Lumen Method Details. Because of the ease of application of the lumen method which yields the average The lumen method is based on the definition of a footcandle, which equals one lumen per square fg Form. illumination in a room, it is usually employed for larger areas, where the illumination is substantially ur footcandle lumen striking an area (107.2) quare feet of area In order to take into consideration such factors as dirt on the luminaire, general depreciation in lume output of the lamp, and so on, the above formula is modified as follows footcandle lamps/ luminaire× lumens/p×CU×LLF (1073) area/luminaire ethod, the following key steps should be taken a. Determine the required level of illuminance. b. Determine the coefficient of utilization(CU)which is the ratio of the lumens reaching the working plane to the total lumens generated by the lamps. This is a factor that takes into account the efficiency and the distribution of the luminaire, its mounting height, the room proportions, and the reflectances of the walls, ceiling, and floor. Rooms are classified according to shape by 10 room cavity numbers. The cavity ratio can be calculated using the formula given in Eq. (107. 1). The coefficient of utilization is selected from tables prepared for various luminaires by manufacturers C. Determine the light loss factor (LLF). The final light loss factor is the product of all the contributing loss factors. Lamp manufacturers rate filament lamps in accordance with their output when the lamp is new; vapor discharge lamps(fluorescent, mercury, and other types )are rated in accordance with their output after 100 hr of burning d. Calculate the number of lamps and luminaires required: footcandles×area no of lamps (1074) lumens/p× CU x LLF no of lamps no of luminaires- lamps/luminair (1075) e. Determine the location of the luminaire--luminaire locations depend on the general architecture, size ys, type of luminaire, posi Point-by-Point Method. Although currently light computations emphasize the zonal cavity method, there is still considerable merit in the point-by-point method. This method lends itself especially well to calculating the illumination level at a particular point where total illumination is the sum of general overhead lighting and supplementary lighting. In this method, information from luminaire candlepower distribution curves must be applied to the mathematical relationship. The total contribution from all luminaires to the illumination level on the task plane must be summed. Direct Illumination Component. The angular coordinate system is most applicable to continuous rows of fluorescent luminaires. Two angles are involved: a longitudinal angle a and a lateral angle B. Angle a is the angle between a vertical line passing through the seeing task (point P)and a line from the seeing task to the end of the rows of luminaires. Angle a is easily determined graphically from a chart showing angles a and p e 2000 by CRC Press LLC© 2000 by CRC Press LLC Lumen Method Details. Because of the ease of application of the lumen method which yields the average illumination in a room, it is usually employed for larger areas, where the illumination is substantially uniform. The lumen method is based on the definition of a footcandle, which equals one lumen per square foot: (107.2) In order to take into consideration such factors as dirt on the luminaire, general depreciation in lumen output of the lamp, and so on, the above formula is modified as follows: (107.3) In using the lumen method, the following key steps should be taken: a. Determine the required level of illuminance. b. Determine the coefficient of utilization (CU) which is the ratio of the lumens reaching the working plane to the total lumens generated by the lamps. This is a factor that takes into account the efficiency and the distribution of the luminaire, its mounting height, the room proportions, and the reflectances of the walls, ceiling, and floor. Rooms are classified according to shape by 10 room cavity numbers. The cavity ratio can be calculated using the formula given in Eq. (107.1). The coefficient of utilization is selected from tables prepared for various luminaires by manufacturers. c. Determine the light loss factor (LLF). The final light loss factor is the product of all the contributing loss factors. Lamp manufacturers rate filament lamps in accordance with their output when the lamp is new; vapor discharge lamps (fluorescent, mercury, and other types ) are rated in accordance with their output after 100 hr of burning. d. Calculate the number of lamps and luminaires required: (107.4) (107.5) e. Determine the location of the luminaire—luminaire locations depend on the general architecture, size of bays, type of luminaire, position of previous outlets, and so on. Point-by-Point Method. Although currently light computations emphasize the zonal cavity method, there is still considerable merit in the point-by-point method. This method lends itself especially well to calculating the illumination level at a particular point where total illumination is the sum of general overhead lighting and supplementary lighting. In this method, information from luminaire candlepower distribution curves must be applied to the mathematical relationship. The total contribution from all luminaires to the illumination level on the task plane must be summed. Direct Illumination Component. The angular coordinate system is most applicable to continuous rows of fluorescent luminaires. Two angles are involved: a longitudinal angle a and a lateral angle b. Angle a is the angle between a vertical line passing through the seeing task (point P) and a line from the seeing task to the end of the rows of luminaires. Angle a is easily determined graphically from a chart showing angles a and b footcandle lumen striking an area square feet of area = footcandle lamps/luminaire lumens/lp CU LLF area/luminaire = ¥ ¥ ¥ no. of lamps footcandles area lumens/lp CU LLF = ¥ ¥ ¥ no. of luminaires no. of lamps lamps/luminaire =