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Lighting Calculations

Chapter 20. Lighting Calculations. 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.

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Lighting Calculations

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  1. Chapter 20 Lighting Calculations

  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 1Solving for Illuminance(E) Level • Room 10’ x 15’ • 5 down lights • 1000 Lumens each down light • E = Lms ÷ A • 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 2Solving 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 2Solving 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 • 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 • 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 • 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 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.

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