The principal direction of natural light is from overhead. The combination of direct sunlight and diffused sky light produces a very strong downward direction to natural light. However, the reason we can see green plants or rocky hills around us is because they are reflecting light toward us. The direct light from above is much stronger than light reflected off surfaces at ground level, but the reflected light can provide a significant amount of ambient light. However, even in the shadiest conditions, the brightest areas tend to be above us. This feature of daylight is one element that can make a particular direction of light feel natural or not.
A very strong uplight illuminating a person has an unnatural quality, partly because it is not seen as a natural direction for light. Of course, it is not impossible to have that situation replicated in the real world. Sunlight reflected from a reflective surface such as water or a smooth wet rock can produce intense uplighting. However, this situation is rare enough for the brain to apply its experience filter to any uplit scene and class it as an unnatural light direction.
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Even in deeply shaded areas the norm is for the brightest part of a view to be above us.
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Although a completely overcast sky with total cloud cover can look as though it is completely uniform, there can be significant variations. The average luminance of a totally overcast sky is around 42º above the horizon. At 5º above the horizon the sky has only half the luminosity of the average. The zenith light from directly overhead can be 30% brighter than the average. This variation in sky luminance can be important when designing glazing systems. Even on overcast days, roof glazing that allows in zenith light could allow in around 2½ times as much natural light per square yard of opening compared to vertical glazing that only allows in near-horizontal light.
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Average sky luminanc
may take place too slowly for us to be aware of the slight color shift, but they are there nonetheless and the quality of natural light can supply us with an intuitive awareness of the time of day.
Over the course of a day there is a large fluctuation in the color of natural light. We are very aware of the changes from sunrise to midday to sunset, but there is a more subtle but constant variation throughout the whole day. There is a qualitative difference between morning light and afternoon light that goes beyond the change of the sun’s position in the sky. The changes
Light can also be scattered by dust and pollen in the lower atmosphere. As with scattering in the upper atmosphere, blue light is affected most strongly, leading to the familiar blue mist that sometimes obscures distant mountains. Artists call this effect “aerial perspective”, and it can be a strong visible signifier of distance in the natural world.
Rayleigh scattering in the upper atmosphere scatters blue light most strongly, giving us the blue sky, a feature that we associate most with clear skies and bright sunlight in the middle of the day.
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Before the sun begins to set it often takes on a subtle golden quality.After the sun has set, natural light is still provided by the sky and clouds. The very high-level clouds in the top right of this image are illuminated by sunlight. There is a definite shift toward a cool blue light in these conditions.
While Rayleigh scattering affects blue light most strongly, it scatters all the colors of visible light. The light from the setting sun has to travel through much more atmosphere than the overhead light at midday. By the time its light has reached the viewer, all the blue light has been scattered, leaving sunlight that gets progressively redder as the sun sinks below the horizon.
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Even on a cloudy night there is still some light in the sky, and a long- exposure photograph can capture its dusky blue quality two hours after the sun has set.
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For interior spaces, the direction in which glazing faces has an impact on the color quality of the natural light that enters a space. In this image the warm light of an early-morning sun enters through an east-facing window and illuminates the wall around the picture. The left-hand side of the wall is only illuminated by the very cool light of a north-facing window. While this is an extreme example, window placement should take into account the color quality of light from different directions.Below left
This gallery in the Musée d’Orsay, Paris, by ACT Architecture Group, is toplit with natural light from a skylight. The light is controlled by a square louvre at a high level with deep baffles at a lower level that prevent any direct light hitting the sensitive artworks. The resulting light in the gallery is reflected from several surfaces to produce an even spread of light across the display surfaces. Even during daylight hours the natural light is backed up with electric light from luminaires mounted in the top of the deep baffles. The addition of electric light in this situation allows for a relatively constant illumination level to be maintained by tracking the available daylight, and dimming or increasing the electric light in response.
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Cathedral architecture is all about mastering the control of natural light. High-level clerestory glazing allows direct light into the building from a celestial high angle. Meanwhile, light at low level is filtered through stained glass to infuse the interior with color. Left
We can use our experience of the natural world to estimate the time of day and time of year at which a photograph was taken. Although the sun is not visible in this image, it is clear from the deep shadows that it is low in the sky to the right of the frame. Normally, the sun being so low in the sky would suggest a time either shortly after sunrise or just before sunset. Yet the quality of light seems to be neither early nor late in the day. The color is wrong: at sunrise or sunset the light would have an obvious warm color tint. Therefore, the most logical answer is that the photograph was taken in winter, when the sun can be low in the sky while still retaining a cool white light. This kind of assessment of natural light is an innate human ability that allows us to make these kinds of judgments very quickly without even being aware that we are making them. We are constantly applying our knowledge and past experience of natural lighting conditions to make judgments about what we are seeing.
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