E5 Natural Light and Lighting

Topic

The amount of light a surface receives—its illuminance—depends on the luminous flux distributed across its area; this flux decreases as the source moves further away and as the angle of incidence becomes more oblique—a behavior analogous to other physical quantities that diminish with distance and the angle of incidence.

To assess how well a space utilizes natural light, the interior illuminance is compared to the exterior illuminance available under a standard overcast sky to determine the daylight factor: the higher this factor, the less reliance there is on artificial lighting during the day, whereas a lower factor implies a greater dependence on it, even during daylight hours. The amount of solar radiation driving that exterior illuminance depends, in turn, on the sun's altitude above the horizon, which varies according to the time of day, the season, and the location's latitude.

Beyond the quantity of light received, the appearance of the illuminated objects matters: a light source's color rendering index—calculated by averaging its performance against a set of reference colors—summarizes how faithfully it reproduces the true colors of objects. Lighting standards combine both criteria—light quantity and quality—by establishing minimum recommended illuminance levels based on the space's function; these requirements are stricter for offices and classrooms than for circulation areas.

Thus, this module—which begins by discussing illuminance in relation to distance and angle—concludes by demonstrating that designing with natural light involves more than simply maximizing light intake; it requires balancing that quantity with color rendering quality and the minimum levels actually required for the space's specific use.

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