Lighting & Acoustics Chapter 8
Zonal Lumens The âZonal Lumen Summaryâ table (Figure 8.4d) provides a quick overview of lumens in two zones. Because the luminaire has a direct distribution of light, all the light is emitted in the zone between 0 and 90 degreesâthat is, in the downward zone. The âZonal Lumen Summaryâ table also indicates the efficiency of the luminaire and lamp combination, which is 51.7 percent (Figure 8.4, label D). In addition, the chart provides âLuminance Summary (cd/m 2)â data (Figure 8.4, label E). For this luminaire, the luminance levels are higher at the lower vertical angles. For example: - Luminance level is 3612 with a vertical angle of 45 degrees. - Luminance level is 338 with a vertical angle of 75 degrees. The remaining table in the chart, âCoefficients of Utilization (%)â (Figure 8.4, label F) is explained in the âLumen Methodâ section under âCalculationsâ later in this chapter. As illustrated in Figure 8.4, luminaires and reflector lamps with a symmetrical distribution are often illustrated on only one side of the graph because each half will be identical. It is important to note that indirect/direct luminaires have a candlepower distribu - tion curve above and below nadir. If the luminaire distributes a high level of illumina - tion toward the ceiling, to avoid annoying brightness or glare, it might be necessary to locate the fixture at a considerable distance from the ceiling. Luminaires that emit light from the top and sides of the fixture will have candlepower distribution graphs for vertical and horizontal light angles. Calculations Calculations for determining illuminance can be done by hand or by using a lighting software package. This section reviews both methods. As mentioned at the beginning of this chapter, understanding how the calculations are performed is critical to knowing the variables that affect the quantity of light on a work plane and in a space. To ensure a quality lighting environment, it is critical to recognize that the illuminance level identified through the calculations is only one of the variables to be considered in arriving at the final specifications. All the criteria discussed throughout this textbook and the uniqueness of a site and its users must be synthesized for the final lighting plan. Lumen Method This section focuses on the âCoefficients of Utilization (%)â data provided in the previously discussed photo metric chart (Figure 8.4) as well as Figure 8.6 and Tables 8.1 and 8.2. The lumen method described in this section is an abbreviated method used to determine the average illuminance on horizontal surfaces in a room. These data can be useful in the initial stages of the lighting design process because they provide an estimate of the number of fixtures that should be used to achieve a uniform distribution of light in a space. To determine more accurate illuminance calculations, refer to IES (2011) or advanced lighting software programs. To perform illuminance calculations, several elements must be identified: - The proportions of a room - Luminaires and lamps - Location of work surfaces - Distance between work surfaces and luminaires - Reflectance values of ceilings, walls, and floors The calculations also require: - Room-cavity ratios (RCR) - Coefficient of utilization (CU) for luminaires - Light loss factor (LLF) - Lamp lumen depreciation (LLD) - Luminaire dirt depreciation (LDD) RCR and CU The RCR is a formula designed to take into account the proportions of a space and the potential distance from the luminaires to the work surface: HâHeight of room or distance from luminaire to work surface LâLength of room WâWidth of room The room-cavity ratio (RCR) is used to determine the coefficient of utilization (CU) (Figure 8.4, label F). The CU depends on the space to be illuminated and the design of the luminaire. The CU indicates the ratio of initial lamp lumens to the lumens on a work surface for a particular luminaire, lamp, location of the task surface, and space. CU percentages are available from luminaire manufacturers (Figure 8.4, label F). The âCoefficients of Utilization (%)â table in is based upon a floor cavity reflectance of .20 (Figure 8.4, label F). The ceiling reflectance percentages in the table are 80, 70, and 50 and the wall reflectance percentages are 70, 50, 30, and 10. Light Loss Factor (LLF) LLF indicates the illuminance that is lost as a result of the type of lamp, the temperature of the space, time, input voltage, ballast, lamp position, interior conditions, and burnouts. There can be a 25 percent loss of lumens because of dirt, dust, and lamp depreciation. IES has identified recoverable and nonrecoverable LLFs. The recoverable LLFs include room surface dirt depreciation, LLD, lamp burnouts factor, and LDD. Nonrecoverable factors include ambient temperature, input voltage, ballast factor, and luminaire surface depreciation. LLD