Electric Lighting Calculation Methods PDF

3. ILLUMINANCE CALCULATION 3.0 LUMEN METHOD The idea of a lumen method of interior design had been proposed earlier, but it was not until 1920 that M/s. W. Harrison and E.A. Anderson introduced a standardised procedure which include both direct and reflected lumens. This gives a simp...

3. ILLUMINANCE CALCULATION 3.0 LUMEN METHOD The idea of a lumen method of interior design had been proposed earlier, but it was not until 1920 that M/s. W. Harrison and E.A. Anderson introduced a standardised procedure which include both direct and reflected lumens. This gives a simple solution to the problem of providing average horizontal illuminance. By definition, Illuminance (lux) = Luminous flux/Area (Lumens/Sq.Mtr.). To obtain this average illuminance we need to determine the total luminous flux reaching the work plane. This flux is composed of two components: Flux which comes directly from the luminaire to the work plane (direct component). Flux which is reflected from room surfaces and reaches the work plane (reflected component). In the Lumen method the fraction of the initial lamp lumen which ultimately reaches the work plane both directly and from reflection is called co-efficient of utilisation (C.O.U.). It should be realised that C.O.U. includes both, the efficiency in room surface in redirecting these lumens to the work plane. The C.O.U. is the function of three factors: Room surface reflection. Room size and proportion. Luminaire (efficiency and intensity distribution). For example, if a particular luminaire has C O.U' of 0.70, then 70% of the lumens produced by the lamp would reach the work plane and 30% of the lumens would be lost, some would be trapped inside the luminaire and others would be absorbed by the room surfaces, such as walls, floors. These factors will be examined individually. Room surface reflectance All surfaces in the room (walls, ceilings, floors, furniture, machines, and people) absorb and reflect light. If these surfaces are highly reflective, less light is absorbed and more light is reflected back into the space. Since some of the reflected light will eventually reach the work-plane, luminaires will perform more efficiently in rooms with high reflectance surfaces than in rooms with surfaces of low reflecting value. Room size and proportions Except from a totally indirect system, light travels directly from the luminaire to the work-plane while other light travels to the walls where some is absorbed and some reflected towards the work-plane. In small rooms, a larger percentage of the total light produced by the luminaires strikes the walls than in large rooms (fig. 1). Note that in the small room a large part of the light from the luminaire strikes the walls wheras in the large room, some light from the end luminaire hits the wall, but the luminaires in the centre send all their light to the work-plane. This means that luminaires have a higher C.O.U. in large rooms than in small rooms since a larger percentage of the total light produced by the systems goes directly to the work plane. 21 Luminaire (efficiency and intensity distribution) The design and light distribution characteristics and efficiency of the luminaire also affect the C.O.U. It depends upon the following factors: Light distribution of the luminaire. Efficiency and light output ratio. Spacing to mounting height ratio. Determination of C.O.U.: 1. Get the C.O.U. table from the luminaire manufacturer for a particular luminaire, 2. Find out the room index (K) of the room: L x W K = ––––––––––– Hm ( L + W) where L = length of the W = Width of the room Hm = Mounting height (Distance between the centre of the luminaire to the working plane 3. Select reflectance factors depending upon the colour of the walls, ceiling or working plane. 4. Knowing K and wall reflectance, ceiling reflectance and work plane reflectance, C.O.U has to be read from the photometric data sheet which is given below: Coefficient of utilization (C.O.U.)