Lighting Design Calculation in a building

In professional field proper lighting design is very important because an under lighting arrangement will decrease the efficiency of the task for which the lightings were designed and an over lighting arrangement will result in over expenditure of the company. On small scale this difference is not too much to worry about but in large buildings, plants, factories, etc it becomes very significant in today electrical wiring installations.

The simple and basic approach for calculating the lighting requirement is to divide the total light requirement of the room by light output (lumen) provided by a single lamp. Although this is the basic approach for an average household room, but it’s not practically accurate.

In practical there are several other parameters which are necessary to be considered in the calculation because nothings Ideal. For example the luminaries lumen output won’t be the same throughout the entire life span, dust deposition on lamps will also reduce their output over time which means cleanliness is also an important parameter. A bright painted room reflects more light than a dark coloured room so they both have different lighting requirements.

So it is important to first understand few basic terms about lighting design before beginning the calculations.

Room Index- It is based on shape and size of the room. It describes the ratios of the room’s length, width and height. It’s usually between 0.75 to 5.

Where “l ” is the length of the room,

“w” is the width of the room and,

h_wc is height between work plane i.e. Bench to Ceiling

This formula for Room Index is applicable only when room length is less than 4 times the width.

Maintenance Factor:

It is ratio of the lamp lumen output after a particular interval of time as compared to when it was new. The lumen output of a light fitting decreases with time because of aging of many of its components by internal (saturation of elements) or external factors (dust deposition). For example maintenance factor of a light fitting used in a cool dust free area will be better than the light fitting used in hot and dusty area.

It is less than or equal to 1.

Typical values used for the lighting calculation are:

• 0.8 – For offices/classroom
• 0.7 – For clean Industry
• 0.6 – For dirty Industry

Room Reflections

The room is considered to consist of three main surfaces:

1. The ceiling
2. The walls
3. The floor

The effective reflectance’s of these 3 surfaces affect the quantity of reflected light received by the working plane. Light colors like white, yellow will have more reflectance compared to dark colors like blue, brown.

Utilization Factor

Utilization factor (UF) is the ratio of effective luminous flux to the total luminous flux of light sources. It is the measure of the effectiveness of the lighting scheme.

It depends upon

• The efficiency of luminaire
• The luminaire distribution
• The geometry of the space
• Room reflectance’s
• Polar curve

Space to Height ratio

It is the ratio of distance between adjacent luminaires (centre to centre) to their height above the working plane.

Where,

• H_m = Mounting height
• A = Total floor area
• N = No. of Luminaires

It should not exceed maximum SHR of the luminaire as provided by the manufacturer.

Note: A normal living room requires 20 lm/ft² i.e. 215 lm/m²

For Studying room i.e. Classroom 300 lm/m² is required.

(Note that for different environment and conditions there are different standards. For example companies like many MNC’s should maintain 600 lm/m² in the Office’s for people working in night shifts)

Now let’s start with the steps. Consider the following layout of a particular floor of the School and analyse the lighting requirements of different sections of the floor.

For ease of the calculation all the light fittings and their ratings taken into account are of Phillips make. You can check the various fixtures and their specification here provided by Philips.

Lighting Design Calculation for Classroom

Cross section area of classroom = 6×9 = 54 m², h = 3m

Lumens required = 54×300 = 16200 lm

The below table is a reference table for calculating Utilisation factor for light fittings. It differs from model to model and make to make. For just understanding the concept we are using a single reference table for all the light fittings. The actual table is provided by the manufacturer and can be little different from the one below.

Room Reflectance			Room Index
C	W	F	0.75	1	1.25	1.50	2.00	2.50	3.00	4.00	5.00
0.70	0.50	0.20	0.43	0.49	0.55	0.60	0.66	0.71	0.75	0.80	0.83
	0.30		0.35	0.41	0.47	0.52	0.59	0.65	0.69	0.75	0.78
	0.10		0.29	0.35	0.41	0.46	0.53	0.59	0.63	0.70	0.74
0.50	0.50	0.20	0.38	0.44	0.49	0.53	0.59	0.63	0.66	0.70	0.73
	0.30		0.31	0.37	0.42	0.46	0.53	0.58	0.61	0.66	0.70
	0.10		0.27	0.32	0.37	0.41	0.48	0.53	0.57	0.62	0.66
0.30	0.50	0.20	0.30	0.37	0.41	0.45	0.52	0.57	0.60	0.65	0.69
	0.30		0.28	0.33	0.38	0.41	0.47	0.51	0.54	0.59	0.62
	0.10		0.24	0.29	0.34	0.37	0.43	0.48	0.51	0.56	0.59
0.00	0.00	0.00	0.19	0.23	0.27	0.30	0.35	0.39	0.42	0.46	0.48

source: http://www.electricaltechnology.org