Let there be light

The advent of relatively cheap electricity in the mid-1960s, together with advances in lighting technology, shifted the balance in building design away from the traditional use of daylighting towards more reliance on artificial lighting. This change reflected society’s newly acquired love affair with technology and the belief that the environment within a building should be controllable and isolated from the exterior. More recently, attitudes have started to shift again with the increasing realisation that artificial lighting and mechanical ventilation can negatively affect their occupants (‘sick building syndrome’).

Visible light amounts to almost 500 W/m2 or 100 000 lux on a sunny day, but constitutes only 42% of the solar radiation reaching earth. The remainder is made up of ultra-violet and infra-red radiation, both invisible.

The level of available light depends greatly on the time of day, season, location and weather conditions. In general in the northern hemisphere, more variation between seasons is experienced in the more northern latitudes, where the total light available is lower. Surfaces facing south usually receive more light than those facing north. East-facing surfaces will get most light in the morning and west-facing surfaces in the afternoon.

Definitions

• Beam radiation: the direct sunlight reaching a surface. Diffuse radiation is the sunlight reflected from clouds and sky. The combination of both is the global (total) solar radiation.

• Illuminance: the measure of light intensity, in units called lux. For natural light, this can range from 5000 lux for an overcast sky to over 100 000 lux for a clear one.

• Glare: occurs when there is large variation in luminance distribution in a room, typically caused by lamps, windows and surfaces appearing too bright in comparison with the general background.

• Daylight factor: the ratio of internal room illuminance to the unobstructed outside illuminance. An average value of 5% shows that the room will be cheerfully lit, a value of 2% shows some daylit character, but supplementary artificial lighting will be needed. It is calculated from room geometry and the view of the sky through the windows.

• Luminous flux: the amount of visible light energy emitted, measured in lumens.

• Luminous efficacy: the ratio of light intensity emitted by a lamp to its power consumption.

  A 100 W bulb giving out 1200 lumen of light has a luminous efficacy of 1200/100, ie 12. The higher the value the more efficient the lighting. The luminous efficacy of daylight is approximately 100 lm/W, which is superior to most forms of artificial lighting, although m odern tubular fluorescent lamps are often over 80 lm/W.

• Prismatic glazing: glazing that changes the direction of light passing through it so that it can be diverted, for example to the ceiling.

Daylighting

When solar radiation strikes glass, part of it is reflected, part absorbed, and the remainder (about 80% depending on glass type and thickness) can pass through. Multiple sheets will reduce light levels entering a building: a triple-glazed window may only allow 56% of incident radiation through. As windows are often the thermal weak point in a building envelope, the loss of light has to be balanced with the improved thermal insulation properties of multiple panes. Various coatings are available to affect the amount of light allowed through the glass.

The following factors must be considered to maximise daylighting:

• Predominant local daylight conditions: some areas have a high proportion of clear skies and beam radiation, while others will have much higher levels of cloud cover and therefore more diffuse radiation

• The surroundings of the proposed building location: nearby buildings or topography can cause shading

• The orientation of the building: this directly affects the amount of daylight reaching the building.

Simulation software can calculate the available light levels on each face of the building. Daylight factors can then be calculated for each room to give an indication of their level of illumination. Daylighting distribution and uniformity for particular rooms is also calculated to determine the minimum window height to give good levels of lighting deep into the room. Reflecting and refracting systems can improve light penetration.

The level of discomfort glare from an electric lighting installation can be evaluated by its glare index. This can be calculated from the size and position of the luminance relative to the background luminances. There are recommended maximum values for certain applications. Some measures to reduce glare are:

• Preventing direct sunlight from reaching occupants by using prismatic glazing and other redirecting techniques

• Brightening the surfaces around windows to avoid uncomfortable contrasts between a bright window and its dark surroundings

• Reducing the reflectiveness of surfaces within the room.

Windows and performance

Windows perform various functions, and their optimum size, positioning and manufacture should reflect the priorities for a specific project. If, for example, ventilation is the primary role, a window should be positioned high up where the warm air accumulates, but if good external views are required a low window position is preferable. The British Standard BS 8206: Part 2: 1992 Lighting for Buildings Part 2 Code of Practice for Daylightings recommends a minimum amount of window area, to provide a view, for a given room depth. This ranges from 20% of the wall area for rooms less than 8 m deep, to 35% for rooms more than 14 m deep. The best position for the windows depends on the size and shape of the room, the type of view desired, and the mobility of the

occupants, but as a general rule the top of the window should be above standing eye level of 1.8 m.The various factors affecting window design are:

• Area: the total fenestration area of a building always represents a compromise: high areas can lead to problems of thermal comfort and glare, whereas low areas may result in poor natural lighting levels, especially if the daylighting potential is reduced due to overcast skies or shadowing from adjacent buildings. For a given window area, however, a larger number of smaller windows can create a better distribution of light than fewer but larger windows

• Shape: can have major effects on the light distribution in a room. Wide windows, for example, produce little difference in light distribution during the day (if facing south), with little glare, and give a panoramic view of the outside. High, narrow windows will produce a band of light that will traverse the room during the day, giving highly variable levels of lighting. The lighting will penetrate further into the building, but may produce more glare. They will limit horizontal views but allow a greater depth of view, encompassing foreground, middle-and long-distance views.

