A wide range of solutions can meet the diverse needs of site illumination.

Properly preparing a lighting design for outdoor loading, storage, and fabricating areas is usually a difficult task. The lighting designer must consider whether stacked materials or heavy machinery will interfere with light distribution, or whether work activities will obstruct or block some of the light distribution.

Picking lighting equipment for the job is sometimes more difficult with an outdoor installation than with an indoor area, because fewer fixtures contribute their light to a given area. Generally, this means there's little margin for error in an outdoor lighting design (see sidebar below).

Although you can install lighting equipment on any high structure, pole mounting offers the most versatility. Luminaires on poles can provide illumination in every direction at distances of two to two and half times the mounting height from the pole. Thus, luminaires on a singe pole can serve an area of about four times the mounting height - squared. For example, a 50-ft pole can cover about 40,000 sq ft and a 150-ft pole about 369,000 sq ft. You can use narrow beam floodlights to light a flat area extending to five times the mounting height from the pole. However, at distances greater than two times the mounting height, uniformity and system efficiency drop off considerably.

You can see the effectiveness of minimizing shadows with a given mounting height in Fig. 1, on page 28 (not available online). This figure shows that the relationship between the length of shadows and the luminaire mounting height follows the law of similar triangles. For a horizontal distance from the pole of twice the mounting height, the length of the shadow will be twice the height of the object casting the shadow.

Once you establish the luminaire locations and mounting heights, determine the quantity and type of luminaire. If you select tall poles, you can use higher wattage lamps, which are more efficient than lower wattage light sources. Generally, 1000W or 750W high-pressure sodium (HPS) or metal-halide (MH) lamps are the choice for high mast applications.

In addition to choosing the lamp type, wattage, and number and location of luminaires, a designer must consider the beam spread, or the candlepower distribution pattern the luminaire provides. High mast luminaires and floodlights offer symmetrical and asymmetrical beam spreads. Naturally, a high mast luminaire delivers most of its light directly downward. But, you can vary the beam spread of a high mast luminaire by vertically adjusting the lamp in the reflector assembly. You can also select reflector-/refractor-type units, which cast light at a high angle. When you rotate the optical assembly of an asymmetrical-beam, a high mast luminaire allows you to shape the distribution pattern of a cluster of these luminaires.

Typically installed on structures and low poles, floodlights have a circular reflector, with the lamp mounted in the center. Floodlight beam spreads and their effective projection distances are classified by a joint IES/NEMA designation. The beam-spread patterns extend from Type 1 to Type 7. Fig. 2, on page 30 (not available online), shows Type 2 to Type 7 beam patterns, which are the most widely used types. In all cases, as the distance from floodlight to the illuminated area increases, the beam spread becomes wider.

Type classification assumes a symmetrical beam shape, meaning that the beam spread angle in the vertical and horizontal axes are identical. Generally narrow projection beams (Type 1, 2, 3, and 4), which are useful for directing a long throw of light, have a symmetrical beam spread. However, outdoor floodlights with Type 5, 6, and 7 beam spread have different beam spread for the vertical and horizontal axes, since they're generally used to project their light output at medium to close distances.

You can use the following simple formula to rapidly figure the number of units needed for a given light level or to determine the light level provided by a certain number of fixtures.

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

where, fc = average maintained illumination level in footcandles

N = number of luminaires

BL = beam lumens of the luminaire

UF = utilization factor (percentage of the beam lumens that fall within the area being lighted)

MF = maintenance factor (light loss factor)

Area = area to be lighted in square feet or square meters.

If the lighting project must satisfy only general criteria, such a simple calculation is sufficient. However, a better method of designing an outdoor lighting system is to use an isofootcandle plot.

An isofootcandle plot graphically represents the light distribution pattern on a horizontal surface. The graph consists of a series of lines, or contours, that represent the same illuminance anywhere on the line, with each line representing a different footcandle. Each contour from the center out represents approximately 50% of the value of the previous contour. The plot is placed over a grid, which you can use to indicate mounting height divisions. An isofootcandle plot can vary in shape from a circle, oval, or triangle, and may be symmetrical or asymmetrical.

Essentially, you can use an isofootcandle curve at the same scale as a plan view of the area to be lighted to determine the contribution of each luminaire to the entire area.

Today, manufacturers have powerful and relatively inexpensive software programs to perform these calculations. Many of these programs perform lighting design calculations based on isofootcandle curves and footcandle tables for each luminaire type.


Sidebar: Why Design Calculations Can Differ from the Actual Installation

When using software, a contractor might find that after the installation, the measured illuminance differs from the computer-predicted illuminance. Why? A number of factors may cause this variation. First of all, site conditions frequently vary from the assumptions used in preparing the design.

Then, consider that a lamp can vary 55% in light output and still be within the manufacturer's tolerances. An HID ballast can vary 57% and still be within tolerance. Thus, it's possible for a lamp/ballast combination to be 12.5% under the predicted output.

You may find that the installer skewed the lamp's arc tube or mounted the fixture slightly out of alignment - resulting in the distribution of light at angles other than those intended.

Another factor could be a reflector or a refractor also mounted slightly off axis, producing similar results. It takes only a few degrees of tilt to produce significant change in the light distribution pattern.

Low voltage at the ballast of the fixtures could also be a problem, resulting from excessive voltage drop in the feeder or branch-circuit conductors. A regulator-type ballast is available for such a situation.