Lighting design for outdoor areas can be challenging, as it requires careful consideration of various factors. From stacked materials to heavy machinery, the lighting designer must account for potential obstructions and ensure optimal light distribution. Unlike indoor installations, outdoor lighting designs offer fewer fixtures, leaving little room for error.

The Versatility of Pole Mounting

While lighting equipment can be installed on any high structure, pole mounting provides the most versatility. Luminaires on poles offer illumination in all directions at distances of two to two and a half times the mounting height. Consequently, a single pole can cover an area approximately four times its own height. For example, a 50-ft pole can illuminate about 40,000 sq ft, while a 150-ft pole can cover up to 369,000 sq ft. Narrow beam floodlights can be used to light a flat area extending up to five times the mounting height from the pole. However, beyond this distance, uniformity and system efficiency decrease significantly.

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Minimizing Shadows

By understanding the law of similar triangles, it becomes clear that the length of shadows diminishes with increasing mounting height. Fig. 1 (not available online) demonstrates this relationship. For instance, at a horizontal distance from the pole twice the mounting height, the length of the shadow will be twice the height of the object casting it.

Choosing the Right Luminaire

Once the luminaire locations and mounting heights are determined, it is crucial to select the appropriate quantity and type of luminaires. Taller poles allow for the use of higher wattage lamps, which are more efficient than lower wattage sources. High-pressure sodium (HPS) or metal-halide (MH) lamps with wattages of 1000W or 750W are commonly used in high mast applications.

Considerations for the beam spread and candlepower distribution pattern of the luminaires are also essential. High mast luminaires and floodlights offer symmetrical and asymmetrical beam spreads. The beam spread of a high mast luminaire can be adjusted by vertically positioning the lamp in the reflector assembly or by selecting reflector-/refractor-type units. Floodlights, typically installed on structures and low poles, provide circular reflectors with the lamp mounted in the center. Beam spreads and their effective projection distances are classified from Type 1 to Type 7.

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Calculating Lighting Requirements

To determine the number of luminaires needed or the light level provided by a specific number of fixtures, the following formula can be used:

fc = [(N)(BL)(UF)(MF)]/Area

Where:

• fc represents the average maintained illumination level in footcandles
• N indicates the number of luminaires
• BL denotes the beam lumens of the luminaire
• UF corresponds to the utilization factor (percentage of beam lumens within the lighted area)
• MF represents the maintenance factor (light loss factor)
• Area refers to the area to be lighted in square feet or square meters

For general criteria, this simple calculation suffices. However, to design an outdoor lighting system more comprehensively, an isofootcandle plot can be used. This plot graphically represents the light distribution pattern on a horizontal surface and is created by lines, or contours, with each line representing a different footcandle.

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Frequently Asked Questions

Q: Why might the actual installation's illuminance differ from the design calculations?
A: Site conditions and variations in lamp output or ballast performance can contribute to differences between predicted and measured illuminance.

Q: What factors can affect light distribution in an outdoor lighting installation?
A: Skewed lamp arc tubes, misalignment of the fixture, or mounting reflectors or refractors off-axis can all influence light distribution. Voltage drop in feeder or branch-circuit conductors can also impact the performance of the fixtures.

In conclusion, designing outdoor area lighting requires careful consideration of factors such as obstructions, mounting heights, luminaire selection, and light distribution. By understanding these elements, lighting designers can create effective and efficient outdoor lighting solutions.