Lighting Uniformity: Study Shows An Important Advantage of LED Fixtures for Parking Lots

One of the fastest growing segments of the LED commercial lighting market is site, area, street and parking lot lighting. Owners of commercial property and municipalities in particular are looking at LED solutions for new developments or to replace HID fixtures in existing applications.

In this post we will look at occupant perceptions of safety, security and visibility in parking lots. Specifically, we will review a study that uses LED parking lot light fixtures to focus on the relationship between illuminance levels and lighting uniformity and how the two affect occupant perceptions of safety. The results are significant for parking lot lighting design and fixture selection and can be applied more generally to site, area and street lighting as well.

Lighting Design Criteria for Parking Lot lighting

Current design recommendations from the Illuminating Engineering Society of North America (IESNA) for parking lot lighting are found in the latest version of RP-20 (2014).

Illuminance

Illuminance values that match the physical characteristics and unique lighting needs of the parking lot need to be determined. RP-20 gives recommendations.

Uniformity

Lighting uniformity (which translates to the human perception of how evenly illumination is distributed throughout the parking lot) is expressed as the ratio of maximum-to-minimum illumination levels. The current IESNA recommendation is 15:1 (although 10:1 is commonly used). This means there can be 15 times more illumination when measured in one area of the parking lot compared to another.

A 15:1 or 10:1 uniformity ratio does not produce lighting that most people would call uniform. It will result in both bright and dark areas of the parking lot. This unevenness will likely produce feelings of insecurity for people walking to their vehicle. In addition, these dark areas can also encourage illicit behavior.

Lack of lighting uniformity is largely a function of traditional HID fixtures used in parking lots. HID lamps generate light by means of an electric arc between tungsten electrodes inside an arc tube. This arc tube can be considered a point source of light. The fixture design re-directs the light into the desired distribution. The result is often high or higher intensity illumination directly under the HID fixtures and darker areas between one fixture and the next.

With the emergence of LED, the issues around lighting uniformity in parking lot lighting can be addressed in ways that were previously difficult or impossible with HID. LED fixtures inherently provide higher uniformity compared to HID fixtures. The light from LED fixtures is generated not from a single point source, like HID, but from multiple, discrete LEDs. This fact usually allows for much lower maximum-to-minimum uniformity ratios when LED fixtures are used.

Relationship Between Illuminance and Uniformity

The relationship between illuminance and uniformity in parking lot lighting is the focus of a study by the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute. The LRC published results (in 2015) of a research project they conducted of occupant perceptions of the key issues of safety, security and visibility in parking lots under different illuminance levels and two different lighting uniformity ratios: 10:1 (base line) and 3:1 (improved).

The parking lot in the study measured 120 ft. x 250 ft. The researchers were able to switch between six light levels ( 0.2 footcandles to 6.0 footcandles and two uniformity ratios 10:1 (baseline) and 3:1 (enhanced uniformity).

The interviews were conducted with occupants in the parking lot at night with no full moon and minimal contribution of other nearby lighting.

The occupants perceived the lighting as safer when the uniformity was higher (3:1). Significantly, this result remained true even when the illuminance level was increased. Other research has confirmed these conclusions.

Conclusions

LED fixtures in parking lots provide a significant benefit compared to HID for increasing occupant feelings of safety and visibility -largely because of increased lighting uniformity.

This study also demonstrated that the higher inherent lighting uniformity of LED fixtures in parking lots means lower light levels are possible while not compromising and perhaps even enhancing, occupant perceptions of safety.

Read the complete LRC study.