Daylighting in the tropics

Pergamon PII: Solar Energy Vol. 73, No. 2, pp. 111–121, 2002  2002 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 2 ) 0 0 0 3 9 – 7 All rights reserved. Printed in Great Britain 0038-092X / 02 / $ - see front matter www.elsevier.com / locate / solener DAYLIGHTING IN THE TROPICS I. R. EDMONDS and P. J. GREENUP † Centre for Medical, Health and Environmental Physics, Queensland University of Technology, P.O. Box 2434, Brisbane, Q 4001 Australia Received 17 August 2001; revised version accepted 28 March 2002 Abstract—Traditional adaptations of tropical / sub tropical buildings to high ambient irradiance from high elevations are outlined. Generally, these adaptations result in severe shading of window apertures, greatly reducing access to daylight. Some examples of optical systems designed to improve daylighting in tropical buildings are discussed. These include angle selective glazing, light guiding shades, vertical and horizontal light pipes, switchable glazing and angle selective skylights. The simulation of these devices within packages such as RADIANCE is also discussed.  2002 Elsevier Science Ltd. All rights reserved. 1.2. Architectural adaptations to climate in relation to natural lighting 1. INTRODUCTION 1.1. The tropical and sub tropical climates In the temperate regions, with primarily overcast skies, it is desirable to maximise the area of windows to utilise as much of the relatively weak natural light as possible. However, windows are the primary source of heat loss during the cold winters. Thus the compromise between natural lighting and thermal comfort has traditionally favoured smaller windows and thermal comfort over extensive glazing and natural lighting. Older European and North American buildings have relatively small areas of glazing with a correspondingly low daylight factor (ratio of internal to ambient illuminance). The modern adaptation has been to develop highly insulating glazing which can be used more extensively in buildings, increasing the daylight factor while maintaining thermal comfort in winter. In tropical and sub tropical regions, with a mainly direct sunlight climate, the principal objective of window design is thermal comfort in summer. In the sub tropics, traditional buildings have wide awnings or verandas shading small windows which can be opened in summer and closed during the colder, dry winter months for thermal comfort. In the wet, humid areas of the tropics, traditional buildings are of light construction with very wide awnings or verandas shading large windows which can be opened during most of the year throughout the day and night for ventilation. In desert areas traditional buildings are of massive construction with small, heavily shaded windows, open during the day for ventilation and closed at night. In modern air con- The tropical zone extends from latitudes of 10 to 238. As the Tropics of Cancer and Capricorn are approached from the Equator the annual dry season during the winter months becomes progressively longer. The sub tropics extend a further 108 towards higher latitudes to about 358 North and South, and in this region the dry winter season extends to about 8 months of the year. Thus the climate of the sub tropics and much of the tropical area is dry and clear most of the year with an annual average direct sun component typically about 8 h per day. Regions in this zone include Southern USA, the Mediterranean, Northern Africa, the Middle East, India, East Asia, Australia, South Africa and central South America. This climatic region differs markedly from the temperate regions extending from about 35 to 608 latitude including most of Europe and North America. The temperate zone has numerous rainy or overcast days throughout the year and the sky condition is primarily bright overcast during the summer and dark overcast during the winter. The Equatorial zone, the area of the world’s surface within 108 of the equator, has a hot, wet climate with, at most, one or two dry months per year — for example Brazil, Central Africa and South East Asia. The sky condition is primarily bright and overcast. † Author to whom correspondence should be addressed. Tel.: 1 617-38-64-2584; fax: 1 617-38-64-9079; e-mail: i.edmonds@qut.edu.au 111 112 I. R. Edmonds and P. J. Greenup Fig. 1. Tropical buildings (this example — 111 George St. Brisbane) are severely shaded with external shades and absorbing / reflecting glazing to reduce radiant heat gain and glare. reduce glare. The daylight in the interior of this deep floor plan building was measured on a bright summer day and compared with the light level when the artificial lights were on (Fig. 2). It is evident that the contribution from daylight is very small — just inside the windows the daylight contribution is below the design