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CHAPTER 14
LIGHT AND LIGHTING
OVERVIEW
This chapter introduces the physics of light and the application of light to produce
useful visual environment called lighting. Light can be easily demonstrated by the use of
some simple instruments, such as a flashlight, a prism that can be obtained from scientific
instrument houses, colored glasses, colored papers, etc. Lighting fixture manufacturers
also can provide small samples of lighting control mediums, such as diffusers, reflectors,
Fresnel lens, polarizers, etc. A portable light (illuminance) meter is a must for teaching
illumination classes. It costs about $40 for a simple unit to about $200 for a more actuate
one. Illuminance meter may also be used to measure the luminance of a surface with
approximation by reversing the meter positioning from aiming at the light source to
aiming at the task surface. The first reading is the illuminance, the second reading is the
luminance and ratio of luminance to illuminance is per unit reflectance.
Even though the physics of light has been basically taught in high school science or
college physics classes, the application of light to lighting is new. We found this subject
has generated enthusiastic response from most students. For the first time, students begin
to realize how knowledge of light can become such an important tool in the building
design process and how creative lighting design can help to create exciting solutions.
CHAPTER 14
LIGHT AND LIGHTING
QUESTIONS AND ANSWERS
14.1 Natural light is radiant energy from the sun. The visible spectrum ranges
from
_____ ( _____) to _____ ( _____) nanometers.
14.2 Light generated from electrical light sources (lamps) is also radiant energy.
( True ) ( False ).
14.3 The primary colors of light are _____ ( _____), _____ ( _____) , and _____
( _____)
14.4 The three subtractive primaries or light are _____ ( _____), _____ ( _____),
and _____ ( _____)
14.5 What is the unit of measure used for illuminance, or illumination level?
14.6 Daylight under direct sunlight is about _____ fc.
14.7 Daylight under an overcast sky is about _____ fc.
14.8 Color temperature is a method to define the heat generated by various types
of lamps. ( True ) ( False )
False. Color temperature has nothing to do with the heat generated by lamps. It
is a scientific method to describe the color characteristics of some, but not all
14.9 What is the approximate color temperature of a gas-filled incandescent
lamp?
_____ to _____
q
K; a fluorescent lamp? _____ to _____
q
K.
14.10 What is the unit of measure of light power in conventional unit? And in SI
unit?
14.11 What are the normal human responses to various colors?
14.12 What is the meaning of illuminous intensity?
14.13 Color difference also improves visual acuity; even the objects and its
background are at the same luminance level or zero contrast. ( True ) (
False )
14.14 A higher luminance contrast is desired for viewing a fast-moving object,
such as a baseball, but is not important for a stationary task. ( True ) (
False )
14.15 Does a high luminance contrast provide more visual comfort?
14.16 What is glare, direct glare, reflected glare, discomfort glare, disability glare,
and blinding glare?
14.17 The sun’s ray reaching the earth has a balanced energy level of all
wavelengths.
( True ) ( False)
14.18 What wavelength is most sensitive to human’s eyes?
14.19 A white light is a mixture of colored lights. ( True ) ( False )
14.20 What are the basic characteristics of color?
14.21 If 64 lumens reaches a surface of 10 sq ft, what is the illumination level in
foot-candles and in lux?
14.22 What is the recommended illuminance level for office occupancies?
14.23 When is inverse square law and cosine law used?
14.24 A surface 2 feet from a light source is illuminated by a light source having a
candlepower of 20 candela. What will be illuminance on the source?
14.25 If the light source in Question 14.24 is at a 30 degree angle with the normal
of the surface, what is the illuminance?
14.26 What are the means of light controls?
14.27 Light travels in a straight line in a single medium, such as air, water, or
glass.
( True ) ( False )
14.28 Light always bends the same way between media. ( True ) (False )
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14.29 What is the refracted angle inside of the glass if light enters from air into a
glass at 40
q
from the normal axis and the index of refraction of glass is 1.4?
14.30 Refraction is the most effective means of light control. ( True ) ( False )
14.31 The reflected light beams from a parabolic reflector are parallel to each
other. ( True ) ( False )
14.32 What is the primary use of an elliptical reflector?
14.33 What is a Fresnel lens?
14.34 Describe the light control principles used for the following light sources or
lighting fixtures.
a. A fixture with a diffusing glass globe.
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The fixtures utilize the following light control principles:
a. Diffused transmission.
b. Reflection above the lamp bulb and absorption on the sides.
c. Reflection within the fixture and diffusion at the bottom.
d. Reflection within the fixture and refraction at the bottom.
e. Reflection at the bottom and general distribution (no controls) at the top.
14.35 A book is printed in black ink on buff colored paper. The luminance
measured on the paper is 60 cd/m2 and the prints are 4 cd/m2. What is the
contrast of the reading task (prints and its background)?
14.36 A book with 0.80 sq ft in area is placed on a desk having 4 sq ft in area. If
the average light power, directly and indirectly, reaching the desk is 75
lumens. What is the illuminance on the book?
14.37 A lighting system consists of 10 spotlights; one is located directly above a
task and the other 9 are aimed elsewhere. The reflected illuminance from
these 9 fixtures is calculated to be 3 foot-candles on the task. What is the
illuminance on the task if the candlepower at the center of the spotlight is
2500 candela and the spotlight is 10 feet above the task.
From equation 14-5c, illuminance from the fixture directly above,
14.38 What is the luminous exitance of the task if its reflectance is 25%?
14.39 Why does the approaching automobile headlights appear so bright at night?
14.40 How would you describe your visual sensation when the approaching
headlight is on low beam? And on high beam?
14.41 A lamp rated for 1000 lumens light power is installed within a lighting
fixture (luminaire). Can all of the lumens be utilized to produce useful
illumination in the space?
14.42 Suppose the portion of light power escaping a fixture is 80% of the original
light power of the lamp, would all 80% be useful to illuminate the desktop in
the middle of the room?