PÉCLETBuilding Physics
PÉCLET / Disciplines / 05E — lux · DF — %

Daylight

Daylight is the only building service that is free, variable, and impossible to retrofit.

QuantityIlluminanceTransportVisible radiationInteractionDrag to move the sun. Use the aperture control to change the size of the opening.
Fig. 05 — Daylight in sectionAperture size changes how far light reaches
With the aperture at 48 per cent of the wall height and the sun 38 degrees above the horizon, daylight on the floor is strongest about 44 per cent of the way into the room, and the back of the room receives about 63 per cent of that peak. The plotted curve shows the distribution along the floor. Values are relative and indicative, not measured illuminance.
Back of room63 % of peakBrightest at 44 % depth
The interior of a room seen in section, with a single aperture in one wall. Light enters through the opening, forms a bright pool on the floor near the window, and falls away steeply toward the back of the room. Changing the aperture size and the sun angle changes the depth and evenness of the lit zone, with an illuminance falloff curve drawn along the floor.

The position

An aperture does four things at once: it admits light, admits heat, offers a view and loses energy. Sizing it for any one of those in isolation produces a building that fails at the other three.

Depth of plan, ceiling height, surface reflectance and aperture position matter more than glazing area. A smaller, better-placed opening usually beats a larger one.

What we ask

  • 01How far into this plan does useful daylight actually reach?
  • 02What is the contrast between the brightest and darkest part of the room?
  • 03Is the electric lighting ever switched off during the day?
  • 04Does this window give light, view and glare in the proportions intended?

How we answer it

  • 01Daylight availability and distribution studies
  • 02Aperture position, proportion and depth testing
  • 03Glare and contrast assessment
  • 04Surface reflectance and interior finish guidance