Determining Target Light Levels

The Illuminating Engineering Society of North America has developed a procedure for determining the appropriate average light level for a particular space. This procedure (used extensively by designers and engineers) recommends a target light level by considering the following:

•the task(s) being performed (contrast, size, etc.)
•the ages of the occupants
•the importance of speed and accuracy

Then, the appropriate type and quantity of lamps and light fixtures may be selected based on the following:

•fixture efficiency
•lamp lumen output
•the reflectance of surrounding surfaces
•the effects of light losses from lamp lumen depreciation and dirt accumulation
•room size and shape
•availability of natural light (daylight)

When designing a new or upgraded lighting system, one must be careful to avoid overlighting a space. In the past, spaces were designed for as much as 200 footcandles in places where 50 footcandles may not only be adequate, but superior. This was partly due to the misconception that the more light in a space, the higher the quality. Not only does overlighting waste energy, but it can also reduce lighting quality.

Within a listed range of illuminance, three factors dictate the proper level: age of the occupant(s), speed and accuracy requirements, and background contrast.

For example, to light a space that uses computers, the overhead light fixtures should provide up to 30 fc of ambient lighting. The task lights should provide the additional footcandles needed to achieve a total illuminance of up to 50 fc for reading and writing. For illuminance recommendations for specific visual tasks, refer to the IES Lighting Handbook, or to the IES Recommended Practice No. 24 (for VDT lighting).