Quantitative Properties Of Light

QUANTITATIVE PROPERTIES OF LIGHT The Measurement of Light

• In the study of light, it is important to have a basic understanding of the physical concepts and terminology involve. • This pertains to: 1. Luminous Flux 2. Luminous Intensity 3. Illuminance or Illumination 4. Luminance

LIGHT SHINES IN A SPHERICAL MANNER

Luminous Flux • Defined as the flow of light,  • Measured in lumens • A lamp receives watts and emits lumens. The measure of success of doing this is called efficacy and is measured in lumens per watt (lm/W) Luminous Flux is equivalent to Flow in Plumbing.

Luminous Flux

Power

LAMP LUMENS Type of Lamp

Wattage

Luminous Flux in Lumens

Tubular Fluorescent Lamp

40 watts

2500 lumens

Circular Fluorescent Lamp

32 watts

2050 lumens

Compact Fluorescent Lamp (CFL) – 2U

15 watts

750 lumens

CFL – 3U

26 watts

1420 lumens

CFL – 4U

125 watts

6875 lumens

Halogen Lamp

50 watts

1000 lumens

Metal Halide

400 watts

32,000 lumens

Mercury Lamp

500 watts

11,000 lumens

High Pressure Sodium Lamp

400 watts

48,000 lumens

EFFICACY OF VARIOUS LIGHT SOURCES Source

Efficacy in Lumen per Watt

Candle

0.10

Oil Lamp

0.30

Incandescent Bulb

8 - 20

Halogen Lamp

16 - 20

Fluorescent Lamp

32 - 102

Mercury Lamp

30 - 60

Metal Halide Lamp

70 - 100

High – pressure Sodium Lamp

50 - 130

Low – pressure Sodium Lamp

120 - 140

Illuminance • As luminous flux travels outward from a source, it ultimately impinges on surfaces, where it is reflected, transmitted, and/or absorbed • Illuminance on a surface, E is the density of luminous flux incident on that surface • Measured in lumens per square meter • Lumen/m2 is called a lux while lumen/ft2 is called footcandle

ILLUMINANCE Average illumination of a surface is luminous flux per unit area.

Typical Illuminance Levels VISUAL ACTIVITES

RECOMMENDED LEVEL IN LUX (S. I. system)

Lighting for safety

5-20

Lobbies

100

Stairways

50

Reading #2 pencil

300

Reading #3 pencil

500

Inspection (Simple-Difficult)

300-5000

Handcrafts

300-1000

Fine Machine Work Source: IESNA Lighting Education: Fundamental Level

3000

Luminous Intensity • Generally speaking, a light source emits its luminous flux (Φ) in different directions and at different intensities. The visible radiant intensity in a particular direction is called luminous intensity (I). The unit of measurement is the candela (cd).

Luminance • The luminance (L) is the brightness of an illuminated or luminous surface as perceived by the human eye. Unit of measurement is candelas per square meter (cd/m2).

Typical Luminance Values Object

Luminance in Candela per Square meter

Wall brightness in a well – lighted office

100 cd/sq.m.

Moon, Candle flame

4,000 to 5,000 cd/sq.m.

Fluorescent lamp

6,000 to 8,000 cd/sq.m.

100 watt Incandescent bulb

50,000 cd/sq.m.

Sun

2.3 E9 cd/sq.m.

Preferred and Permissible Luminance Item Recommended Road Luminance

Luminance in candela per square meter 1 – 2 cd/sq.m.

Clearly discernable human features

15 – 20 cd/sq.m.

Preferred Wall Luminance

25 – 150 cd/sq.m.

Preferred Ceiling Luminance

50 – 250 cd/sq.m.

Preferred Task Luminance

100 – 500 cd/sq.m.

Permissible Luminaire Luminance

1000 – 7000 cd/sq.m.

Lighting Terminologies and Basic Units Quantity

Quantity is a Measure of

Symbol

Luminous Intensity (Candlepower)

Ability of source to produce light in a given direction

I

Candela (cd) Approximately equal to the luminous intensity produced by a standard candle

Luminous Flux

Total amount of light

Ø

Lumen (lm) Luminous flux emitted in a solid angle of 1 steradian by a 1 candela uniform point source

Illuminance (illumination)

Amount of light received on a unit area of surface (density)

E

Lux (lx)

One lumen equally distributed over one unit area of surface

Luminous exitance

Density of light reflected or transmitted from a surface

M

Lm/m2

A surface reflecting or emitting 1 lumen per unit area

Luminance (brightness)

Intensity of light per unit area reflected or transmitted from a surface

L

Cd/m2

A surface reflecting or emitting light at the rate of 1 candela per unit of project area

1 meter (m) = 3.28 ft; 1 m2 = (3.28 ft)2 = 10.76 ft2;

Unit in SI

Definition of Unit

1 cd/m2 = 3.14 lm/m2 1 cd/in2 = 452 lm/ft2;

1fc = 10.76 lux

Lamp Families and Some Common Lamps Types

* Although shown in this figure, these lamps are not considered efficient energy-saving lamps

End of Presentation