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LIGHTING DESIGN LECTURE 10 – March 29th Lighting Calculations

LIGHTING DESIGN LECTURE 10 – March 29th Lighting Calculations. Why: Predict what lighting equipment will be needed Predict how a lighting system will perform. The two ways in which we consider light Lumen: Measured light flux output from a source in an arbitrary direction

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LIGHTING DESIGN LECTURE 10 – March 29th Lighting Calculations

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  1. LIGHTING DESIGN LECTURE 10 – March 29thLighting Calculations Why: Predict what lighting equipment will be needed Predict how a lighting system will perform. The two ways in which we consider light Lumen: Measured light flux output from a source in an arbitrary direction Candela: Light intensity in a specific direction from a source A Light source has one lumen output value, but can emit different candela values in every direction. We get light output info from fixture cut sheets / diagrams Total lumen output Candella distribution / diagram • Lumen output is the sum of all of the light leaving a light source in every direction.

  2. LIGHTING DESIGN LECTURE 11Lighting Calculations The lumen method of calculating average illuminance levels Average value of Lumens distributed evenly over surface Used for surfaces in large, simple, open areas with simple layouts Yields Illuminance, which is light ONTO a surface. Really only intended to tell you about a specific surface That surface can however be the entire floor, or an imaginary task plane in a room. Based on the unit definition of a Foot-candle 1 FC = 1 lumen / 1 sq.ft. Extract the equation Illuminance = lumens / area Or Lumens = Illuminance x area

  3. LIGHTING DESIGN LECTURE 11Lighting Calculations The Lumen method procedure: Solving for lighting needs (determine how much light is needed then how many fixtures are needed) Determine area receiving light (usually task surface, floor or work plane) Define the desired illuminance (in foot-candles) Lumens needed = Area of space (in sq.ft.) x Illuminance (in FC) Translates “Lumens Needed” into “how many fixtures are needed” Number of fixtures needed = lumens needed / (lumens per fixture) OR…. If things get messy: lumens per fixture can be: Number of lumens per lamp x Number of lamps per fixture

  4. LIGHTING DESIGN LECTURE 11Lighting Calculations • Most architectural fixture cut sheets will list the total lumen output in the “0-90 degree” zone.

  5. LIGHTING DESIGN LECTURE 11Lighting Calculations The two “safety factors” make the Lumen Method procedure more accurate. 1. Coefficient of utilization CU How well the surface you are considering is receiving light Room geometry, fixture orientation (uplight, downlight, indirect, etc.) Sample CU’s to get used to Direct fixture: 85% (0.85 in your equation) Indirect fixture: 50% (0.50) Spot / accent: 95% (0.95) Wash / ambient: 75% (0.75) 2. Light Loss Factor LLF Accounts for depreciation of lighting system over time Lamp Lumen depreciation (LLD), lamp dirt depreciation (LDD), ballast factor (BF) We generally make a safe assumption of 85% (0.85) Both of these factors have a huge impact on predicted effect of your light solution. So our lumen method equations become: Illuminance = (lumens per fixture x No of fixtures x CU x LLF) / Area Lumens needed = (Illuminance x area) / (CU x LLF) Or… describe your solutions in ”Functional Lumens” Use lumen method for solving for fixtures (example)

  6. LIGHTING DESIGN LECTURE 11Lighting Calculations • 1 candela is light intensity leaving a source in a density of one lumen of light per “steradian” of spherical area

  7. LIGHTING DESIGN LECTURE 11Lighting Calculations The point calculation method (using candela values) Candela output is an expression of the intensity of a light source in a specific direction. The “Point Calculation” accounts for this intensity as it is affected by distance (light spreads out) Generally used for situations involving a specific surface lighted from a specific distance by a directional / accent fixture. Based on the candela: lumen output divided by spatial extent (steradians). Diagram: Extract the equation: Illuminance = candella value / distance squared……or E = CD / D^2 Distance must be in feet (squaring will give you square feet) The procedure: Define distance form light source to surface receiving light (estimate) Use elevations or trigonometry (SOHCAHTOA) if necessary Identify candela value of fixture in the appropriate direction (we almost always use the “center beam candle power” (CBCP) because it is the brightest) CBCP of lamp from lamp catalog (if it is a “lamp driven” fixture) Candela value from candela distribution (photometric curve) on fixture cut sheet.

  8. LIGHTING DESIGN LECTURE 11Lighting Calculations • 1 candela is light intensity leaving a source in a density of one lumen of light per “steradian” of spherical area

  9. LIGHTING DESIGN LECTURE 11Lighting Calculations • Examples of locating candela values on cut sheets. We are usually interested in the Center Beam Candela Value or CBCP.

  10. LIGHTING DESIGN LECTURE 11Lighting Calculations Reading Assignment: The Architecture of Light, Chapter 25 Go over assignment project progression: Calculation Spreadsheets

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