Lumen maintenance strategies have only recently become economically viable for general use. Conventional lighting systems are designed to provide more than the required levels of illumination when new; lighting levels gradually drop as the system ages until somewhere in the middle of the effective life of the lamps, the "maintained" lux level of the system is (temporarily) achieved. This overdesign has historically been based on the popular philosophy that being overlighted for part of the time is better than being under lighted.
Fortunately, with the development of dimming electronic ballasts that are competitively priced, systems that take advantage of lumen maintenance techniques can be designed.
The strategy is straightforward: design the system in the standard manner, slightly overlighted initially; then, tune the system to the exact lux level desired. This design requires dimming ballasts in all fixtures that are included in the lumen maintenance zone and a control system that will allow ballasts to increase output as necessary. As the system ages and lamps begin to degrade, the ballast increases voltage to the lamp, which causes the lamp to produce more light.
The energy efficiency of this procedure is exceptional, with initial savings of 50 percent being realized in many instances in comparison to traditional lighting designs. The savings over the life of the lamp are expected to be significant; exact numbers are not available to date. Lawrence Berkeley National Laboratory's lighting research group is currently conducting studies to determine actual savings and effects on lamps and ballasts operated over time in this mode. It is hypothesized that lamps will have longer useful lives and that savings over time could be as high as 40 percent.
There are a number of methods for controlling lumen maintenance areas. They are:
·Installing photoreceptors tied directly to ballasts, controlling the voltage output to the lamps based on the set level of luminance.
·Connecting dimming ballasts directly to an energy management system that has been programmed for the expected depreciation of the lamps; the energy management system directs the output of ballasts as necessary to maintain required lighting levels over time.
·Manually verifying lighting levels in a space by using photometers in specific locations, and then manually setting the system to meet lighting requirements.
Each of these options provides reliable technologies that enable successful lumen maintenance strategies. Designers must select the most appropriate control device for a laboratory's particular needs.