 meters. Parking garage lights should provide at least the minimum amount of illuminance to ensure that the vehicle driver will be able to detect an object within the safe stopping distance and the pedestrian has sufficient time to react to a potentially hazardous event.

Illuminance requirement​

While detection of an object is based on adequate luminance contrast between the object and its background, the purpose of a parking garage luminaire is to produce an adequate quantity of light (illuminance) that falls on an object to create the required luminance (visibility). Targeting and achieving a specified illuminance level is a standard practice often used to select a luminaire. The illuminance or density of the luminous flux incident on a target area depends on the radiant power (light output) of the luminaire and its light distribution pattern and mounting height. Design or selection of parking garage luminaries for a specific project should be based on the illuminance requirement the other way round.

The amount of light that reaches the intended area is critical to creating an appropriate luminous environment for identification of obstacles. Horizontal illuminance which describes the amount of light landing on the floor is the primary design criteria in the vast majority of applications. However, vertical illumination that allows identification of vertical objects is also an important part of parking garage lighting. The vertical illuminance in parking garages should be at least 50 percent of the horizontal illuminance value. The IES recommended minimum horizontal illuminance in parking garages is 10 lux.

Uniformity ratio of horizontal illuminance is the most important factor that defines the quality of illumination in parking garages. A low uniform ratio of maximum-to-minimum illuminance (no higher than 10:1) should be maintained within the parking structure to maximize pedestrian safety and reduce visual fatigue due to continuously adapting between two significantly different luminance levels. This means a parking garage light should be designed to provide maximal illuminance uniformity so that there're no extreme luminance differences in the neighboring areas of lighting installations.

Lighting technology​

In the past, parking garage luminaires had utilized high pressure sodium (HPS), metal halide (MH), or linear fluorescent lamps as the light source. HPS lamps gained their popularity in outdoor applications because of their low cost, high efficacy, and long life. But the poor color rendering index (CRI) of HPS lamps make them obsolete in indoor applications where color vision is important. Therefore, most legacy garage light fixtures had been built around metal halide and fluorescent light sources which have improved color quality but are less efficient in power conversion and shorter in lifespan. When incorporated into light fixtures, the omnidirectional emission pattern of these light sources leads to a poor downward luminaire efficiency (70% - 85%). A significant quantity of the light produced by HID luminaires (HPS, MH) lands directly below the luminaire to form a hot spot. Excess illumination over area directly below the luminaire combined inadequate illuminance around this area yields a poor illuminance uniformity that is intolerable in applications with a low mounting height. Compared with HID luminaires, fluorescent luminaires produce a more uniform light distribution. Nevertheless their low output makes them incapable of providing adequate illuminance for garages with high mounting heights.

The inability to catch up with the ever-changing energy codes and high maintenance costs of traditional lighting systems are driving a massive migration to LED lighting. The ability to offer a source efficacy as high as 200 lm/W and a source life well over 100,000 hours lend compelling advantages to LEDs. The energy efficiency of LED luminaires can be further maximized when the semiconductor based light source is operated by dimmable, programmable drivers which can interpret control signals from sensors or electronic logic circuits. The solid-state nature of LEDs allows luminaires to be engineered with greater resistance to shock, vibration, and wear, thereby increasing luminaire lifespan significantly. Aside from its efficacy, lifecycle and reliability benefits, the impressive optical controllability of LEDs also appeals to lighting designers and architects. The directional nature of LEDs not only high efficiency light extraction as system level, but also enables precise light distribution with unparalleled illuminance uniformity.

Luminaire design​

The physical appearance of parking garage luminaires can vary widely, especially with LED luminaires. On the whole, this type of luminaires can be classified into low bay luminaires which are designed to produce general illumination in spaces with a mounting height of less than 6.1 m (20 ft). Low bay luminaires for garage lighting are either in a linear form factor or in a round or square shape. Although first cost often dominates the priorities and bare strip fixtures have their presence in the market, most linear garage luminaires are enclosed strips, often referred to as "vapor tight" luminaires in the U.S. market or "triproof" lights as coined by Chinese lighting manufacturers. HID garage luminaires rely on reflectors or refractors to control lumen distribution. These products can be retrofitted with high power LED bulbs which are supposed to have the same electrical supply adaptor and similar physical shape for plug-and-play operation. Modern LED luminaries in round or square assemblies come equipped with integrated LED assemblies or modular light engines. Luminous flux from the light source is typically regulated via specialized secondary optics either associated with individual LEDs or with arrays of LEDs.

Optical engineering​

Optical design parameters of garage lighting systems are generally determined on the basis of photometric needs and environmental considerations. Garage lighting systems are either shielded to limit high-angle light between 75° and 90° above the horizontal plane or designed with a dropped luminous diffusing lens over cover element to allow wider spacing. LED garage lights of the non-linear type eliminate physical shields and make use of total internal reflection (TIR) optics to produce uniform illumination with clean beam patterns. The TIR optic collimates the rays emitting from an LED with a refractive lens and then redistributes the collimated rays by a reflector. There are also edge-lit systems which produce uniform, edge-to-edge illumination with no visible LEDs, hotspots, or shadows from LEDs arranged along the side of the luminaire. An acrylic or polycarbonate light guide panel (LGP) extracts, directs and distributes light from the edge mounted LEDs for visually comfortable illumination. Linear vapor tight luminaires simply use the diffusion mechanism of polycarbonate lenses to produce homogeneous light distribution. LED luminaires designed with a semi-indirect light distribution pattern direct some of the light upward toward the ceiling, thereby raising the adaptation level for glare mediation.

