The lighting industry uses many terms to describe various aspects of lighting related to lighting quantity, quality, and efficiency. Although not a complete list, discussion of the following terms will help you become familiar with some of the concepts used in lighting selection.
Lumen is a measure of the lamp’s light output. Most lamps are rated in lumens, and by comparing the lumen ratings of various lamps you can select the one that has the light level you desire.
Footcandles (fc) (or lux – one footcandle = about 10 lux) is a measure of the light levels on a surface or work plane, the place where activities needing light take place. A footcandle meter can be used to measure the light levels. Standard bathroom counter work planes are 2.5 to 3 feet(0.8 to 0.9 m) from the floor, and a reading work plane is typically 2.5 feet (0.8 m). Footcandle levels vary by activity. Footcandle levels for grooming in a bathroom are typically 20 to 50 fc (200-500 lux), but in other areas of the home they may be higher. The goal is to select lighting sources that will provide the appropriate level of lighting for the space.
Candelas or candle power. The intensity of the light beam in one direction is measured in candelas or candle power. This figure is taken into consideration when calculating the lamps needed to achieve a certain footcandle level onto a surface at a certain distance. If 10-foot (3.1 m) ceilings are present, for example, more intense light is needed than with 8-foot (2.4 m) ceilings.
Color temperature is the term used to specify the color of light coming from a lamp (see Figure 7.15). All lamps emit some color ranging from warm oranges to cool blues. Some lamps come close to neutral or white but may still have a small amount of color. The lamp color affects how the colors of objects appear in your room. Color temperature is rated in Kelvin and can range from 1500 to 9000; the higher the number, the bluer the light. Color temperatures in the 3000 to 3600 range are closer to neutral, which create little or no effect on the room colors. Warm lamps in a room with blue hues will make the blue color appear gray. Lamps placed in the same room or area should have the same color temperature to help them blend together. When selecting lamps for a bathroom you want colors that are pleasing to the face.
Color Rendering Index (CRI). Color rendering is the lamp’s ability to accurately show the colors of objects illuminated by that lamp. CRI peaks at 100 and the higher the number, the more accurately the light will reproduce color—the more natural and normal colors appear. Daylight is very close to 100 and that is why many people will take fabrics over to a window to view them. Colors will appear differently under various lamps and is usually even more critical to color appearance than color temperature. Lamps may have the same color temperature but different CRI. Find lamps with the highest CRI as possible, and in most rooms a CRI of 85 to 90 is good. Incandescent lamps typically have the highest CRI but some newer fluorescent lamps are around 90.