Light Redirection (Light from the Side)

As the depth of a room increases (normally 5-7 m), the intensity of the daylight in the room diminishes (see daylight factor curve). Redirecting the light allows rooms to be completely illuminated with daylight, even rooms of considerable depth.

The redirection of the light is based on the principle that the angle of incidence equals the angle of reflection. The aim of this redirection is (- (50)):

- to obtain a more uniform distribution of daylight;
- to obtain better daylight illumination in the depths of the room;
- to avoid glare when the sun is high, and to make use of winter sun;
- to mask out zenith luminance, or to make indirect use of it;
- to redirect particularly diffuse radiation;
- to eliminate the need for additional solar protection (possibly trees) by achieving glare protection on the inside.

Light shelves (reflectors). These can be placed inside or outside the window in the area of the abutment. Mirrored, polished or white surfaces can be used as the reflection plane. They improve the uniformity of the illumination, particularly if the ceiling is shaped to correspond with the redirected light. If necessary, glare protection can be provided in the region between the abutment and the ceiling – (51).

Prisms. Optical prisms can be used to achieve a desired selection of radiation and redirection – (52). Prism plates reflect the sunlight with less deviation, and only allow diffuse light from the sky to pass through. In order to prevent penetration of the sun's rays, the prism plates are mirrored. The prism plates guarantee adequate daylight illumination up to a room depth of approximately 8m.

Outlook, light deflection and glare protection. The illumination in the depths of a room can be improved by redirecting the light and by providing reflecting surfaces on the ceiling - (53). The outlook remains the same, but the zenith illuminance is masked out. Glare protection is only required in winter, but if necessary, a means of enhancing daylight illumination may be provided on the abutment.

Solar control glass, glass bricks and Venetian blinds are used for radiation selection and redirection, and include the following systems (- (54)):

- solar control glass, i.e. mirror reflectors (rigid) between the glass panes cause the light to be reflected in summer and transmitted in winter;
- glass blocks, i.e. polished prisms to increase the uniformity of the light;
- Venetian blinds, i.e. adjustable bright outer blinds to deflect the daylight.

Examples of light redirection in ceiling areas in museums are shown in – (55).

Methods and procedures for determining the level of daylight (D%) in internal areas (side and overhead light) with a clouded sky. A number of methods are available to determine the level of daylight, for example calculation, graphical methods, computer-supported methods and measurement techniques.

In order to arrive at a basis for a decision on the 'room to be built' or the 'building to be erected', an approximate simulation of the daylight levels is recommended. This can be accomplished using drawing methods or with a model.

However, the distribution of the daylight can only be determined and evaluated in three dimensions. Therefore a model of the room or building should be tested under simulated conditions so that the various effects of daylight can be examined.

Experimental method. A model room was built with a suspended bright, matt, translucent ceiling, artificial illumination above the ceiling and a mirrored surface rotating in a horizontal plane which mirrored the surrounding walls. This simulated the actual effect of a uniformly clouded sky – (56).

An illuminance of approx. 2000-3000 lx was adequate. The external illuminance of the artificial sky was measured (Ea = 2000 lx), using a special purpose-made device, on a 1:20 scale architectural model. The illuminance in the inner area of the model was measured by means of a probe (Ei = 200lx). Thus the daylight factor in the internal area had a value of 10% at point P. The variation of daylight in the model was determined using this method – (57).

Different materials can be used to influence the variation in daylight, illuminance, colours effects, room dimensions, etc., but care should be taken that the quality criteria for daylight are maintained. The following materials can be used to experiment with the effects of light on the model: cardboard or paper of various colours, preferably pastels; transparent paper to prevent glare and to generate diffuse radiation; aluminium foil or glossy materials as reflective surfaces – (58).