Technically, luminance thoroughly describes the visual scene and should be the primary design metric. The problem is, luminance is not easily measured and calculations are complex and time consuming. Because luminance depends on the reflectance of the surface, it is highly dependent on exact knowledge of architectural and interior finishes. Most footcandle-based standards are simplified approximations of true luminance-based design and analysis. Luminance should be considered as part of the design. As a basic premise, IES design criteria suggest that wall and ceiling luminance be close to task luminance, the range being dependent upon application. The luminance of the task background—typically white paper—should be used as the basis and room surfaces should not exceed a 10:1 luminance ratio.
Distribution & Spaces Figure 1. Photometrically-accurate computer renderings of the same office using different luminaires. Although each office provides the same average illuminance on the desk, the ceiling and wall luminances vary dramatically. Images courtesy of Leslie M. North.