Design Principles

Defining Daylight

Daylight is calculated throughout Northern Europe using the CIE (Commission Internationale de L’Eclairage) Overcast Sky standard.

In this -

  • Daylight is classified as the natural light from an overcast sky.
  • Sunlight is classified as natural light from a direct sunbeam, not obscured by clouds.

The standard CIE Overcast Sky is defined as -

  • Three times brighter overhead than on the horizon.
  • Of the same brightness in all compass directions.

Using these accepted CIE Overcast Sky definitions, convention assumes that buildings designed for daylighting  will experience significantly enhanced performance when outdoor illuminance is brighter.

Based on records, it is assumed that average outdoor illumination is 5,000 lux for 85% of the normal working day which represents a ‘dull’ day by most standards. By the same standards, a sunny day is assumed to be 100,000 lux, which is taken as a maximum level of illuminance for design purposes.

THE DISTRIBUTION OF SKY LUMINANCE

sky_distributions2.gif
These sky distribution graphs were generated using the RADIANCE synthetic imaging system. To create this, the sun was assumed to be at an altitude of 60° due South. The sky luminance was then mapped between the Southern (0°) and the Northern (180°) horizon passing through the zenith (90°).

Please note - graphs should not be compared to one another because it is not correct to say that uniform and overcast skies always have the same zenith.

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Clear Sky

The luminance of the standard CIE clear sky varies over both, altitude and azimuth. It is brightest around the sun and dimmest opposite it. The brightness of the horizon lies in between those two extremes.

Intermediate Sky

The standard CIE intermediate sky is a somewhat hazy variant of the clear sky. The sun is not as bright as with the clear sky and the brightness changes are not as drastic.

Overcast Sky

The luminance of the standard CIE overcast sky changes with altitude. It is three times as bright in the zenith as it is near the horizon. The overcast sky is used when measuring daylight factors. It can be modelled under an artificial sky.

Uniform Sky

The luminance of the standard CIE overcast sky changes with altitude. It is three times as bright in the zenith as it is near the horizon. The overcast sky is used when measuring daylight factors. It can be modelled under an artificial sky.