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A Holistic Approach to Lighting Design

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Light is fundamental to life. At the simplest level, light enables us to enjoy the world around us and carry out a myriad of visual tasks. It can create a psychological mood of delight or melancholy, or one that is stimulating or soporific. Lighting may affect our performance. It is also believed to affect our health, although the link is not yet well established.

A considerable amount of sensory information is received by our visual system which, of course, requires light to operate. But for the visual system to function well, it requires a composition of light that enables it to operate comfortably and efficiently.

Daylight is often the preferred form of lighting. This may be because of the way daylighting models objects and spaces or its good color rendering properties, or it could be because daylight is composed of shorter wavelengths compared with most electric light sources.

Another reason may be that the ever-changing light pattern associated with the variation in daylight throughout the day creates a stimulating visual effect. Also, daylit areas of buildings often provide an external view, thereby offering a natural visual relief.

BALANCING WELL BEING AND EFFICIENCY
One indication of the importance of daylight, windows, and views is a study of the recovery rate of patients in a hospital in Pennsylvania. In the study, patients received similar treatment by similar staff in similar wards. The only difference was that some patients were in wards looking out onto trees, while others looked out onto a brick courtyard. The study showed that patients with the tree view recovered 10% sooner than those with the courtyard view. Although the results of this study are not definitive, they do suggest the importance of windows, daylight, and views.

There is little other experimental evidence concerning the effects of lighting on well-being, mainly because such effects are extremely difficult to measure. However, it is felt that by providing a visual amenity, people's feeling of 'well-being' will be enhanced and their performance increased. This may reduce time lost to sickness and other absenteeism.

Apart from these considerations, there are significant energy and environmental costs associated with electric lighting. Daylight cannot provide for all our lighting requirements, and it is rare for a building to be totally daylit throughout the working day, even on a bright day. Areas furthest from the windows will receive lower levels of daylight, thus requiring supplementary electric lighting.

Electric lighting consumes valuable energy, often generated by the burning of fossil fuels which contributes to environmental pollution. Thus energy efficiency in lighting is an important aspect of design from both environmental and economic points of view.

The lighting designer needs to balance human requirements with a need to use energy effectively and in a way that is in accordance with both building design and function. Sadly, the designer often considers only the basic requirement of function, neglecting all the other factors that could enhance human performance while using electricity efficiently. To ensure the best possible lighting, a framework for design is required.

A LIGHTING DESIGN FRAMEWORK
In the design of new buildings, the architect needs to ensure that, wherever possible, the potential to use daylight is utlizied to its fullest. Sometimes however, daylight - particularly bright sunlight - can be uncomfortable. For instance, it can overheat the building and can cause visual discomfort or even disability through glare. In such cases the occupants need to be able to make adjustments to minimize problems. This will require some form of daylight or sunlight control such as blinds or louvers. It is important to recognize that daylight is constantly changing and that most windows will require some form of daylight control at some time. This needs to be considered when the building is being designed rather than as an afterthought.

Once the daylighting has been designed it is time to consider the electric lighting needs - but where should you start?

It is important to consider the total requirements and constraints, and for these to be balanced, to reach a satisfactory and acceptable solution. The figure illustrates the basic elements that need to be considered within a design framework. There is no particular order in which the elements should be addressed, nor do they carry equal weight. The optimal order and relative weights will vary depending on the particular application and building design. The framework applies to new buildings or refurbishment, although for the latter, opportunities may be more limited. There is a range of publications on lighting design to assist designers, contractors, and users involved in new construction or refurbishment of all kinds of buildings.

Visual function: It is essential that requirements are discussed with the client and analyzed. These may range from ensuring that people can carry out visually complex tasks, where errors could be disastrous, to merely making sure that they can move around safely. The designer should decide on the amount of light required for each task and how it should be provided. In the UK, for example, the CIBSE Code for Interior Lighting 1994 is a useful source of information and contains examples of appropriate lighting levels for a range of activities and interiors. Similar codes exist in other countries.

Providing sufficient task lighting is not enough. Although the human visual system can operate over a very wide range of light levels, it has difficulty in coping with high levels of brightness. Bright elements within the visual field should be avoided to minimize visual discomfort or disability. This will mean screening direct views of bare lamps or avoiding bright reflected images in the task, such as computer screens. If such measures are not taken the occupant's performance could be impaired and money wasted.

Most lighting operates at normal mains frequency (50 Hz). When discharge lamps are used, the flicker can be annoying or uncomfortable for some people. Electronic ballasts for fluorescent lamps operate at a very high frequency (typically 30 kHz). These circuits eliminate noticeable flicker and also use less energy. They can also be dimmed, although special dimmable ballasts will be necessary.

