Rooflights

• Mirrors can be used in a rooflighting system to direct sunlight into large spaces, such as atria.

• Diffuse light can be reflected to improve daylight factors by 1% or more, compared to a normal, northlight lit room.

• Northlights with a 60 degree overhang and internal silvering have produced uniform lighting conditions with typical daylight factors of 3.5%.

• Micro-louvres can be mounted in rooflights to receive only north light and reject south light and the associated solar gains.

9.5.3 Light Shelves (see fig. 9.2.2)

Light shelves are among the simpler forms of innovative daylighting systems. They do not move and are relatively simple to design and install. They come in a variety forms:

• Internal - External

• Tilted - Horizontal

• Diffuse - Mirrored (specular)

• Solid - Semi-Transparent

Light shelves are not so suitable for use in Northern Europe, as they do not reduce low level sun admittance and hence still give rise to glare problems whilst obstructiong daylight and reducing the overall daylight factor. They do not increase core illuminances significantly, although they do improve uniformity by reducing the light levels near to windows. Their main application is for side windows, not rooflighting.

The most successful light shelves:

• are external;

• cause minimal obstruction to window area;

• have a specular reflective surface;

• are used with reflective, light coloured ceilings;

• are used in rooms with ceilings greater than 3 metres high;

• should be kept clean and in good condition.

9.5.4 Prismatic Glazing (see figs. 9.2.2 and 9.2.5)

This relatively recent development uses the principles of refraction, rather than reflection, to change the direction of sunlight coming through a window. It can be useful for avoiding the use of light shelves whilst illuminating the ceiling and improving daylight uniformity.

The main forms of prismatic glazing are:

• Sunlight-directing triangular prisms;

• Sunlight-excluding triangular prisms;

• Lens systems (diverging);

• Rectangular or triangular prisms mounted in louvres.

The glazing is most effective in sunlight and as a high-level glazing, such as clerestories, as it restricts views out.

Prismatic glazing will not be effective at all times of the year, unless adjustable prismatic louvres are used - which are expensive to install and maintain. It will, however, always stop direct sunlight, provided that it is installed at the correct angle.

9.5.5 Holographic Films

Holographic films are similar to micro-prisms. The prisms are etched onto plastic and are considerably cheaper than prismatic glazing.

Sunlight that is at an altitude greater than 28 degrees is reflected within the film and then to the outside, see fig. 9.2.5. Low level winter sun can still be admitted into the space, where it is angled up towards the ceiling.

Holographic films create interesting patterns as the light passes through them, which may be distracting to people in a sports hall. However, they are cheap and effective at controlling direct sunlight.

The glare factor, as with all innovative systems, still needs to be addressed in sports halls, as they introduce natural light. This is brighter than any artificial illumination in the hall and, in the case of direct sunlight, will be intolerable if viewed directly. Innovative systems might be successful if carefully considered and integrated with other means of light control.

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9.4 Solar Control Glazing

Low Emissivity

The thermal transmission of glass can be altered by the addition of an extremely thin metallic film on the surface of one or more of the window panes (for double or triple glazing). This has the effect of reducing the amount of solar energy entering the room and restricts heat loss from the room, by reflecting long wave radiation back into the room.

Some types of ‘low-e’ glass can absorb heat whilst others are heat reflecting. They do not significantly alter the amount (10% reduction) or colour of visible light entering a room and so are most suitable for use in daylighting schemes. Low-e glass is now preferred for the majority of glazing applications as it is more effective thermally and visually.

Tinted

This glass was popular in the 1980s for glazing large office blocks with, for example, brown or mirrored glass. It tends to significantly change the colour of the external view and restrict daylighting levels.

9.5 Innovative Glazing and Control Devices

Innovative systems have been developed to improve the distribution of light within buildings. Most of these are for specialist applications, due to their design and costs, whereas some may be suitable for use in sports hall design. Three aspects have been identified that designers of innovative daylighting systems should consider:⁷⁷

• Can the system increase lighting levels at the back of the room (for side lit rooms)?

• Can the system improve uniformity?

• Can the system control direct sunlight?

Tests on three of the systems discussed below; prismatic glass, mirrored louvres and holographic film, showed that there was no significant increase in light levels. However, the prismatic glass and holographic film did increase the illuminance levels at the back of the room in sunny conditions. All systems considered improved the uniformity of lighting.

Some of the systems discussed are not particularly suitable for the UK climate, as they rely on sunlight rather than daylighting. The designer should therefore be able to judge whether or not the choice of an innovative daylighting system is going to work, or be worth using in a sports hall.

9.5.1 Heliostats and Light Pipes (see fig. 9.2.2)

Heliostats are mirror systems which track the sun by pivoting on one or two axes. They are complex to control correctly, and require direct sunlight. The heliostat is used to collect the sunlight and feed it into a light pipe. This is a mirrored shaft or optic fibre, used to collumnate and channel the sunlight inside a building.

Heliostats suffer from several technical problems:

• One 8m² heliostat is needed to light 1000 m² of floor;

• Transmission efficiencies of only 20%;

• Dirt reduces effectiveness;

• Tracking motors need to be maintained and calibrated.

Manchester airport, despite not receiving constant levels of sunshine, has used heliostats to light decorative chandeliers in the departure lounge. In this case it is not absolutely necessary for constant light levels, as the chandeliers are not strictly functional.⁷⁸ The costs have to be carefully considered in relation to the energy savings and the architectural objectives.

Light pipes are more reliable than heliostats, mainly because they do not have moving parts. They need to be kept clean to allow maximum light transmission.

There are now light pipes available where diffuse skylight is gathered rather than sunlight. They consist of a perspex dome, a silvered shaft and a ceiling diffuser. Light pipes such as these are more suitable to the UK climate and can be used to provide daylight in rooms where windows are not normally suitable, such as internal corridors and stores. They are relatively expensive and there will still need to be an artificial light for night time use.

9.5.2 Mirror Systems (see fig. 9.2.2)

To avoid the complications of heliostats, simple mirrors can be used locally, such as outside windows. There are four main categories:

Fixed Louvres (see fig. 9.2.4)

• Maintenance can be difficult;

• Internal louvres are more reliable and easier to maintain;

• Horizontal louvres are best for south facing windows;

• East and west facing windows can have diagonal louvres.

Movable Louvres

• Provide better year-round lighting;

• Can be in the form of silvered Venetian blinds;

• Simpler tracking mechanisms possible.

Reflective Sills

• Glare can be a problem;

• Tilting sills can solve glare problems;

• Diffuse and spectral lighting is possible;

• Diffuse provides a more even spread of light.