In 2002, we briefly reviewed the benefits of bi-level stairwell lighting, which uses built-in ultrasonic occupancy sensors to dim light levels and cut wattage while stairwells are unoccupied. Studies at various facilities found occupancy as low as 2% with variations depending on type of building (e.g., apartment house, dormitory, commercial offices).

Such products have now been in service long enough to provide a high level of confidence, and independent studies are available supporting their use to cut energy and maintenance costs. Some states offer incentives for installation of such energy-saving devices.

How much could be saved? That depends on existing illumination levels and how far they may be safely reduced. Units are available to cut lighting levels by 50% to 99% with wattage savings ranging from 50% to almost 80%. Some units turn fixtures off completely, but fire and building codes should be closely checked to avoid violations.

An application in which light output is cut by 90% and wattage cut by about 80% would, at an average of $.10 per kWh, yield about a seven year payback (assuming no scaffolding or new power wiring is needed). Bi-level fixtures are standard with electronically ballasted T-8 or compact fluorescent lamps, so greater savings (and shorter paybacks) are possible where existing lighting is still magnetically ballasted with T-12 lamps (or the full light level can be reduced). Costs may be lower if emergency battery backup or vandal resistance is not needed. Where financial incentives are available, payback periods are further reduced.

Bi-level lighting using motion-sensing technology may also be applied in library book stacks, storage rooms, warehouses, and other irregularly occupied areas where full shutoff could create safety or security problems. Depending on the manufacturer, sensitivity and cycling time may be adjustable to minimize false starts (e.g., due to updrafts in stairwells) and maximize savings.

Issues To Consider

Fire and safety codes may require a minimum light level regardless of occupancy. The latest NFPA Life Safety Code 101 calls for 10 foot-candles (fc) on stair steps, but allows bi-level lighting down to 1 fc when stairwells are unoccupied. Check to be sure if and when the new code will apply to your facilities.

A possible conflict exists when using photoluminescent (PL) paint, markings, or exit signs that glow in the dark. Such materials are designed to guide occupants during evacuation when emergency lighting has failed or runs out of backup power. Existing PL products generally require a minimum of 5 fc of illumination for at least one hour to be properly activated when lights go off. Where PL materials are in use, dimming below 5 fc could cause them to glow too dimly when needed. Due to a more stringent code being developed for New York City high-rise buildings, it is likely that more powerful PL materials will soon be available that can be activated by 2 fc applied for two hours. It should be noted that the fc levels in this case may need to be measured in the vertical plane where markings are to be applied to walls, and not just on horizontal surfaces.

New fixtures may have a different light distribution than that existing. As with all new technologies, remember: it's always best to test before you invest.

Case Studies

Two independent studies of this technology were published in 2004. When the California Energy Commission's Public Interest Energy Research (PIER) Program reviewed an application of bi-level lighting in a 10-story university building with multiple stairwells, it found a payback (at California electric rates) of less than four years. That analysis may be downloaded at: www.archenergy.com/lrp/products/-brochures/deliverable_6.2.5_CaseStudy_5.1.pdf.

A similar study (for stairwells in a New York City apartment building) was performed under the Delta Program of the Lighting Research Center. It may be found at: www.lrc.rpi.edu/programs/DELTA/pdf/-DELTAsnapshotStaircaseLighting.pdf. ES