level of 500 lux. This example illustrates the fact that daylight levels in shaded sub tropical buildings are well below the levels achieved in buildings with unshaded windows in more temperate climates. A solution to this problem is to adapt the structure of an external shade so that the shade itself becomes an optical system that guides some of the light incident upon the shade deep into the building. The light guiding shade (LGS) consists of an external shade with a diffusing glass aperture at its outer edge. The shade is formed from an upper planar reflector and a lower parabolic reflector designed to direct diffuse light from the input aperture through the body of the shade and into the building so that the output light lies within a specified angular range (Fig. 3). Usually the angular range is designed to extend from the horizontal up to an elevation of about 608. The lower elevation is set at horizontal to avoid any glare to occupants. Thus, to occupants, the output aperture of the LGS appears dark. The LGS is fixed over the window in the same way as a conventional external shade and the LGS acts ditioned office buildings the necessity to minimise radiant heat gain results in severe externally shaded windows (Fig. 1) or highly reflective glazing with severe internal shading. With severely shaded windows the utilisation of natural light in buildings is minimal even though ambient illuminance levels are very high. Thus daylight factors in buildings in the tropics are typically several times lower than commonly achieved in European and North American buildings. Accordingly, simulations show that artificial lighting is the major contributor to peak cooling load in high rise office buildings in the tropics (Lam and Li, 1999). Recent adaptations to this problem of daylighting buildings in the tropics is the subject of this article. 2. WINDOW DAYLIGHTING SYSTEMS 2.1. Daylighting through shaded windows Fig. 1 shows a typical example of a severely shaded office building in sub tropical Brisbane, latitude 278. The building uses external shades to reduce radiant heat gain and absorbing glass to Fig. 2. Measured horizontal illuminance versus distance from window in the building of Fig. 1 during bright ambient conditions close to mid day. Illustrates the potential for daylighting deep plan buildings in the tropics. Daylighting in the tropics Fig. 3. The light guiding shade (LGS) adapts the form of a conventional external shade to an optical system which provides shade as well as near optimal daylighting. both as a shade to reduce radiant heat gain and as a daylighting device. Sunlight is incident upon the LGS from a wide range of directions. However, as the input aperture is diffusing the directional dependence of the input light is not transferred to the output light which remains spatially constant. As the light entering through the input aperture is diffuse, it is possible to use the principles of non-imaging optics (Welford and Winston, 1978) to design the light guiding reflectors so that the output light falls within an exactly defined angular range (Edmonds, 1992). The output angular range can be as narrow or as wide as desired. However, the constraints of thermodynamics imply that a narrow output angular range requires a long reflective light guide and a small input aperture to output aperture ratio. Thus, for a narrow output range which directs light precisely and deeply into room interiors the system is constrained to collect only a small fraction of the light incident on the shade and the potential for a major contribution from daylight is reduced. A compromise is required between the precision with which light is directed into the room and the amount of light being directed into the room. Since the daylight contribution is minimal in severely shaded rooms the best compromise is to direct the light into a relatively wide output angular range, for example, 0 to 608, and use the larger input aperture to output aperture ratio, approximately in the ratio 1 to 2, to maximise total daylight input. There is a considerable energy benefit in using an LGS. Conventional external shades reduce the 113 daylight input very significantly and are designed to exclude all direct sunlight. Typically the average daylight level in a deep room with a severely shaded window is less than 50 lux (Fig. 2). For a shaded window 1.5 m high and 1 m wide, an LGS will usually cover the upper 1 / 3 of the window. The output aperture to the window is therefore 0.5 m high and 1 m wide and the input aperture at the outer edge of the shade is typically 0.25 3 1 m. Thus the luminous flux of direct sunlight incident on the input aperture can be as high as 25 000 lumens in tropical climates (ambient 100 000 lux). The efficiency of the LGS is typically about 50% and assuming that 