Bioclimatic (Daylighting)

The optimal use of natural daylight, especially in buildings used mainly by day, can, by replacing artificial light, make a significant contribution to energy efficiency, visual comfort and the well-being of occupants. Such a strategy should take account of the potential for heat gain and conservation, energy savings by replacing artificial light and the more subjective benefits of natural light and external views enjoyed by the occupants.

A good daylighting system has a range of elements, most of which must be incorporated into the building at an early stage in its design. This can be achieved by consideration of the following in relation to the incidence of daylight on the building:
  • the orientation, space organisation, function and geometry of the spaces to be lit
  • the location, form and dimensions of the openings through which daylight will pass
  • the location and surface properties of internal partitions which will reflect the daylight and play a part in its distribution
  • the location, form and dimensions, etc., of movable or permanent devices which provide protection from excessive light and glare
  • the optical and thermal characteristics of the glazing materials.
Good daylighting design will not only reduce energy costs related to artificial lighting but will also diminish the need for mechanical devices to cool rooms overheated by lowefficiency electric lighting appliances.

Achievement of comfortable lighting conditions in a space depends on the amount, distribution and quality of the light there. Enough illuminance, indicated by a sufficiently high daylight factor, should be provided to allow relevant objects to be seen easily, without fatigue.

The light distribution in the space should be such that excessive differences in relative illumination which could give the impression of inadequate lighting are avoided.

Sufficient contrast should, however, be retained for the relief of each object to be brought out. Window openings and artificial light sources should be placed in such a way that glare is minimised.

Finally, particular care should be taken over the quality of the light to be provided. Both the spectral composition and light consistency should be appropriate for the task to be performed.

Illuminance
Although the human eye is extremely adaptable, it can nevertheless only perform visual functions within a small range of illuminance levels. For a particular task, the range is affected by the visual performance required, the light
distribution in the room and the luminance of the walls and other surfaces. Recommended optimal illuminance values for the workplace for different types of task, are given in the Building Energy Code published by the (UK) Chartered Institution of Building Services Engineers (CIBSE).

Daylighting devices.
 Daylighting devices

Contrast
Contrast is the difference between the visual appearance of an object and that of its immediate background. It can be expressed in terms of luminance, illuminance or reflectance between surfaces. The amount and distribution of the light (and hence the amount of contrast) in a room is very dependent on the reflectivity of the walls and other surfaces. Surface finishes should, therefore, be chosen with regard to their reflectances (the ratio of overall reflected radiant energy to incident radiant energy). In general, to achieve good luminance distribution, light colours should be used for large surfaces.

Glare
Glare is caused by the introduction of an intense light source into the visual field. It can be mildly distracting or visually disabling for the occupant. Whatever its level, it always produces a feeling of discomfort and fatigue. Glare can be caused directly, indirectly or by reflection. Direct glare occurs when a light source with a high luminance enters directly into one’s field of view. It can be experienced with interior lighting or when the sun or clear sky is seen through windows either directly or after reflection from an exterior surface. Indirect glare occurs when the luminance of walls is too high. Reflected glare is caused by specular reflection from polished interior surfaces. Glare can be reduced by careful design and choosing light sources and backgrounds of suitable luminances.

Light control
Penetration of solar radiation into a building contributes much to the quality of the lighting there - as long as the sun’s rays do not reach the occupants’ eyes directly or by specular reflection. The penetration of natural light can be controlled by reducing the incident flow, the amount of contrast and the luminance of the windows. Control of direct or diffuse sunlight is important to comfort because it reduces glare. It can be achieved either by incorporation of permanent or movable exterior devices into the building design to reduce the view of the sky or by using movable interior screens to reduce the luminance of the window.

Health effects
Besides being needed for visual perception, light also regulates metabolic processes in the human body, and affects the immune system and psychological and emotional states. Daylight is involved in setting the "biological clock" and its associated rhythms. A lack of light (particularly in winter at high latitudes) can lead to seasonal affective disorder (SAD) with symptoms of lethargy and depression. This effect could be enhanced in the occupants of deep-plan buildings where artificial light levels are insufficient to trigger physiological responses. Daylight also provides clues for spatial and time orientation which, when removed, lead to psychological discomfort and loss of productivity. Humans evolved in an environment of purely natural daylight and it seems likely that it has other, hitherto unknown effects on the human
mind and body.

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