Steps In Doing a Lighting Layout

Module: Module 2 - Doing a Lighting Layout

Activity: Overview

(1) Figure out how many footcandles (average) you need

Let us say this is an industrial establishment where the workers are doing intricate electrical soldering. You decide you need 150 footcandles. However, you can provide 60 footcandles of ambient lighting and the balance through task lighting.

(2) Take the area of the room in square feet and multiply it by the footcandles (which is lumens per square feet). This gives you the total lumens falling on the work plane

Let us say the space is 50 feet x 60 feet = 3000 square feet. Since each square foot must receive 80 lumens, the total space will have 3000 x 60 = 118,000 lumens falling on it.

(3) How many primary lumens do you need from the lamps?

Not every lumen coming out of a lamp is going to reach the work plane. There will be losses in the fixture, from bouncing off walls and other objects in the room, and so on. A conservative estimate is that half the light from the lamps finds its way to the work surface. So, in order to have 180,000 lumens on the work plane, you need 360,000 lumens from the lamps.

This number--the percentage of light from the lamps that is going to find its way to the work plane--is called the coefficient of utilization. It depends on factors like the shape of the room. the reflectance of the walls, the fixture beam pattern, and so on. Our number here of 50% is a good average representation.

(4) Pick the approximate minimum number fixtures you will use

This will be determined by the height of the ceiling and how uniformly you wish to light the space. If this is a 10 foot ceiling you probably don't want to space the fixtures apart by more than about 15 feet. Every fixture manufacturer specifies a recommended spacing to mounting height ratio for that fixture. If you space your fixtures too far apart you will get dark areas and hot spots even thought your average light level may be OK.

If you put 20 fixtures on approximately a 5 x 4 grid, you will get en even coverage for the room. You could use more fixtures but probably not less.

(5) Pick a lamp (type, color and wattage)

You could get 360,000 lumens from three 1000 watt HPS lamps. You would get the average footcandles right but it would be a terrible choice. The color is bad for indoor lighting and secondly you will have a terrible time getting the light distributed evenly.

Since you have already decided on 20 fixtures, you know that each fixture must have lamps accounting for 18,000 primary lumens (18,000 x 20 = 360,000)

An HID source like a 250 watt metal halide lamp produces about 22000 lumens initially. A 4-lamp fluorescent fixture produces about 12,000 lumens initially. Note that the lamp lumens depreciate with burning. so you want to start out with more than 60 footcandles if you want to maintain an average of 60 footcandles.

If you went with metal halide lamps you would probably use a fixture with a spread lens to diffuse the light, 20 fixtures would be OK. If you went, instead with fluorescent fixtures, you would need more than 20 and they would have to be more closely spaced to accommodate them in the room. Let us say you went with fluorescent fixtures and decided to use 32 of them laid out in an 8 x 4 grid.

(6) Lay out the grid of fixtures

Make sure there are fixtures close to each wall to get light on the walls. Also remember, you have ensured that the spacing of the fixtures is not greater than the manufacturer's recommendations.

You have now completed a primary and simple lighting layout. Congratulations!