Do not use the energy code lighting power density (LPD) maximum as your lighting goal—it is easy to reduce the LPD by at least 15% below the code maximum. For example the CA 2005 and the ASHRAE 90.1-2-004 Energy Codes stipulate a prescriptive maximum of 1.5 watts per square foot for retail buildings; however a large retail chain account recently committed its projects to an LPD of 0.95 watts per square foot—a 30% reduction. High efficient lamp/ballast combinations provide a 15% savings in electricity consumption without reducing light output. Careful design with efficient light fixtures, and strategies incorporating lower ambient light levels with appropriately placed task lighting can further reduce the LPD.
Lighting research suggests that the human eye perceives cooler temperature lighting (lamps rated at 5000 K or greater) to be brighter than standard warmer temperature lamps (3500 K or less). The lamps containing more blue in the spectrum (5000 K or greater) will be more visually efficient than lamps with less scotopic content (white light with a higher bluish content) even if they have the same lumen and efficacy values. The use of scotopically enhanced lighting can therefore be used at lower energy levels while maintaining equal visual effectiveness.
Scotopically enhanced lamps have an energy savings potential of 17-24% compared to 835 lamps (lamps with a Color Rendition Index of 85%, and a color of 3500K) and 22-30% compared to 735 lamps (lamps with a Color Rendition Index of 78%, and a color of 3500K).
An additional advantage of utilizing 5000 K lamps is the integration of the lamp color with natural daylight. Daylight, in general, is cool in color temperature, which gives it a bluish-white appearance. Noontime sunlight has a color temperature of approximately 5000 K, this compares to incandescent lighting at 2800 K, warm fluorescent at 3000 K, and cool fluorescent at 4100 K. 5000K or 6500K lamps may be too "blue" or "cold" for some applications, but proportional savings can be achieved by using 3500K or 4100K lamps in place of 3000K lamps.