Designing with Light

How much is enough?

LIGHT LEVELS are partly a matter of individual preference. Some people grow accustomed to brightly lit offices and want similar uniform illumination in their homes. Others feel more relaxed and secure with relatively low levels, preferring to focus on the area where they’re reading, working, or dining. The current thinking is toward bright, efficient lighting in task areas, with surroundings more softly lit.

So how much task light is “enough?” When planning, consider these factors:

* How difficult is the task to be performed?
* How much speed and accuracy does the task require?
* How much color contrast is there among materials involved in the task?
* How good is the eyesight of the person who will be engaged in the activity?

If an older person will be doing embroidery on a dark cloth with richly colored thread, lots of light will probably be required; the task calls for a high degree of accuracy, and the thread is hard on older eyes. For less demanding visual activities, such as reading the newspaper or watching television, light levels can be much lower.

Measuring lumens

One method for measuring and planning light levels involves adding up the amount of light - measured in lumens – emitted by all the bulbs in a certain area. If you look at the sleeve around a light bulb, you’ll see that it states both the bulb’s wattage (the amount of electricity used by the bulb) and the number of lumens (amount of light) that the bulb produces. Lumen outputs vary from one manufacturer to another, and they diminish as bulbs age.

As a rule of thumb, the most difficult visual tasks, such as embroidery, require a total of at least 2,500 lumens in an average-size room, with the greatest number of lumens concentrated at the work location. A casual task, such as watching television, requires from 1,500 to 2,000 lumens. To figure total lumens, just add up the lumen outputs of all the bulbs in the room.

For reference purposes, a standard 40-watt incandescent A-bulb (the familiar pear shape) puts out about 455 lumens, a 75-watt bulb casts 1,190 lumens, and a 150-watt bulb produces 2,880 lumens.

Measuring footcandles

Lighting designers and engineers have determined standard footcandle levels needed to perform ordinary household tasks. The chart below includes both a high and a low number of footcandles; the higher level is recommended for older people, the lower level for younger eyes. While these are recommended levels, individuals may prefer more or less light. Remember that dimmers allow you to dial light up for one use, and down for another.

Experts measure footcandles with a special footcandle meter, but you can use the light meter built into a 35mm camera; for directions, see "Testing Your Ideas." Providing enough light for task areas is of primary importance, but care should also be taken to provide the surrounding areas with ambient light. In rooms with task lighting, the recommended ambient light level is 20 footcandles or about a third of the task area's footcandle value, whichever is less. For rooms where the main activity is entertaining or relaxing, a level of 5 to 10 footcandles is recommended. Entries, stairs, and passageways should also have a general light level of 5 to 10 footcandles.