50% of the light directed over the ceiling is diffusely reflected into useful illuminance onto work surfaces, the luminous input available to internal work surfaces is 25 000 3 0.5 3 0.5 5 6250 lumens. If the room is 10 m deep this corresponds to an average work surface illuminance of 625 lux. The corresponding radiant heat input through the 1.5 square metre of window is only 125 W. In overcast sky conditions the average illuminance would be about five times smaller, i.e. 125 lux. This illustrates the potential for energy conservation from this system. In practice, gains achieved depend on the ambient conditions, the shape, size and orientation of the window, the reflectance of the ceiling, walls and floor, and the electric light control and HVAC systems used. The LGS has been used on large buildings such as the Brisbane Herbarium and has been adapted for use on domestic homes. The optical element of the LGS may also be reduced in scale almost indefinitely without affecting the optical performance. Thus a panel comprising an array of several micro LGS elements may be formed. Such panels are around 80 mm thick and may be installed in buildings in the same manner as conventional shade panels. 2.2. Light deflecting glazing — light shelves The light shelf is a standard daylighting device effective in redirecting down-coming light toward the ceiling of a room. As well as improving daylight penetration and thereby reducing radiant heat gain, the light shelf reduces glare on work surfaces near windows. The light shelf can be highly effective in high direct sunlight climates as it provides both a shading and daylighting function. However a light shelf is difficult to incorporate in window openings and has a tendency to accumulate dust which reduces performance over time. Various forms of prismatic glazing have been used to perform a similar function to the light shelf. However the light deflection power of 114 I. R. Edmonds and P. J. Greenup prismatic materials is low and there is a tendency for prismatic glazing to accumulate dust that is difficult to remove. The laser cut panel (LCP) is a powerful light deflection system which may be mounted as the primary glazing or as a second internal glazing in the upper part of a window to perform the same function as a light shelf (Fig. 4a). Incorporation in the window as a double glazing is simply a matter of clipping the panel to the interior of the existing window framing. The panel, with all external surfaces vertical, does not accumulate dust. Thus the disadvantages in installation and deterioration of the light shelf are removed while retaining the performance. The LCP is an optical material produced by making parallel laser cuts in a thin panel of clear acrylic material (Edmonds, 1993). The surface of each laser cut becomes a small internal mirror which deflects light passing through the panel. The principal characteristics are: (a) very high proportion of light deflected through a large angle ( . 1208), (b) maintenance of view through the panel, and (c) flexible manufacturing method suitable for small or large quantities. When a thin panel has been divided into an array of rectangular elements by laser cutting, light is deflected in each element by sequential refraction, reflection and refraction (Fig. 5a). As each deflection works in the same direction, the deflecting power is high — much higher than in prismatic glass. The optics of the LCP is simple (Edmonds, 1993). The important performance characteristic, the fraction of light deflected as a function of incidence angle, is illustrated in Fig. 5b. From Fig. 5 it is evident that a vertical LCP strongly deflects light incident from higher elevations, ( . 308) into the upward direction, while transmitting light at near normal incidence with little disturbance — thus maintaining view. Also shown in Fig. 5b is the dependence of the fraction deflected on the ratio of cut width to cut depth. As there are no rounded surfaces produced in the panel during the laser cutting, the amount of light scattered by the LCP is insignificant and the glare arising from the LCP itself when in direct sunlight is very low. For large areas of LCP the cost approaches US$100 per square metre. Recently, light deflecting systems based on the same optical principle but produced by different methods have become available (extrusion, ‘Inglas’, and moulding and lamination, ‘Serraglaze’). The LCP may be used as an external glazing if the cuts extend only partly through the panel or if the cut surface is protected by lamination within Fig. 4. (a) The laser cut light deflecting panel (LCP) provides, in a simple vertical panel, the same function as a light shelf. (b) Combining LCP with venetian blinds provides integrated daylighting and shading functions. thin glass sheets. More