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Daylight factor and its components

The daylight factor is defined as: the ratio of the daylight illuminance formed by directly or indirectly receiving sky light with an assumed or known brightness distribution at a point on a specified plane to the daylight illuminance formed by receiving light from the unobstructed hemisphere of the sky on a horizontal plane. Both illuminance values do not include direct sunlight.

The natural light at a point in a room can be considered to be composed of three components:

(1) light directly from the sky;

(2) light reflected from outdoor surfaces to the point;

(3) light reflected from indoor surfaces or indoor equipment to the point.

Sky component It is defined as: the ratio of the illuminance generated by the part of natural light directly obtained from the sky with an assumed or known brightness distribution at a point on a specified plane to the illuminance obtained from the unobstructed hemisphere of the sky on a horizontal plane. Both illuminance values do not include direct sunlight.

The outdoor reflection component is defined as the ratio of the illuminance directly generated by the outdoor surface at a point on the specified plane and the illuminance obtained on the unobstructed hemisphere of the sky on a horizontal plane. The illuminance generated by direct sunlight on the outdoor surface and the illuminance generated on the comparison plane are not included.

The outdoor reflection component is the "equivalent sky component" of the outdoor obstacles at the reference point, that is, the brightness of these obstacles corresponds to the brightness of the part of the sky that is blocked. The brightness of the outdoor surface seen from the indoor is generally about 10% of the sky brightness, so the outdoor reflection component is equal to the sky component of the same sky area multiplied by 0.1.

The indoor reflection component is defined as the ratio of the natural light illuminance (the sky is assumed or known to have a brightness distribution) formed by the reflection of the indoor surface at a point on the specified plane and the illuminance formed by the unobstructed hemisphere of the sky on a horizontal plane. This does not include the illumination caused by direct sunlight on indoor reflective surfaces and the illumination caused on comparative planes.