In architecture, a daylight factor (DF)[1] is the ratio of the light level inside a structure to the light level outside the structure. It is defined as:
where, Ei = illuminance due to daylight at a point on the indoors working plane, Eo = simultaneous outdoor illuminance on a horizontal plane from an unobstructed hemisphere of overcast sky.
To calculate Ei, requires knowing the amount of outside light received inside of a building. Light can reach a room via through a glazed window, rooflight, or other aperture via three paths:
Direct light from a patch of sky visible at the point considered, known as the sky component (SC),
Light reflected from an exterior surface and then reaching the point considered, known as the externally reflected component (ERC),
Light entering through the window but reaching the point only after reflection from an internal surface, known as the internally reflected component (IRC).
The sum of the three components gives the illuminance level (typically measured in lux) at the point considered:
Illuminance = SC + ERC + IRC
The daylight factor can be improved by increasing SC (for example placing a window so it "sees" more of the sky rather than adjacent buildings), increasing ERC (for example by painting surrounding buildings white), increasing IRC (for example by using light colours for room surfaces). In most rooms, the ceiling and floor are a fixed colour, and much of the walls are covered by furnishings. This gives less flexibility in changing the daylight factor by using different wall colours than might be expected[2] meaning changing SC is often the key to good daylight design.
A study of daylight factors within a single storey building resulting from different perimeter glazing and rooflight designs and glass types. Undertaken using the IES Radiance software Module.
Architects and engineers use daylight factors in architecture and building design to assess the internal natural lighting levels as perceived on working planes or surfaces. They use this information to determine if light is sufficient for occupants to carry out normal activities. The design day for daylight factor calculations is based on the standard CIE overcast Sky for 21 September at 12:00pm, and where the Ground Ambient light level is 11921 Lux. CIE being the Commission Internationale de l´Eclairage, or International Commission on Illumination.
Calculating daylight factors requires complex repetition of calculations and thus is generally undertaken using a complex software product such as Radiance. This is a suite of tools for performing lighting simulation, which includes a renderer as well as many other tools for measuring simulated light levels. It uses ray tracing to perform all lighting calculations. One failing in many of these calculations is that they are often completed without wall hangings or furniture against the walls. This can lead to higher predictions of the daylight factor than is correct.[3]
To assess the effect of a poor or good daylight factor, one might compare the results for a given calculation against published design guidance. In the UK this is likely to be CIBSE Lighting Guide 10 (LG10-1999), which broadly bands average daylight factors into the following categories:[4]
Under 2 – Not adequately lit – artificial lighting is required all of the time
Over 5 – Well lit – artificial lighting generally not required, except at dawn and dusk – but glare and solar gain may cause problems
↑ Simm, S. and Coley, D., 2011. The relationship between wall reflectance and daylight factor in real rooms. Architectural Science Review, 54 (4), pp. 329-334.
↑ Simm, S. and Coley, D., 2011. The relationship between wall reflectance and daylight factor in real rooms. Architectural Science Review, 54 (4), pp. 329-334.
Daylighting is the practice of placing windows, skylights, other openings, and reflective surfaces so that direct or indirect sunlight can provide effective internal lighting. Particular attention is given to daylighting while designing a building when the aim is to maximize visual comfort or to reduce energy use. Energy savings can be achieved from the reduced use of artificial (electric) lighting or from passive solar heating. Artificial lighting energy use can be reduced by simply installing fewer electric lights where daylight is present or by automatically dimming or switching off electric lights in response to the presence of daylight – a process known as daylight harvesting.
A light meter is a device used to measure the amount of light. In photography, an exposure meter is a light meter coupled to either a digital or analog calculator which displays the correct shutter speed and f-number for optimum exposure, given a certain lighting situation and film speed. Similarly, exposure meters are also used in the fields of cinematography and scenic design, in order to determine the optimum light level for a scene.
Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing daylight. Daylighting is sometimes used as the main source of light during daytime in buildings. This can save energy in place of using artificial lighting, which represents a major component of energy consumption in buildings. Proper lighting can enhance task performance, improve the appearance of an area, or have positive psychological effects on occupants.
A foot-candle is a non-SI unit of illuminance or light intensity. The foot-candle is defined as one lumen per square foot. This unit is commonly used in lighting layouts in parts of the world where United States customary units are used, mainly the United States. Nearly all of the world uses the corresponding SI derived unit lux, defined as one lumen per square meter.
A heliodon (HEE-leo-don) is a device for adjusting the angle between a flat surface and a beam of light to match the angle between a horizontal plane at a specific latitude and the solar beam. Heliodons are used primarily by architects and students of architecture. By placing a model building on the heliodon’s flat surface and making adjustments to the light/surface angle, the investigator can see how the building would look in the three-dimensional solar beam at various dates and times of day.
