Lighting Design Procedures The best option available for lighting design is the use of standard lighting software. If no such facility is available, any one of the following techniques can be used for the design ο· ο· ο· Short hand method Long hand method Simplified method Short hand method In this method it is assumed that the average maintained quantity of lumens arriving on the work plane will be half the quantity of the total new lamp lumens. This method assumes “normal” sized rooms, “normal” surface reflectance and “normal” dirt conditions”. A normal sized room is the one in which the mounting height (distance from the luminaire bottom to the work plane) is less than half of the smallest room dimension. It is also assumed that conventional fixtures are used. A high bay fixture is used when mounting height is over 6 m. Otherwise low bay fixtures are recommended. The total number of fixtures (luminaires) to be used is then calculated as follows. Step-1 Obtain the required level of illumination in lux. Step-2 Choose a particular lamp and obtain the lam lumens form the lamp data. Step-3 Delivered maintained lamp lumen (DMLL) = 0.5 x Rated lamp lumens. Step-4 Number of fixtures (Luminaires) = π πππ π΄πππ π₯ πΏπ’π₯ π·ππΏπΏ Select the nearest round figure. Step-5 π πππ π΄πππ Area per fixture = Spacing between fixtures = √π΄πππ πππ πππ₯π‘π’ππ ππ’ππππ ππ πππ₯π‘π’ππ Step-6 Note: Spacing should not be more than the mounting height (MH) for high bay fixtures and 1.5 times the mounting height for low bay fixtures. Check for conformity. If the spacing is too large, go back to step-2 and start again with a lower lamp rating. Long hand method This is a more accurate and effective method of estimating the number of light fixtures (luminaires) and their spacing, because it takes into account the differences in Photometric performance caused by room geometry and system lumen depreciation. The core idea of this method is the determination of Coefficient of Utilization (CU) on the concept that the area to be lighted has three cavities or spaces that have effective reflectance with respect to each other and the work plane. The given space to be lighted is divided into three cavities as shown below. The space between the ceiling and the bottom of the luminaire is known as the Ceiling Cavity (CC) and the distance is h cc or the height of ceiling cavity. The space between the work plane and the bottom of luminaire is Room Cavity (RC) and the distance is hrc or the height of room cavity. The space between the work plane and the floor is Floor cvity (FC) and the height is hfc or the height of floor cavity. The room cavities are shown in the figure below. The cavity ratios represent the geometric properties of the ceiling, room and floor cavities and can be found using the formula: πΆππ£ππ‘π¦ πππ‘ππ = 2.5 π₯ β π₯ (πππππππ‘ππ) π΄πππ Where: h = hcc, for Ceiling Cavity Ratio (CCR) h = hrc, for Room Cavity Ratio (RCR) h = hfc, for Floor Cavity Ratio (FCR) Ceiling Cavity hcc Fixture Fixture Pc Room Cavity hRC Pw Work plane hFC PF Step-1 Study the site plan and elevation of the installation. Determine the lighting needs in the consultation with standards and the end user. Step-2 Select the light sources and luminaire, appropriate to the installation geometry, nature of virtual task and energy criteria. Step-3 Calculate the ceiling, room and floor cavity ratios and using Table for “Percentage Effective Ceiling and Floor Reflectances ” to find out the ceiling and floor cavity reflectances. Step-4 Use the given chart for the coefficient of utilization, the effective ceiling and floor reflectances and RCR; find out the value of effective coefficient of utilization. Step-5 Select the light loss factor (LLF0, this factor was previously referred as maintenance factor (MF), accepted values are AC rooms, clean rooms, etc. Industrial environment Dusty areas 0.8 0.7 0.6 Step-6 Use the formula to arrive at the quantity of luminaires (fixtures) Number of luminaires = π΄πππ π₯ πΌπππ’πππππ‘πππ πΆπ π₯πΏπΏπΉ π₯ πΏπ’πππ ππ’π‘ππ’π‘ ππ ππ’πππππππ Step-7 Arrange the luminaires symmetrically giving due considerations to spacing criterion to achieve uniformity of illumination. π΄πππ πππ ππ’πππππππ (πππ₯π‘π’ππ) = π πππ ππππ ππ’ππππ ππ πππ₯π‘π’πππ πβπππππππ, πππ₯π‘π’ππ π ππππππ = √π΄πππ πππ πππ₯π‘π’ππ Note: Spacing should not be more than the mounting height (MH) for high bay fixtures and 1.5 times the mounting height for low bay fixtures. Check for conformity. If the spacing is too large, go back to step-2 and start again with a lower lamp rating. Simplified design procedure A simplified design procedure for lighting system design by lumen method for a simple interior is given below (Assumption: C.O.U. chart are based on room index is available) Step-1 Study the site plan and elevation of the installation. Determine the lighting needs in consultation with standards and the end user. Step-2 Select the light sources and luminaire, appropriated to the installation geometry, nature of visual task and energy criteria. Step-3 Calculate the room index (RI) using the formula: π πππ πππππ₯ (π πΌ) = πΏππππ‘β π₯ π΅πππππ‘β (πΏππππ‘β + π΅πππππ‘β)π₯ πππ’ππ‘πππ βπππβπ‘ Step-4 Based on this, coefficient of utilization value can be obtained from the C.O.U. table for the selected luminaire. These values are generally based on the RI and room surface reflectances which are generally 70% for light colored, 50% for average and 30% for dark walls and 50%, 30%, and 10% for corresponding ceilings. Step-5 Select the light loss factor (LLF), this factor was previously referred to as maintenance factor (MF). Generally, accepted values are: AC rooms, clean rooms, etc. Industrial environment Dusty Areas 0.8 0.7 0.6 Step-6 Use the formula to arrive at the quantity of luminaries: ππ’ππππ ππ ππ’ππππππππ = π΄πππ π₯ πΌπππ’πππππ‘πππ πΆπ π₯ πΏπΏπΉ π₯ πΏπ’πππ ππ’π‘ππ’π‘ ππ ππ’πππππππ Step-7 Arrange the luminaires symmetrically giving due considerations to spacing criterions to achieve uniformity of illumination. π΄πππ πππ ππ’πππππππ(πππ₯π‘π’ππ) = π πππ π΄πππ ππ’ππππ ππ πππ₯π‘π’πππ πβπππππππ, πππ₯π‘π’ππ π ππππππ = √π΄πππ πππ πππ₯π‘π’ππ Note: If spacing is too wide, repeat the process with a fixture with lower wattage of lamps. Normally, spacing should not be more than the mounting height for high bay fixtures and 1.5 times the mounting height for low bay fixtures. Arrangement of fixtures Fixtures shall be arranged from the middle of the room to the outside. A square array is the best option but rectangular array also will work satisfactorily as long as the spacing to mounting height ratio is not violated. With an odd quantities of fixtures of rows or columns, there will be a fixture line on the center line. With even quantities of rows or columns, the locations are half the spacing off the center line. The closest fixture to a wall should be one half the spacing or less.