It is essential to ensure that the building can be evacuated safely in the case of a mains supply failure. Most public buildings must have emergency lighting. This must switch on automatically in the event of a mains supply failure and should provide sufficient light to allow people to exit the building safely.

Visual amenity: One often refers to the "quality of a lighting installation", but what does this mean? Research shows that people prefer spaces to appear 'light' and that this sensation relates to the brightness of surrounding surfaces. Since eyes generally view from a horizontal plane, walls are most important, but "light" ceilings can also have an effect. The "lightness" of a surface is created by a combination of the amount of light incident on a surface and its reflectance. For a surface to appear 'light' it will require a high reflectance finish.

Other studies indicate that people prefer a lit scene to have some degree of illuminance variation. In general, people find uniform light patterns bland and uninteresting. If possible, the designer needs to introduce some light variation into the design, making some areas brighter than others. This must be done in a controlled way to avoid visual discomfort, and the pattern of light created should provide a logical complement to the architectural design. A simple solution is to highlight some of the walls, perhaps walls displaying material relevant to the company or a textured wall that forms an architectural feature. Another possibility is to highlight a group of plants or some other object of visual importance.

Architectural integration: Lighting needs to form an integrated part of the building. This applies not just to the lighting equipment but also to the light pattern it produces. The eye is drawn to the brightest part of a scene, so the lighting can be used to direct people to particular areas or down certain routes. For example, brighter lighting at the end of a corridor will encourage people in that direction.

The appearance of the lighting equipment and the manner in which it is installed also needs to be considered. Luminaires mounted onto the ceiling often produce a cluttered appearance. This is why many architects prefer ceiling recessed luminaires. However, it is essential to consider the lighting performance of luminaires as well as their appearance.

Lighting can sometimes be integrated into architectural features such as cornices or coves. Sometimes it is necessary for the architect to provide features to enable the lit effect to appear relevant, such as a change in ceiling or floor level. It is important for the lighting designer to work closely with the architect to ensure a satisfactory solution.

Since daylighting and electric lighting should be integrated, it is necessary for the lighting to be switched or dimmed to supplement the daylighting. This means planning the installation to allow for this to happen in terms of the daylight distribution. Modern lighting controls with daylight sensing will help. The choice of light-source color performance will also need to be considered. Although the appearance of the daylight color will vary depending on time of day, season, and weather conditions, the use of lamps with an 'intermediate' color appearance will usually be an acceptable compromise, i.e., a light source that is neither excessively "warm" nor excessively "cool" in color appearance.

Daylighting and artificial lighting must be considered with respect to the other building services and factors, particularly the thermal performance of the building.

Energy efficiency: To minimize global energy consumption as well as client operating costs, all lighting installations need to use electricity effectively and efficiently. Lighting equipment should be used that converts electricity into light as efficiently as possible, and the lamps selected should be appropriate for the particular task(s). "Efficacy" is the term used to describe the efficiency of this, and clearly, lamps should have a high efficacy. For example, incandescent lamps have a low efficacy (approximately 12 lumen/watt), while fluorescent lamps have a much higher one (ranging from 50-100 lumen/watt, depending on rating and color).

A further consideration is the efficiency of the luminaire. This is described by its light output ratio (LOR), which is the proportion of the lamp light output that emerges from the luminaire. Another unit used is the utilization factor (UF), i.e. the proportion of the lamp light output that arrives on the working plane. Both these figures need to be considered with respect to the operation of the installation, including both visual function and amenity.

Electric lighting should be used only when needed. People will usually switch lights on when they are required, but rarely switch them off when they are no longer needed. This is because our visual system can effectively adapt to a wide range of conditions. However, automatic controls can help reduce unnecessary lighting. Time switches, light sensors connected to dimmable ballasts, and occupancy sensors (presence detectors) can all be used to limit the unnecessary use of lighting.

Lighting controls must relate to the user's requirement and be user-friendly. It will nearly always be necessary to provide a manual override to the automatic control system. This will give the occupants a feeling that they control the lighting rather than that the lighting is controlling them.

Lighting maintenance: Maintenance is often overlooked at the design stage. The light output of an installation will depreciate with time. One reason for this is that, except in the case of incandescents, lamp light output generally decreases with hours of burning. In addition, light output depreciation can be caused by dirt buildup on the lamps, reflectors and other luminaire surfaces. Room-surface depreciation also affects lighting levels since most lighting installations include some room surface reflection component. Hence, if the surfaces become dirty they will be less effective. Lamp replacement must also be considered.