Architectural lighting design is a field of work or study that is concerned with the design of lighting systems within the built environment, both interior and exterior. It can include manipulation and design of both daylight and electric light or both, to serve human needs.
Light tubes are structures that transmit or distribute natural or artificial light for the purpose of illumination and are examples of optical waveguides.
Solar gain is the increase in thermal energy of a space, object or structure as it absorbs incident solar radiation. The amount of solar gain a space experiences is a function of the total incident solar irradiance and of the ability of any intervening material to transmit or resist the radiation.
Glare is difficulty of seeing in the presence of bright light such as direct or reflected sunlight or artificial light such as car headlamps at night. Because of this, some cars include mirrors with automatic anti-glare functions and in buildings, blinds or louvers are often used to protect occupants. Glare is caused by a significant ratio of luminance between the task and the glare source. Factors such as the angle between the task and the glare source and eye adaptation have significant impacts on the experience of glare.
The Chartered Institution of Building Services Engineers is an international professional engineering association based in London, England that represents building services engineers. It is a full member of the Construction Industry Council, and is consulted by government on matters relating to construction, engineering and sustainability. It is also licensed by the Engineering Council to assess candidates for inclusion on its Register of Professional Engineers.
Often task lighting refers to increasing illuminance to better accomplish a specific activity. However, the illuminance level is not the only factor governing visibility. Contrast is also important, and a poorly positioned light source may cause contrast reduction, resulting in loss of visibility. The most important purpose of task lighting in the office is not increasing illuminance, but improving contrast. General lighting can be reduced because task lighting provides focused light where needed.
In photography and cinematography, a reflector is an improvised or specialised reflective surface used to redirect light towards a given subject or scene.
A light shelf is a horizontal surface that reflects daylight deep into a building. Light shelves are placed above eye-level and have high-reflectance upper surfaces, which reflect daylight onto the ceiling and deeper into the space.
In lighting design, the lumen method,, is a simplified method to calculate the light level in a room. The method is a series of calculations that uses horizontal illuminance criteria to establish a uniform luminaire layout in a space. In its simplest form, the lumen method is merely the total number of lumens available in a room divided by the area of the room. In order to perform this calculation, many factors, coefficients, lamp lumen data and other quantities must be gathered. Despite the scientific impression of the lumen method equations, there are inaccuracies and assumptions built into the method. Therefore, the lumen method should not typically be used as a standalone, final solution; it should be used as a tool in particularly uniform settings of lighting design if a simple, rough technique of illuminance quantification is desired.
Daylight harvesting systems use daylight to offset the amount of electric lighting needed to properly light a space, in order to reduce energy consumption. This is accomplished using lighting control systems that are able to dim or switch electric lighting in response to changing daylight availability. The term Daylight Harvesting has become the standard in the fields of lighting, sustainable architecture, and active daylighting industries.
Anidolic lighting systems use anidolic optical components to light rooms. Light redirected by these systems does not converge to a focal point or form an image, hence the name.
Sustainable lighting is lighting designed with energy efficient light sources. "There are simple design strategies and some materials that can facilitate the energy saving advantages of natural light. Light colored interiors and open floor plans are good choices. This approach also augments artificial light efficiency. Energy efficient lighting is not simply finding the most light for the least wattage or the longest lasting light bulb. Proper sizing of the light to the needs of the location and the tasks that will be performed, called task lighting, is an energy saving strategy."
A skylight is a light-permitting structure or window, usually made of transparent or translucent glass, that forms all or part of the roof space of a building for daylighting and ventilation purposes.
The artificial sky is a daylight simulation device that replicates the light coming from the sky dome. An architectural scale model or 1:1 full-scaled aircraft is placed under an artificial sky to predict daylight penetration within buildings or aircraft that subjects to different situations, complex geometries, or heavily obstructed windows. The concept of the artificial sky was derived due to heliodon’s limitation in providing a stable lighting environment for evaluating the diffuse skylight component.
Climate based daylight modelling (CBDM) also known as dynamic daylight metrics is a calculation methodology first developed in the late 1990s to assess daylight quality and quantity. It is used by Building Design engineers and architects to predict luminance and/or illuminance within buildings using standardised sun and sky condition climate data for a given geographical location. It is a different design metric to Daylight factors which only considers the ratio of the light level inside a structure to the light level outside the structure from an overcast sky. With CBDM, if used considerately, the facade design of a building can be optimised to maximise useful daylight whilst excluding excessive daylight, which otherwise might cause issues with glare, visual discomfort, and/or solar gains which can cause thermal comfort issues. At the same time reducing reliance and operation of artificial lighting. CBDM calculations are calculated within Building simulation modelling software tools for each and every hour of the year, or sometimes for smaller increments, which allows for daily and seasonal profiles to be tested and optimised
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