is a metric measurement of the loss of lumens due to the design of a lamp. Table 8.1 provides a list of LLD for selected lamps. LDD accounts for the loss of light caused by dirt and dust accumulation. Important considerations for LDD are the design of the luminaire, the atmosphere of the space, and how often the lamps are cleaned. Table 8.2 and Figure 8.6, produced by IES, illustrate maintenance categories for various luminaires, atmosphere considerations, and dirt conditions. This information serves as a reference for determining LDD. As defined by IES, the categories include âvery cleanâ (VC), âcleanâ (C), âmediumâ (M), âdirtyâ (D), and âvery dirtyâ (VD). Lamps mounted in an exposed luminaire, in an environment that has a great deal of dust, such as a woodworking studio, would have to be cleaned very frequently to reduce significant light loss. For an abbreviated method of determining the average illuminance on horizontal surfaces in a room, LLF can be calculated by: LLD x LDD = LLF Zonal Cavity Calculation A simple method for determining average illuminance is the lumen method, also referred to as the zonal cavity calculation (Box 8.1). This method provides only the average illuminance in a space and does not factor in variation in light levels. Note that when the LLF and the CU are factored in, illuminance levels are decreased. This reflects characteristics of luminaires, interior architecture, and environmental factors in the space. Without these two considerations, the illumination level would be approximately 75 fc at the initial installation, and would not take into account what could occur in the space throughout the life of the installation. This notation helps to illustrate why it is important to consider all the systemic factors that affect illuminance. Point-by-Point Method The basic point-by-point method determines the fc (lux) level for a focal point. This method uses the inverse square law and cosine law, also referred to as Lambertâs Law. The inverse square law is utilized only for point sources The inverse square law formula is: I/d squared = E Key: Eâillumination (fc) Iâluminous intensity (cd) of the source (Available from the lamp manufacturer. Refer to the lampâs candlepower distribution chart.) (Figure 8.7). dâdistance from the light source to a surface The inverse square law is based on the principle that the illumination level on a surface decreases the farther the surface is from the light source. According to the formula, the factor by which the illumination on a surface is decreased is equal to the square of its distance from the source. Therefore, the illuminance on a surface 2 feet away from its source is one-fourth as much as the illumination 1 foot from the light source (Figure 8.8). The inverse square law formula can be utilized for determining the illuminance on a point from a light source that is located directly above the surface (Figure 8.9). Example:
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Connections to LEED Certification For a checklist of how you can apply quantifying light in creating a LEED-certified building, see the box, âSustainable Strategies and LEED,â and Appendix D. Chapter Concepts -> Professional Practice These projects are based on the principles of brain- based learningâa theory developed from research that examines how the brain functions and learns (see Preface). Projects can be completed individually or via group discussions. Practice ApplicationsâLumen Method As a lighting designer, you have been hired to determine the average maintained illuminance on a work surface in a libraryâs study room. The space has the following characteristics: - Dimensions: 60 feet à 60 feet with a 12-foot ceiling- - - Work surfaces: 2 feet 6 inches (AFF) - Distance from luminaires to work surfaces: 5 feet 6 inches - Reflectances in space: 70 percent ceiling, 50 percent walls, and 20 percent floor - Clean space with lamp cleaning three times a year - 24 surface-mount luminaires (Figure 8.4) with two F32T8 lamps in each luminaire. Initial lamp lumens: 2800 - LLD â see Table 8.1 - LDD â see Figure 8.6 â maintenance category III Use the following formula to determine the average maintained illuminance on the work surfaces in the library (see Box 8.1): Practice Applicationsâ Illuminance for Artwork You are working with a client to illuminate artwork on a wall in a hotel lobby. The entire composition requires five luminaires. Each luminaire has a 85W reflector BR40 spot lamp (see Figure 8.7). Luminaires are aimed at an angle of 30 degrees and are nine feet six inches from the artwork. Determine the illuminance for specific locations by using the cosine law for a vertical surface: I/d 2 X sin θ = E Internet ExplorationsâSky Chart Software packages can help designers to specify quality lighting. Explore how SkyCalc can help designers to optimize daylighting by reviewing The Skylighting
