Increasing energy costs and concern about global warming, there is considerable interest in using daylight as the major light source in buildings. Unfortunately, there is little point in doing this if daylighting causes problems to the occupants of buildings. The possibility of daylight causing problems to occupants may seem unlikely given the well established desire of people to have natural light wherever possible, whenever available. However, a short walk around any city will reveal numerous well-glazed office buildings where the blinds on many windows are permanently closed. Such behaviour demonstrates the existence of a failed daylighting design for at least some people within the building. Nonetheless, unless there is a good reason why there should be no daylight in the building, daylighting should always be encouraged.
To make the best use of daylight it is first necessary to recognise that daylight can have both positive and negative effects on people.
- Physically, daylight is just another source of electromagnetic radiation in the visible range. Physiologically, daylight is an effective stimulant to the human visual system and the human circadian system. Psychologically, daylight and a view out are much desired and, in consequence, may have benefits for human well-being.
- The performance of tasks limited by visibility is determined by the stimuli the task presents to the visual system and the operating state of that system. Daylight is not inherently better than electric light in determining either of these factors. However, daylighting does have a greater probability of maximising visual performance than most forms of electric lighting because it tends to be delivered in large amounts with a spectrum that ensures excellent colour rendering.
- There can be no guarantee that daylighting will always be successful in maximising visual performance. Daylight can cause visual discomfort through glare and distraction, and it can diminish the stimuli the task presents to the visual system by producing veiling reflections or by shadows. The effectiveness of daylighting for visual performance will depend on how it is delivered.
- People will take action to reduce or eliminate daylight if it causes discomfort or increases task difficulty.
- The performance of both visual and non-visual tasks will be affected by disruption of the human circadian system. To avoid such disruption, the exposure to bright light during the day and little light at night is necessary. Daylighting is a convenient means to deliver bright light during the day to people who have little opportunity to go outside.
- Different lighting conditions can change the mood of occupants of a building. However, there is no simple recipe for what lighting conditions produce the most positive mood. Windows are strongly favoured in work places for the daylight they deliver and the view out they provide, as long as they do not cause visual or thermal discomfort, or a loss of privacy. Therefore, the presence of well-designed windows is expected to enhance positive mood and their absence to increase negative mood, although whether it is the view out or the admission of daylight that provides the benefit is unclear.
- Exposure to daylight can have both positive and negative effects on health. The strongest effects occur outdoors. Exposure to daylight outdoors can cause tissue damage, which is bad, and generate vitamin D, which is good. Daylight and sunlight delivered through glass will have less ultra-violet radiation than the same radiation outdoors, but can still have adverse effects on people who are sensitive to ultra-violet radiation. Daylighting that makes what
needs to be seen difficult to see can cause eyestrain. Conversely, daylighting that makes what needs to be seen easy to see can reduce eyestrain. Windows that provide a view out as well as daylight can reduce stress.
These conclusions imply that good daylighting design is not simple. Thought needs to be given to the amount of daylight available, the view out, the control of glare, the light distribution in the space, solar heat gain and integration with electric lighting. When done well, daylighting can make a very effective and attractive space but when done without thought or with thought limited to the external appearance of the building, daylighting can cause discomfort to occupants and add to the energy consumption of the building.
KEY POINTS TO UNDERSTAND DAYLIGHTING
Daylighting is an excellent light source for almost all interior spaces. On the hand, it is not the only but the best known “Natural Lighting Source” compared to other lighting sources. Daylighting usually needed for highly utilized places of human usage likewise offices, schools, and workspaces requiring a lot of light and for public spaces such as malls, airports, and institutions. Windows, skylights, and other forms of fenestration are used to bring daylight into the interiors of buildings. Daylight is highly desirable as a light source because people respond positively to it.
The amount of available daylight varies according to time of day, time of year, weather, pollution levels, and so on. The maximum amount of daylight is about 10,000 foot-candles on a sunny summer day. For energy efficiency in
buildings, however, only about 5% of the daylight, or a peak of about 500 footcandles, should be allowed into a building; more will generate so much heat that energy will be wasted in air conditioning.
The color of daylight varies as well. The color temperature of the setting sun is as low as 2000K, and the normal sun-and-sky color temperature at noon on a sunny day is 5500/6000Kelvin. The cold blue light from the winter north sky is over 10,000K. The color quality (CRI) is excellent. However, daylight has a relatively high ultraviolet (UV) light content, which has potential negative side effects such as sunburn and skin cancer. Extreme care must be exercised when using daylight in places such as museums where damage, called photodegradation, causes bleaching of pigments and other harm to irreplaceable art and antiquities. Ordinary interiors may also experience photodegradation in the form of fading fabrics.
From an energy efficiency perspective, daylight enjoys a significant advantage over electric light. At least 2.5 times as much air conditioning is needed to cool the heating effect of the most efficient electric light producing the same lighting level as daylight. Thus, if daylight is employed at light levels comparable to or even 50% higher than electric lighting and electric lights are extinguished, a building can be illuminated while saving all of the electric lighting energy and about half of the energy needed to cool the building load ordinarily created by electric lights.
