This document discusses two methods for calculating lighting levels: the lumen method and point calculation method. The lumen method determines average light levels over large, open areas by calculating illuminance (foot-candles) based on the total lumens of lights and the area of the space. The point calculation method determines light levels at specific points by calculating illuminance based on the candela output of lights and their distance from the calculation point. The document provides examples of calculations using both methods and discusses factors like light loss and coefficient of utilization that affect calculations.

1. Chapter 20

2. Lighting Calculations

Lumen Method – determines average
light levels in large open areas

Point Calculations – determines light
levels at a specific point on an object or
surface

3. Lumen Method Calculations
1 foot candle (FC) = 1 Lumen (Lms)
divided by 1 square foot (area / A)
 1 FC = 1 LM ÷ A
 Illuminance (E) (amount of light falling
onto an object measured in foot
candles) = Lumens divided by Area (in
sq. ft.)

 E = Lms ÷ A (sq. ft.)

4. Lumen Method Example 1
Solving for Illuminance(E) Level



Room 10’ x 15’
5 down lights
1000 Lumens each down light

E = Lms ÷ A
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

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Illuminance = Lumens divided by Area
A = 10’ x 15’ = 150 sq. ft.
1000 Lumens x 5 down lights = 5000 lumens
E = 5000 Lms ÷ 150 sq. ft.
E = 37.5 Lms per sq. ft.
E = 37.% foot candles (FC)

5. Lumen Method Example 2
Solving for Lumens or Luminaires
needed
Room 10’ x 15’ (A = 150 sq. ft.)
 Illuminance level (E) = 60 foot-candles
 Need Lumens / Luminaires
 E = Lms ÷ A - original equation
 Lumens needed = Illuminance x Area

 Lms = E x A
○ Lms = 60 E x 150 sq. ft.
○ Lms = 9000 Lumens

6. Lumen Method Example 2
Solving for Lumens or Luminaires
needed

Lumens needed = Illuminance x Area
 Lms = E x A
○ Lms = 60 E x 150 sq. ft.
○ Lms = 9000 Lumens
○ 550 lumens = per luminaire

Luminaires needed = Lumens needed
divided by Lumens per luminaire
 Luminaires (X) = 9000 (lumens) ÷ 550 Lumens
per Luminaire
 9000 ÷ 550 = 16.36
 Always round up with any point.
 Luminaires needed = 17

7. Lumen Method Safety
Factors

Light loss factor (LLF) – light loss due to
deteriorating lamps
 Industry standard of .85 for a light loss factor

Coefficient of Utilization (CU) – effectiveness of
luminaires working on surfaces of the space to
deliver light to the target surface.




Direct fixture or downlight – CU = 85% (0.85)
Indirect fixture – CU = 50% (0.50)
Spot or Accent – CU = 95 (0.95)
Wash or ambient – CU = 75% (0.75)

8. Lumen Method Calculations
with Safety Factors
Illuminance = Lumens ÷ Area
 Illuminance = (Lumens x LLF x CU) ÷ Area
 Lumens needed = Illuminance target x Area
 Lumens needed = (Illuminance target x
Area) ÷ (LLF x CU)


9. The Point Calculation
Method
Estimate location of lighting equipment
 Light density = Candle power = Candela
 Candela information given by Manufacturer
 Center Bean Candle Power – CBCP –
center of a directional light
 Illuminance = Candela value ÷ Distance
(squared)(in feet)

 E = CD ÷ D2

10. Point Calculation Example 1
10’-0” ceiling
 Accent Fixture
 Above a plate on a table 3; above the finished
floor (A.F.F.)
 Illuminance = Candela value ÷ Distance2

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E = CD ÷ D2
CD = 10,000 Candelas
D2 = 7 ft 2 = 49 square feet (units get squared too)
E = 10,000 CD ÷ 49 sq.ft.
E = 204 Foot-candles

11. Point Calculation Example 2
10’-0” ceiling
Accent Fixture
Above a plate on a table 3; above the
finished floor (A.F.F.)
 Candela value needed = Illuminance level
desired x distance squared

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
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

CD = E x D2
E = 150 Foot-candles
D2 = 7 ft2 = 49 sq. ft.
CD = 150FC x 49 sq. ft.
Candela value needed = 7350 Candelas

12. Luminaire and Lamp Type
Candela represented in form of a
distribution diagram
 Provided by manufacturers
 When sources is not perpendicular (90
degree angle to surface)

 Candela value needed = (Illuminance level
desired x distance squared) ÷ cosine of the
angle
 CD = (E x D2) ÷ cosine of angle
 E = (CD x cosine of angle) x D2

13. Point Calculation Example 3

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
10’-0” ceiling
Accent Fixture
Above a plate on a table 3; above the finished floor
(A.F.F.)
30 degree angle for fixture
100 foot-candles
 Candela value needed = (Illuminance level desired x distance


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
squared) ÷ cosine of angle
CD = (E x D2) ÷ cosine of angle
E = 100 FC (foot-candles)
D2 = 8.1 feet = 65 sq. ft.
Cosine of angle = cosine of 30 degrees = 0.87
CD = (100 FC x 65 sq. ft.) ÷ 0.87
Candela value needed = 7471 Candelas.