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Illuminance Measurements in Practice Interior designers conduct calculations for new construction as well as for the remodeling of existing spaces. Both applications utilize the calculation methods discussed in this section. For existing spaces, an interior designer will frequently want to obtain current fc (lux) levels by taking illuminance measurements in the field using a light meter (Figure 8.11a). This instrument indicates the fc (lux) levels for any area within the space, as well as the reflectance values of surfaces. To determine a foot-candle (lux) level at a given location, an interior designer places the light meter in the specified location and reads the result. Usually, an interior designer will want the fc (lux) levels for both ambient and task lighting within a room. To obtain the ambient illuminance, an interior designer will establish a grid, take readings at each cross-section of the grid, and then average the results. Foot-candle (lux) readings for task lighting can be obtained by placing the light meter on each work surface. To determine an approximation of the reflectance of a surface, an interior designer will place the light meter approximately 4 inches from the surface and record the fc (lux) level (Figure 8.11b). Lighting Software Packages Lighting software packages are available for basic and advanced illuminance calculations using daylight and electrical light sources. Basic programs, including AutoCAD extensions, will predict the brightness of surfaces and patterns of light distribution on vertical and horizontal surfaces. Advanced programs are able to calculate illuminance in rooms with unique shapes, including sloped ceilings (Figure 8.12). Software that can calculate the potential effects of daylighting is especially useful because of the varia - bility associated with natural conditions. Calculations are generally performed by using several factors for a particular location, such as daylight conditions (e.g., clear and overcast), time of the year (e.g., winter, equinox, and summer), and time of the day (e.g., 8:00 AM, noon, and 4:00 PM). There are software packages that are useful for calculating daylighting, such as Sensor Placement + Optimization Tool (SPOT / ) and Skycalc. SPOT / software (2015) can assist a designer to quantify an existing or a proposed electrical lighting and annual daylighting characteristics of a space (www.daylight inginnovations.com/spot-home). The program can also help to identify optimal photosensor placements in the space based on annual performance and energy savings (SPOT / , 2015). SkyCalc / (2015) is a tool that can help designers to conserve energy for lighting as well as HVAC (heating, ventilation, and air-conditioning), but the software is limited to skylights (http://energydesign resources.com/resources/software-tools/skycalc .aspx). From a broader perspective, the U.S. Department of Energy (DOE) (2015) has a free tool, Commercial Lighting Solutions , that enables designers to estimate the energy savings of a new lighting system against a specified energy code (http://energy.gov). The program can also be used to compare a proposed lighting upgrade to an existing commercial interior This chapter explores what is involved in determining the quantity of illumination in an interior. Many factors relating to the environment and the users of the space are covered, within the context of recommendations provided by IES and other international organizations. As reviewed in Part One of this book, quantity of lighting is only one of the factors that must be considered in designing a quality lighting environment. Part Two describes how designers can apply the information from Chapters 1 through 8 to professional practice.
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Lamp: 85 W reflector BR40 spot lamp Location: Directly above the surface and 7 feet 6 inches from the lamp Candelas (cd): 3000 (see Figure 8.7) Calculation: 3000 (cd) /7.5 2 (distance from light source to surface) = 53 fc (E) The formula for the cosine law is I/d 2 à cos θ = E, where θ (theta) is the angle between a ray from the luminaire falling on a point and a line perpendicular to the plane upon which that point is located. The cosine law indicates that the illuminance on a surface will vary according to the cosine of the angle of incidence. Cosine and sine are used for horizontal and vertical surfaces, respectively, and figures can be found using a calculator. Determining the illuminance for specific locations can be very complex because of the variety of areas within a space and the interdependence of the factors that affect lighting. For this reason, interior designers and engineers will generally use lighting software to perform the calculations. To facilitate a conceptual understanding of the process and the factors that are