usually the panel is simply fixed inside existing glazing. It is possible to make the cuts at an angle to the normal to gain more control over the direction of the deflected light. In the simplest application, an LCP fixed Daylighting in the tropics 115 similar way to a light shelf. An application more suited to the tropical climate results when laser cut panels are combined with venetian blinds (Fig. 4b). Here, when the venetians are tilted to exclude direct gain through the lower 2 / 3 of the window, the LCP in the upper 1 / 3 of the window deflects sunlight through the venetians onto the ceiling of the room, providing a useful diffuse source from there to work surfaces. This combination provides a high shading coefficient and good daylight transmission. Energy savings depends on the application. For example, an LCP fixed in the upper third of an open window to deflect light more deeply into a room may increase the average level of natural light deep inside the room by 10 to 30% depending on sky conditions (Edmonds, 1992). If the window is shaded with venetians as described above, the daylighting gain can be much greater. If the panels can be tilted to the outside by incorporation in a hung window, both the amount of light collected and the penetration of this light into the building can be dramatically increased. 3. ANGLE SELECTIVE GLAZING FOR RADIANT HEAT CONTROL Fig. 5. (a) The laser cut panel (LCP) is produced by making parallel laser cuts in a sheet of transparent acrylic sheet, producing a deflected fraction, fd, with the remaining fraction, fu, transmitted without deflection. The cut spacing ratio is given by D/ W. (b) The fraction of light deflected, fd, as a function of incidence angle for LCP cut spacing ratios D/ W 5 0.3, 0.5 and 0.7. Sunlight incident in the normal plane at 458 elevation angle on a 6 mm thick panel with laser cuts 4 mm apart and right through the panel (D/ W(0.67) will be 75% deflected towards the ceiling while 25% will be transmitted without deviation. The effects of Fresnel reflection at the panel surfaces are not included in these results, see Edmonds (1993) for details. vertically in a window (Fig. 4a) will deflect nearly all light incident from above 458 and transmit most light incident from angles below 208. Thus a high fraction of high elevation light is deflected by the panel onto the ceiling which then acts as a secondary source of diffuse illumination in a Direct sunlight is the primary tropical sky condition. In the interior of a building, direct sunlight is the source of significant disability glare and of limited illuminating function unless carefully redirected and diffused. Sunlight directs up to 1 kW/ m 2 of radiation through windows into the building interior and is the major cooling load in buildings which are not severely shaded. Thus, the primary concern in tropical buildings is radiant heat control. Sunlight comes from higher elevations during the hottest time of day and year. Therefore, angle selective glazing which transmits low elevation light and rejects high elevation light has considerable potential. The external shade is the simplest and most effective form of controlling heat gain through windows. However, as discussed in the previous section, the potential for daylighting is severely reduced. Most window wall buildings in the sub tropics and tropics use highly reflective or tinted glass and venetian blinds to control radiant heat gain. Venetian blinds can be closed to reflect sunlight outwards or opened to admit low angle daylight and reflect sunlight inwards. However, when closed, venetians severely restrict view and reflective / tinted glass reduces the potential for daylighting. Angle selective glazing based on the direction 116 I. R. Edmonds and P. J. Greenup dependent absorbing properties of thin film coatings of columnar metal (aluminium and silver) deposited on glass have been developed (Smith et al., 1998). However, these have not been commercially applied. The laser cut panel can be produced in sheets as thin as 2 mm and therefore may be regarded as an angle selective glazing which transmits near normal light and transmits and strongly deflects light at higher angles of incidence (Fig. 5). The very strong light deflecting power of the laser cut panel makes it possible to use the panels in various glazing configurations to provide effective radiant heat control as well as improved daylighting function. 