Good Lighting: Key to Optimum Warehouse Performance
A poorly illuminated warehouse is an environment ripe for errors. Most warehouse facilities have a variety of functions performed within their walls—from picking operations and packaging to shipping and receiving, light assembly and even office work. These multiple tasks make lighting design a challenge because more than one task may be performed within the same area, requiring different light levels.
Good lighting is essential in a warehouse to promote worker visibility, safety and performance. An efficiently illuminated facility will also reduce operating costs, which will impact the bottom-line.
Selecting a lighting system
Whether the application is a new facility or a lighting retrofit, the question often arises as to what type of lighting system is best—a high intensity discharge (HID) system or fluorescent.
If efficiency is the primary concern, a high-pressure sodium (HPS) system is the most efficient light source available, providing the most lumens per watt. HPS lamps have a longer life than standard metal halide systems, which translates into lower maintenance costs.
However, many warehouse owners and managers are reluctant to use HPS systems because of the lamps’ yellowish color. In warehouse applications where storage, shipping and receiving are the main functions, color may not be an issue.
But, in applications where employees are doing light assembly work or boxing product, color identification may be essential. Color rendering is the ability of the light source to represent the true colors in an object. The closer the color rendering index (CRI) is to 100, the more natural product colors will appear. The CRI of metal halide lamps is 65, with HPS lamps at 20. The CRI for fluorescent lamps is 80.
Many warehouse owners and managers tend to prefer the white light provided by metal halide or fluorescent systems. Even though the measured footcandles may be nearly the same as for an HPS system, employees believe they can see better under a metal halide system because the environment appears brighter.
Pulse start metal halide lamps are the most efficient metal halide lamps on the market. They not only provide the light levels needed, but they offer more light over the life of the lamp. A pulse start lamp will produce 110 lumens per watt (LPW) compared to 80 LPW provided by a standard metal halide lamp. Pulse start metal halide lamps also warm up faster to full brightness and have quicker restrike times. Although pulse start lamps cost more initially than conventional metal halide lamps, warehouse owners and managers benefit from lower installation, operating and maintenance costs (See Figure 1).
New luminaire and lamp technology has also made fluorescent systems better suited to warehouse environments. The new systems provide the illumination levels needed in facilities with higher ceilings and they use less energy than a metal halide system. For example, one 400-watt metal halide high bay fixture may be substituted with six 32-watt fluorescent fixtures for a total wattage of 192 vs. 400 (not including ballast losses).
Fluorescent systems also offer the advantage of “instant on.” All HID systems have a warm up time. During a power surge or outage, for example, a standard metal halide luminaire will require 10 to 15 minutes to cool down, plus an additional five to seven minutes to heat up. Fluorescent systems, on the other hand, will re-light immediately.
Fluorescent systems operate cooler than HID luminaires. The higher the wattage lamps, the more heat an HID system will produce, which could lead to higher HVAC costs. If a facility’s air conditioning costs are unusually high and the lighting system uses high wattage lamps, the system may be retrofit with fluorescent luminaires or lower wattage HID lamps to reduce the operating costs.
The down side of fluorescent systems is that they are heat sensitive. A fluorescent system may not be suitable for a warehouse in Texas that has no air conditioning during the summer.
Providing needed light levels
The light levels and visibility required within a warehouse will depend upon a number of factors, including the tasks performed, the age of the workers and the type of space—whether it is an open space or has racks. The more active the area—such as a loading dock or staging area—the higher the light level requirements. Illumination levels will also be affected by the size of the items that are being handled. For example, an active area with small items (and small labels on containers) will require 20 to 50 footcandles on average. An active area with large items will require only 10 to 20 footcandles. An inactive area, such as a cold storage facility, will require only 5 to 10 footcandles.
The above recommendations refer to the work plane. In warehouses, this is likely the vertical stack surface, and at times, the horizontal at document reading height. Adequate levels of vertical illumination are essential to read the labels on cartons, read signs within the facility (including exit signs), and to drive a forklift.