important to consider when determining illuminance for specific points, two representative examples follow. Illuminance on a Horizontal Surface The cosine formula (I/d 2à cosθ= E) can be used to determine the illuminance on a horizontal surface when the luminaire, or the point to be lighted, is at an angle (Figure 8.10a). Example: Lamp: 85 W reflector BR40 spot lamp Location: aimed at a 30 degree angle to the horizontal surface and 7 feet 6 inches from the lamp Candelas (cd): 200 (see Figure 8.7) Calculation: 200 (cd) / 7.5 squared (distance from light source to surface) à .866 (cosθ) = 3 fc (E) Illuminance on a Vertical Surface The corresponding formula for determining the illuminance on a vertical surface is: I/d squared X sinθ= E (Figure 8.10b). For example: Lamp: 85 W reflector BR40 spot lamp Location: aimed at a 30 degree angle to the vertical surface and 7 feet 6 inches from the lamp Candelas (cd): 200 (see Figure 8.7) Calculation: 200 (cd) / 7.5 squared (distance from light source to surface) à .500 (sinθ) = 2 fc (E)
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SUMMARY - Measuring the quantity of lighting in an environment is based on the principles of radiometry and photometry. - The basic categories for measuring lighting include luminous intensity, luminous flux, illuminance, luminance, and luminance exitance. - To determine the direction, pattern, and intensity of light from reflector lamps and luminaires, interior designers refer to photometric data reports, such as those provided by lamp and luminaire manufacturers. - A simple method for determining average illuminance is the lumen method, also referred to as the zonal cavity calculation. - The basic point-by-point method determines the fc level for a focal point. - There are many strategies for integrating the sustainable principles related to quantifying light with LEED-certified buildings
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The luminaire efficacy ratio (LER) is a ratio of the lumens per watts consumed for the entire luminaire system, which includes the total lamp lumens, ballast factor, and photometric efficiency. Illuminance (E) The unit of measurement used to determine the total amount of light falling on a surface is illuminance (E). The SI symbol is E , and it is measured in lux (lx) or foot-candles (fc) in the metric and customary systems, respectively. An illumination of 1 lux is produced by 1 lumen of light shining on an area of one square meter. Ten lux equals approximately one foot-candle. Thus, a recommendation for 400 lux of illumination on a work surface would be equivalent to 40 foot-candles. By way of context for these measurements, the foot-candle levels for a full moon and for sunlight at noon are .01 fc and 10,000 fc, respectively. Many work areas in homes, offices, and conference rooms utilize 30 to 50 fc (300 to 500 lx). The bulleted list below illustrates ranges of target illuminances recommended by the Illuminating Engineering Society (IES) (2011): - Orientation, relatively large-scale, physical (less-cognitive) tasks (0.5â60 lux) - Common social activity and large and/or high- contrast tasks (20â400 lux) - Common, relatively small-scale, more cognitive or fast-performance visual tasks (150â1500 lux) - Small scale, cognitive visual tasks (500â4000 lux) - Unusual, extremely minute and/or life-sustaining cognitive tasks (1500â20000 lux) As discussed in previous chapters, there are many factors to consider in determining illumination levels, including the lamp, design of the luminaire, maintenance procedures, reflectance values, and distance and angle from the light source to the task. Luminance (L) and Luminous Exitance Luminance (L) is a measure of the objective brightness of a light source. It indicates the amount of light in the eyes of users of the space after reflection or transmission from a surface. Thus, illuminance and reflectance affect luminance. The unit of measurement is candela per square meter (cd/m 2), and L is the SI symbol for the term. Brightness is the term used to refer to an individualâs perception of the light in a space and, being somewhat subjective, it is not a measurable property. Luminance affects the apparent brightness of a surface or material and is dependent upon the location of the user as well as colors, textures, and interior architecture. Luminous exitance is another term associated with luminance. This is a measure of the total quantity of light reflected and emitted in all directions from a surface or material. Luminous exitance is measured in lumens per square foot (lm/ft 2). Photometric Data This section focuses on the photographs and photometric data provided in Figures 8.3 and 8.4, respectively. Photometric data are especially important to sustainable designs because they provide the information that is needed to have the appropriate level of light where it is needed.