3.1. Fixed angle selective glazing If an array of narrow laser cut panels is mounted horizontally in a window (with the face of the panels horizontal), sunlight from higher elevations is deflected back to the outside (Fig. 6). This system is unique in excluding sunlight while being open for viewing. The narrow array of panels may be incorporated in the space between double glazing or may be incorporated in the window opening by replacing the slats of venetian blinds with narrow laser cut panels. The appearance of this type of angle selective glazing as seen Fig. 6. An array of laser cut panels fixed, venetian style, between double glazing provides both radiant heat control and improved daylighting. Fig. 7. The view looking through angle selective glazing of the form in Fig. 6 is similar to the view through open venetian blinds. by the occupants of an office room is illustrated in Fig. 7. The theory of this type of glazing has been formulated by Reppel and Edmonds (1998). The most useful form of predicted performance is the daily time variation of irradiance through North, East and West windows for different seasons of the year (Fig. 8). These figures show that in the tropics (here latitude 278), this type of angle selective glazing is most effective for radiant heat control when installed on East or West facing windows. North facing windows in the tropics (South facing in the Northern hemisphere) receive only a small radiant input during the summer months (Fig. 8b), whereas East and West facing windows provide most of the unwelcome summer radiant heat. Referring to Fig. 8a, it is evident that at mid summer the glazing rejects more than 75% of solar energy incident on East facing windows between 7 am and noon and on West facing windows between noon and 5 pm. The horizontal view is relatively unobstructed, (Fig. 7). As most of the useful diffuse daylight (daylight which penetrates deeply into a room) comes from the lower elevations, the daylighting performance of this type of glazing is far superior to the reflective or absorbing type of glazing commonly used in window wall office buildings in the tropics. This is illustrated in Fig. 9 which compares the daylighting performance under a diffuse sky, of angle selective glazing (similar to that illustrated in Fig. 7) and 20% transmitting reflective glazing (Reppel and Edmonds, 1998). The results for each type of glazing are given relative to the horizontal and vertical illuminance obtained with a clear glass window. The performance of the angle Daylighting in the tropics Fig. 8. (a) Irradiance versus time of day through an East facing window in Brisbane (latitude 278), for mid summer (S), equinox (E) and mid winter (W). The broken lines correspond to the irradiance through an open window. The full lines correspond to the irradiance through angle selective glazing as illustrated in Fig. 7. For example, at 8 am at mid summer, the irradiance through angle selective glazing, 30 W/ m 2 , is much less than the irradiance through an open window, 600 W/ m 2 . (b) Irradiance versus time of day through a North facing window in Brisbane (latitude 278), for mid summer (S), equinox (E) and mid winter (W). The broken lines correspond to the irradiance through an open window. The full lines correspond to the irradiance through angle selective glazing as illustrated in Fig. 7. selective glazing is about five times better in the deeper parts of the room. 3.2. Tiltable angle selective glazing Additional functionality may be added to the fixed radiant control glazing described above by making the panels able to be tilted by incorporating thin panels (20 mm) panels in the form of a venetian blinds or by incorporating wider panels (150 mm) in the form of the louvre window common in Australia. The three modes of opera- 117 Fig. 9. Relative horizontal illuminance (a), and relative vertical illuminance (b), calculated at workplace height through an angle selective glazing (full lines) in comparison with that through 20% reflective glazing (broken lines) in a deep plan building under overcast skies. Displayed results are relative to the illuminances obtained with a clear glass window. tion are illustrated in Fig. 10. The summer mode (a) maintains the panels horizontally in the open position, acting as an angle selective system to reject radiant heat while being open for viewing and for maximum ventilation. The winter mode (c) has the panels fully closed, deflecting sunlight to the ceiling. In this mode all incident sunlight is transmitted to the interior and most is deflected over the ceiling providing heat gain and improved daylighting. An intermediate mode (b) provides for deflection of sunlight more deeply into the room. The principal application of tiltable angle selective glazing in the tropics is in the louvre configuration applied to homes or schools where cross ventilation is important in summer. The summer mode works well on windows when combined with a limited amount of external 118 I. R. Edmonds and P. J. Greenup Fig. 10. Three modes of operation of louvre style angle selective glazing. LCP replace conventional clear glass panels in a louvre window. With sunlight incident from the