07.04 Natural lighting

Categories: Lighting

Introduction
It cannot be stressed too highly how central is natural lighting to memorable architecture. Designers who rely too heavily upon electric lighting for illumination very often produce buildings which are spatially simple, uninteresting and somehow two dimensional. Why is this the case? How can it be that in an effort to provide good daylighting a designer, it must be admitted sometimes unwittingly, produces buildings which people perceive as quality architecture?

Consider the buildings you regard as memorable. Is it true that they too, much depend either upon the way natural light interacts with the building form and texture or upon utilisng daylight within the interior of the building ?

Responses to daylighting will of course depend much upon personal experience, but in any effort to daylight a building, the designer is forced to consider a number of aspects which will crucially affect the form , spatial layout and solidity of the façade.
 

The changing nature of natural light is a part of life
The seasonal rhythms of natural light affect the life cycles of plants and animals. Both flora and fauna respond to the changing length of dayand the reduced levels of daylight seen in Winter. Although in humans the life cycle is no longer so closely bound to the seasons they also can be affected by the low levels of light experienced in Winter. This effect is referred to as Seasonal Affective Disorder and is believed to be the result of changing levels of melatonin in the body.

The diurnal rhythms of daylight are so much a part of daily life that they are taken for granted. Daylight resets our body clock each day and our state of arousal is partly determined by our exposure to high light levels experienced during the day. Depriving people of exposure to daylight without proper compensation for that loss can disturb the physiology of the human body. The psychological need for daylight and its rhythms is even less well understood, although its need is widely recognised in the words of poets.

The colour of daylight can vary widely from the warm rosy hues of the setting sun to the cool blue of a clear sky. Although plants react to different hues in the spectrum of light there is little direct evidence that humans respond to different colours. However there are those who believe that the colour of light does affect our mood and health and that the range of colours provided by daylight promotes health. Certainly, daylight is a continuous broad spectrum light source and therefore colours and materials appear well under it. For this reason natural light is often the preferred light source in art galleries and museums.
 

Utilising daylight embraces the heart of architecture
Introducing natural light forces designers to think in 3 dimensions. Light must enter through the building envelope, be it either the wall or the roof. The supporting structure may be minimised in order to aid the passage of light to an internal area, utilized as a screen to filter the light or positioned to intercept the light in order to create visual interest in itself.

A cloudy sky is an extended light source and there are few ways of controlling and redirecting its light successfully. A small light source such as a lamp can be focused and redirected by lenses and mirrors; this is also possible with sunlight because it too is a relatively small source of light. However, this is not the case with light from cloudy skies and in order to accentuate the daylight level in one area it is necessary to reduce it in another area. This applies to all scales within the building, from the design of small louvres within a window to the massing of blocks of building.

Because the main methods of controlling and redirecting light are obscuration and plane reflection it is the architectural elements themselves which create the patterns of light. Thinking about lighting automatically ensures consideration of architectural form.
 

The drama of space is mediated through the flow of light
Light may come firom any direction of the envelope. Decisions as to where openings in the envelope are positioned will determine the direction from which light arrives in the interior. This flow of light will naturally lead to a particular grading of light level through the room, it may be great or small depending upon the relative position and areas of the windows. Not only will the flow of light affect the appearance of the room's surfaces, but objects within the room will respond to the different directions and strengths of modelling throughout the room.

The flow of light may be clearly defined. A single window will produce a simple and consistent flow of light where the observer is conscious of how light is changing throughout the space. The indirect daylighting of baroque churches produces a much less distinct flow of light and this can lead to a lighting effect which is almost ethereal. Such effects are quite appropriate to reverie and religious buildings but may be perceived as soporific in a working interior.

A room of white surfaces will reflect the light around so that no matter from where the light originated the resulting light will be omnidirectional. This will soften if not completely eliminate shadows created by the original flow of light into the room. When considering the interior decor it therefore needs to be remembered that surface reflectances not only determine the appearance of a surface but also change the nature of light in a room.
 

The sense of enclosure is inseparably linked with windows.
A window opening out to the world beyond increases the sense of spaciousness within interiors. This is not only the result of the window physically allowing the eye to gaze beyond the confining surfaces of a room, but it is also in part the result of a perceived link with outside world which is not necessarily directly related to the actual view possible through an opening.

The experience of Architecture is intrinsically about a sense of enclosure and therefore it is essential that building designers appreciate the way in which windows modulate that perception of space.
 

Solid or light, textured or plain, the windows create the façade.
Windows are a major element of the façade. This is clearly important, but it too often is the only aspect of the window which is considered in depth. In bringing order to the façade it may be necessary to add, omit or change the proportions of windows and in so doing compromise other design considerations.
 

Natural light
Real skies change continuously. The sky becomes brighter towards noon and its light distribution changes as clouds gather, thicken and disperse. These changes occur, partly in response to the predictable movement of the sun across the sky and partly as a result of the rather less predictable cloud cover. There is therefore good reason to try and simplify matters in order that designers can appreciate how their buildings will respond to natural lighting.

