Of equal importance to the quantity of light is the quality of light. The quality of light is dependent on both the properties of the light and how that light is delivered to the space. The fundamental quality issues include all of the IESNA “design issues“ listed in The Right Quantity of Light with special consideration given to:

• Glare
• Uniformity of luminance
• Color temperature and color rendition

The eye does not see absolute levels of illuminance; the amount of light shining on a surface. It sees differences in luminance, the amount of light reflected back from the surface. Eyestrain and fatigue are caused when the eye is forced to adapt continually to different luminances. Therefore, it is important not only to provide the right level of light but also to ensure that light is evenly distributed across the task area. Balancing light levels also ensures that task lighting levels will be adequate throughout the space. Uniformity on vertical surfaces should also be maintained to avoid a gloomy, cavelike atmosphere.

IESNA recommends as good design practice an average luminance ratio of no more than 3 to 1 for close objects and 10 to 1 for distant objects and outdoor applications (IESNA Lighting Handbook, Sect. 11—Office Lighting). In other words, the difference in light level between the task area and the background should be less than a factor of three. While some designers use illuminance variation as an organizing theme, such as defining hallways leading to open offices, or as a highlighting strategy, such as in retail and merchandising locations, large footcandle variations within a workspace should be avoided.