Illuminance and uniformity standard 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 values that match the physical characteristics and unique lighting needs of the parking lot need to be determined. RP-20 gives recommendations.
Lighting uniformity (translated into human perception of the uniform distribution of lighting throughout the parking lot) is expressed as the ratio of the maximum lighting level to the minimum lighting level. The current IESNA recommendation is 15:1 (although 10:1 is usually used). This means that when measuring in one area of the parking lot, its illuminance is 15 times that of another area.
A uniformity ratio of 15:1 or 10:1 will not produce what most people call uniform illumination. This will result in bright and dark areas of the parking lot. Such unevenness may make people who walk into the car feel insecure. In addition, these dark areas can encourage illegal behavior.
The lack of uniformity of lighting is largely a function of the traditional HID lamps used in parking lots. HID lamps generate light through the arc between tungsten electrodes in the arc tube. The arc tube can be regarded as a point light source. The luminaire design redirects the light to the desired distribution. The result is usually to illuminate high-intensity or high-intensity light directly under the HID lamp, but in the darker area between one lamp and another.
With the advent of LEDs, the problem of uniformity in parking lot lighting can be solved in a way that was difficult or impossible before HID. Compared with HID lamps, LED lamps inherently provide higher uniformity. The light emitted by LED lamps is not produced by a single point light source (such as HID), but by multiple discrete LEDs. When using LED lamps, this fact usually allows for a lower maximum-minimum uniformity ratio.