Planning for the proper amount of lighting in a home or work environment requires a detailed analysis of how the space will be used. Also needed is a working knowledge of the vocabulary of light used by lighting architects and engineers. Illuminance, formerly known as illumination, is one of the factors to consider when designing a lighting system.
A light source emits a flow of energy measured by a unit of power called a lumen. The density of light that falls on a surface is measured in foot candles, or lux. A foot candle is one lumen of light density per square foot. A lux is one lumen of light density per square meter. The foot candle is an older British measurement that is largely being replaced by the metric-based lux. One lux equals 10.752 foot candles.
Outdoor light levels, as measured by a light meter, can range from 107,527 lux in bright sunlight to 107 lux on a very dark day. A full moon provides .108 lux, while an overcast night might produce only .0001 lux. Recommended light levels (in lux) for indoor work spaces are 500 for personal computing work; 750 for supermarkets; 1,500 to 2,000 for detailed drawing work; and 10,000 to 20,000 for visual tasks in extremely low contrast using small-sized objects. Direct, intense lighting is required for the latter tasks.
Creating good lighting is not as simple as increasing lux levels. Other factors must also be considered. Light direction, distribution, temperature and colour all affect visibility. Illuminance levels are influenced by the complexity of the tasks performed within the space, how the light is reflected, the incidence of shadows, the age and condition of the eyes of workers and whether the job requires speed and accuracy.
If a space is underlit, any resultant energy saving may be offset by a reduction in human productivity. As eyes age, lux levels must be increased to allow the older worker to perform effectively. A classroom designed for adults may require twice the illuminance as one designed for young children. General room lighting can be effectively supplemented through the use of task lighting on office desks or directed pot lights in a kitchen workspace.
Since 1958, illuminance recommendations have been published in table form by the Illuminating Engineering Society. A four-step process determines proper lighting. The visual task is defined, an illuminance category is established, a range of lighting is determined and the result is weighted according to the ages of the occupants, the reflectance of the environment (i.e., lighter paint colours reflect more light) and the importance of speed and/or accuracy of the tasks performed.
Valuable art works and rare museum pieces may require less light to avoid damage due to fading. A dark stairwell may require more than the recommended level of lighting to ensure worker safety. Advertisers wishing to draw attention to a product increase lighting for visual effect. Whatever the application, lux level calculation is an important part of the lighting engineer's or architect's job.