Environmentally Responsible Electric Lighting Design

A systems approach to electric lighting is needed to achieve maximum energy - effective and energy - efficient design. This is the energized system that includes luminaires and controls. Luminaire is the technical names for the complete lighting unit that consists of the housing, lamps, ballasts, and transformers as well as light - controlling elements such as reflectors, shielding devices, and diffusing media.

In making any lighting decision, illumination needs must be established and trade - offs between electric lighting options need to be assessed. The cost - value benefit analysis for each option includes energy cost, lighting - system costs, operating costs, and lighting - quality issues. Lighting - quality issues cover a range, including employee productivity and absenteeism, security and safety, business image and environmental “mood,” and accommodation of spatial changes.

EFFICIENCY RATINGS

Efficiency of the electric lighting system is dependent on characteristics of each individual component as well as how the components work together to produce electric illumination. There are different measures of efficiency related to different components of the system.

• Lamp efficacy is the technical term to describe how efficient a lamp converts electricity of visible light. This is stated as lumens per watt (LPW).
• Luminaire efficiency is the ratio between the light output from the fi xture and the light output generated by the lamps it houses. This is stated as a percentage.
• Coefficient of utilization (CU) is concerned with the amount of light that reaches the work surface relative to the amount of light produced by the lamp. This measure of efficiency is affected not only by the characteristics of the luminaire but also by the size and shape of the room, as well as the reflectance of the ceiling, walls, and fl oor. Standardized procedures are used by the manufacturer to establish a CU table of values for each luminaire.
• Visual comfort probability (VCP) is a value that indicates how much glare a luminaire is likely to produce. The room dimensions infl uence this rating. A VCP of 80 or higher is considered necessary for highly computerized offices. This number means that 80 percent of the users located in the least desirable spot in the space would not be bothered by direct glare from an even pattern of luminaires mounted on or in the ceiling.
• Ballast factor (BF) is the relative light output produced by a lamp and ballast system relative to the manufacturer ’s rated light output of the lamp itself. A high ballast factor means fewer lamps and ballasts are needed to achieve a specific level of illumination. A low BF ballast would permit lowering the light level in an overilluminated space without replacing or rearranging existing luminaires.

COLOR RATINGS

Effective lighting is dependent on perception of the appearance of the light as well as the general appearance of the space, especially related to color. Two additional ratings are used to identify the color appearance of the light source and its effect on surfaces in a space:

• Color temperature (CT) or correlated color temperature (CCT): CT is used for lamps with fi laments, including standard incandescent and halogen sources. CCT is used for nonfi lament light sources, including fl uorescent and metal - halide lamps. This measure, stated in degrees Kelvin (K), is based on the color change of a test wire as it is heated. The color of the wire goes from yellow to orange/red to white to blue as it increases in temperature. Sunny daylight at noon is about 5,500K, an overcast sky is about 7,000K, a 100 - watt incandescent lamp is about 2,800K.
• Color rendering index (CRI) provides an estimate of how “ natural ” or expected a standard set of colors appear when seen under a specifi c lamp relative to their color appearance under the standard test source with the same CCT. This latter appearance is rated as 100 CRI. Current energy codes defi ne a rating of 70 CRI as the minimum value for lamps used in most interior environments.

These two color ratings are linked to energy effi ciency in recent research findings. As CCT increases, the blue content of the light increases. A 5,000K light source has been found to provide more contrast and better resolution of details. With this bluer light, it is possible to design with lower foot - candle levels to achieve a perception of the same brightness in a space.

Light sources with a higher CRI have been linked to energy efficiency. The IESNA Lighting Handbook reports that “ lamps with color rendering indexes of 70, 85, and 100 require about 10%, 25%, and 40% lower illuminance levels than lamps with a CRI of 60, respectively, to achieve impressions of equivalent brightness ” (Rea 2000). Thus, the higher the CRI number of the light source, the brighter a space should appear with the same energy use.