The basic simplification adopted by designers is to initially consider

only the two extremes of natural skies;

• sunlight with a clear blue sky,
• the completely cloudy sky.

In these notes, the term 'natural' will be used to describe the lighting under real skies and encompasses all the many unpredictable variations that occur in reality, 'sunlighting' will be used to describe the combined effects of sunlight and a clear blue sky, and 'daylighting' will be used to describe the lighting effects under a completely cloudy sky, image.
 

Sunlighting
Sunlight will be considered in detail later. However, it will not be promoted as a working illuminant, but as a means of enhancing the quality of our environments. Sunlight can contribute to the working light within a room but it is difficult both to control the light and to estimate the extent of its contribution. For these and other reasons it is only recently that techniques of prediction and analysis have progressed sufficiently to incorporate sunlight illuminances into design calculations. However, it is usual to concentrate upon the sun’s qualitative contribution to lighting within and around buildings and to see any increase in light level as a welcome but unaccounted contribution.
 

Daylighting
An important reason for considering the extreme of a completely cloudy sky is that such skies may be considered as a worst case. A building designed to be satisfactory under such skies is considered to provide some minimum level of daylight and to benefit from improved lighting under less extreme conditions e.g. under partly cloudy and clear skies with sunlight. The specification of some minimum level of daylight at the back of a room was prompted by a desire to ensure adequate lighting to perform various visual tasks in schools and factories. Analysis of daylighting was developed to enable designers to assess whether or not the lighting met those new design criteria.

The lighting within a room can be accurately investigated using scale models and these will be considered in a later section. However, even a completely overcast sky in reality changes in brightness from one direction to another and this can make it difficult to compare the effects of different designs of window when measurements are made under real skies. Therefore to enable the comparison of lighting effects in buildings with different windows, there is an internationally agreed standard sky which is used in place of a real cloudy sky. There are a number of standard skies, each of which has a specified brightness pattern.
 

Standard cloudy skies
For the moment only two types of standard sky will be considered and both of these skies are assumed to be rotationally symmetric about the zenith, image. Under these skies the daylighting will not be affected by a building's orientation.

The first standard sky proposed was the Uniform Luminance Sky in which the brightness of the sky was assumed to be constant with the angle of altitude, image. This type of standard sky is still used in legal cases of Rights of Light and also quite well describes that part of the clear blue sky where there is no sun.

The second standard sky is the CIE Overcast Sky. The Commission Intemationale de L‘Éclairage is an international body which oversees standards relating to light sources, vision and lighting design. The Overcast Sky quite well models completely cloudy skies in temperate climates such as the United Kingdom and western Europe. A principal feature of the sky is that the sky at zenith is much brighter than at the horizon, image.
 

Measurements of daylight
Horizontal illuminances from a diffuse sky have been measured over a number of years by recording the light level from the whole sky except that part of the sky through which the sun passes. This is achieved by using an occluding ring whose position is changed every few days so that direct sunlight never falls upon the photocell measuring the light level, image. The results from a number of years may be used to predict the average horizontal illuminance from the sky that occurs at a particular time of day for each day of the year, image.

This figure may be used to estimate the number of hours during the year when particular levels of daylight are exceeded. Plotting the hours that given levels of light are exceeded generally results in a series of points which may be reasonably represented by a straight line, image. Normally only times within the working year are considered because it is assumed that commercial and industrial buildings will not be occupied outside working hours. It will be found that the relationship becomes somewhat non linear where there is no daylight for a large part of the working year.

This probability curve for diffuse daylight is fundamental to the prediction of when daylighting will provide sufficient light within buildings. However, as a simplification, it should be noted that a horizontal daylight illuminance of 5000 lx is exceeded for approximately 85% of the working year, and if a building is designed for such a sky it will be considered to be well daylit. A 5000 lx sky is typical of the average illuminance from a mid Winter sky.
 

Daylight Factor
Because the illuminance from an unobstructed overcast sky varies the daylight illuminance indoors will also change. If the distribution of light from the sky is assumed to be constant then any change in light level will be directly related to the change in illuminance from the unobstructed sky.

From observation it will have been noticed that the impression of daylight within a room is dependent more upon the relative distribution of daylight than its absolute level. This effect can be accommodated by considering the daylight indoors relative to the light from the sky. This may be quantified in the Daylight Factor (DF) which is defined as;

Daylight factor  =  ( illuminace in room / horizontal illuminance from an unobstructes diffuse sky ) x 100%

It should be appreciated that if several designs are going to be compared it is important that only the changes in design will affect the indoor light level. This strictly will only be true if a constant sky is used so that the distribution of light is constant and this will never be the case with a real sky. Therefore to properly define the daylight factor the type of sky needs to be clearly specified. Usually it is assumed that the sky is a CIE Overcast Sky.
 

