Lighting Warehouse Aisles

By Siva K. Haran, P.E., LC, Senior Electrical Engineer, A. Epstein and Sons, Chicago -- Consulting-Specifying Engineer, 6/1/2006

At the risk of making it sound too simple, lighting design for warehouse aisles can be boiled down to two steps: 1) determining the quantity of illumination required at the task and 2) selecting the lighting system by considering functionality or performance, energy and economics.

The primary standard to reference, of course, is the Illuminating Engineering Society of North America (IESNA) Lighting Handbook, which should be used in conjunction with IESNA Design Guide for Warehouse Lighting DG-2. The latter is presently under revision.

Adequate horizontal illumination must be provided not only for safety, but also for navigation convenience and a staff's ability to read documents. Variables to consider are that rack storage areas can vary greatly, according to the material handled, and that visual tasks in warehouse aisles occur on both vertical and horizontal planes.

Regarding the latter, the handbook lists footcandle requirements for active areas where staff handles: 1) small items with small labels; and 2) large items with large labels. These requirements are at the work plane level, which is 2 ft., 6 in. above finished floor level. The horizontal maximum-to-minimum uniformity ratio for a rack area should also be specified.

As for the vertical plane, the Design Guide specifies the vertical maximum-to-minimum uniformity ratio for a rack. Generally, when required uniformity ratio is provided, vertical footcandle requirements will be satisfied.

Be aware that the actual optimum lighting level for a facility could be higher or lower than IESNA recommendations, depending on a multitude of factors, including type of product, the surrounding area's reflective surfaces and age of the employees.

OK, how about lamp types? In the 1960s and 1970s high-bay lamps were primarily high-pressure sodium (HPS). From 1980 through the early '90s, it was a combination of HPS and metal halides. By the late '90s, pulse-start metal halides became popular, finally giving way after 2000 to T8 and T5HO lamps.

For T8 and T5 fluorescents, first and maintained costs are similar, and efficacy is comparable. System functionality and performance should be the deciding factors, including the following considerations:

Controls. T5HOs with programmed start ballasts are often specified because lamps are not adversely affected by repetitive cycling and occupancy sensor controls.

Ambient temperature. T5HOs perform better in non-conditioned spaces, while T8s perform better in refrigerated spaces.

Mounting heights. T5HOs and high-lumen T8s are comparable. Both can be used up to 45 ft.

Project economics. A system with controls will generate the greatest savings opportunity.

Fluorescents provide a better color-rendering index (CRI) than metal halides. Studies have shown that photopic lumens (raw, measurable light) may be less important to our ability to make visual distinctions than scotopic lumens, which take into account the eye's ability to perceive colors more accurately when they are well-rendered.

Finally, higher color temperature (CT) lamps make the environs brighter, because the rod nerve endings in the periphery of our retina are sensitive to higher CT wavelength. This provides better peripheral vision.