There is more to lighting than just being able to read comfortably. Visual comfort should not be neglected just to save electricity. Some defining principles for every illumination retrofit include:
1. Luminaire efficiency, measured in terms of “coefficient of utilization,” should be 0.55 or higher for tasks requiring a high visual comfort. Such a quality luminaire will diminish veiling reflectances in the space.
2. If veiling reflectances are not critical to performance in the space under consideration, but if visual comfort is critical, then the coefficient of utilization should be 0.63 or more. This will provide medium to high visual comfort.
3. If no critical tasks are done in the space, and if visual comfort is not overriding, then the luminaire selected can have a coefficient of utilization of 0.70 or more.
Lighting designers regularly select fixtures that have the highest possible coefficient of utilization — since this is an indication that the most possible light reaches the work plane from the luminaire. Granted, such fixtures are the most energy-efficient, but visual comfort is sometimes sacrificed.
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Excerpt from Energy Conservation Projects.