Many factors influence a proper lighting system design. Among these are the costs of system installation, operation and maintenance. Additionally, the lighting quality for each task environment can have a significant effect on productivity, safety, efficiency and, ultimately, on the quality of the end product. Careful selection of lamp type, source and control can lead to significant energy savings while still providing the best possible visual environment.
Flexibility is a critical need in many plant environments. Changes in tasks or work zones; modifications of assembly, inspection or packaging areas; and plant expansion are among the concerns a lighting system should be able to accommodate. Key factors that influence a good lighting design also include the following:
- Occupancy type: A picking warehouse, a bindery and a small parts assembly line each has different layout and control needs.
- Tasks performed: This is a function of the visual acuity necessary for large machines with moving parts, fine detail performed manually or color rendering being critical.
- Plant environment: This addresses whether its unconditioned, the presence of harsh chemicals or explosive vapors, cold storage or processing.
Quality breeds quantity
Recent studies show a correlation between higher-quality lighting and better worker performance. In addition, when lighting quality improves for a particular task environment, the overall quantity (measured in foot-candles at the task surface) can be reduced without adversely affecting performance. Therefore, introducing lamps having a higher color rendering index (CRI), reducing glare, properly directing the illumination, and increasing a workers control over lighting levels can result in a system that consumes less wattage than an equivalent system without these considerations. Managing contrast and uniformity for the work area also has an effect.
If contrast is inadequate, a task can become much more difficult to perform.
Tuning your facility begins with an evaluation of each task type and environment that exists or might exist in the planned future. The Illuminating Engineering Society of North America (IESNA) recommends illumination levels based on the task, speed, accuracy, contrast and size. Using these guidelines, you can intuitively recognize that a warehouse requires significantly less illumination than an inspection station.
The best method for achieving flexible control is by continuous dimming, which permits lighting levels to be varied to specific levels. Fluorescent dimming ballasts typically can dim between 100% to 10% output, and T5HO linear lamps can dim as low as 1%. HID continuous dimming ballasts cant be dimmed much below 50% output.
Step-dimming permits lighting levels to be dimmed incrementally, typically at full, 50% and off. Step-dim ballasts are readily available for fluorescent and HID lamps, and control is by means of two switch legs rather than a dimmer.
Quality improves efficiency
Visually enhanced lighting (VEL) is an innovative way to offset total lighting energy demand. In 2004, the U.S. Department of Energy established a Spectrally Enhanced Lighting Program¹ that has funded several building retrofits to study the savings potential of using fluorescent lamps with a higher correlated color temperature (CCT). It showed that lamps with an 82 CRI and 5,000° CCT provide the same perceived brightness at a reduction of 20% to 46% of connected load when compared with standard lamps.
This is possible because light at higher CCT is cooler or bluer, and the human eye perceives this as brighter². In a retrofit setting, substandard illumination levels can be offset by a direct exchange of existing lamps for spectrally-enhanced lamps. Alternatively, energy consumption can be reduced by replacing existing ballasts with new electronic low-ballast-factor units while still maintaining perceived brightness. For a retrofit, expect a return of one to four years on the upgrade investment. In new construction, implementing a VEL design can provide additional efficiencies by reducing fixture quantities, power distribution requirements, air-conditioning load and peak demand.
Many existing plant environments use high-pressure sodium (HPS) lamp sources for general illumination. HPS lamps are indeed more efficient than their HID and fluorescent counterparts. However, the quality of light is very poor, particularly where work is being performed for extended periods (warehouse shift, for example). The Visual Effective Illuminance (VEI) of fluorescent and pulse-start metal halide lamps is significantly higher than for HPS, which results in HPS being a net efficiency loser. As an example of potential savings, exchanging HPS lamps for T5HO fluorescents can reduce energy consumption while improving the visual environment, even though the lumen output is as much as 18% lower (Table 1).
Table 1 - Click to enlarge
Among the advantages of T5HO high-bay luminaires are improved lumen maintenance, longer lamp life, better color temperature and lighter luminaire weight (20 pounds vs. 50-plus pounds). For safety and reduced worker impact, the failure of a single lamp in the luminaire doesnt result in a total blackout. Also, if power is interrupted or if automatic shutoff controls are in use, the fluorescent lamps restrike instantly when energized. Battery ballasts can be provided for life safety without the need for separate dedicated emergency luminaires. As mentioned earlier, dimming and switching options for fluorescent high-bays are vastly greater than for HID or HPS sources. An option for a task area might include a six-lamp high bay with an occupancy sensor-driven four-lamp ballast and an emergency battery ballast for the two remaining lamps. This permits multi-level switching of 33%, 66% and 100%, with automatic shutoff of four lamps when the area is unoccupied, yet still ensure that the area is never in total darkness if power is interrupted.