Daylight varies in both amount and spectrum with sun altitude and atmospheric transmission. This means that the availability of daylight will vary with the season of the year and the nature of cloud cover. Picture given below shows how the illuminance on a horizontal plane provided by daylight varies with the time of year and the time of day. Details on the availability of daylight can be found in SLL Lighting Guide 10 and Littlefair and Aizlewood (1999).
Of course, these illuminances are measured on an unobstructed horizontal plane. In a building, the amount of daylight available will depend on the position and size of the windows or rooflights. The contribution of daylight inside a room is given by the daylight factor in conjunction with the daylight availability. This can indicate a minimum, a range or an average. The daylight factor is defined as the ratio of the illuminance at a point within a building to the illuminance on an unobstructed horizontal surface at the same position. Daylight factor is usually expressed as a percentage.
For determining the minimum contribution of daylight to an interior, it is usual in temperate climates like that of the UK to assume the luminance distribution of the sky follows the CIE Standard Overcast Sky. This assumption eliminates sunlight from consideration. For the Standard Overcast Sky, daylight factor is the sum of three components; the sky component, the internally reflected component and the externally reflected component. The sky component is the light that reaches the measurement point directly from the sky. The internally reflected component is daylight that arrives at the measurement point after reflection inside the room. The externally reflected component is daylight that arrives at the measurement point after reflection outside the room.
PROBLEMS OF DAYLIGHTING
The visual problems of daylighting are glare and veiling reflections. Glare is caused by a direct view of either the sun or the bright sky. Glare is usually experienced when facing a window in a façade receiving direct sunlight. Veiling reflections are most commonly experienced when sitting with ones back to a window, when the high brightness impinging on a computer screen reduces the contrast of the display.
The first step in overcoming glare is to ensure that the differences in luminance between the window or rooflight and the immediate surroundings are minimised. This can be done either by decreasing the luminance of the sky or by increasing the luminance of the window surround or both. The luminance of the sky can be reduced by fitting tinted, reflective or fritted glass. This can be effective for a bright sky but not for direct sunlight.
Shading devices can be either passive or active. Passive shading devices restrict daylight at all times, active shading devices do not. Passive shading devices consist of light shelves, overhangs and louvres. Active shading devices, such as louvres and awnings, are located on the exterior of a building. Motorised louvres can be effective at maximising the amount of daylight available, whilst reducing the penetration of the sunlight. Movement of louvres can be distracting. They also impose a maintenance requirement. The same concerns apply to motorised awnings but in
addition, there is a need to sense wind speed so that the awnings can be retracted if necessary. Screening is usually provided by some sort of blind fitted to the window. Blinds can be used to reduce glare and direct radiation but in so doing they may also restrict daylight and view out. Some blind materials, such as perforated fabric, allow a degraded view out to be retained while limiting daylight admission.
Others, such as venetian and vertical blinds allow the user to adjust blind coverage and the angle of the blades to preserve a limited view out while restricting the admission of sunlight. Yet others, such as roller blinds allow the view of the sky to be restricted while preserving a view of the ground outside. While such adjustments are possible in principle, in practice human inertia usually means that blinds are adjusted rarely with the consequence that the amount of daylight in the interior is less than expected by the designer. Such inertia can be overcome by using motorised rather than manual blinds linked to sunlight on the façade but this is expensive and is another maintenance issue. All blinds should have a reflectance of at least 0.5. Where they are likely to be subject to direct sunlight, blinds should have a transmittance of less than 0.1.
2. Thermal Problems
Daylight admitted to a building represents a heat load. In winter this may be useful but in summer it can represent an additional cooling load. Therefore, when considering the energy balance of the whole building, it is essential to consider the contribution of daylighting. On a local scale, sunlight directly incident on people near a window can cause thermal discomfort. This is a good reason for not positioning workplaces close to a window but rather to use this space for circulation. When selecting shading devices, consideration should be given to these effects.
3. Privacy problems
Extensively glazed buildings can present privacy problems, particularly on the ground floor. Concerns about privacy can lead to blinds being closed at all times with a consequent lack of daylight and view out. There is little that can be done about the admittance of daylight but a degraded view out can be preserved without sacrificing privacy by using blinds made from perforated fabric, particularly when the outside face of the blind is of high reflectance and the
inside face is of low reflectance. An alternative solution is to move workplaces away from the windows and to use this space for circulation.
MAINTENANCE OF DAYLIGHTING SOLUTIONS
Dirt will build up on the exterior and interior surfaces of windows and rooflights. This will reduce the transmittance of the glass and therefore the amount of daylight entering the building. The degree to which this will occur will depend largely on the inclination of the glass and the air quality of the local environment. A busy urban environment will produce more dirt than a rural one. To minimise the problem a regular window cleaning programme is needed,
which will require easy and safe access to the windows. Without this, window cleaning will be expensive and is likely that it will not be carried out as often as necessary.