• The position within a wall: high windows are good for light distribution away from the window, whereas low windows provide a better view. Windows will produce less glare if in the corner of a room, but better light penetration if in the centre of a wall

• Orientation: affects the levels of light at different times of the day. South-facing windows will deliver the most light and give medium variation throughout the day. Energy gains in the winter (when they are most required) will be high, and medium in summer. East/west facing windows will give high variation through the day, with low energy gains in the winter and high gains in summer. North-facing windows will give low but constant levels of lighting throughout the day, with low energy gains.

Windows are passive devices, so a control mechanism of some type is often desirable to limit the excesses of lighting and heat gain, either fixed (light shelves over windows), directly controllable by the user (blinds) or operated automatically (louvres adjusted by intelligent control systems).

Windows are not the only form of fenestration: internal and external atria can be very effective lighting the centre of buildings.

Artificial lighting

Although ideally most work would be conducted by daylight, this is of course not feasible, particularly in countries in northerly latitudes like the UK. Different activities require different levels of illuminance (as shown in the table, left), provided by artificial lighting for at least part of the day.

The general principles for daylighting (uniform and even lighting with no glare) apply equally to artificial lighting, but the onus on energy-efficiency makes the design of lighting systems particularly important.

Choice of lamps and luminaires

A luminaire comprises the mechanical, optical and electrical apparatus for artificial lighting. Its chief functions are:

• Mechanical: support for the lamp casings and electrical infrastructure, protection of the lamps and provision of mounting facilities

• Optical: spatial distribution of the light, directional focus of the lighting, lighting colour, control of glare, luminaire efficiency

• Electrical: to provide the required voltage and current for the lighting, while ensuring electrical safety, provision of dimming.

Light control from luminaires can be by:

• Obstruction: enclosing the lamp with only a limited opening to allow the light through

• Diffusion: enclosing the lamp in a translucent material

• Reflection: making use of materials, ranging from highly polished reflectors to very matt surfaces, which allow light to be collected and redirected as required

• Refraction: using a prism material, generally plastic, to bend the light and direct it.

The incandescent lamp, or light bulb, is probably the most commonly used source of artificial light. A current is passed though a tungsten wire, heating it up to approximately 2700 K, and making it glow. It is cheap but because the filament has to be heated to such a high temperature, it has a low luminous efficacy of only 12 lm/W. Lifetime is only 1000 hours due to degradation of the filament, which also causes blackening of the bulb and reduces light output through its lifetime. Reflectors can be built into the bulb to achieve a more directional light output.

Tungsten halogen lamps are similar in design but have a bromine compound added to the gas in the bulb which allows them to operate at a higher gas pressure and temperature and to avoid blackening. The luminous efficacy is increased to between 16 and 18 lm/W and the average lifetime to between 1000 and 2000 hours. Their smaller size allows for greater freedom in luminaire design, making them ideal for narrow-beam spotlights.

Tubular fluorescent lamps operate by passing a current through a gas containing a mercury compound. The mercury atoms become excited and emit radiation, which is turned into visible light by a coating on the tube. As they operate at low temperatures they have a much higher luminous efficacy of between 60 and 96 lm/W and a much longer lifetime of up to 12 000 hours. They are suitable for working environments as they tend to be large and do not create strong shadows. Compact versions are now available that can be used in place of incandescent lamps, but their compact shape means their luminous efficacy is reduced to 40-80 lm/W, and their life span shortened to 6000-10 000 hours. Fluorescent lamps have a higher capital cost than incandescent ones, but are cheaper when life-cycle costing is considered.

• Energy efficient controls

  Case studies have shown that well-applied lighting control systems can reduce the use of electric lighting by 30-40%. Control strategies can be broken down into four basic forms:

• Manual: occupants will initially turn on lighting according to daylight levels, but will often turn on all the lighting and not turn it off again until everyone has left. This can be the best strategy for small rooms with only one or two occupants, but for larger more occupied spaces energy can be wasted as occupants tend to leave the lights on all day irrespective of improved natural lighting.

• Timed systems: lights can for example be turned off automatically at set times. Occupants can then switch them on again, but if natural lighting levels have improved they rarely do so.

• Photoelectric switching: in its simplest form, lights are switched on or off according to daylight levels. This may result in continual switching on and off if natural lighting levels are oscillating, due to partly cloudy conditions for example. Energy savings are not substantial unless the system also monitors room occupancy, or gives some level of manual control. These systems are better suited for well-lit perimeter areas where the lights are normally off during the day.

• Photoelectric dimming: these systems also monitor natural lighting conditions, but use this information to alter electric lighting levels to provide constant illuminance. They save more energy than the switch on/off system and are less obtrusive, but are expensive and cannot work with standard fluorescent lighting.

Statutory controls

Application of part L of the Building Regulations is limited to new commercial installations of over 100 m2. The requirements are that lighting installations:

• Use no more power than ‘reasonable in the circumstances’. The document describes two satisfactory scenarios: either 95% of installed lighting incorporates high-efficiency lamps or, alternatively, the light efficacy of all the lights is at least 50 lm/W.

• Make reasonable provisions for their control, via local switching together with other controls (time-switching or photo-electric switching) where appropriate.

© Mónicia Grinfeld and Ryan Southall 1999

Bibliography

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Baker, N, Fanchiotti, A, Steemers, K Daylighting in Architecture – A European Reference Book, (1993), James & James, ISBN 1 873936 21 4

‘Biceps Module, Lighting and Energy Efficiency’, BRECSU

Littlefair, P J, Designing with Innovative Daylighting, (1996) BRE

Thermie Report, Commission of the European Communities Directorate-General for Energy. ‘Energy Efficient Lighting in Buildings’, (1992)’ BRECSU