left, the three modes of operation of this type of glazing are: (a) the radiant heat rejecting summer mode, where most incident sunlight is deflected back outside and the louvres are open for ventilation; (b) an intermediate mode providing for deep daylight penetration; and (c) the winter mode when the louvres are closed and incident sunlight is deflected into the room and towards the ceiling. horizontal shading. However the winter (closed) mode can be problematic. The radiant flux on a North facing window in winter is high (Fig. 8b). With the panels closed, all incident flux is transmitted to the interior and most is deflected upwards. This simply over-daylights a room which becomes very ‘glary’. The solution appears to be to restrict the use of LCP to the upper 1 / 3 of North facing windows with clear glass panels in the lower 2 / 3. 3.3. Angle selective skylight and atrium glazing Simple skylights strongly transmit high elevation light and weakly transmit low elevation light. Thus, in clear sky conditions in the tropics the radiant energy transmitted to the interior through conventional skylights varies strongly during the day with an intense maximum near noon. The high radiant input near noon overheats interiors during summer making it difficult to use skylights in tropical climates. A more suitable skylight for tropical climates is formed by incorporating a triangular or pyramid configuration of LCP within a skylight to provide an angular selective transmission (Fig. 11). This skylight admits considera- bly more low elevation light while rejecting most high elevation light, thereby reducing overheating near noon. The performance of the angular selective skylight depends on the cut spacing of the laser cuts in the panel, the tilt angle of the pyramid or triangle configuration of the panels, the well depth of the skylight, the time of day and season and the sky conditions (Edmonds et al., 1996). A useful measure of performance is to compare the irradiance through an angular selective skylight Fig. 11. With laser cut panels mounted in triangular or pyramid form in a skylight or atrium, high elevation light is deflected by one panel across the skylight to the other panel and deflected to the outside. Conversely, low elevation light is deflected more effectively down into the interior. Daylighting in the tropics Fig. 12. Irradiance through the roof apertures of an angular selective skylight (full lines) and a clear skylight (broken lines) as a function of time of day for mid summer and mid winter in Brisbane (latitude 278), under clear sky conditions. Note that the radiant heat gain near noon at midsummer may be reduced to 10% of that through a conventional clear skylight. with the irradiance through a conventional clear skylight as a function of the time of day (Fig. 12). The ratio of irradiances is the shading factor of the skylight. The results of Fig. 12 relate to a skylight with zero well depth or, effectively, to the radiant input through the roof aperture of the skylight. The relative improvement in performance of the angular selective skylight at low elevation angles increases rapidly as the well depth increases, due to the deflection of low elevation light more directly down the skylight thereby reducing reflection losses within the skylight well. 119 The pyramid form of this type of skylight as applied to a school building is illustrated in Fig. 13. The severe shading of the windows of this building reduces glare and radiant heat gain but also minimises the potential for daylighting through the windows. A comparison made between two exactly similar, cross ventilated school buildings, one with angular selective skylights and the other with no skylights, demonstrated that sufficient daylight may be safely admitted by angular selective skylights so that use of electrical lighting is eliminated during class time (9 am–3 pm) for most of the year and for most sky conditions (Edmonds et al., 1996). This type of skylight has been applied commercially in Australia from Sydney (latitude 348) to Thursday Island (latitude 118). In another application of the high light deflecting power of the LCP, the panels may be incorporated as an inverted triangular or pyramid form at the ceiling level of skylights. This light spreading skylight (Fig. 14) deflects light coming down the well of a skylight over the ceiling on either side of the skylight. This redirects the light from the area directly below the skylight into a more even distribution within the room, thereby improving daylighting performance. 3.4. Light piping systems Vertical light pipes are an effective means of transferring natural light from the roofs of buildings to floors at lower levels. It is evident that simple light pipes will perform well in direct sunlight from high elevation. For this reason light pipes have been a commercial success in sub tropical climates. Conversely vertical light pipes Fig. 13. Application of pyramid style angular selective skylights on a severely shaded school building in Brisbane (latitude 278). 