328 Electrical Power Systems Technology
Four-Way Switch
Lighting Branch Circuits
Lighting Fixture
Luminaire
Coefficient of Utilization (CU)
Room Ratio
Depreciation Factor (DF)
Light Output
CHARACTERISTICS OF LIGHT
In order to better understand lighting systems, you should know
something about the basic characteristics of light. Light is a visible form of
radiation that is actually a narrow band of frequencies along the vast elec-
tromagnetic spectrum. The electromagnetic spectrum, shown in Figure
13-1, includes bands of frequencies for radio, television, radar, infrared
radiation, visible light, ultraviolet light, x-rays, gamma rays, and vari-
ous other frequencies. The different types of radiation, such as light, heat,
radio waves, and x-rays, differ only with respect to their frequencies, or
wavelengths.
The human eye responds to electromagnetic waves in the visible light
band of frequencies. Each color of light has a different frequency, or wave-
length. In order of increasing frequency (or decreasing wavelengths), the
colors are red, orange, yellow, green, blue, indigo, and violet. The wave-
lengths of visible light are in the 400-millimicrometer (violet) to 700-mil-
limicrometer (red) range. A micrometer (mm), which is also called a mi-
cron (μm), is one millionth of a meter, and a nanometer (nm) is 1 × 10 -3
micrometer. Angstrom units (Å) are also used for light measurement. An
angstrom unit is one-tenth of a nanometer. In order to avoid confusion,
use the conversion chart given in Table 13-1.
Visible light ranges from 4000 Å to 7000 Å. The response of the hu-
man eye to visible light exhibits a frequency selective characteristic, as
shown in Figure 13-2. The greatest sensitivity is near 5500 Å. The poorest
sensitivity is around 4000 Å on the lower wavelengths, and 7000 Å on the
higher wavelengths. Our eyes perceive various degrees of brightness, de-
pending on their response to the wavelengths of light. The normal human
eye cannot see a wavelength of less than 4000 Å (<400 nm), or more than
7000 Å (>700 nm).