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
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.)
Lumen Method Example 1
Solving for Illuminance(E) Level
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Room 10’ x 15’
5 down lights
1000 Lumens each down light
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E = Lms ÷ A
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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)
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
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
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.
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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)
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)

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
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
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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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
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
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.