Illumination - Method of calculation
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There are three main methods for lighting calculations: 1) Watts per square meter, 2) Lumen or light flux, and 3) Point to point or inverse square law. The lumen method is used to estimate illumination patterns from diffuse lighting sources over a broad area. It determines the total installed luminous flux (Φinst) needed to provide the required received flux (Φrec), taking into account the maintenance factor and utilization factor. The utilization factor depends on luminaire properties, room reflectance, distribution of light sources, and the room index. The number of lamps required is calculated as the product of the required flux and area divided by the lamp lumen output and adjustment factors.
Illumination - Method of calculation
- 1. Vijay Balu Raskar (BE Electrical)
- 2. Methods for calculating illumination There are three method for Lighting calculations:- 1)Watts per square meter method 2)Lumen or Light flux method 3)Point to Point OR Inverse – Square law method Vijay Balu Raskar (BE Electrical)
- 3. Watts per square meter method Principal based on “Rule of thumb”. Very handy for rough calculation or checking. Illumination based on assumption and consists in making an allowance of watts per square meter of area. Vijay Balu Raskar (BE Electrical)
- 4. The problem of the lighting designer at the functional level • To determine how many lights and • Where to place them to get the correct level of illumination for a given activity. Vijay Balu Raskar (BE Electrical)
- 5. Calculations can be divided into two types • Accent or task lighting. • To determine the illumination at a small specific location in the room from a point light source. • Simply get polar plot for luminaire and work out the illumination on surface for a given lamp. • Called the point illumination method. Vijay Balu Raskar (BE Electrical)
- 6. The lumen method • To estimate the illumination pattern from a set of diffuse lighting sources over a broad area in a room. • This method gives rough and reasonable estimates of the lamps / luminaires needed. • Is used in areas where a uniform light intensity is required for the work area. • Used for rectangular rooms with gridded luminaire pattern. • Also called as Light Flux Method Vijay Balu Raskar (BE Electrical)
- 7. The lumen method • Illumination level required at the work surface is obtained by, rec E= A • E = illumination level required at the work surface • A = total area of the plane where the work is done. • Φrec = flux of light received on the working surface. • It is to determine how much flux needs to be installed, i.e. Φinst, to get the required amount of received flux Φrec. Vijay Balu Raskar (BE Electrical)
- 8. The lumen method • First determine how much flux is to be received. • Multiply the illumination required by the surface area. • Φrec = E A • The received flux is related to the installed flux by a formula rec = MF UF inst • where • MF = Maintenance factor (Light Loss Factor LLF) • UF = Utilization factor • Φ = Total Lumen Vijay Balu Raskar (BE Electrical)
- 9. Maintenance Factor • Gives an estimate of how lighting conditions will deteriorate through use. • Some factors are dust and dirt inside luminaire surfaces. • Aging of light bulbs emitting less light • Cleaning of room surfaces, e.g. ceiling • Without detailed knowledge of a maintenance plan, MF is assumed to be = 0.80. Vijay Balu Raskar (BE Electrical)
- 10. Depreciation factor • Light emitted by source reduces due to dust or dust decomposition on light. • So, quantity of light reflected from ceiling also get reduced. • This reduction of light can be minimized by cleaning the light fittings or white washing. • But absolute cleanliness is not possible hence we need to consider depreciation factor. • D. F. = 1/ M. F. Illumination under ideally clean condition D.F. = Illumination under normal working condition Vijay Balu Raskar (BE Electrical)
- 11. Utilization Factor (η) • Is the ratio of effective luminous flux to the total luminous flux of light sources. • Always less than one Fn Effective luminous flux U.F.= = F Total luminous flux of all the sources Vijay Balu Raskar (BE Electrical)
- 12. Effective Luminous Flux (Fn) • The luminous flux incident on the working area. • Rest luminous flux is used for lighting the walls and ceilings and is partly absorbed by the fittings. 3 2 2 1 Working plane Vijay Balu Raskar (BE Electrical)
- 13. U. F. Depend on the following • The luminaire properties. • The light output ratio (LOR). • How much of the light emitted by the lamps actually leaves the luminaire? • The reflection factors of ceiling and walls. • Reflectance are available from manufacturers of paints and furniture finishing. Colour Reflectance White, Off-white, light shades of gray, 75-90% brown, blue Medium green, yellow, brown or gray 30-60% Dark gray, medium blue 10-20% Dark blue, green, wood Balu Raskar (BE Electrical) Vijay paneling 5-10%
- 14. U. F. Depend on the following • Distribution of light sources in the room • It is usual to make the reflectance of the ceiling highest, walls slightly lower and the floor darker. • Typically recommends in offices Light C eiling 70-90% Wall 50-70% Floor 20-50% • Do not choose very dark wood, or bright surfaces. Vijay Balu Raskar (BE Electrical)
- 15. U. F. Depend on the following • The geometric shape of the room i.e. length, breadth and height of a room will affect the UF. • A factor called the room index (RI) is defined from the horizontal vertical areas of the room. Vijay Balu Raskar (BE Electrical)
- 16. Room index • The horizontal areas are Area H = 2 Length Width =2 L W • The vertical areas are Area L = 2 Length Width Height Lum =2 L W Hm • Hm = mounting height = Vertical distance from the work place to the luminaire. • The Room index is Area H L×W RI = = Area V(BE Electrical) Vijay Balu Raskar (L+W) × H m
- 17. For uniform illumination • As a rule of thumb, to achieve uniform lighting spacing between the luminaries should be less than 1.5 times the mounting height. • Lumnaire Spacing < 1.5 Hm Vijay Balu Raskar (BE Electrical)
- 18. No. of lamps required rec = MF UF inst rec E×A inst MF UF MF UF Φ So, No. of lamps required = inst Lumen o/p So, No. of lamps required = E×A Lumen o/p MF UF Vijay Balu Raskar (BE Electrical)