The long life and energy efficiency of Light Emitting Diodes (LEDs) has sparked excitement from consumers, manufacturers, and energy conservation groups. In fact, the US Department of Energy predicts that by gradually switching to LEDS over the next 20 years, the nation could save around $250 billion in energy costs and cut our lighting electricity consumption by half. That’s a big deal!

As LED technology continues to evolve, so too grows the efficiency and lifespans of LED lamps. Because LEDs have such long lives (estimated at 35,000 to 100,000 hours), it’s rather challenging to measure precisely how long they really do last — few groups have the patience or resources to measure an LED in various environments for 5 or more years. However, the National Institute of Standards and Technology (NIST) is currently working on such a project.

So, how do we measure service life of LED systems? Well, to understand the process, it helps to know how lamp life is measured in traditional bulbs…

Measuring Life of Conventional Bulbs

The lighting industry uses the term ‘Average Rated Life’ or ARL when describing how long it takes for 50% of lamps in a particular test batch to fail. For instance, if 100 fluorescent bulbs are tested and 50 bulbs die after 10,000 hours then that type of bulb has an ARL of 10,000 hours. Although some bulbs may have failed before or after that mark, 10,000 hours was the point where 50% had died, so it becomes the ARL.

Traditional lights, such as incandescent, fluorescents, and HIDs have all been used for many decades and their various modes of failure (burned filaments, cathode burnout, electrode wear, etc.) are highly predictable. Thus, even when a new lamp comes on the market, those in the industry can accurately estimate its service life by referencing accepted statistical projections.

However, LEDs turned this whole measuring process on its head. This is because they do not have a consistent type of failure and never really burn out but simply become dim.

LED Life is Gauged by Lumen Depreciation

Without a predictable failure point, manufacturers decided to define LED life as the amount of time it takes for the light to fade to a certain percentage of its original strength. This designation is represented by the letter ‘L’ followed by a set of numbers such as:

L70 = time to 70% of original light output. There are other ratings, but this is the most common rating and is universally accepted as the standard to measuring LED life.

Still, even these numbers are highly variable depending on how and where the LED was operated. Things that may influence the LED’s life are junction temperature, operating current, the type of luminaire and the quality of the material used. These differences make defining a LED’s service life even more challenging, since the lamp’s environment can drastically influence its lifespan.

Put simply, we can’t determine the service life of an LED without considering its housing and application. This is the major reason why the NIST is conducting their LED study. They are monitoring LEDs in various scenarios with the goal of uncovering a reliable method of projecting how long LEDs will last.

Getting the Most Out of Your LED

Uncertainties aside, given the right conditions, we do know that LEDs have the potential to far out last conventional lamps. For example, compare the typical lifespans of common lamps:

Incandescent = 750 to 1,000 hours
Halogen = 2,000 to 3,000 hours
Metal Halide = 7,500 to 10,000 hours
Fluorescent = 15, 000 to 20,000 hours
Mercury Vapor = 16,000 to 24,000 hours
High Pressure Sodium = 15,000 to 25,000 hours
High Power LEDs =35,000 to 100,000 hours

Some things you can do to get the maximum life from your LEDs are to buy products that use high quality LED chips and a quality heat sink that properly dissipates heat away from the LED. Although quality LEDs which assist in reducing heat may cost extra, you’ll find you get much more service life for your money. Also, fluctuations in supply currents can shorten an LED’s life, but again, a high grade bulb should eliminate this problem.

Although the lighting industry is still learning about the efficiency of LEDs, what we currently know holds real promise for major energy savings in the future. And while the LED bulb you pick off the store shelf may not entirely live up to its 100,000 hour rating, it will certainly outperform most traditional bulbs.

One of the strongest 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.