ObjectivesBe aware of the energy implications of lighting.Understand why buildings require lighting.Understand what is meant by the term ‘daylight factor’.Be able to identify a number of innovative daylighting technologies.Be able to identify the advantages of daylighting.Understand what is meant by the terms general lighting, localised lightingand local lighting.
LightingLighting within the indoor environment can be considered to have three basicpurposes: To enable the occupants to work and move about in safety. To enable tasks to be performed. To make the interior look pleasant.There are two principle ways in which a building can be lit. These are: Naturally - by daylight received from the sky. Artificially - by electric lamps or other artificial light sources.Daylight as two distinct sources of light: Sunlight – That part of solar radiation that reaches the earth’s surface as parallel rays after selective attenuation by the atmosphere. Skylight – That part of solar radiation that reaches the earth’s surface as a result of scattering in the atmosphere.Sunlight and skylight may therefore be considered as the direct and diffusecomponents of daylight.
DaylightingDaylight (both sunlight and skylight) is usually admitted into a building by themeans of windows and skylights.The quantity of daylight obtained within a room will be dependent upon: The orientation, geometry and space planning of the spaces to be lit. The dimensions and orientation of the openings through which daylight will pass. The location and surface properties of any internal partitions which may reflect and distribute the daylight. The location, form and dimensions of any shading devices which will provide protection from too much light and glare. The light and thermal characteristics of the glazing materials used. Atrium Rooflight Clerestorey Borrowed light Window
Controlling daylight A range of methods are available to control the amount of daylight that penetratesinto the building. Fixed external – Permanently obstructs skylight and is maintenance free, but is architecturally dominating. Variable external – Allows the maximisation of skylight but can suffer from maintenance problems. Variable internal – Absorbs solar radiation and acts as a secondary heat source within the building. Effective for visual comfort. Various methods of controlling daylightIn addition, further control can be provided by the choice of glazing type.
Daylight factorInterior daylight is measured using a parameter known as the Daylight Factor (DF).The daylight factor is defined as: Interior illuminance Daylight factor (%) = ×100 Simultaneous horizontal unobstructed exterior illuminanceThe Daylight Factor is a combination of 3 separatecomponents: The sky component (SC) – the light received directly from the sky. The externally reflected component (ERC) - the light received directly by reflection from buildings and obstructions outside the room. The internally reflected component (IRC) - the light received from surfaces inside the room. DF = SC + ERC + IRC
Glare from daylightGlare from daylight can be reduced by: Using solar control devices - such as external screens and louvres, glass of low transmittance, or internal blinds and curtains Other methods of decreasing the contrast between the interior and the view of the sky - such as ensuring that the window wall is light coloured. Designed to eliminate the use of artificial light on normal days. Average daylight factor of 2% over 80% of the office space. Atrium has rooflights which allow natural light to enter the building. PV façade designed to produce 25-33% of the offices electricity needs. Designed to achieve a minimum 2% daylight factor over the office area. Solar shading provided on South façade via motorised external translucent glass louvres.
Innovative daylighting technologiesA number of innovative daylighting technologies are available which arecapable of redirecting the incoming sunlight or skylight to the areas where it isrequired. Mirror systemThe technologies available include: Sunlight tracking systems - have mirrors and/or lenses that follow the sun and redirect its light to a required location. − heliostat and light pipe systems. − mirror systems. Daylighting systems - redirect diffuse skylight and usually sunlight as well. They generally modify or supplement an existing window or rooflight. − light shelves.
Innovative daylighting technologiesLight shelvesThese can be used to redirect sunlight and skylight deep intoa space. Both interior and exterior light shelves are available.Exterior shelves can also function as a shading device.The performance of light shelves depends upon the proportionof the shelf which is situated inside or outside the space.They can also be used to control sunlight and reduce glare. External light shelf Without light shelf No light shelf With light shelf Internal light shelf
Energy implications of daylightingTo achieve good daylighting, the glazing must be designed so that there is a correctbalance between the heat gains and losses resulting from the transmission ofthermal radiation in and out of the building and the light entering the building. Daylight – Reduces artificial lighting load Solar gain – Reduces winter heating load Solar gain – Increase summer cooling load Conduction Convection Radiation Air leakage The energy implications of daylightingCorrect daylighting design will not only reduce energy costs related to artificiallighting, but also reduce the possibility of having to cool rooms overheated by lowefficiency lighting appliances.However, although large glazed areas may provide sufficient daylighting at somedistance into the building, they can also cause glare, overheating and highheating and cooling energy costs.
Advantages of daylightingThe utilisation of daylight in buildings has a number of advantages, namely: It can make a significant contribution to energy efficiency. It has a variability and subtlety which is more pleasing than the relatively monotonous environment produced by artificial lighting. It helps to create optimum working conditions by bringing out the natural contrast and colour of objects. Windows and skylights give occupants contact with the outside world. The presence of natural light can bring a sense of well-being and awareness of the wider environment. It is also claimed that exposure to natural light can have a beneficial effect on human health.However, due to its uncertainty and variability, daylighting cannot provideadequate illumination of the internal environment all of the time. Therefore, artificiallighting systems must always be incorporated into buildings in order tosupplement daylighting when this is required.
Illumination• Tasks to be performed in the space• Desired light levels based on the tasks performed in the space• Room size and dimensions• Structural obstructions such as beams• Layout of furniture and obstructions such as partitions• Room and object surface colors and reflectances• Special concerns such as safety and security• Hours of operation• Assessment of normal operating conditions• Possibility or known existence of abnormal operating conditions• Cleanliness of the area during operation• Maintenance schedule• Availability of daylight
Methods of IlluminationInverse Square LawIt states that illumination at any given surface due to a given light source is inversely proportional to square of distance between them.where• E – Illuminance (Lux)• I - Luminous Intensity (Lumens/m2 )• r - distance of surface from light source (m)
Cosine Law• E= I cos θ d²where• E – Illuminance (Lux)• I - Luminous Intensity (Lumens/m2 )• r - distance of surface from light source (m)• θ - angle of incidence Coefficient of Utilization Method F= E X A______ CU X LLD X LDDWhereF= Lumens generated by light (lm)E= Illumination level (Lux)A= Area of work plane (m2)CU= Coefficient of Utilisation (%)LLD= Lamp lumen Depreciation (%)LDD= Lamp Dirt Depreciation (%)
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