The potentiality of reflected sunlight through Rawshan screens

Rawshans has been one of the traditional architectural devices that are still being used in recent buildings as an environmental controlling device. Beside their use as an architectural component, they have been used to control natural ventilation and light. The most critical portion of the Rawshan, as light control concern, is the front screen. A systematic analysis of this portion has not yet been taken care of. However, pervious work of this component was investigated by the author for the effect of direct sunlight transmission. The attention of this paper is to check the effect of the reflected lighting component for the screen cells. The typologies of the screen cells were reviewed; and selected types of screen geometry were tested using laboratory experimentations. 1. WHAT IS A RAWSHAN? Rawshan is an architectural device made of a combination of wood strips and screens, which is commonly used, for large external openings. It is also defined as a projected bay window with decorative wooden screens as an enclosure. (Saini, 1991). The word Rawshan originated in India where it is known as" rushandan" which means to give light, (Ashraf, 1983). There is no difference between the types of Rawshans in different countries as far as their functional principles are concerned. They are generally considered to fulfill their common functions such as cross ventilation, light control, humidity control, cooling of water in clay jars, and ensure social privacy for occupants, (Spencer, 1990). However, there are various kinds of Rawshans, which vary in appearance that arose from the taste of the client and the skills of the craftsmen. Their sizes also depend on the size of the opening, which is mainly influenced by the type of climate. The Rawshans can be classified in to three kinds: cantilevered, screen panels, louvered timber walls and louvered windows, (Aljofi, 1995). 2. THE ENVIRONMENTAL ISSUES It is common to read information in articles and architectural magazines about Rawshans and their contribution as ventilators or light controllers. However, it is almost lacking in any critical evaluation of performance. There are a few researchers who have dealt scientifically with the thermal performance of the Rawshan, particularly, the potentials of its natural ventilation. An investigation of air movement through different areas of the traditional house in Jeddah city centre indicated that air velocity is about 0.3 to 1.3 m/s due to the room location and wind speed (Allyali, 1990). Humidity may also be reduced in internal spaces as saturated air passing through the screen loses some of its humidity molecules as the wooden screen absorbs them (Hassan, 1988). Because of the high intensity of the atmosphere, screens act as a light filter to reduce the glare effect (Spencer, 1990). Saini stated that screens not only diffuse sunlight but also distribute it uniformly through the internal space (Saini, 1991). These comments about the use of screens as a glare controller are found in many other articles. However, there is no further explanation of their physical behavior. 818 International Conference “Passive and Low Energy Cooling for the Built Environment”, May 2005, Santorini, Greece 3. TYPOLOGY OF SCREEN GEOMETRY The most effective components of the Rawshan as far as daylighting is concerned are the screen panels. They contain various forms of gaps and strips, which function differently, to avoid direct beam penetration and to reduce glare effect. Few researchers have mentioned the size of the screen gaps in their works Spencer stated the names of some of the various design pattern of the screens such as Sahrigi (square grid) and Musadass (Hexagon), (Spencer, 1990). Depending on the craftsmen's skills, there are screens of very complicated shapes composed of up to 2000 pieces to one square yard (Spencer, 1990). The most common patterns of the screen strips are the cress crossed; diagonal crossed and rounded strips (Ashraf, 1983). Figure 1 shows some of these types (Jean, 1976). 4. THE LIGHT CHARACTERISTICS OF