Measuring Daylight Factors
An approximation of the daylight factor may be measured in a room under a completely cloudy sky, but such measurements are not at all easy to make because of the infrequency of truly overcast conditions and the varying level of light from the sky. Where daylight factors are to be measured in real rooms then it is important that external conditions are carefully recorded and measurements are made when the sun is in a part of the sky which would preclude it from entering the room if there were a clear sky, image.

The above difficulties mean that it is often more accurate and usually more convenient to take measurements of daylight factor within a scale model room under the standard conditions provided by an Artificial Sky. Particularly at the early stages in a design it can be beneficial to investigate daylighting using models because changes can be made quite simply and at minimal cost. Also, scale models allow an assessment of the visual appearance of a room as well as providing the vehicle for quantitative measurements of daylight factor.

It should be noted that if the model is constructed properly to scale then the lighting measurements will be as they would have been in a real sized room. The reflectances of surfaces should be the same as should be the transmittance of any glazing materials. This latter point is more difficult to achieve and often measurements are made without glass and a correction factor is applied later. The reason for scale models correctly modelling full sized buildings lies with the fact that the illuminance on any surface is determined by the angular size of the light source and the angle of incidence of the light onto a plane. Both of these quantities are non dimensional and are unaffected by the scale of the model.

Daylight factors measured in models are often greater than daylight factors measured in real spaces. There are a number of reasons for this: smaller window frames are often used in models, a too high transmittance is assumed for the glazing, furniture is often omitted in models, incorrect reflectances are used and the smoother textures used in models can considerably increase reflectances. Careful attention to detail is necessary if accurate measurements of daylight factors are to be made in models. Also, it should be realised that we are not sensitive to small differences in daylight factor. Therefore, it is rarely necessary to quote daylight factors to more than one or two significant figures.

When making assessments of model interiors it is important that the viewer is correctly adapted to the level of light in the model. If the room is being viewed through a hole in the side of the model, then light should be prevented from entering through the hole, and viewers should lower their adaption level by covering their heads and the side of the model with something like blackout cloth.
 

Daylight Factor Contours
If the daylight factor is measured at a number of points on a regular grid within a room, a series of contours can be drawn which connect together those points which have the same daylight factor, image. Such daylight factor contours show how the daylight varies within the room. They better relate to the visual impression within the room if they are drawn at levels at increasing intervals e.g., 0.5%, 1.0%, 2.0%, 5%, 10% and 20%.
 

Average Daylight Factor
The quantitative analysis of daylight was originally developed in order to investigate the minimum daylighting at the back of a room and estimate the loss of light due to the erection of new buildings. However, it has been found that the average daylight factor in a room gives a good indication of how well lit that room appears to be. This is a most useful finding because the average daylight factor can be calculated using quite simple calculations. Also, if a particular average daylight factor is to be provided, it is easy to establish the area of glazing required. There is a simple formula which can be used to calculate the average daylight factor

DF_av  =  q A_W  t  MF  GBC / [ 2 A_T ( 1 - r_av )]  %
 

The development of this formula is provided in Section 07.05.

More detailed calculations are needed if the designer wishes to determine the daylight factor at specific positions in a room. There are numerous techniques to do this.
 

Daylighting Schedule
The daylight factors recommended for different spaces within buildings are given in the CIBSE Applications Manual 'Window design'. The primary concern here is to ensure that there is sufficient light to perform particular visual tasks. Values recommended for some spaces are shown in the following table.
 
 

Planning and Site layout
The provision of daylight is an important consideration for most buildings and therefore at an early stage of design it is often necessary to consider whether or not a building development will be capable of meeting daylighting design requirements. Developments should be designed to ensure that their general form and disposition on site allow:

• Rooms in the proposed building to be daylit,
• The daylight within existing buildings to be safeguarded,
• The potential for daylighting on adjacent sites to be maintained.

There are a number of alternative rules which if adhered to will ensure that these aims are achieved. Some of these require quite involved calculations, but others are quite simple and are based upon the following guidelines:

i) Potential for daylighting will be provided if, image;

• no obstruction, measured in a vertical section perpendicular to the main face, from a point 2m above ground level, subtends an angle of more than 25º;
• all points on the main face on a line 2m above ground level are within 4m (measured sideways) of a point which has a vertical sky component of 27% or more.

ii) Existing buildings may suffer if, image;

• the vertical sky component is less than 27% at the centre of the existing main window and is less than 0.8 times its former value;
• The area on the working plane receiving direct light from the sky is reduced to less than 0.8 times its former value.

iii) Proposed developments will be safeguarded if, image;

• no new building, measured in a vertical section perpendicular to the boundary, from a point 2m above ground level, subtends an angle of more than 43' to the horizontal;
• all points 2m above the boundary line are within 4m, measured along the boundary, of a point which has a vertical sky component of more than 17%.