The best way to determine the required horizontal and vertical illumination levels is to consider the average to minimum ratios. The horizontal average to minimum ratio for a rack area should not exceed 3:1. If the average light level in an aisle is 20 footcandles (fc), the minimum horizontal levels should be 6.7 fc or greater. The vertical average to minimum ratio for this same area should not exceed 10:1. This means that if the average footcandle level is 15, the minimum vertical footcandle level should never be less than 1.5 fc.
Contrast and the size of objects involved in a task will also influence the light levels required. Tasks are easier to see when the contrast between an object and its background is greatest. For example, black lettering on a white label will have a higher level of contrast than dark blue lettering on a medium blue or medium gray background. Higher illuminance will help compensate for poor contrast and smaller objects.
The age of the workers is another consideration when determining the light levels needed. As workers grow older, they require more light to compensate for degraded vision and the decreased size of the pupil. Attaining retinal illuminance for older workers equal to that of younger workers requires greater luminance on the task. (See Figure 2).
Light levels will be affected by the reflectance of surfaces within the facility, such as the walls, ceiling, floor, racks, pallets and containers. A black or dark colored wall or ceiling will not be as reflective as a white wall. For example, a space with two brown walls and two white walls may require six luminaires to provide the light levels required. The same space with four white walls will require only four luminaires. Keep in mind that shiny metal racks will reflect light while brown-colored boxes and other containers will absorb light.
If glare is a potential problem, consider using enclosed HID luminaires or a fluorescent system with a bottom lens. Installing luminaires with an element of uplight will also help reduce glare.
Lighting uniformity within a warehouse space is essential. Forklift drivers and others must be able to look up and down the stacks of product without constantly having to adjust their eyes. The human eye functions more comfortably and efficiently when the luminance within the field of vision is fairly uniform.
Providing an element of uplight will help create a more uniform environment. Most glass luminaires will provide 15 to 20 percent uplight, which illuminates the ceiling and eliminates the cavern effect that can occur when a ceiling is dark. With a white or light colored ceiling, uplight will bounce off the ceiling to create a more uniformly illuminated environment.
Luminaire placement will affect lighting uniformity. Warehouse managers typically try to get the most use of the space they have within their facilities. For example, if the ceiling height is 30 feet, products in the racks or on pallets may be stacked to within a foot of the ceiling. Mounting the luminaires in the middle of the aisles instead of directly above the racks or pallets will minimize shadows and will help assure maximum lighting uniformity.
Lighting fixtures should be placed above the tallest rack or pallet for the best lighting distribution—which may make the case for using a low profile luminaire. Low profile luminaires may also be better suited to environments where forklifts are used to move merchandise so the forklift masts do not strike and break the luminaires.
A luminaire’s distribution pattern will contribute to a system’s uniformity. Fluorescent fixtures or HID luminaires with an asymetric (long and narrow) pattern are best for aisleways, whereas luminaires with a symmetric pattern are more applicable for open storage and shipping areas. In open areas where multiple tasks are performed, the ambient lighting may need to be supplemented with additional fixtures. For example, in an open storage space where light assembly is performed, supplemental fluorescent luminaires may be installed directly above the task to boost the light levels.
Lamps may burn constantly in areas that are continually in use. However, in peripheral aisles that are used only part of the time, warehouse operating costs can be reduced by turning HID lamps to lower levels when a space is unoccupied—or turning the fixtures off completely. A bi-level switching system may be installed to reduce the input wattage by 50 percent for metal halide lamps, or 35 or 50 percent for high-pressure sodium lamps.
Fluorescent systems may be wired with multiple switching so that the luminaires within a certain area can be turned on and off as needed. This capability increases the system’s flexibility and allows the facility to take advantage of skylights and daylight harvesting to reduce operating costs.
Warehouse managers, owners and designers have a variety of options when selecting the lighting system for a warehouse environment. But, no matter what type of system they choose, lighting uniformity is essential. A uniformly illuminated environment will not on promote worker safety but will improve performance. An efficiently illuminated facility will also impact the bottom-line.