Lighting is usually considered the most important safety and security feature in a parking facility; it deters crime and vandalism and creates a sense of personal safety. And since parking lots and garages apply interior, roadway, and pedestrian lighting techniques, anything that can guide a specifier through the design process is valuable.
IESNA's Recommended Practice for Parking Facilities highlights some of the more important factors in the design of a parking facility lighting system. The last 25 pages of this 40-page publication include reference data for topics like the color rendering index of lamps, methods for measuring illuminance, and making photometric evaluations. It also provides help for selecting luminaires.
Light levels. The guide's illuminance recommendations for a parking lot are given in Table 1. These values should be measured or calculated on a clear section of pavement without any parked vehicles. While the data in Table 1 offers illuminance values, or the amount of incident light falling on a surface or object, luminance values may also be considered. Luminance is what a person sees or perceives. It's described as the light reflected off an object and the detection of that object based on adequate luminance contrast between it and its background.
Generally, tenants in an apartment building have the same security needs as a shopper at a retail mall. However, a shopping center owner may want an illuminance level even higher than what's recommended in Table 1 because of a relatively high volume of traffic or a perceived need for increased security at that location. In this case, the design should consider a driver's ability to adapt to the lower light levels of the adjacent streets after leaving the higher light level of the parking lot.
A designer should calculate the uniformity using the maximum to minimum ratio given in Table 1. In some cases you may want to consider alternate designs that either use higher wattage lamps at greater spacing or change the mounting height to reduce energy consumption or trim capital costs. Annex B offers a test grid for quickly figuring out the horizontal foot candle level on a road surface.
Vertical illumination is important in parking lots for making curbs, poles, fire hydrants, and people easily visible. Thus, one lux of vertical illuminance is recommended, and the vertical lux value at the lowest point between luminaires with the minimum horizontal illuminance should be at least one-half the minimum horizontal value given in Table 1. Preferably, it should be greater.
Both the light pole locations and luminaire mounting height should be coordinated with the landscaping plan and/or existing trees because landscaping is typically located in boundary strips, parking row end islands, and curved strips between parking modules in most parking lots.
In open parking areas, light delivered above the light source instead of toward the ground is often called “spill light,” which wastes energy because it puts illumination where it isn't wanted or needed. Parking lot lighting shouldn't shine onto nearby homes. Therefore, the initial design should include some shielding at the edge of the parking lot.
Lamp selection is based on the efficacy, color rendition, and lumen maintenance of the light source. Additionally, local weather conditions, such as extremely low temperatures in the winter, may require the specification of a starting aid for gas-discharge lamps or jacketed lamps in the case of a fluorescent source.
Luminaire considerations. Parking facility lighting luminaires are selected based on photometric distribution to specific areas of the lot surface.
Architectural luminaires blend with the total architectural design of the facility. Generally, they achieve efficient and uniform light distribution through reflectors and refractor lenses.
Pole-mounted luminaires can provide illumination at distances of two to two-and-a-half times the mounting height from the pole.
Post-top luminaires offer symmetrical and asymmetrical distribution, in either direct or indirect design. The mounting height is usually limited to no more than 26 feet.
Indirect type luminaires shield the light source from normal view, thus providing excellent glare control along with a good aesthetic appearance. These types of luminaires are used for parking lot lighting and hold lamps of 250W or greater, with a mounting height of 20 feet or more.
High mast systems have a mounting height of 66 feet or more and can cover a large area with a minimum number of poles. Suitable for nonresidential areas, the luminaires of a high mast system can provide symmetrical or asymmetrical distribution using refractors and/or reflectors, or the distribution can be totally indirect.
Roadway fixtures provide a wide range of symmetrical and asymmetrical distributions. The wall-mounted unit is applicable to a narrow area between buildings. The mounting height is usually no more than 26 feet. A wide variety of distribution types are available, generally divided in cutoff and semi-cutoff distribution design.
Covered parking facilities. The illuminance recommendations for a garage are given in Table 2 on page 18. Low ceilings, wide fixture spacing, shallow viewing angles, the need for signage, and the difficulty in keeping adequate reflectance on the various surfaces call for adequate initial and maintained illuminance.
Maximum to minimum illumination uniformity is a primary concern in a covered parking facility because people traverse the driving lanes going to and from their parked vehicles and the exits, elevators, and stairwells. Thus, the spacing between luminaires shouldn't create dark areas. At the same time, the fixture layout should represent a balance between safety and the lighting system's total operating cost.
Low ceilings of a typical parking garage require precisely controlled light distribution with minimum glare in the direct viewing zone. The luminaire profile should be shallow and direct enough light up to eliminate shadows, make the ceiling appear higher, and create a sense of spaciousness. This can be accomplished using compound reflectors.
During the day, the portal to the parking structure should have a transition area from bright sunlight (outdoors) to the darker garage interior. Electric lighting, in addition to sunlight in the entrance area, should meet the uniformity criteria and minimum illuminance criteria of Table 2. Dimming or switching used with automatic controls, which doesn't adversely alter lighting uniformity, should be used.
Luminaires for garages have photometric requirements very similar to those for parking lots. They can be categorized as cutoff and non-cutoff. Cutoff luminaires limit high angle light and usually have a flat lens. A dual-reflector system offers excellent glare control and maximizes fixture spacing. Non-cutoff luminaires generally have a dropped diffusing lens to allow wider spacing between fixtures.
Lamps. Parking lighting applications generally employ two types of high-intensity discharge (HID) lamps: metal-halide and high-pressure-sodium lamps. Additionally, modern luminaire optical systems usually use clear (uncoated) lamps.
Occasionally, low-pressure-sodium (LPS) lamps are used in parking facilities. Although higher in efficacy than any of the HID lamp types, the poor color properties of the LPS lamp have limited its acceptance.
Previously, fluorescent lighting systems were widely used for garage lighting because of their low unit brightness per linear foot. An advantage of the fluorescent lamp is its quick restart capabilities following a momentary power outage. The continuing development of biaxial, quad, and other “compact” fluorescent lamps favor their use, especially in stairwells, ramps, and some parking bay applications. In all cases, a luminaire exposed to sunlight, wide temperature ranges, humidity, salt, vehicle emissions, wind-driven rain, and vibration must be able to withstand these conditions.