120 I. R. Edmonds and P. J. Greenup Fig. 14. Application of laser cut panels in inverted triangular form to produce a light spreading skylight with improved spatial distribution to a wide plan building with external shading on side windows (Brisbane Herbarium). perform poorly when the solar elevation is low, due to multiple reflection losses as the light traverses the pipe. Simple light deflection systems (metal reflectors and LCP deflectors) increase the performance of long, vertical light pipes in the early morning and late afternoon and during winter (Edmonds et al., 1995). However, with the use of silvered aluminium sheet (reflectivity 5 0.95) becoming more common in the construction of light pipes, the gains achievable are significantly less than can be achieved in the older form of aluminium light pipe (reflectivity 5 0.85). As the performance of a light pipe falls exponentially with length, it is probable that horizontal light pipes equipped with means for deflecting near zenith light along the axis of the pipe will outperform vertical light pipes in office buildings in the tropics (Chirarattananon et al., 2000; Garcia-Hansen et al., 2001). Systems combining light redirection and light piping are being developed to address the problem of deep plan office buildings in temperate climates (Beltran et al., 1997; Courret et al., 1998) and it seems likely that a similar approach may prove successful for buildings in tropical climates. 4. ELECTRICAL CONTROL OF GLAZING TRANSMITTANCE Electrochromic or ‘smart’ windows potentially provide the means to maximise energy savings from daylighting (Selkowitz and La Sourd, 1994). Transmission is maximised (clear state) when direct sunlight is not incident on windows or when the sky is overcast, and minimised (coloured state) when direct sunlight falls on the window. In the tropics, the primary function of electrochromic glazing is for radiant heat control to reduce cooling loads. The high sunlight intensity and ambient temperature in the tropics requires that electrochromic glazing be used as the external panel in double glazed windows to reduce convective and radiative heat transfer to the interior as the glazing absorbs radiation and is heated to high temperatures (Bell et al., 1999). While the potential for energy saving is high, electrochromic glazing is not currently used in tropical buildings. 5. SIMULATION OF DAYLIGHTING SYSTEMS The daylighting and radiant heat control systems described above rely on refraction and / or reflection in optical elements or arrays of optical elements to redirect light. It is difficult to incorporate such systems in simple daylighting simulation packages. However, more advanced simulation packages such as RADIANCE (Ward, 1994) provide material types which can be adapted to model light deflecting materials such as the LCP. Greenup et al. (2000) provide a simple algorithm based on the RADIANCE ‘prism2’ material which allows quite complex arrangements of LCP to be simulated. Several new algorithms have been developed which have extended the ability of RADIANCE to model advanced daylighting systems in the tropics (Greenup and Edmonds, 2001). These algorithms include models of the laser cut panel, angular selective skylight, light spreading skylight, light guiding shade and micro-light guiding shade panels. Sensitivity studies and model validations have been performed on all of these devices. Improved sky models, applicable to the tropics, have also been created for use in RADIANCE. Daylighting in the tropics 6. CONCLUSIONS The severe shading characteristic of buildings in the tropics presents a challenge to the designer endeavouring to improve natural lighting. However, the frequency and intensity of direct sunlight in the tropics provides a very significant potential if it can be effectively utilised while avoiding glare and excessive heat gain. A major practical challenge in the tropics is the natural lighting of deep floor plan office buildings. Modern office buildings have either severe external shading as in Fig. 1 or highly reflective window wall glazing and internal blinds. Such buildings make little use of the extremely high levels of ambient illuminance ( . 120 klux) common in the tropics. Angle selective glazing, light guiding shades, smart windows and other devices have been applied with some success to relatively small buildings. Systems combining light redirection and horizontal light piping may prove successful for buildings in tropical climates. REFERENCES Bell, J. 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