How much light do you need?
The various activities carried out in the average home require not only different types of lighting, but also different amounts of light. In technical terms the requirements are expressed in lumens per square metre. (To convert them to lumens per square foot, divide by 10.) For general use a living room should have a light intensity of 100 lumens per square metre (10 per square foot). Light reading requires 200, prolonged reading (or studying) 400. Sewing or close work calls for 600 lumens per square metre.
You could achieve this necessary versatility by having a ceiling light fitting which provides sufficient light for general purposes, supplemented by spots or table lamps. Additional light will be needed near or above chairs or work surfaces. The average requirements for other rooms are kitchens-200 lumens per square metre (particularly over work surfaces); bedrooms about 50 lumens per square metre with provision for a 200 lumens headboard light for the late night reader.
Most other rooms including landings and halls need about 100 lumens. Stairs, at their treads, ought to have 50-100 lumens to be absolutely sale with control switches at both the top and bottom of the staircase. It is simple to calculate the wattage required to provide these standards of lighting as long as you know the efficiency with which the lamp converts electrical energy into light-in other words, the number of lumens per watt. A fluorescent tube provides about 50 lumens per watt. So, for example, if the light intensity needed in a kitchen is 20 lumens per square foot the wattage required using a fluorescent tube will be 20 + 50 =0.4 watts per square foot. Thus, one 40 watt fluorescent lamp will provide the equivalent of 20 lumens per square foot over an area of 100 square feet (0.4 x 100). Metallic filament bulbs are far less efficient than fluorescent lamps, producing approximately 12 lumens per watt. Daylight has hundreds of lumens per square foot but it is generally easy on the eye because it is so evenly distributed.