Room Proportion
There is no universal answer to the question: “How deep can a room be effectively illuminated with natural light?” Room proportion determines light distribution, not size. For example, given the same proportion and opening, orientation, and reflectances these two rooms will have the same luminance distribution.
As ceiling height increases, the light distribution becomes more even for both toplighting and side lighting. Shown in the side lighting sections below, as the ceiling height increases from 8 to 16 m, the ratio of average light levels to minimum light levels decreases by a factor of 6. The flatter the illumination gradient, the more even the distribution of light.
Room Reflectances

Light distribution is highly dependent on room reflectance. In general, the ceiling is the most important light-reflecting surface. The IESNA guideline of 70/50/20 (ceiling/walls/floor) for minimum surface reflectance follows this principle. Because most tasks “see” light reflected from the ceiling, it will be a significant light source, especially in deep, wide, sidelit rooms. In toplighting and smaller rooms, the side walls become increasingly important.
In diagrams below, various combinations of flat black and matte white surfaces are placed opposite a window wall. The reduction of daylight on the desktop surface illustrates the relative importance of each surface for a space with this light source and proportion. The percentages show illuminance according to all white surface conditions rated at 100%.

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Architectural Lighting by M. David Egan and Victor W. Olgyay