Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

364 Chapter 12 Electric Current and Related Parameters

many factors such as activity, safety, and task. Sometimes the lighting system is designed to

draw attention to a feature or something special in the building, then the engineer designs for

so called accent lighting.

As you know, there are many types of light bulbs and fixtures. According to the U.S. Depart-

ment of Energy, incandescent lights account for 85% of lights used in homes (2009). Unfortu-

nately, the incandescent lights have very low efficacy values (10 –17 lumens /watt). They also have

short service life (750 – 2500 hours). Another important factor in choosing a lighting system for

an application is its source color. As shown in Figure 12.15, in a incandescent lamp, the electric

current runs through the lead wires and heats up the filament (a tiny coil of tungsten wire), which

in turn makes the tungsten to glow or produces light. The light produced in this manner is a yel-

lowish color. In general, the colors of light sources are classified into warm or cool categories. The

yellow – red colors are considered warm, whereas the blue – green colors are considered cool. For

a light source it is common to define a color temperature in Kelvin. The higher Kelvin tempera-

tures (3600 –5500 K) are considered cool, while lower color temperatures (2700 – 3000 K) are

considered warm. Warm light sources are preferred for general indoor tasks. Be careful, with the

counter-intuitive way the warm and cool light sources are defined (high temperatures are cool,

whereas low temperatures are warm!). How true the colors of an object appear when illuminated

by a light source is more important than color temperature of light source. For this reason, a vari-

able called color rendition index (CRI)is defined. The CRI provides a measure of how well a light

source renders true colors of an object as compared with direct sunlight. The color rendition index

has a scale of 1 to 100 with a 100-W incandescent light bulb having a CRI value of approximately

100. For most indoor applications, light sources with CRI of 80 or higher are preferred.
     There are different types of incandescent light bulbs. The standard incandescent light is
     referred to as a screw-in-A-type. There are also Tungsten halogen and type R incandescent light
     bulbs. The Tungsten halogen lamps have higher efficiencies than A-types, because they have
     inner coatings that reflect heat, consequently require less energy to keep the filament hot at a
     certain temperature. The type R incandescent lights also spread and direct light over a specific

Filament

Nitrogen or

argon gas

Lead wires

■Figure 12.15

A schematic of incandescent lamp.

Source:DOE’s Office of Energy

Efficiency and Renewable

Energy.

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