( Typical data for a luminaire provided by manufacturer) C.O.U. calculated in accordance with IS: 3646 (Part 3) – 1968 Room Room Index reflectance F C W 0.60 0.80 1.00 1.25 1.50 2.00 2.50 3.00 4.00 5.80 10.56.62.67.71.74.78.79.82.83.85 10 70 30.52.59.63.68.71.75.78.79.82.83 10 50 50.55.61.66.70.73.76.79.80.82.83 30.52.58.63.67.70.74.76.78.80.82 30.52.58.62.67.69.73.75.77.79.80 10 30 10.49.53.60.64.66.71.74.76.78.79 C.O.U = D.C + U.C D.C = Downward co-efficient U.C = Upward co-efficient C.O.U = ( LFU x DLOR) + (UFU x ULOR) LFU = Lower flux utilance 22 DLOR = Downward light output ratio UFU = Upward flux utilance ULOR = Upward light output ratio In the IS 3646 Part II for each type of fitting classified under the BZ class (BZ1 to BZ10) the values of LFU as well as UFU are given for particular room index values. The LFU values and UFU values are different for different reflectance factors for ceiling , floor and walls. Type of fitting should be given by the manufacturer. Generally a metal halide high bay fitting comes under BZ1 class and fluorescent tube fitting falls between BZ5 and BZ6. The classification of the BZ class is based on the intensity distribution based on the polar curve. The values of DLOR and ULOR is the ratio of downward output fraction to upward output fraction which depends on the luminaire. The C.O.U value is now calculated. INITIAL LUMEN OUTPUT Luminous flux x C.O.U. Illuminance = ––––––––––––––––––––––––. Area This illuminance corresponds to the initial value of the brand new lighting installation. Maintained illuminance We are interested in average maintained illuminance rather than average initial illuminance, we must include a light loss factor (maintenance factor) in our calculation. Hence, the equation becomes: Lamp Lumens x C.O.U. x M.F. Illuminance = ––––––––––––––––––––––––––––––. Area Maintenance Factor (Light Loss Factor) Total light loss is divided conventionally into unrecoverable and recoverable losses and the light loss factor should include as many of each of these as are quantifiable. Recoverable losses There are four recoverable losses: a) Lamp Burnouts (LBO) b) Lamp Lumen Depreciation (LLD) c) Luminaire Dirt Depreciation (LDD) d) Room Surface Dirt Depreciation (RSDD) a) Lamp Burnouts It may not be feasible to replace a lamp every time one burn out. If burnt-out lamps are not replaced immediately, the average illuminance level will be tolerated in a given installation before they are replaced. Then the LBO factor is the ratio of the number of lamps remaining lighted to the total number of lamps, when the maximum number of burnouts is reached. 23 b) Lamp Lumen Depreciation The Lumen output of a lamp decreases as it ages. The LLD factor is generally taken at the 70% life point for high intensity discharge lamps, and fluorescent lamps. Lamp lumens at 70% life is divided by the initial lamp lumens to obtain the LLD. c) Luminaire Dirt Depreciation The greatest loss of light output is mainly attributed to the dirt on the lamp and the luminaire reflecting surface. It depends upon following two parameters: Type of the luminaire (either the luminaire is enclosed or open, whether or not it is having louvres or diffusers etc). Degree of dirtiness of the luminaire environment (very clean, clean, medium dirty, very dirty). d) Room Surface Dirt Depreciation Dirt on room surfaces reduces the reflected component of luminous flux and thus the illuminance on the work plane. The resulting RSDD factor is a function of the dirtiness of the environment, the time between cleanings, the luminaire flux distributions, and the room proportions. Unrecoverable factor a) Luminaire Ambient Temperature (LAT) Fluorescent lamp and luminaire ratings are established in still air at 25°C. Furthermore, fluorescent luminaire photometry is relative in that the luminaire output is adjusted for a 25°C lamp operating temperature. In an actual installation, the luminaire operating temperature is likely to differ from the test condition. Operation above or below the test value decreases lumen output. For example, if the operating temperature for a fluorescent luminaire is 30°C, the lamp output would decrease by about 7.5%, giving a temperature correction factor of 0.925. There is no significant variation with ambient temperature for incandescent and HID Lamps. b) Voltage at the Luminaire (VL) Deviations in supply voltage from rated value can affect the light output of all types of lamps. A 1% change in voltage causes a 3% change in-lumen output for incandescent lamps. c) Luminaire Surface Depreciation (LSD) Effects such as yellowing of plastic with age and pitting and discoloration of enamel overtime can cause a reduction in light reflected within the luminaire and transmitted through the diffuser. No numbers are available for this factor. d) Ballast Factor (BF) Ballast factor is the ratio of the lamp lumens when the lamp is operated on its ballast to the rated lamp lumens determined on a standard lamp and ballast testing circuit. Commercial Ballasts do not generally perform as efficiently as test ballasts. 24 Illuminance Calculation -------------------------------------- 25