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To determine the direction, pattern, and intensity of light from reflector lamps and luminaires, interior designers refer to candlepower distribution curves ,as provided by lamp and luminaire manufacturers (Figure 8.4). Interior designers often use the term âbat-wing graphsâ to refer to specific curves that resemble the shape of a batâs wings. These curves result from a bat-wing lens. On a polar candlepower distribution curve, zero, or thenadir (straight down), is the location of a light source. The concentric circles on the graph indicate the intensity expressed in candelas and the radiating lines are the angles of distribution (Figure 8.4, label B). Photometric Data Chart Figure 8.4 illustrates a photometric data chart of a direct luminaire with two fluorescent T8s. Directly above the graph are summary data related to the luminaire (Figure 8.4, label B): D (direct) = 100% (indicates luminaire is a direct fixture with 100% of the light directed down) I (indirect) = 0% (indicates luminaire does not given any (0%) indirect light) Spacing criteria (SC): Along 1.1; Across 1.3 Lamp lumens: 2950 Input watts: 59 Spacing Criterion (SC) The spacing criterion (SC) is a metric measurement that indicates luminaire locations for spaces requiring consistent illumination levels. The SC for the length of the luminaire is referred to as the âparallelâ or âalongâ (1.1 in Figure 8.4, label B) and the short side of the luminaire is the âperpendicularâ or âacrossâ (1.3 in Figure 8.4, label B). The height of direct luminaires is measured from the bottom of the fixture to a work surface that is 2 feet 6 inches above the floor (Figure 8.5). The measurement for indirect luminaires is from the ceiling to the work surface. Deviations from the recommended SC locations may result in 174 Part One Principles of Lighting illumination levels that are too high in some areas or too low in others. Reflected light can be put to use. To maximize the amount of illumination derived from reflectance, luminaires should be installed close to walls, but not so close that they cause excessive brightness on the walls. Generally, the ideal distance from a wall to a fixture is half the center-to-center distance between fixtures. The formula for calculating the center-to-center distance between fixtures is: SC ratio à mounting height (MH - distance from the bottom of the luminaire to the work surface) = spacing intervals (SI) Example : The calculations below illustrate the spacing intervals for a luminaire that is 7 feet 6 inches (7.5 in calculation) from a work surface. (Remember: 1.1 and 1.3 were provided by the manufacturerâs photometric data sheet. See Figure 8.4.) 1.1 (along length of fixture) à 7.5 (MH) = 8.25 (SI) or 8 feet on center along the length of the fixture 1.3 (along the short side of fixture) à 7.5 (MH) = 9.75 (SI) or 10 feet on center along the short side of the fixture Candlepower Distributions To read the candlepower distribution graph on the photometric chart, identify a specific angle of view, and then read the associated candelas (Figure 8.4, label A). For example, for the direct linear luminaire (Figure 8.3b) the emitted candelas at a vertical angle of 30 degrees are approximately 1160 (Figure 8.4, label A). This number is also available in the âCandlepower Summaryâ table (Figure 8.4, label C). Moving to the right in the same row will provide the emitted candelas at horizontal angles of 22.5 degrees, 45 degrees, 67.5 degrees, and 90 degrees. For example, at a vertical angle of 30 degrees, the emitted candelas at the 45 degrees horizontal angle are 1233 (Figure 8.4, label C). The last column in the âCandlepower Summaryâ table indicates the output in lumens at various vertical angles. For example, at a vertical angle of 25 degrees, the output in lumens is 598 (Figure 8.4, label C)
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Units of Measurement Determining the appropriate quantity of illumination is a key factor in designing a quality environment. This requires interior designers to understand the interna - tional system of units as well as photometric data. International System of Units Measuring the quantity of lighting in an environment is based upon the principles of radiometry and photometry . Radiometry is a scientific discipline dealing with the measurement of radiant energy in the form of electromagnetic waves. Radiant energy is heat energy transferred through space. Photometry is a science derived from radiometry that includes the human response to a source of illumination. The worldwide standard for the units of measurement is the International System of Units (SI). The properties measured include luminous intensity (I) luminous flux (F) illuminance (E), luminance (L) and luminous exitence Luminous Intensity (I) and Luminous Flux (F) In photometry, luminous intensity is the intensity of a light source, measured in candelas (cd). The SI symbol or abbreviation is I . One candela represents the luminous intensity from a source pointing in a specific direction on a solid angle called the steradian (Figure 8.2). Originally, a candle was used to measure luminous intensity, but it was impossible to arrive at a standard with so many types of candles; hence the international use of the candela, from which other units of measurement are derived. Candlepower and candela are considered interchangeable terms. Luminous flux (F) is the total amount of illumination emitted by a light source, measured in lumens (lm). The SI symbol or abbreviation is F. Lamp manufacturers provide this information about their products. As discussed in Chapter 3, the number of lumens produced by a lamp per watt of electricity consumed determines the lampâs efficacy. .
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