All these factors should be considered at the design stage to ensure that the lighting works well throughout the maintenance cycle, i.e. the period from installation to the point at which maintenance should take place (luminaire and lamp cleaning, room surface cleaning and lamp replacement). Appropriate lighting equipment and a maintenance schedule for the client should be specified. An important factor is the accessibility of lighting equipment. Equipment that is difficult to reach is rarely well maintained. In cases where poor accessibility cannot be avoided, lamps of very long life should be specified.

Lighting cost: (equipment and operation costs) There are several costs to be considered: the capital cost of the design and installation, the cost of the lighting equipment, including lamps, luminaires, and lighting controls, and the cost of the installation, including the wiring.

There is also the operational cost to consider, including the cost of the electricity and the cost of installation maintenance.

All costs need to be considered together if a cost-effective installation is desired. However, it is also important to consider the cost of the lighting with respect to the cost of the building and the cost of the people it serves. For a commercial building, the lighting installation typically amounts to about 1-4% of the total cost of the building. This means that the capital costs are relatively small. However, lighting energy costs can amount to as much as 30-50% of the total energy costs. It is important to minimize the running costs by using an energy efficient design, although, when both capital costs and running costs are compared with the cost of the staff then it may be minimal. For the UK, it has been estimated that the capital cost of an office-type building account for 7.5%, its running costs for 7.5%, and staff salaries for 85% of the sum of the building capital cost and the total operating cost for the first ten years, including staff cost. This means that any savings in lighting costs that result in reduced staff performance could actually increase overall costs.

LIGHTING DESIGN - WHERE TO NEXT?
Lighting installations that work well for people, enhance buildings, and use energy efficiently will lift lighting out of the doldrums and into the new millennium.

Many lighting installations use a regular array of the same luminaire to provide a uniform horizontal-plane illuminance, usually with an acceptable glare control through a limiting glare index. Recently attention has shifted towards minimizing reflected glare on visual display screens. This has led to the use of luminaires with low brightness louvers, which in terms of the task, have often performed well. However, such luminaires tend to produce a gloomy interior because they shield the light at high angles. At times the room can also appear under-lit, even though the recommended level of illuminance for the task has been reached.

People generally prefer interiors to appear "light" and to have some degree of light variation. The lighting designer also faces the challenge of reducing energy use without reducing quality. Many modern installations fall short of these requirements. Perhaps it is time to resurrect the idea of task and background lighting?

Research has indicated that by highlighting the task area separately from the surrounding area, visibility can be enhanced. The idea is that there should be a relatively modest level of horizontal-plane illuminance, topped-up to the required task illuminance by local lighting in the work area. Lighting then becomes non-uniform and therefore more interesting. Such a design would also reduce energy use.

According to a recent study in Canada, users prefer this form of lighting over more traditional regular array lighting systems, and energy savings of at least 30% can be realized.

Taking this a step further - if the background lighting could relate more to the building by lighting surfaces, particularly walls, then it could make spaces appear brighter as well. Perhaps the new approach should be called task and building lighting to encourage the idea that it is illuminating the task area and the building surfaces.

FLEXIBILITY IS THE KEY

There are many ways of achieving tomorrow's lighting requirements, but it will take the ingenuity of the lighting designer, working closely with the architect and interior designer, to produce a satisfactory result with regard to the whole lighting design framework. To some extent the solution will be dictated by user requirements and the building design. It could involve the use of lighting equipment fitted to surfaces, such as walls and ceilings, or to the furniture, both integrated or free standing.

Alternatively, it could be provided from floor-standing devices incorporating both the task lighting element and the building lighting. Flexible, user-friendly lighting controls will form an important aspect of this approach. The task lighting may only need to be used when the desk or workstation is occupied. This means that the room will appear attractively lit at all times, without the need for all task lighting to be on at all times. This could lead to significant energy savings through the installation's design and the use of automatic controls with manual override.

A further benefit of this approach is that, if the task lighting can be easily moved with the workstation, it could provide for a more flexible use of space. This would allow users to reorganize their space as required, without having to make major changes to the wiring, provided there is a flexible, floor-recessed electrical power installation. The approach has many potential benefits, but requires a greater degree of consideration at the design stage to achieve a satisfactory solution.

David Loe
Paul Davidson

David Loe is Honorary Research Fellow at the University College London and Consultant to BRECSU, the Building Research Establishment (BRE)
Paul Davidson is Resource Manager at BRECSU



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