Thermal management​

A common misconception regarding the lifespan of LED luminaires is to equate it with the life of LEDs. The lifetime reported by LED manufacturers is predicted on data collected from tests with limited lengths of duration at controlled operational and environmental conditions. When LEDs are incorporated into luminaires they can be affected by more challenging and complicated operational and environmental conditions. One of the variables that leads to short lifespan or performance degradation is high junction temperature. LEDs throw most of the consumed electrical power away as waste heat which can raise the operating temperature if allowed to accumulate at the semiconductor junction. Excessive junction temperature will cause phosphor thermal quenching and degradation, deterioration of packaging materials, nucleation and growth of dislocations in the active region of the semiconductor diode.

LED garage lights typically use passive cooling technique to address heat dissipation requirements. Thermal management of non-linear garage luminaires is executed primarily by a die cast aluminum housing which extracts heat away from the LED packages via additional thermal interface layers such as a metal core printed circuit board (MCPCB) and/or a thermal interface material (TIM). The heat is then dissipated to the air around the aluminum housing by natural convection. Linear lighting systems usually have an aluminum or steel geartray which serves as the heat sinking device for the linear LED modules. This type of thermal design may not be as efficient as that of luminaires that dissipate heat using exposed aluminum housing. The geartray of an LED triproof light is usually enclosed by a fiberglass reinforced polyester housing which is less efficient in transferring heat from the geartray for subsequent convective heat transfer. However, there are linear fixtures that come with an extruded aluminum housing providing high efficiency thermal conduction and convection.

LED driver​

The LED driver is the heartbeat of any LED system. When selecting or designing an LED driver for LED garage lights which typically requires a power input in the 20 - 100 W range, these specifications should be considered: efficiency, reliability, safety, power factor, feedback features, ripple, noise, and dimming. The challenge for the LED driver is to convert AC power to regulated DC output power with a high power factor (PF) and low total harmonic distortion (THD) over wide variations in line voltage as efficiently and economically as possible.

At its bare minimum, an LED driver should provide constant currents to operate the array of LEDs for consistent performance regardless of changes in LED forward voltage as well as any fluctuations that may occur to the AC power being fed to the driver. The driver is the most active factor that affects the reliability of LED luminaires. Overdriving LEDs can cause efficiency droop and premature failures because of an increased volume of waste heat and a high probability of electromigration that can leads to short circuit failures. The driver should also be configured to prevent transient electrical surges such as electrical overstress (EOS), electrostatic discharge (ESD) and in-rush current, from reaching the LEDs.

The LED driver is usually the first component of a luminaire to fail. The highest efficiency LED drivers are switched mode power supply (SMPS) systems which adjust the frequency or duty cycle of the switching to produce a desired output. In contrast, linear power supplies have a significant drive loss because the output current is regulated by throwing away dropout voltage, which is the minimum voltage differential between the input and output required for regulation. The main problem of the SMPS drivers their lifetime is determined by reactive components. Electrolytic capacitors are the most used reactive component in SMPS systems for their high energy storage density. However, the electrolyte in the capacitor has a high evaporation rate under high temperature conditions inside the driver. To ensure the lifetime of the electrolytic capacitors will match that of the lighting system, efficient thermal management of the driver and the use of electrolytic capacitors with high thermal stability is of paramount importance.

System protection​

LED garage lights should be completely sealed against moisture and environmental contaminants in order to survive a hostile corrosion- and emission-prone environment. A product rated as IP66 or above would be better protected against the ingress of moisture and vehicular emissions which are two extrinsic causes of LED failure. The moisture presence in LED packages can cause cracking and discoloration of silicone encapsulant. Sulfur compounds in vehicular emissions diffused in the LED packages will react with the silver lead frame plating of PLCC LED packages, which results in accelerated lumen deprecation and even premature LED failures. The luminaire housing and exposed lens should be engineered to withstand the corrosive environment generated by humidity, salt, and vehicular emissions. Typically, the aluminum housing receives a UV and abrasion resistant, electrostatically applied, thermally cured, TGIC polyester powdercoat finish. Low mounting heights dictate that impact- and vandal-resistances should be a serious design consideration for garage luminaires.

Lighting control​

A tremendous opportunity remains to maximize energy savings when LED garage lights are paired with lighting controls. Occupancy sensors (motion detectors) are often incorporated into garage lighting systems to control light output based on the presence of vehicular and pedestrian traffic. Occupancy sensors are usually of the passive infrared (PIR) type or the ultrasonic type, but a combination of both is also used. Daylight harvesting is often used in conjunction with occupancy sensors in above-ground garages to significantly increase energy conservation opportunities. Modern lighting control of garage light fixtures are generally designed as networked systems which allow remote operation, interactive control, and interoperation with other building automation systems.