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.
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 Sky Illuminance topic.
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.
|Direct Sun (low altitude)||90 lm/w|
|Direct Sun (high altitude)||117 lm/w|
|Direct Sun (mean altitude)||100 lm/w|
|Diffuse Sky (clear)||150 lm/w|
|Diffuse Sky (average)||125 lm/w|
|Global (average of sky and sun)||115 lm/w|
|Incandescent (150 w)||16-40 lm/w|
|Fluorescent (40 w, CWX)||50-80 lm/w|
|High Pressure Sodium||40-140 lm/w|
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 apertures through which each space will connect to light from outside. For more details, see the Daylight Strategies topic.