Placing material into storage for later retrieval has been an important component of both the manufacturing and distribution industries for decades. But in recent years, the concept of “warehousing” has expanded to include new procedures and technologies, such as bar coding, shrink wrap packaging, high-rise storage, automatic placement and retrieval of material, and radio frequency identification tagging (RFID) of cartons, boxes, and modular containers.
Thus, the Illuminating Engineering Society of North America (IESNA) Design Guide for Warehouse Lighting DG-2 describes the current lighting methods to achieve efficiency and safety in warehouses and storage areas. This 21-page design guide should be used in conjunction with other IESNA Recommended Practices, especially the ANSI/IESNA Recommended Practice for Lighting Industrial Facilities RP-7-01, which discusses the appropriate quantities of light for various tasks as well as the quality issues that are equally important in providing a comfortable and safe work atmosphere.
Specific visual tasks in a warehouse can be quite varied.
Loading dock areas require general illumination for safety and efficient movement of materials. Although this typically calls for overhead lighting, supplemental lighting can provide illumination into a trailer or container.
Workers in shipping and receiving areas need to read shipping documents, so good quality lighting with shielding provided by lenses, louvers, and other glare reducers is required. Task lighting may be necessary at individual workstations.
Open storage areas for bulky items calls for general area lighting that offers a balance of vertical and horizontal illumination. Aisles that are formed by the movement of materials may be constantly reconfigured, and the quality of illumination on the vertical surfaces of stored goods is essential.
A rack storage area can vary greatly according to the material handled. The required light can be defined as general area lighting with a balance of vertical and horizontal illumination.
Despite those differences, it's possible to get started designing a suitable lighting system by following this five-step process:
Determine the quantity of illumination required at the task or process.
Determine the lighting required for safety, and ensure all three of the factors — quality, quantity, and safety — are properly considered in the final design.
Select listed or approved lighting equipment that satisfies each requirement, including mechanical performance.
Prepare a lighting fixture layout that considers maintainability and safety.
Review the energy, economic, and operating characteristics of the lighting system.
The light levels and visibility required within a warehouse depends on a number of factors, including the task performed, the age of the workers, and whether the space is open or obstructed. The more active the area — such as a loading dock or staging area — the higher the light levels need to be.
Illumination levels are also determined by the size of items handled in the area. An active area where small items with small labels are handled will require 20 to 50 horizontal footcandles on average. An active area where large items are handled require only 10 footcandles to 20 footcandles. An inactive area, such as a cold storage facility, needs about 5 footcandles to 10 footcandles.
These recommendations refer to the horizontal footcandle levels on a work plane, but other lighting should be considered. Employees read signs in the facility and labels on cartons, and personnel maneuver material handling equipment, which makes adequate vertical illumination important as well.
In a warehouse, determining the needed horizontal and vertical illumination levels also involves considering the average to minimum ratios. The horizontal average to minimum ratio for a rack area should not exceed 3:1. So if the average light level in an aisle is 20 footcandles, the minimum levels should be 6.7 footcandles or greater. The vertical average to minimum ratio for the same area shouldn't exceed 10:1. So if the average level is 15 footcandles, the minimum should be 1.5 footcandles or greater.
Providing an element of uplight helps to create acceptable illumination ratios. Most glass- and plastic-lensed HID fixtures and industrial fluorescent fixtures provide 15% to 20% uplight. If the ceiling is light in color, the upward light component from the luminaire will be redirected downward. In general, the colors used on the interior surfaces are important; a black or dark colored wall or ceiling may require 33% more luminaires than if the surfaces were light colored.
Active stacking aisles should be considered as rooms for the purpose of lighting calculations. The “room” then has the width and length of the aisle, and the actual ceiling height above the aisle is the height of the room.
HID luminaires with symmetrical light distribution spaced reasonably close together are usually specified for this type of long and narrow room. This spacing can be extended to two times the mounting height if the luminaires have a substantial uplight component and the ceiling zone has a highly reflective surface.
Asymmetrical distribution HID luminaires, which can be spaced farther apart, can also be used. If aisles are 20 feet wide or greater, low-bay luminaires with prismatic optics and symmetric distribution are a good selection.
When maximum spacing is used to gain proper illuminance levels, the designer should consider the possibility of source glare that could adversely affect personnel looking straight up. Shielding and diffusing media will block lamps from direct view (to prevent glare) and diffuse or redirect the light. The media include diffusers, lenses, and louvers.
For open areas that hold pallets and other materials, it's common to specify a high-bay metal halide (MH) luminaire with either an open-bottom specular aluminum reflector or a prismatic refractor housing with a 15% uplight component and a field adjustable lamp position for widest light distribution. In industrial and warehouse facilities, where previously the high-pressure-sodium (HPS) lamp was widely used, the MH lamp is now the overwhelming choice.
Industrial fluorescent luminaires in a continuous row or individually mounted units are also specified in warehouses; 4-foot or 8-foot T8 lamps with electronic ballasts within luminaires that provide a 15% uplight component are most common.
More recently, the T5 line of fluorescent lamps, which consists of the standard T5 lamp and the T5 high-output (HO) lamp, are also being specified. Straight tube T5 lamps are available in nominal 2-foot, 3-foot, 4-foot, and 5-foot lengths.
While the T5 and T5HO are the same diameter and length, the 4-foot T5 is rated at 2,900 lumens, similar to the lumen-per-watt output of a T8 lamp. On the other hand, the 4-foot T5HO lamp is rated up to 5,000 lumens, offering twice the maintained light output of a T8 lamp. This means that on some projects a designer can use fewer fluorescent fixtures or lamps, thus providing certain savings on installation and long-term maintenance. In addition, the T5HO is suitable for almost any application with ceiling heights greater than 15 feet. Refer to the Table on page 26 for more differences between T8, T5, and T5HO lamps.
Regardless of the light source used in a warehouse, a high/low dimming system controlled through passive infrared (PIR) detectors provides efficiency, flexibility, and economy for storage areas that are continuously occupied. One type of PIR has a wide beam, suitable for large open areas, while other types have long narrow viewing areas, making them ideal for an aisle.
The construction of a new warehouse offers lighting designers an opportunity to include skylights and gain the advantage of daylight illumination. To establish the greatest lighting uniformity, space skylights on a uniform grid no more than 1.5 times ceiling height. Diffusing the light is important to prevent glare and excessive light levels. The optimum number and size of skylights needed is determined by many factors, such as glazing material, building design and operation, local energy costs, and local climate conditions.
Automatic photo control systems make it possible to reduce power usage when skylights are used. Most facilities place a photosensor facing upward in the skylight well. Sensing only the daylight levels, these “open loop” systems are simple, relatively inexpensive, and reliable. The photosensor sends a signal to a controller that determines a response to the available daylight by slowly dimming or switching lamps.
An automatic switching system is the most common control procedure for warehouses with skylights. Circuiting individual fixtures in three groups is recommended for MH luminaires. The first group should switch off when the daylight level exceeded the electric light level by 40% for five minutes or more. The second circuit turns off when daylight level exceeds the electric level by 80%. The third circuit remains on during occupied hours. A time delay of at least 10 minutes (or at least twice the restrike time) should be used before energizing the circuit when the daylight level drops.
Fluorescent lamps can be automatically switched within a 4-lamp luminaire. Four levels of output are gained by switching lamps individually for greatest uniformity, or pairs of lamps can be switched for greatest economy. A time delay of at least five minutes should be used before switching on or off.
The term “warehouse” may bring to mind images of dark, cavernous spaces, but it doesn't have to be that way. With the right lighting schemes, it can be a comfortable — and safe — place to work.