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Daylighting Overview Square One Research Natural lighting refers to the admittance of light
from the sky into internal spaces and is a key factor in the design of
energy efficient commercial buildings. Properly used, it can result in
substantial energy savings by reducing the need for artificial lighting. The
primary aim of natural lighting is to provide sufficient light under all
circumstances for the tasks performed within a space. If such a lighting
level cannot be achieved by natural light alone, then localized artificial
task lighting can be used to supplement.
Figure 1 – Guggenheim Museum, New York (Frank Lloyd Wright). On a clear summer day, outside light levels can be as high as 100,000-120,000 lux on a horizontal surface, whilst on a dark overcast winter day this might fall to around 4,000-5,000 lux (depending on the latitude of the location). The required light levels inside a building range from 100 lux in an access corridor, 300 lux on the desktop in an average office, 800 lux on a drawing board, and up to 1200 lux for display cases in a supermarket. With some thoughtful and innovative design, natural lighting can potentially provide more than enough light for most applications in almost any type of building. Daylight vs Sunlight Daylight refers to the level of diffuse natural light coming from the surrounding sky dome or reflected off adjacent surfaces. Sunlight, on the other hand, refers to direct sunshine and is very much brighter than ambient daylight. The Sun’s position in the sky varies markedly
throughout the day and, when viewed from any particular point, is often
obscured by clouds, trees or other buildings. It also experiences
significant changes in intensity at different times of the year. Thus it
does not make a very reliable light source with which to light the inside of
a building. Also, its intensity is such that it can be a significant source
of glare when falling on a work surface or reflected off a computer screen.
As a result, direct sunlight is rarely included in architectural daylighting
calculations.
Figure 2 – The difference between a sunlit and daylit space. Daylight, however, can be a very effective light source, even on the most dark and overcast day. Daylight levels can also be quite variable and depend on the amount or type of cloud in the sky and the time of day. However, there exist a range of mathematical models that allow the calculation of how bright different parts of the sky will be under different sky conditions. These models allow us to choose a set of worst-case situations around which to design the building. For more information, see the Luminous Distribution topic. Luminous Efficacy Light and heat normally come together, however the
amount of heat produced by different lights for the same lighting intensity
can vary significantly. It turns out that, in terms of the number of
lighting lumens per watt of heat energy, diffuse daylight is about 5 times
more efficient than a normal incandescent globe and as much as twice as
efficient as a fluorescent tube. In a typical office building, turning the
lights off and substituting daylight alone can reduce overall heat loads by
as much as 40%, principally by reducing over-illumination near peripheral
windows.
Table 1 - Efficacy of various forms of daylight and electric lamps. Table 1 above shows that the luminous efficacy of direct sunlight is also much greater than that of most commonly used electric alternatives. However, it is also considerably brighter so it will introduce significant heat gains if allowed to enter the building directly at the wrong time of year. Obviously in many climates this heat gain may be welcomed in winter. This requires the careful use of shading devices and light diffusers to properly protect against direct summer sun penetration whilst distributing natural light deep into each space. Careful selection of glass type is also an important factor. However, the most important factor in daylighting design is the selection of the most appropriate type of appertures through which each space will connect to light from outside. For more details, see the Daylighting Systems topic. Related Links Daylight Advanced Lighting Guidelines: 2003 Edition Energy Efficient Technologies: Daylighting Design EcoAdvisor - Daylighting Daylight and Natural Air Flow Daylighting: Lighting Naturally Lighting - the Electronic Textbook
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